X-Planes from the X-1 to the X-60: An Illustrated History 9783030863975, 9783030863982

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Michael H. Gorn and Giuseppe De Chiara

X-Planes

from the X-1 to the X-60 An Illustrated History

X-Planes from the X-1 to the X-60 An Illustrated History

Michael H. Gorn and Giuseppe De Chiara

X-Planes from the X-1 to the X-60 An Illustrated History

Michael H. Gorn Thousand Oaks, CA, USA

Giuseppe De Chiara Caserta, Italy

SPRINGER-PRAXIS BOOKS IN SPACE EXPLORATION Springer Praxis Books ISSN 2731-5401     ISSN 2731-541X (electronic) Space Exploration ISBN 978-3-030-86397-5    ISBN 978-3-030-86398-2 (eBook) https://doi.org/10.1007/978-3-030-86398-2 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Cover design: Jim Wilkie Project Editor: Michael D. Shayler Cover Image Credits: (Top) The X-15A-3 rocket plane flies over Edwards Air Force Base during a mission in the 1960s. This aircraft subsequently crashed in 1967, killing pilot Major Michael J. Adams. According to a NASA report, the X-15A-3 crashed “due to a stable, albeit non-robust adaptive controller.” (Archive photo courtesy of NASA). (Bottom) The X-29 at high Angle of Attack (AoA), with smoke generators. (NASA Dryden Flight Research Center photo collection, image EC-91-491-07, 1991) This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Contents

Dedication �����������������������������������������������������������������������������������������������������������������������������������������������������  vii Foreword ������������������������������������������������������������������������������������������������������������������������������������������������������  viii Preface������������������������������������������������������������������������������������������������������������������������������������������������������������  xi Acknowledgements���������������������������������������������������������������������������������������������������������������������������������������   xii Editorial Note�����������������������������������������������������������������������������������������������������������������������������������������������  xiv Prologue: A long and winding road ����������������������������������������������������������������������������������������������������������������  xvi Part I X-Planes in the Cold War, 1945−1990 1 High-Speed and High-Altitude Flight: The First X-Planes�����������������������������������������������������������������������������������   2 The X-1s���������������������������������������������������������������������������������������������������������������������������������������������������������������������   2 X-1A���������������������������������������������������������������������������������������������������������������������������������������������������������������������������   8 X-1B���������������������������������������������������������������������������������������������������������������������������������������������������������������������������  11 X-1C���������������������������������������������������������������������������������������������������������������������������������������������������������������������������  12 X-1D���������������������������������������������������������������������������������������������������������������������������������������������������������������������������  12 X-1E���������������������������������������������������������������������������������������������������������������������������������������������������������������������������  15 X-2 �����������������������������������������������������������������������������������������������������������������������������������������������������������������������������  16 X-3 �����������������������������������������������������������������������������������������������������������������������������������������������������������������������������  18 X-7 �����������������������������������������������������������������������������������������������������������������������������������������������������������������������������  20 X-9 �����������������������������������������������������������������������������������������������������������������������������������������������������������������������������  22 X-10 ���������������������������������������������������������������������������������������������������������������������������������������������������������������������������  24 2 Specific Improvements: Technology Demonstrators���������������������������������������������������������������������������������������������  27 X-4 �����������������������������������������������������������������������������������������������������������������������������������������������������������������������������  27 X-5 �����������������������������������������������������������������������������������������������������������������������������������������������������������������������������  29 X-6 �����������������������������������������������������������������������������������������������������������������������������������������������������������������������������  32 X-13 ���������������������������������������������������������������������������������������������������������������������������������������������������������������������������  34 X-14 ���������������������������������������������������������������������������������������������������������������������������������������������������������������������������  36 X-16 ���������������������������������������������������������������������������������������������������������������������������������������������������������������������������  38 X-18 ���������������������������������������������������������������������������������������������������������������������������������������������������������������������������  40 X-19 ���������������������������������������������������������������������������������������������������������������������������������������������������������������������������  42

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Contents X-21A�������������������������������������������������������������������������������������������������������������������������������������������������������������������������  44 X-22A�������������������������������������������������������������������������������������������������������������������������������������������������������������������������  46 X-25 ���������������������������������������������������������������������������������������������������������������������������������������������������������������������������  48 X-26 ���������������������������������������������������������������������������������������������������������������������������������������������������������������������������  48 X-27 ���������������������������������������������������������������������������������������������������������������������������������������������������������������������������  52 X-28 ���������������������������������������������������������������������������������������������������������������������������������������������������������������������������  54 X-29 ���������������������������������������������������������������������������������������������������������������������������������������������������������������������������  55

3 Prelude to the High Frontier: Early Space Vehicles ���������������������������������������������������������������������������������������������  59 X-8 �����������������������������������������������������������������������������������������������������������������������������������������������������������������������������  59 X-11 ���������������������������������������������������������������������������������������������������������������������������������������������������������������������������  61 X-12 ���������������������������������������������������������������������������������������������������������������������������������������������������������������������������  63 X-15 ���������������������������������������������������������������������������������������������������������������������������������������������������������������������������  64 X-17 ���������������������������������������������������������������������������������������������������������������������������������������������������������������������������  75 X-20 Dyna-Soar ���������������������������������������������������������������������������������������������������������������������������������������������������������  78 X-23A�������������������������������������������������������������������������������������������������������������������������������������������������������������������������  84 X-24A, B, C ���������������������������������������������������������������������������������������������������������������������������������������������������������������  87 Part II X-Planes Since the Cold War, 1990−2021 4 Flight Testing for Combat: Military Vehicles���������������������������������������������������������������������������������������������������������  96 X-31A�������������������������������������������������������������������������������������������������������������������������������������������������������������������������  96 X-32 and X-35 �����������������������������������������������������������������������������������������������������������������������������������������������������������  98 X-44A������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 104 X-45A������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 106 X-46 (X-45N)������������������������������������������������������������������������������������������������������������������������������������������������������������� 107 X-47 ��������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 109 X-49 ��������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 111 X-50A������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 113 X-51A������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 115 X-55A������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 116 X-56A������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 118 X-60A������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 119 5 Aerodynamics and Engines: NASA’s Research Agenda��������������������������������������������������������������������������������������� 121 X-36 ��������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 121 X-43A������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 122 X-48B and C��������������������������������������������������������������������������������������������������������������������������������������������������������������� 125 X-53 ��������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 127 X-54A������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 128 X-57 ��������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 130 X-59 ��������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 132 6 Beyond the Horizon: Access to Space��������������������������������������������������������������������������������������������������������������������� 135 X-30 ��������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 135 X-33 ��������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 138 X-34 ��������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 141 X-37A and B��������������������������������������������������������������������������������������������������������������������������������������������������������������� 143 X-38 ��������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 148 X-40A������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 151 About the Authors�������������������������������������������������������������������������������������������������������������������������������������������������������������  154 Bibliography ��������������������������������������������������������������������������������������������������������������������������������������������������������������������� 155 Index�����������������������������������������������������������������������������������������������������������������������������������������������������������������������������������  159

To Christine, with Love and Admiration Michael H. Gorn To the women of my life: my wife Annamaria and my daughter Nicole, for their love and understanding. Giuseppe De Chiara

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Foreword

Everyone thinks they know the story. After all, they saw it in that movie that they can’t quite remember the name of about daring fliers who “pushed the envelope” of aircraft and space capsules and flew higher and faster than ever before. They remember the orange bullet-shaped airplane and the gallant Air Force pilot who flew it faster than the speed of sound. He had a fractured rib and his friend had to saw off a broom handle to use as a lever to close the aircraft door. He nonchalantly asked for a stick of Beeman’s chewing gum as he got into the cockpit to break the “sound barrier,” as if it was actually a barrier at all. It was all so exciting, and romantic, and nostalgic. And some of what we remember is even true. Of course, in the fall of 1947 humans first flew faster than the speed of sound, approximately 767 mph in the X-1, the first of the X-planes and a major part of this stunning book about the various X-planes by Michael H. Gorn and Giuseppe De Chiara. Air Force captain Chuck Yeager flew that first supersonic flight on November 14 in a specially designed X-1 research aircraft investigating the transonic/supersonic flight realm. That opening sequence of the feature film “The Right Stuff” (1983) conjures an heroic set of emotions, leaving out the hardheaded engineering aspects of the endeavor. Yeager had “The Right Stuff,” sold in the film as the mix of duty, honor, and bravery necessary to dare the unknown. It made us feel good about the process of pushing back the frontiers of flight. It conveyed, however, very little of what actually happened in the context of aeronautical experimentation in the post-World War II era of the X-planes. I wish the film had focused on the engineering accomplishments as much as it did perceived heroism. Regardless, the X-planes built immediately after World War II followed an already impressive period of advancement, and set the stage for a remarkable twenty years in advancing flight capabilities thereafter. During the first century of powered flight, aeronautics fired the world’s imagination with three words: speed, altitude, and distance. The years between 1945 and the middle part of the 1960s were remarkable both for their advances in aeronautical technology and for the development of rocketry and the possibilities of spaceflight. The X-plane research of the era was one major element of this advance, with the NACA—and later NASA—at the center of efforts to fly ever higher, faster, and farther. The X-1 program represented a major success; research with it led to the interceptor built for North American defense by Lockheed, the F-104 “Starfighter.” The original rationale behind the X-planes had been to explore a flight regime that wind tunnels could not simulate. However, by the time the first X-planes flew, researchers had figured out how to extend ground test facilities into this realm. Therefore, the real value of the research X-planes lay in the comparison of the groundbased techniques with actual flight results to validate theories and wind tunnel results. The fact that the first transonic flights showed nothing particularly unexpected—dispelling the myth of a sound barrier—was of great relief to the researchers. Through this effort a difficult transonic zone had been reduced to an ordinary engineering problem. Although few people were thinking about it at the time, the results from these experiments would also be instrumental in developing space vehicles later on. viii

Foreword

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By far the most storied of all of the X-planes are the X-1, which as noted above has become the stuff of legend, and the X-15, the remarkable hypersonic plane that flew for a decade beginning in the latter 1950s. Like a Maserati parked on a street, the X-15 exuded high performance even when it sat idly on the tarmac. Through 199 flights the X-15’s principal rationales included (1) verifying existing theory and wind tunnel techniques about high-speed flight; (2) studying aircraft structures under high (1200 degrees Fahrenheit) heating; (3) investigating stability and control problems in flight and reentry; and (4) learning the biomedical effects of both weightless and high-g flight. It achieved all of those goals and more. The X-15 actually achieved Mach 6.7, an altitude of 354,200 feet, a skin temperature of 1350 degrees Fahrenheit, and dynamic pressures over 2200 pounds per square foot. The X-15 is widely considered the most successful experimental aircraft ever built. Two of the three X-15s— one crashed in 1967 with the loss of the pilot, USAF Maj. Michael J. Adams—are now in museums. The first X-15 is in the National Air and Space Museum in Washington, D.C., and the other is in the United States Air Force Museum in Dayton, Ohio. The program yielded over 765 research reports using data from its 199 flights over almost a decade. For all of this success, I have always found the little things profoundly interesting. NASA research pilot Joe Walker’s first flight was memorable. As the powerful rockets pushed him back into his couch as the X-15 accelerated, he exclaimed, “Oh, my God!” His flight controller jokingly responded, “Yes? You called?” That exchange became famous in NASA. Through 25 flights behind the controls of the X-15, Walker had other famous moments. He reached 4104 mph (6605 km/h) (Mach 5.92) during Flight 59 on June 27, 1962. He also made three X-15 flights into suborbital space, 62 miles (100 km). The first was Flight 90 on July 19, 1963, to 66 miles in altitude (106 km), and the second, Flight 91 on August 22, 1963 at 67 miles (108 km). He then did more, setting an unofficial world altitude record of 354,200 feet, or 67.08 miles, on August 22, 1963. This marked the highest altitude ever flown in the X-15. Throughout Walker remarked that he gained a new appreciation for the world below, and like every other NASA research pilot followed mission requirements meticulously, descending as required, slowing as needed, and setting down on the Muroc Dry Lakebed. He said he always had to overcome the adrenaline rush of the flight even as he voiced thankfulness for returning safely to Earth. Joe Walker’s experiences were repeated many times by many different research pilots flying the various X-planes. The many other X-planes had a dazzling array of purposes. Some were intended to go higher, farther, and faster, of course, and these have garnered the majority of the accolades. The X-43, for example, was an automated test vehicle that in 2004 set hypersonic speed records. But there have been many others. The X-45, X-46, and X-47 vehicles tested drone capabilities. The X-25 explored autogiro technologies. So broad-based and various were the X-planes that they sometimes defy categorization and firm description. Indeed, the earliest X-planes are the easiest to deal with since they were all piloted vehicles that pushed the altitude and speed envelope. We identify with them, and the pilots who flew them. Other X-planes are harder to comprehend and make sense of, although the authors of this book do so admirably. One other X-plane that I want to feature here is the X-33, not because if was successful but because it suggested a path for the future of human spaceflight. It pointed toward full-fledged public/private partnerships to develop technology that had the potential to move beyond the Space Shuttle for space access. NASA and Lockheed-Martin instituted a cooperative agreement to develop the X-33 experimental spaceplane in 1995, known also as the Advanced Technology Demonstrator Program, which had an ambitious timetable to fly by 2001. NASA invested approximately $1 billion while Lockheed contributed approximately half that amount. Once the X-33 technology reached some maturity, Lockheed vowed to scale up the X-33 into a human-rated vehicle, VentureStar™, which could serve as the Space Shuttle’s replacement. As it turned out the program was far more challenging both technologically and politically than originally envisioned. Among the technologies it would demonstrate were reusable composite cryogenic tanks, graphite composite primary structures, metallic thermal protection materials, reusable propulsion systems, autonomous flight control, and advanced electronics and avionics. Given the problems experienced on the X-33 program, with delays of more than a year because of difficulties with critical elements such as the fuel tanks, NASA terminated the program in 2001 before any flight whatsoever. Nonetheless, the NASA/Lockheed Martin partnership was pathbreaking in a fundamental way: before the X-33

x

Foreword

the space industry had rarely expended significant resources on its own for launcher development. The industry contribution to X-33 development amounted to $442 million dollars through 2000. In an era of declining government space R&D budgets the importance of that investment cannot be underestimated, and it seems obvious that although a sizable government role in the development of future launchers would be required, this X-plane program proved that there was ample reason to pursue additional cooperative projects. More important, the X-33 program was an early object lesson in NASA’s efforts to initiate so-called “new ways of doing business.” It used expeditious acquisition procedures, streamlined bureaucracy, limited oversight, and allowed less investment by NASA. Industry became more of a partner than a contractor in this effort, an enormously important lesson for future human spaceflight initiatives in the twenty-first century. It paved the way for the partnerships between NASA and SpaceX, and other corporations, that enabled the replacement of the Space Shuttle as a human launch vehicle. Not all important results from the X-planes programs over the years since the end of World War II have been technological. Most are, of course, as demonstrated by a lot of the projects discussed in this book. But others yielded lessons in other areas. All are examples of innovation and discovery, advancement and progress. Even the failed programs demonstrated significant results that have shaped the trajectory of aerospace. This X-planes book offers excellent descriptions of these programs, and their results. The authors deserve great credit for telling these stories and more importantly relating their significance for a twenty-first century audience. Dr. Roger D. Launius Former NASA Chief Historian August 31, 2021

Preface

For the past 75 years, a period when the U.S. government’s spending priorities fluctuated wildly, funding for a costly program of advanced aerospace research remained relatively constant. X-planes: From the X-1 to the X-60, An Illustrated History asks a straightforward question about that commitment: What has America and the world gained from the long investment in the exotic and expensive X-planes? In order to answer this question, our book surveys the entire period from the X-1, first flown in 1946, to the X-60, a project of the present time. But our aim of full inclusion had its limits. As we were writing and illustrating our book, two new X-planes − the X-61 and the X-62 − came into being. Because they had just gotten underway and the reference material relating to them is scarce, we decided to omit them from our current work and leave them for a future publication. Despite that decision, X-planes is still uniquely comprehensive − but it is also more than that. Its original and highly detailed illustrations show the X-planes in ways not possible even with the best photography: in crosssection, in flight profiles, and in intense close-ups, to name just a few advantages only possible with artwork. Second, we tell the story historically, rather than as a series of discrete technological advances. So while our approach certainly gives full due to technical developments, it also emphasizes such factors as the broader historical trends, the context of the historical moment, and the influence of politics, budgets, inter-agency conflicts, and personalities, among others. In the end, the book attempts something that we hoped to achieve in our recent work, entitled Spacecraft: 100 Iconic Rockets, Shuttles, and Satellites That Put Us in Space. In that work, we presented 100 essays in such a way that each one stood alone, but at the same time we attempted to offer a story that could also be read horizontally, from cover to cover, by linking them through common themes, incidents, and chronologies. X-planes is organized in two sections. The first covers the aircraft and spacecraft (especially the X-1 and X-15) that embody the Cold War struggle between the U.S. and the U.S.S.R. The second consists of a survey of postCold War vehicles, typified less by charismatic, bold designs than by incremental improvements that tested one or two specific technologies. (The X-30 National Aerospace Plane represents several obvious exceptions.) At the same time, we attempted to write and illustrate it with sufficient breadth and depth to appeal to enthusiasts, as well as to professionals in the field. Michael H. Gorn Giuseppe De Chiara May 2021

xi

Acknowledgements

I would like to begin by expressing a huge debt of gratitude to my former mentor at the University of Southern California, the late Professor John A. Schutz, who not only taught his students the discipline and craft of history, but also drummed into them the importance of clear, vigorous, and appealing writing. I hope that he would not be too disappointed with the narrative presented here. My partner in this project, Giuseppe De Chiara of Naples, Italy, contributed the 87 hand-drawn images that illustrate this book. In a real sense, they represent its essence. The briefest examination will inform the reader of his work’s exceptional refinement and fidelity to detail. It has been my privilege to team up with Giuseppe on this, our second collaboration, and to be the beneficiary of his generosity and friendship. Thanks are also due to the exceptional people at the NASA History Office in Washington, D.C. In particular, archivist Colin Fries answered my many reference questions with customary grace and good humor. I also owe a debt to the NASA History Office for the magnificent book series that it has sponsored over the years, many of them helpful to this project. At the other end of the country, at the Armstrong Flight Research Center in California, the always helpful Chief Librarian Karl Bender shared manuscripts, books, articles, newspapers, and other sources in his collection, in addition to materials found in online databases. Two books proved to be bedrock sources for X-planes. Along with more specialized studies (such as Curtis Peebles’ wonderful history of the X-43, entitled Road to Mach 10), Jay Miller’s The X-Planes: X-1 to X-45 provided the backbone for many of the vehicles covered here. It is a lavishly detailed and carefully considered work. Additionally, Dennis Jenkins’ American X-Vehicles: An Inventory, X-1 to X-50 gives concise, capsule descriptions of each of his subjects. (Jenkins also wrote the closely related X-Planes Photo Scrapbook.) Above all else, I would like to thank my wife Christine M. Gorn for enabling the writing of this and other works. For her encouragement and her confidence in me, and her willingness to part with my company (such as it is!) for long periods of time, I can only express deep gratitude, appreciation, and love. Michael Gorn June 2021

xii

Acknowledgements xiii First, I would like to thank our Springer-Praxis team – especially Associate Book Editor Hannah Kaufman, textual editor Mike Shayler, and cover designer Jim Wilkie – without whose support and skill our book would have been so much poorer. In addition, I would like to recognize my collaborator and friend Michael H. Gorn, who provided a true “story” for my artworks. I also want to acknowledge author and friend Davide Sivolella, who introduced us to SpringerPraxis. My parents, Nicoletta Sangiglio and my late father Antonio De Chiara, always encouraged me to pursue my dreams. I am indebted to three mentors: the late Professor Luigi “Gino” Pascale (who also designed the Tecnam P-2006 that became the X-57 Maxwell); the late Professor Rodolfo Monti (former president of the Mars Center); and Professor Francesco Saverio Marulo, my graduate advisor and friend. Finally, I must also mention the late Steve Pace, who introduced me into the world of professional aerospace artworks, and my friend and former colleague Stefano Tempesta for all the times we spent together having technical talks about X-Planes and spacecraft. Last but certainly not least, I want to thank my wife Annamaria Terlati, the love of my life, and my daughter Nicole De Chiara for their continuous support as I worked in my limited spare time to realize the artwork for this book. Giuseppe De Chiara June 2021

Editorial Note

Coverage Although this book covers the entire range of X-planes, it excludes three classified projects: the X-39 Future Aircraft Technology Enhancements Program, the X-41 Maneuvering Reentry Vehicle, and the X-42 Expendable Liquid Propellant Upper Stage Rocket. In addition, Air Force officials − responsible for designating the X-planes − made the conscious decision to leave two numbers blank: the X-52 (skipped to prevent a mix-up with the B-52 bomber); and the X-58 (unfilled to avoid mistaking it for the Kratos XQ-58 unmanned combat drone). Moreover, because this book concentrates on the X-planes themselves, it only treats in passing a number of other high profile experimental projects (such as the D-558 Skystreak and Skyrocket, the pioneering lifting bodies flown by the NACA and NASA, and other prominent aircraft and spacecraft conceived and tested by the military services). The authors invite others to explore these vehicles in the depth they deserve. Institutional Designations Many governmental institutions receive mention in this book, which may cause confusion as their names and roles evolved over time. The following organizations played their parts in the X-planes story. 1. The U.S. Air Force and its antecedents: U.S. Army Air Service U.S. Army Air Corps U.S. Army Air Forces (AAF) U.S. Air Force (USAF)

August 1918−July 1926 July 1926−June 1941 June 1941−September 1947 September 1947 to the present

2. The National Advisory Committee for Aeronautics (NACA) and its successor, the National Aeronautics and Space Administration (NASA): NACA NASA

March 1915−September 1958 October 1958 to the present

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Editorial Note xv 3. The NACA and NASA organizations devoted to flight testing the X-planes on Muroc/Edwards Air Force Base: NACA Muroc Flight Test Unit NACA High-Speed Flight Research Station NACA High-Speed Flight Station NASA High-Speed Flight Station NASA Flight Research Center NASA Dryden Flight Research Center NASA Armstrong Flight Research Center

September 1946−November 1949 November 1949−July 1954 July 1954−September 1958 October 1958−September 1959 September 1959−March 1976 March 1976−March 2014 March 2014 to the present

Prologue

A long and winding road In a plot too strange even for Hollywood, the X-planes originated not in the halls of Congress, nor in the corridors of the Pentagon, nor even at a military air field, but surprisingly, at a parade. This parade occurred in the air, rather than on city streets. It celebrated an aeronautical facility completed after three years of utter misery, characterized not only by relentless heat and humidity, but by the numbing monotony of chopping trees, blasting stumps, and dredging mud. Novelist Tom Wolfe, who worked on the site himself and wrote about it in Look Homeward, Angel, described it as a landscape overflowing with, “the muddiest mud, the weediest weeds, the dustiest dust, and the most ferocious mosquitoes.” The dedication of this place in late spring 1920 attracted a mixed audience of Washington officials, local dignitaries, and air-minded enthusiasts. They saw an impressive display of American air power, as Brigadier General William “Billy” Mitchell, the Army Air Service’s Director of Military Aeronautics, unleashed a mighty armada of 25 combat aircraft that flew in formation over the proceedings. Rear Admiral David Taylor, the Chief Constructor of the Navy, predicted in his remarks that the new complex would become a “shrine to which all visiting aeronautical engineers and scientists will be drawn.” But the onlookers that day saw no shrine, just a set of half-finished buildings separated by muddy pathways. It bore the name of Samuel P.  Langley, the former Secretary of the Smithsonian Institution and a one-time rival of the Wright brothers. This less-than-imposing campus constituted the initial home of an obscure federal agency known as the National Advisory Committee for Aeronautics (NACA). The NACA began in extraordinarily humble circumstances. Its empowering legislation, tucked unobtrusively into the Naval Appropriations Act of 1915 as a twoparagraph rider, anticipated the need for federally-­sponsored aeronautical research as World War I approached. But it authorized a skeletal budget of only $5,000. At first, federal parsimony did not matter, because the NACA took its cue from a low-budget, voluntary organization called the British Advisory Committee for Aeronautics, founded in 1909 to enlist civilian scientists and engineers to counsel the U.K. government about aeronautical developments. Like its model, the NACA appointed unpaid subject area experts to panels corresponding to their technical specialties (aerodynamics, materials, propulsion, and so on). Also like its British counterpart, the NACA assumed the role of enlightening politicians and other policy-makers about advances in the field. It differed, however, in that the NACA had no experimental facility (like the Royal Aircraft Factory at Farnborough) for actual research − that is, until the completion of the Langley Memorial Aeronautical Laboratory in Hampton, Virginia, the site of Billy Mitchell’s aerial parade. In issuing the NACA’s birth certificate, Congress gave it an unambiguous mission: to concentrate not on theoretical work, but instead “to supervise and direct the scientific study of the problems of flight, with a view to their xvi

Prologue xvii practical solution, and to determine the problems which should be experimentally attacked, and to discuss their solution and their application to practical questions.” (Author’s italics.) Under this charter, a cadre of youthful engineers began to arrive at Langley, where they launched a series of aerodynamics studies of great concern to aircraft designers of the time, concentrating on such areas as the effects of air pressure d­ istribution on flight surfaces, the problem of stability and control, and the efficacy of encasing engines in cowlings. The laboratory staff sought answers to these and other questions through two means: by utilizing cast-off military aircraft for actual flight tests, and by analyzing data obtained from scale models mounted in the many wind tunnels constructed at Langley from the 1920s to the 1950s. The results − disseminated worldwide through the NACA’s series of technical reports − burnished the national and international reputation of the fledgling organization, which became well known and respected long before America’s entry into World War II. Congress recognized the NACA’s value by appropriating funds for four new NACA facilities, beginning in 1940 with the Ames Aeronautical Laboratory in Sunnyvale, California; followed by the Aircraft Engine Research Laboratory in Cleveland, Ohio, in 1942; the Pilotless Aircraft Research Station in Wallops Island, Virginia, in 1945; and the NACA Muroc Flight Test Unit in California’s western Mojave Desert, in 1946. Although the NACA made many notable discoveries in aerodynamics and aeronautics during its first 20 years, one problem − perhaps the most tantalizing of all − eluded researchers. It involved a vexing phenomenon called compressibility, which first manifested itself during the 1920s. As increasingly powerful engines yielded higher and higher aircraft speeds, pilots experienced the consequences of compressibility: turbulence and buffeting at the propeller blade tips as they rotated supersonically. Two scientists at the National Bureau of Standards (NBS) decided to investigate. Drs. Hugh L. Dryden (the chief of the Bureau’s Aerodynamics Section and later the director of the NACA), and Lyman J. Briggs (the future director of the NBS), devised a clever research plan. In the absence of wind tunnels capable of generating supersonic air speeds, they relied instead on large centrifugal compressors at the General Electric plant at Lynn, Massachusetts. In a seminal 1925 NACA technical report entitled Characteristics of Airfoils at High Speeds, they published some of the earliest experimental observations of aerodynamic drag during transonic flight (just below, at, and just over the speed of sound). They also explored the effects of compressibility on aerodynamic lift and drag, and recommended design modifications for propeller manufacturers that reduced turbulence. But the findings of Dryden and Briggs constituted merely the opening salvo in a long campaign to understand the fundamentals of transonics. Three years after their paper, a young Massachusetts Institute of Technology (M.I.T.) engineering graduate named John Stack arrived at the Langley Laboratory as a novice aerodynamicist, and he soon gained a reputation for impatience and brilliance in equal measure. Stack worked for an experienced and respected NACA researcher named Eastman Jacobs, the section chief of the Variable Density Wind Tunnel group. The two men joined forces during the late 1920s and early 1930s to probe the characteristics of transonic flight, at first using an 11-inch (28 cm) high-speed tunnel, which they paired with Schlieren photography in order to visualize airflow. But when they transferred their project to Langley’s bigger tunnels, they encountered a seemingly insurmountable obstacle. There, they discovered a frustrating phenomenon: when technicians raised the air speed over Mach 0.7, the air that flowed around the test subjects became “choked” in the throat of the tunnels. It seemed that as shock waves ricocheted off the models, they collided with the tunnel walls and rebounded back onto the subscale aircraft, making it impossible to collect any reliable data. Unable to surmount the problem at this juncture (later successfully achieved by other Langley engineers who placed slots in the throats of tunnel test sections), the hard-driving Stack wanted answers immediately, not later. So in 1933 and 1934 he took a bold step. Stack sketched out and published plans for a piloted, full-scale research aircraft capable of flying up to and through Mach 1 − a costly and daring project under the best of circumstances. This ambitious answer to the choking problem waited in the shadows until World War II. Then, in 1942, Stack presented his concept for a high-speed test vehicle to his boss, George W. Lewis, the courtly, canny, and experienced director of the NACA. Lewis authorized Stack’s research, but nominally, as a low profile, “back-of-theenvelope” job that consumed few resources. Stack accepted Lewis’s restrictions gratefully, knowing that Langley often pursued new, high-risk ventures “off the books.” Stack also realized that with all of the war work flooding into its hangars at this time, the laboratory lacked the wherewithal for a full-blown assault on Mach 1. As Stack assembled a team to consider this unprecedented aircraft, the Army Air Forces (AAF) got wind of it. Around the same time, American military intelligence discovered that German scientists planned to advance

xviii Prologue

beyond the recent turbojet revolution (embodied by the Messerschmitt Me 262) and eventually deploy combat aircraft powered by rocketry (which the Soviets accomplished with the Bereznjak-Isajev B1). This alarming prospect prompted General Frank Carroll, the Chief of the AAF’s Engineering Division at Wright Field, to consult one of the world’s leading authorities on aeronautics and missiles, Caltech’s Hungarian-born professor Theodore von Kármán, to determine the technical feasibility of supersonic flight. After mulling over the question one weekend in early 1943, Kármán told Carroll that, in his estimation, the technology existed to fly aircraft at speeds up to 1,000 miles (1,609 kilometers) per hour. From this point forward, the NACA and the AAF formed an alliance – often a testy and contentious one − to build and fly an aircraft that could exceed Mach 1. Tensions between the two rose and fell. They ran high during the negotiations to establish the partnership, levelled off during the period of aircraft fabrication, and increased again later on during the flight research phase. The friction stemmed from the contrasting interests of the NACA and the AAF, typified respectively by in-depth research versus practical application. John Stack and his compressibility group wanted to delve deeply into the underlying aerodynamics of the transonic region. Carroll and his able civilian project manager Ezra Kotcher sought a more direct outcome; namely a prototype that demonstrated supersonic flight, leading to a combat version that manufacturers could reproduce on an assembly line. As the project gained urgency under the threat of German advances, NACA director George Lewis responded by opening an expanded Compressibility Research Division at Langley, with Stack as its leader. With this new status, Stack and his subordinate Robert Gilruth launched a series of daring flights to gather preliminary transonic data. During these missions, NACA pilots put powerful P-51 fighters − equipped with miniature airfoils mounted vertically on the aircraft’s real wings − into plunging, hair-raising dives. These test fixtures, wired with highly sensitive instruments, measured and recorded transonic and supersonic forces up to Mach 1.4. Meanwhile, the AAF pursued its own objectives. Impressed by the work of Bell Aircraft in producing the XP-59A Airacomet (America’s first jet-powered aircraft, which the company finished in just one year), the Army Air Forces naturally favored Bell as a partner in the supersonic challenge. But before the military got too far ahead of itself, all concerned parties gathered at Langley in May 1944 at the first meeting of the Research Airplane Program Committee, a group that eventually became the primary incubator of federally-sponsored experimental flight. Here, the NACA and the AAF arrived at a compromise that, paradoxically, unified the project by enabling each side to pursue its individual aims. It gave Stack and his researchers permission to contract separately with industry for a transonic turbojet aircraft (funded by the Navy, a frequent collaborator with the NACA); and it gave Kotcher and his team the authority to pursue a rocket-propelled demonstrator capable of flying at and over supersonic speed. By early 1945, it became clear that the path laid out the previous summer at Langley had gained traction. Kotcher and his associates at Wright Field signed an agreement with Bell to fabricate a research aircraft for the Army (the X-1), and the Navy Bureau of Aeronautics awarded a contract to Douglas Aircraft for Stack’s transonic vehicle (the D-558 Skystreak). Despite the competitive feelings that continued to exist, the Solomonic decision of the Research Aircraft Program Committee to divide the project prompted a more cooperative atmosphere among the Army, the Navy, and the NACA. As a sign of the increasing − if temporary − good will, the NACA retained a full partnership in the activities, despite the hard fact that the military funded almost all of it. True, the NACA’s mandate to probe the limits of practical flight assured it at least some hand in defining the specifications of the new vehicles and in overseeing their test programs, but its role proved to be far wider. Stack and his young lieutenants − not the AAF − produced the fundamental X-1 design that materialized on the factory floor of Bell’s plant in Buffalo, New York. Moreover, as fabrication of the Bell and Douglas aircraft progressed, the NACA devised a complicated, roughly 300-pound (136-kilogram) suite of advanced instrumentation that would be installed on the X-1s and D-558s to capture an exhaustive record of their flight behavior. On the military side, despite the intense and historic rivalry between the Army and Navy air services, the Bureau of Aeronautics maintained close contact with Army personnel at Wright Field, and communication flowed freely among Bell, Douglas, and the government’s engineers, pilots, and technicians. In fact, in two stunning examples of cross-service cooperation, Navy representatives (such as the famed rocket designer Captain Robert Truax) shared their on-going XLR11 rocket engine research with the Army, and the AAF reciprocated by transferring its practical experience with air launch technology to Douglas and Navy officials. Although the level of teamwork and sharing certainly improved during this time, the Navy and the AAF still vied to be the first to fly. Stack and his engineers lost some early momentum as they weighed whether to use

Prologue

xix

straight wings on the D-558, or to take a leap and equip it with swept wings, a new design feature developed in Germany and the U.S. almost simultaneously during the war. In the end, they decided to make two different aircraft: the D-558 Skystreak with straight wings and the D-558 Skyrocket with swept. But this decision and its ramifications took time. As a result, Kotcher and the AAF, which envisioned the X-1 solely as a straight-­wing planform, got the lead on the timeline and launched a series of unpowered test flights beginning in early 1946. The first NACA/Navy vehicles took to the skies about 15 months later. The initial trials of the AAF’s experimental airplane took place at Pinecastle Army Air Field, near Orlando, Florida, with extensive NACA participation. A group of bright young NACA engineers, led by the Langley Laboratory’s Walter C. Williams − himself only 26 years old and a favorite of Stack due to their shared ambition and strong will − traveled to Pinecastle to handle the instrumentation of the aircraft and the collection of flight data. Starting on January 25, 1946, the AAF’s bullet-shaped research aircraft embarked on a three-month program of ten unpowered flights, launched from an NB-29 mother ship (see image on page 4) and flown by Bell’s chief test pilot Jack Woolams. Dropped from the bomber at 25,000 feet (7,600 meters), Woolams and his vehicle glided in at speeds up to 400 miles (640 kilometers) per hour. He found the plane a joy to fly and easy to maneuver, but he also discovered some flaws. By the end of the test flights on March 6, it became clear to everyone that the approach and landings had been tricky and risky, resulting in many rough and off-target touchdowns. The reasons hinged on several factors, many unique to Florida: persistent cloudy conditions that inhibited visual tracking; low contrast skies that made it hard for Woolams to pick out the flight line; and the terrain around Pinecastle, with its trees and a varied landscape that offered too many pilot distractions. This realization brought the AAF, the Navy, and NACA officials to a single conclusion: the upcoming rocketpowered flights, with much higher speeds and a much higher chance for accidents, required an entirely different climate and terrain, one already familiar to the Army and to Bell as the site of the flight test program for the P-59A Airacomet. At this distant location, in the high desert of southwestern California, the X-planes would make their debut − and remain rooted there for the next 75 years, even until the present day. Despite their later celebrity, America’s experimental aircraft got their start under circumstances lacking entirely in glamor or publicity. It began when two individuals from the NACA’s Langley Laboratory arrived in the Mojave Desert on September 15, 1946. Both of them volunteered to be a part of a seminal step in the history of flight: the launch of an aircraft, still hidden from the public eye, capable of achieving as yet unattainable speeds. Not surprisingly, the life that these men found in California bore almost no resemblance to what they left behind in Hampton, Virginia. Before they went west in late summer, they soaked in the ambience of the Langley campus, by now well established since its founding in 1920, with its solid brick structures, deep green lawns, and thick stands of oak and pine common to Tidewater Virginia. When they arrived at their destination, they, like many others who followed them, may well have wanted to reverse course and go home. Compared to the land they had just left, the desert looked like stark desolation; a monochrome, barren landscape dotted with meager sprays of sage, juniper, and creosote, not to mention the widely scattered and weirdly shaped Joshua Trees. Most of the people who made the journey took the train across country and disembarked at a dusty, arid railway platform in hardscrabble Barstow, California, where the U.S. Army had trained young men at nearby Fort Irwin for service in World War II. Only a two-hour drive by car to downtown Los Angeles, the new surroundings in which the NACA contingent found themselves seemed as distant to L.A. as the Moon. The contrast in the weather struck the newcomers even more acutely than the scenery. The trip ended at the Muroc Army Air Field (the present Edwards Air Force Base) where, on the very same September 15, the two new recruits encountered a sweltering temperature of 95 degrees F (35 degrees C), inconceivably low humidity (with a dew point of just 37.7 degrees F (3.2 degrees C)), and maximum sustained wind speeds of over 26 miles (41.8 kilometers) per hour. If any of them had called home that night, family members might have told them about the weather at Hampton: 73 degrees F (22.7 degrees C), dew point of 52.5 degrees F (11.4 degrees C), and top winds of 10 miles (16 kilometers) per hour. But one pair of numbers reminded them why they had traveled across the continent. That September 15, the average person in Hampton could see for eight miles (12.8 kilometers); at Muroc, visibility that same day extended out to 26 miles (41.8 kilometers) − more than three times as far, and a critical advantage for safe and effective flight testing. Add to that the unobstructed blazing blue skies from horizon to horizon and a degree of solitude that assured privacy, and the group (reluctantly) saw the sense of it.

xx  Prologue

For all of its good qualities as a place for experimental aeronautics, however, working in the Mojave Desert posed an equal number of liabilities. The migrants who arrived at Muroc discovered the positive and negative almost immediately. They marveled at the shimmering white expanse of the massive Rogers Dry Lake, a hard sea of compacted silt roughly 12.5 miles long and 5.5 miles wide (20 x 8.8 kilometers) − the main geographical feature of Muroc and, not accidentally, suitable for emergency landings. They saw incredibly long runways and witnessed magnificent dawns and sunsets, but they also experienced the daily grind of incessant wind and excessive heat. Indeed, when they bunked down that first evening − perhaps in makeshift barracks in an area of the base called Kerosene Flats, named for the fuel that burned in the space heaters and cook stoves − they probably fell asleep listening to the air whistling through the cracks in their building. When they got up the next morning, they found the proof of their new existence: a layer of desert sand that accumulated on their sheets and blankets. Fresh from the settled town life they left behind in Hampton, they not only lived with these primitive physical conditions, but found no real community, no entertainment, and no places to eat other than at military canteens. In short, they now lived in a place with nothing to do but work. The NACA’s senior leader at Muroc − the restless and wildly ambitious Walter Williams, whom Langley designated as engineer in charge − had two main goals after his arrival on September 30, 1946: getting his team ready for the flight research project that had brought them there in the first place; and, even more urgently, finding off-base housing for a Langley contingent about to expand significantly. “I am in a shock,” Williams told Mel Gough, his boss back in Hampton, about the lack of places to live. So he took to the road repeatedly, driving from Tehachapi in the north to Palmdale in the south and back again looking for apartments or houses for his employees to rent. While the search went on, Williams fought simultaneous bureaucratic wars with base officials for hangar space, offices, and clerical equipment. After much struggle and against the odds, Williams largely succeeded. By 1947, Muroc began to witness a steady influx of NACA employees. At the same time, Bell Aircraft engineers and technicians streamed in from the company’s New York factory, and the Army assigned promising young officers and enlisted men to Muroc from the AAF’s Engineering Division at Wright Field and elsewhere. In the end, three local and three global ingredients coalesced to make the world’s first supersonic flight possible. On Muroc itself, strong Air Force and NACA personalities met and clashed, but still managed to carry out the mission; the desert landscape and climate acted as protagonists in the drama; and the X-1 itself finally arrived from Bell. On the global scale, three factors combined to enable what happened on the base in October 1947: the Cold War rivalry between the U.S. and the U.S.S.R. became inescapable; a budgetary “horn of plenty” opened up to meet the Soviet threat; and the post-war world waited to witness the culmination of technologies first auditioned during World War II. But the outcome in that month and year did not seem inevitable to contemporaries, either before or after the actual event. Because the X-1 and the X-planes that followed existed precisely to challenge the dogmas of flight, their overall record represents a mixed history of successes and failures; of triumphs, but also of crashes, injuries, and deaths. Amid these gains and losses, there has been one constant factor: the hard-won flight data, which continues to be collected, and which informs and animates the X-planes up to the present day.

Part I X-Planes in the Cold War, 1945−1990

1 High-Speed and High-Altitude Flight: The First X-Planes

The X-1s X-1-1, X-1-2, X-1-3 Despite its heroic reputation, the world’s first supersonic flight did not arise in an atmosphere of great and lofty purpose. Instead, it happened in a climate of conflict and controversy.1 The actors in this drama, who came together initially from fall 1946 to summer 1947, included personnel from a small federal agency called the National Advisory Committee for Aeronautics (NACA); a contingent from Bell Aircraft, a relatively obscure aircraft manufacturer from Buffalo, New York; and a cadre of officers and enlisted men drawn mostly from the Army Air Forces (AAF) Engineering Division at Wright Field, Ohio. They gathered at Muroc Army Air Field, a remote AAF base in the western Mojave Desert of Southern California and a former target zone for artillery practice. Each of the three participating institutions had its own motivation for contributing to the X-1: the NACA sought to conduct practical aeronautical research, continuing to pursue its mission since its founding in 1915; Bell wanted to expand its reputation in the industry and to make a solid profit; and the AAF hoped to come away with a Mach 1 prototype that could be adapted for combat. These newcomers to Muroc arrived at a time of high tension on the base. Those already posted there found  Please refer to the Prologue, which covers the origins of the X-planes and the X-1. 1

themselves under heavy pressure to flight test a long queue of post-war combat aircraft, among them the XP-80A, XP-83, XP-84, and XP-86 fighters, and the XB-45 and XB-46 bombers. Added to that challenge, the incoming X-1 workforce stretched the already limited local resources to the maximum, pitting the NACA, Bell Aircraft, and the AAF against one another for housing, office space, and the basic necessities of life and work. Into this hothouse environment came three indispensable but volatile X-1 figures. Walter Williams, a driven young NACA engineer, arrived at Muroc in September 1946 to lead a small team from the organization’s Langley Memorial Laboratory in Hampton, Virginia. His group became known as the NACA Muroc Flight Test Unit. Robert Stanley, a highly capable contractor test pilot, acted as Bell’s primary X-1 representative, and Williams and Stanley locked horns from their first encounter. They not only disagreed about the essential direction of the project − Stanley wanted it completed without delay in order to maximize his company’s gain; Williams, in keeping with the NACA’s tradition, insisted on maximum data derived from carefully instrumented flight tests − they also contested each other personally. Both men exuded supreme confidence and neither tolerated dissent, which set up a classic clash of wills. As if this mixture lacked sufficient explosive power, a short fuse appeared in the person of Captain Charles (Chuck) Yeager, chosen by the AAF to fly the Mach 1 mission. Yeager brought with him the bona fides of a

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 M. H. Gorn, G. De Chiara, X-Planes from the X-1 to the X-60, Springer Praxis Books, https://doi.org/10.1007/978-3-030-86398-2_1

2

 legitimate World War II hero: a recipient of two Silver Stars, veteran of 64 combat missions in Europe, and an ace who defeated 13 enemy aircraft. If all that failed to distinguish him, after being shot down over France he scaled the Pyrenees Mountains, evaded capture, and later returned to combat. Yeager’s temperament complemented his achievements. He showed almost no patience with authority, made no secret of his high admiration for his own flying skills and technical instincts, and held the NACA’s engineers and pilots in low regard. For months on end, the program simmered in this cauldron of mismatched personalities and institutional cross-­purposes. Williams and Yeager often crossed swords about the importance − or lack of it, in Yeager’s view − of the NACA’s instrumentation equipment. Williams insisted that no X-1 be flown without a full recording of the aircraft’s behavior; Yeager became infuriated by the many scheduled tests being delayed only “because some instrument wouldn’t work.” Stanley and Williams also came to rhetorical blows in October 1946, when the newlyarrived Bell leader ordered the first X-1 glide flight at Muroc on a single day’s notice. Williams did more than protest; he asked the military chiefs in Dayton (who paid most of the X-1’s bills) whether they really wanted to risk losing all data on a flight that had no telemetry coverage. The AAF ruled in Williams’ favor. Some of the tension lessened during summer 1947, when the NACA engineers and the military brass agreed to a division of resources and tasks that clarified roles and quelled some of the friction. The accord that they arrived at actually set the standard for many future X-planes projects. It laid out guidelines for the two X-1s then under fabrication by Bell: X-1 number 1 (X-1-1) for the AAF, designed to break Mach 1; and X-1 number 2 (X-1-2), made for the NACA to pursue transonic research. Under the terms of the accord, the NACA agreed to provide the AAF with technical advice on both X-1s as needed, and although the Army Air Forces took the lead on all X-1-1 flights, it conceded that it would do so in coordination with the NACA. Additionally, both sides signed on to flying the X-1-1 with a minimum of two pieces of equipment: the NACA’s direct recording instruments, and its six-channel telemeter. The two parties also committed themselves to outfitting X-1-2 with the full NACA instrument suite. Moreover, Williams and his team won the tasks of installing and servicing the telemetering and data recording machinery on both the X-1-1 and X-1-2, and

The X-1s  3 performing overall maintenance on both of the X-1s during the early testing. Finally, Williams retained the role of resident NACA Engineer-in-Charge; AAF Captain Jack Ridley, a Caltech graduate, assumed a parallel role for the AAF; and Yeager, the pilot chosen to surpass Mach 1, got the added task of checking the airworthiness of X-1-2 prior to the NACA accepting it from Bell. Although these compromises failed to stifle the animosity among the different factions at Muroc entirely, they did at least establish clear rules of engagement. With this new accord in hand, the X-1 itself took center stage. Although its profile sorely disappointed those who expected a long, sleek aircraft to top Mach 1, no one could argue with its straightforward purpose. The X-1-1 rolled out of the Bell plant in Buffalo in December 1945, a month prior to the glide tests at Pinecastle, Florida (see the Prologue for coverage of these flights). Not yet painted in the bright orange hue that would distinguish it from the blue skies and the brown floor of the California desert, it otherwise represented the final form of this historic machine. Its straight, thin wings, mounted at about the midpoint on the fuselage, measured just under 28  feet (8.53 meters) from tip to tip, with a surface area of only 130 square feet (12 square meters). They almost looked unfit for the task of lifting the bulky, barrel-like vehicle into the air. Indeed, the NACA and Bell engineers, uncertain about the stress loads and buffeting that the plane faced in the transonic region, decided to pattern the aircraft on some well-known contours already accustomed to speeds over Mach 1: those of a .50 caliber machine gun bullet, but scaled up to almost 31  feet long (9.42 meters) and nearly 11  feet (3.30 meters) tall. As a second precaution, in order to feel sure that the as yet ill-­defined stresses encountered at and over Mach 1 would not destroy the aircraft or kill the pilot, the NACA instructed Bell to make the structure strong enough to endure 18 times the weight of the X-1 and its contents, a standard more than two and one-half times that required for contemporary fighter aircraft. The X-1 weighed 12,250 pounds (5,557 kilograms) loaded fully, and about 7,000 pounds (3,175 kilograms) empty. Development of the X-1’s propulsion system progressed in parallel with its airframe. It so happened that Reaction Motors had its XLR11 engine under development for the Navy, the first American-made liquid propellant rocket developed just for aircraft. Bell

4  High-Speed and High-Altitude Flight: The First X-Planes

 subcontracted with Reaction for the XLR11. Fueled by ethyl alcohol and liquid oxygen, it produced a combined 6,000 pounds (2,722 kilograms) of thrust from four combustion chambers, none of which could be throttled, but all of which could be turned on or off independently. The Army funded the fabrication of the four XLR11s, which became the mainstay of all of the X-1s. According to the terms of the NACA-AAF agreement, Williams and his team lost the battle to outfit X-1-1 with a full suite of NACA instrumentation, but they did manage to cover the essentials. Using the six-channel telemeter, they could record airspeed, altitude, the elevator, stabilizer and aileron positions, stick-force, and normal acceleration. They also got approval to equip Yeager’s aircraft with four strain gauges, capable of measuring air loads and vibration. With all of these measures in place, the Muroc contingent braced for the main event. During the six months of spring and summer 1947, two individuals flew the X-1-1 towards its ultimate objective. First, Bell test pilot Chalmers “Slick” Goodlin − a World War II pilot who served initially with the Royal Canadian Air Force then later as a Navy test pilot − began a series of nine contractor flights. He tried the aircraft for the first time on April 10, 1947, in a glide familiarization flight. The following day the program crossed an historic threshold, when Goodlin and the X-1-1 dropped from the belly of the B-29 mother ship for the first time and he lit the XLR11 engine. Goodlin made three more powered flights between late April and early May to assess the plane’s handling qualities, and another three in mid- to late May to test the X-1-1’s buffet boundary. Finally, on June 5, he conducted a demonstration flight for the Aviation Writers Association. Captain Chuck Yeager at last got his turn at the controls on August 6, 1947, when he took the X-1-1 for a ride with the XLR11 switched off. Thrilled by its light and responsive performance, Yeager called it “the best damn airplane I ever flew.” During the following two days he notched two more familiarization glide flights, but three weeks later, on August 29, his enthusiasm and self-confidence got the better of him. In his first

The X-1s  5 mission with the XLR11 engaged, Yeager put the rocket plane into a 90-degree climb at Mach 0.85. His elation did not last long. Back on the ground, Walt Williams reminded him − almost assuredly with an acidic tone − that the test plan had called for no more than Mach 0.80, because the technicians could not record any telemetry data above that speed. Having exceeded it, Yeager bristled at being corrected by a group of “college boys” (as he liked to call NACA engineers), but he had no choice but to accede to Williams’ demand that he fly the mission again. This repeat flight, in the Mach 0.80 to 0.89 range, occurred on September 4. After he landed, Yeager’s pent-up frustration boiled over when he discovered that the NACA’s telemetering instruments had failed to collect stability data. Still, this flight had its uses: it seemed to indicate that the X-1-1 operated largely as expected up to Mach 0.89. But missing data again bedeviled the project when Yeager flew the next time, on September 8. After he returned to Edwards, the NACA’s engineers found that the instruments on board the X-1-1 had collected nothing whatsoever. The cause may have been pilot error − Yeager may have forgotten to turn on the system. On this occasion, however, the telemetry did work, and it showed serious problems: heavy buffeting in turns above 2 g and a tendency at Mach 0.88 for the nose to pitch up. Things continued to be worrisome on September 10 and 12, at Mach 0.91 and 0.92, respectively, when Yeager encountered tuck-under tendencies. These unwanted phenomena triggered investigations regarding buffeting, as well as stability and control. Even more concerning during the September flights, along with those on October 3, 8, and 10, elevator control declined sharply beginning at Mach 0.88, just as the aerodynamic shock wave first broke over the wings. It continued to be ineffective as the wave moved rearward and the aircraft accelerated. Only at Mach 0.94 did the elevator function normally. Fortunately, the X-1 design staff at Langley anticipated problems with the elevator, based on its relationship to the X-1-1’s fixed horizontal stabilizer, and they advised the engineers and technicians at Muroc to free the horizontal stabilizer. After that modification, the team felt almost sure that the errant elevator would cease to be a threat to the Mach 1 attempt.

6  High-Speed and High-Altitude Flight: The First X-Planes

 Having satisfied themselves that they now understood the basic transonic idiosyncrasies of the X-1, the combined NACA/Air Force group2 felt reasonably confident of success. They chose the morning of Tuesday, October 14, 1947, to mount the assault on Mach 1. After climbing in the B-29 mothership to about 20,000 feet (about 6,100 meters), Yeager and the X-1-1 fell away from the bomber, at which point he ignited the XLR11. The aircraft roared upwards to 42,000 feet (12,800 meters), where he leveled off at Mach 0.94. At this point Yeager lost full elevator control momentarily, but above Mach 0.96 it became normal again. As he rose to Mach 0.98 he felt a burst of speed, and as the shock wave finally passed over and behind the aircraft, the needle on the Machometer froze, and then disappeared from view. A cable from Muroc Base Commander Signa Gilkey to Wright Field told the story in three lines: XS-1 BROKE MACH NO ONE AT 42,000 FT ALT. FLT CONDITIONS IMPROVED WITH INCREASE OF AIRSPEED. DATA BEING REDUCED AND WILL BE FORWARDED WHEN COMPLETED.3 In the weeks after this historic achievement, Yeager’s luck cooled. He flew six times in rapid succession, but experienced an electrical power outage in one flight and telemetry failures in two others. Only in the seventh did he enter the record books again. On November 6, 1947, he piloted the aircraft to Mach 1.35 and reached a top altitude of 51,434 feet (15,677 meters) − itself an astounding leap over the October 14 triumph. After that, the X-1-1 continued in service through 1948, 1949, and the first half of 1950. It flew 43 more times during that period (14 piloted by Yeager), conducting transonic, supersonic, and high-altitude research. Meantime, as the USAF marched toward Mach 1, the National Advisory Committee for Aeronautics began to realize John Stack’s vision of transonic flight. In July 1947, it took possession of the X-1-2 from Bell, designated specifically for NACA use. The day after Walt Williams’ team received the aircraft, they began to  The Army Air Forces passed out of existence with the National Security Act of 1947. In its place, the U.S. Air Force came into being as an independent military service on September 18, 1947. 3  In the end, Yeager reached Mach 1.06 at about 43,000  feet (13,106 meters). 2

The X-1s  7 swap out the manufacturer’s four instrument panels with ones designed at the NACA’s Langley Laboratory. Aside from this relatively minor modification, the X-1-1 and X-1-2 differed little, except in their airfoil profiles. The Air Force model possessed an eight percent thickness-to-chord ratio; the NACA aircraft incorporated a wider, ten percent cross-section, reflective of Langley and Stack’s concentration on the transonic, rather than the supersonic range of speed. Prior to the X-1-2’s arrival at Muroc, Bell’s Slick Goodlin flew the aircraft a total of 17 times from October 1946 to May 1947 in contractor flights and in trial missions for the Air Force. Then came the crucial airworthiness check on September 25, 1947, with Chuck Yeager in the cockpit (as stipulated in the AAFNACA agreement forged in the summer of that year). Even though the XLR11’s number four chamber burned out during this run, the NACA agreed to accept the X-1-2 from Bell. The NACA began to fly the X-1-2 on October 21, 1947 − just one week after the fall of Mach 1 − when the NACA’s Herbert Hoover flew this and all but one of the first 14 missions up to March 1948 (his colleague Howard Lilly took the controls once in January 1948). Following NACA tradition, the speed of each flight rose by conservative increments, and at each step NACA engineers and technicians used the full array of specialized instrumentation equipment to determine the aircraft’s performance. On his ninth flight on March 4, 1948, Hoover completed the NACA’s first ever supersonic attempt when he reached Mach 1.029. In a stability and loads test two and one-half weeks later, he attained Mach 1.172, the top speed for the X-1-2 in the 1947 to 1948 timeframe. But the work did not end there. The NACA continued to fly the X-1-2 from March 1948 until October 1951, logging 42 more flights that investigated such phenomena as stability and control, pressure distribution, and stabilizer effectiveness, among others. Fresh expectations came into the X-1 program in July 1951, just as the X-1-2 neared the end of its lifespan. During that month, the X-1-3 appeared on the Edwards flight line for the first time. It differed from Chuck Yeager’s X-1-1 and the NACA’s X-1-2 in having greater fuel capacity (due to the addition of a new turbopump system, which replaced the earlier X-1 system of high pressure nitrogen to feed the fuel lines). With this advantage, its proponents hoped that it might fly as fast as Mach 2.4. But at the same time, Air Force leaders, who had authorized the X-1-3  in

8  High-Speed and High-Altitude Flight: The First X-Planes the first place, thought of canceling it outright as the more advanced X-1A, B, C, and D became available. They reconsidered when the NACA offered to flight test it. From the first, the X-1-3 seemed ill-fated. The program began with contractor familiarization on July 20, 1951, with Bell pilot Joe Cannon in the cockpit. Rather ominously, the plane’s nose wheel collapsed on landing. Then disaster struck. On November 9, 1951, during the X-1-3’s second outing, it flew a captive-carry mission with the B-50A mother ship to test the propellant jettison system. Just after the planned activities ended and Cannon turned back to base, the aircraft exploded under the belly of the bomber, possibly the result of a break in the high-pressure nitrogen gas storage system. The pilot suffered serious injuries and the incident destroyed both of the aircraft. During almost six years of combined flight research and 157 total missions, the X-1-1 and X-1-2 earned a place of unmatched prominence among experimental aircraft. They not only proved beyond question that supersonic travel could be achieved, but just as importantly, they enabled designers of civilian and military vehicles alike to apply the underlying data to aircraft of the future. The X-1s also provided a concrete example, and indeed a model, on which future X-planes projects could pattern themselves, drawing both from the technical experiences as well as from the management practices worked out in the California desert. X-1A Rather than follow the X-1 flights with a second, entirely new generation of experimental aircraft, Air Force leaders decided instead to re-fashion the existing one. This choice reflected the realities of the time: sharply reduced post-World War II defense budgets, massive outlays for ballistic missile development, and high expenditure on the Korean War. Responding to these conditions, the USAF set contracts with Bell Aircraft to modify the X-1 for higher speed and altitude, and to construct four models: the X-1A, B, C, and D. Bell delivered the X-1A to Edwards Air Force Base on January 27, 1953. It distinguished itself principally from the X-1 in its greater size and weight: a 36-foot (11-meter) fuselage that measured 5 feet (1.5 meters) longer than the X-1; and, at 14,750 pounds

(6,690 kilograms) fully loaded, about 2,500 pounds (1,134 kilograms) heavier than the earlier aircraft. The X-1A also differed in having greater fuel capacity, an ejection seat, a taller, more conventional cockpit canopy than that of its flat-topped predecessor, turbo-driven fuel pumps (instead of the X-1’s nitrogen under pressure), and an improved XLR11 propulsion system (although still lacking a throttle and still powered by four independently ignited combustion chambers). By the time flight testing on the X-1A got underway in February 1953, the NACA/Air Force formula for collaboration (that had been so hard to work out) lost out as the old, competitive habits resurfaced. A spirit of cooperation did extend to the X-1 flights that immediately followed Yeager’s Mach 1 success, but after that mission, Walt Williams and his team concentrated almost single-mindedly on the NACA’s own high-speed aircraft, initiated during the first meeting of the Research Airplane Program Committee in May 1944 (see the Prologue). The committee empowered the USAF to pursue the X-1 with Bell, while the Navy Bureau of Aeronautics − the NACA’s longtime patron and ally − got the go-ahead to fund two entirely separate experimental aircraft for the NACA. The first of these models, fabricated by Douglas Aircraft to NACA and Navy specifications, represented the fulfillment of John’s Stack’s ideal experimental airplane. Known as the D-558-1 Skystreak, this 35-foot-long (10.66 meters), straight-winged, turbojet-powered vehicle conducted research on the nuances of the transonic regime of flight. That explains its reliance on jet, rather than on rocket power. Thanks to its turbine engine, the Skystreak could stay aloft for extended periods, far longer than the short bursts provided by the XLR11. Moreover, the three D-558-1s actually flew for a significantly longer period − from April 1947 to June 1953 − than either the X-1-1 or X-1-2. In all, they completed 101 missions, but the Skystreak proved to be problematic, especially from Mach 0.75 to Mach 1. Going upwards through that range, buffeting and vibration caused the aircraft to become increasingly difficult to control. Further tarnishing the Skystreak’s reputation, NACA research pilot Howard “Tick” Lilly died in the crash of D-5581-2 on May 3, 1948. A component inside the engine’s compressor section failed, hurling a cascade of debris into the fuselage and bringing down the aircraft and its pilot. Lilly became the first NACA pilot to perish in an experimental flight.

X-1A  9

10  High-Speed and High-Altitude Flight: The First X-Planes It took only a little time after the NACA and Navy accepted the Skystreak’s successor − the Douglas D-558-2 Skyrocket − for a rivalry to develop with the Bell X-1. The Skyrocket differed vastly from the Skystreak. Capable of flying by rocket or turbojet, its designers gave it a sleek, low profile and swept wings, in contrast to the bulkier Skysteak’s straight wings. In addition, it measured 42  feet (13 meters) in length, a full 7  feet (2.1 meters) longer than the Skystreak. Incredibly, its launch weight of nearly 16,000 pounds (7,257 kilograms) roughly doubled that of its predecessor. Like the Skystreak, the D-558-2 suffered from a single, predominant problem, but an entirely different one. The pilots found them prone to pitch-up, especially at high altitudes and at high angles of attack. Despite this serious deficiency, over the course of its career it far outstripped the X-1  in both number of flights (296) and in operational lifespan (February 1948 to August 1955). The paths of the Skyrocket and the X-1 converged in November 1953. Approaching the 50th anniversary of the Wright brothers’ first sustained, powered flight on December 3, 1953, Walt Williams and NACA pilot Scott Crossfield − with the full support of the Navy Bureau of Aeronautics − decided to make a play for the next great aeronautical speed record, in a conscious effort to preempt the Air Force.4 On November 20, Crossfield and the Skyrocket dropped from the bomb bay of the B-29 mother ship, after which he accelerated in a climb to 72,000 feet (21,946 meters). He leveledoff at first, and then, in a shallow dive to about 62,000 feet (18,898 meters), he fired all four chambers of the XLR11 for 45  seconds. Although Crossfield expected to accomplish his mission − as with most test pilots − he admitted to a sense of astonishment when he looked down at his instrument panel and saw the needle on the Machometer holding steady at Mach 2.005. The Mach 2 flight sent a shockwave through the Air Force. Chuck Yeager and the X-1A team, in particular, took the news hard. By the time of the NACA’s triumph, only six X-1A contractor flights had taken place. Bell pilot Jean Ziegler started out with a glide mission on February 14, 1953, and made a second one on February 20 after problems with the plane’s propellant 4  The NACA ordinarily pursued its research in a steady, incremental, deliberate manner, with an aversion to fuss or headlines. But it made a notable exception in the case of Mach 2, fed in part by the long-standing antipathy between Chuck Yeager and Walt Williams.

system prevented him from firing the engine. Starting the next day and ending on April 25, he made four powered flights, which uncovered some problems. On April 10, Ziegler noticed a low frequency buzz from the elevator at Mach 0.93, at which point he stopped accelerating. Despite an investigation, Ziegler encountered the same phenomenon at the same speed on the sixth and last contractor flight. He ended the series with an unrelated incident when the X-1A’s turbopump involuntarily speeded up, prompting him to cut power and drop the plane’s remaining fuel. While engineers and technicians studied these troubles, the X-1A remained out of service for almost seven months − until Scott Crossfield and the NACA made their attempt on Mach 2. On the very next day, November 21, Yeager took the X-1A’s controls for the first time on a familiarization flight that reached Mach 1.15. He returned on December 2 and 8 and tested the aircraft even more strenuously, attaining Mach 1.5 and 1.9, respectively. Then, on December 12, in a bold but reckless effort to take the speed crown back from the NACA, Yeager proceeded despite dire warnings. Project managers − armed with flight data, wind tunnel tests from Langley, and simulations from Bell Aircraft − cautioned him that he must avoid flying over Mach 2.3. They predicted a drastic decline in the aircraft’s directional stability above that point. But Yeager’s supreme confidence guided him in this, as in other decisions, and on the day of the flight he pressed the X-1A beyond its limitations. Arriving at Mach 2.44 as he flew above 75,000  feet (22,860 meters), he discovered the truth of the warnings. In this moment, Yeager came to grips with low dynamic pressure, and as he fought for control he found that the aircraft failed to react normally. Consequently, Yeager tried to compensate when it rolled gently to the left, which brought on a rapid roll to the right. Correcting again, the plane jerked violently to the left. At this point the X-1A ceased to respond. It plummeted towards the desert floor, falling for 51  seconds, plunging about 50,000  feet (15,240 meters), and decelerating from 1,600 to 170  miles per hour. Yeager, meanwhile, got tossed around in the cockpit and cracked his helmet on the glass canopy. Only when the plane went into an inverted spin did his previous experiences enable him to regain command, convert it into a standard upright spin, and reestablish level flight before gliding to a landing. On this day, Yeager cheated death. As if tainted by this catastrophic event, the X-1A underwent more failure than success during its

X-1B  11

remaining career. It flew 14 high-altitude missions during spring and summer 1954, but only three made the record books, with the rest being aborted due to a variety of malfunctions. Air Force Major Arthur Murray flew the aircraft to 87,094 feet (26,546 meters) on May 28, 1954, the highest altitude by any aircraft up to that time. He surpassed that to 89,750  feet (27,356 meters) on June 4 but, like Yeager, he encountered uncontrollable instability that ultimately sent the aircraft tumbling into a 20,000-foot (6,096-meter) freefall before the pilot recovered. On August 26, Murray reached 90,440  feet (27,566 meters). The USAF then transferred the X-1A to the NACA for further testing. The X-1A met its end on its second NACA flight. As pilot Joe Walker prepared his aircraft for the drop from the B-29 mother ship on August 8, 1955, a small internal explosion aboard the X-1A caused its liquid oxygen tank to burst, spraying out a shower of debris. Walker climbed out of the stricken aircraft unhurt and took shelter in the B-29’s bomb bay. The crew considered trying to land with the X-1A, but after assessing its poor condition, ruled that out. Instead, they released it, watched it fall toward the desert floor, and saw it erupt on impact in a ball of orange flame.

X-1B Conceived by the Air Force and Bell Aircraft as a sister ship to the X-1A, the X-1B (as well as the X-1C and D) looked almost indistinguishable, and indeed shared virtually the same dimensions and weight. But as soon as it arrived at the High-Speed Flight Station on Edwards Air Force Base in 1954, the X-1B began a physical transformation. After a series of familiarization flights from September to December of that year, the USAF transferred it to the NACA, whose program managers had in mind a specific set of experiments. They shipped the aircraft to the NACA’s Langley Laboratory in Hampton, Virginia, where technicians installed 300 thermocouples and other equipment. It returned to Edwards in summer 1955 where it assisted briefly in the X-1A accident investigation. Then, from August 1956 to July 1957 the X-1B participated in America’s first major flight research study of aerodynamic heating. In six successive flights, NACA pilot Jack McKay flew the aircraft to a top speed of Mach 1.936  in an effort to obtain data on aircraft skin temperatures during flight in the Mach 2 range. Results showed a wide range of readings, depending on their collection point: from 185 degrees Fahrenheit (85 degrees Celsius) at

12  High-Speed and High-Altitude Flight: The First X-Planes the nose, down to 50 degrees F (10 degrees C) nearest the liquid oxygen tank. These results and others conformed closely to pre-flight estimates. In a series of experiments of even greater consequence, the X-1B served as the world’s first testbed for reaction controls, an indispensable ingredient for future spaceflight. Standard aerodynamic systems failed in the low dynamic pressure of space, so engineers developed small thruster jets that could be fired as needed to maintain a spacecraft at the correct attitude. To get the feel of this unconventional new technology, NACA pilots practiced extensive pitch, roll, and yaw maneuvers in an iron frame simulator whose dimensions, inertial characteristics, and jet thrusters approximated what they would experience on the X-1B. NACA technicians installed the reaction control devices on the actual aircraft in November 1957, and the system underwent flight trials at the High-Speed Flight Station during winter 1957/1958. In a role that foreshadowed his future prominence in spaceflight history, NACA pilot Neil A. Armstrong flew all three of the X-1B reaction control missions: the first on November 27, 1957; the second, a low altitude, low Mach flight on January 16, 1958; and finally, a more ambitious attempt at Mach 1.5 and 55,000 feet (16,764 meters) on January 23, 1958. Before more work could proceed, however, cracks discovered in the X-1B’s fuel tanks ended the project and forced the retirement of the X-1B itself. But the research continued on a Lockheed NF-104 Starfighter, which ultimately trained future X-15 pilots on reaction control techniques. X-1C Although planned originally as one of the four successor aircraft to the X-1 (along with the X-1A, B, and D, all contracted in April 1948 by the Air Force with Bell Aircraft), the X-1C never got to the flight line. It bore the same overall size and weight as the other three (roughly 36 feet (11 meters) long, with a mass of about 14,750 pounds (6,690 kilograms) fully loaded), but Bell did make some modifications in keeping with the X-1C’s projected role. Conceived as a testbed for armament and munitions in the high transonic and supersonic ranges, the company’s engineers added two large yaw-damping fins − one on each wing, installed vertically through their upper and lower surfaces − in addition to a retractable ventral fin beneath the rear part of the fuselage. The manufacturer also redesigned the

nose compartment in order to accommodate a variety of armament. Despite these provisions, the X-1C soon fell out of favor with the Air Force. The arrival of two new transonic aircraft into its inventory − North American’s F-86 Sabre and the F-100 Super Sabre − rendered the X-1C obsolete. Since the USAF no longer required an experimental aircraft for high speed armament and munitions research, it terminated its contract with Bell for the X-1C during the mock-up phase of construction. X-1D Despite its alphabetical designation, the X-1D actually flew first among its X-1A, B, and C family members. The Air Force procured the X-1D from Bell Aircraft with the hope that it would become the first to achieve the next major milestone in aeronautics, the conquest of Mach 2 (a feat actually accomplished on November 20, 1953, by the D-558 Skyrocket, as noted in the section on the X-1A). The USAF put its faith in the X-1D and its stablemates, which surpassed the original X-1 in size at 5  feet (1.5 meters) longer; weight at 2,500 pounds (1,134 kilograms) heavier; and capacity, being equipped with a turbo-driven pump that made more fuel available than the X-1’s pressurized nitrogen system. Bell’s Jean Ziegler piloted the first X-1D attempt (an unpowered familiarization flight), on July 24, 1951, but it did not go particularly well. The nose landing gear broke on touch-down. After repairs, the Air Force accepted the aircraft. Determined to win the Mach 2 prize with no delay, Air Force officials put overt pressure on the designated USAF pilot, Lieutenant Colonel Frank K. “Pete” Everest, to get the job done. When he made the second flight on August 22, 1951, they asked him to fly it “wide open,” despite the fact that the aircraft had not flown under the power of the XLR11 even once. Both good and bad luck intervened. After the combined B-50A mother ship and X-1D took off, they had to return to Edwards due to mechanical difficulties. But as they flew back, the X-1D exploded and caught fire during pressurization, which preceded the off-loading of propellant. Everest took shelter in the bomb bay as the crew dropped the X-1D onto the Mojave, where it blew up on impact. No one sustained injuries and the B-50 managed to land with no damage. The race for Mach 2 remained open.

X-1D  13

14  High-Speed and High-Altitude Flight: The First X-Planes

X-1E  15 X-1E When the X-1-2 aircraft completed five years of testing between October 1946 and October 1951, NACA authorities decided to extend its useful life by revamping it with major modifications. They hoped to recover some of the research opportunities lost in the 1951 explosions of the X-1D and the X-1-3. The success of this project required not only technical ingenuity, but significant inter-agency diplomacy. The first step involved an appeal from the cash-strapped NACA Headquarters to the USAF’s Wright Air Development Center in Dayton, for money to remove the X-1-2’s ten percent thickness-to-chord wings and to substitute them with much thinner, four percent replacements. (The X-1-1, X-1-2, as well as the X-1A, B, C, and D, all used eight percent wings). This plea, made in 1951, fell on deaf ears. In the meantime, aerodynamicists at the NACA’s Langley Laboratory conducted wind tunnel tests that showed a clear correlation between slender airfoils and reduced buffeting. With this consequential evidence in hand, NACA Director Hugh L. Dryden − a prominent scientist whose pioneering World War II research on guided missiles gained him widespread respect and recognition among the military brass − argued the case personally with the generals at Wright Field. He apparently piqued their interest. Dryden’s intervention also prompted the contractor (Bell Aircraft) to reduce its earlier cost estimates, now agreeing to do the job for $900,000. The funding finally arrived from no less a source than the Secretary of Defense’s emergency reserves. Although Bell set a target delivery date of 1953, the four percent wings did not finally materialize until January 1955. Somewhat unexpectedly, the actual modification of the aircraft, now renamed the X-1E, occurred not at Bell’s plant in Buffalo, New York, but by a mixed team of contractor and NACA personnel at the High-­Speed Flight Station on Edwards Air Force Base. In addition to the new wings, technicians there also re-­fashioned the X-1-2’s cockpit and canopy to accommodate an ejection seat requested by the NACA. The wings themselves (actually manufactured for Bell by the Stanley Aviation Corporation) measured a slight 3 3/8 inches (85.7 millimeters) at their thickest point. The airfoils further departed from those of the X-1-2  in another way. Formerly 28 feet (8.5 meters) from tip to tip (like the rest of the X-1 family), the new wingspan measured just 22  feet 10 inches (6.9 meters). The X-1E team also installed 343 strain and temperature gauges

to record structural loads and aerodynamic heating, 200 pressure distribution orifices, and the updated low-pressure turbo pump fuel system common on the X-1A through D (replacing the high-­pressure nitrogen feed used on the X-1s). The fuselage measured 31 feet (9.4 meters), almost 6 feet shorter than the X-1A, B, C, and D. The X-1E underwent ambitious and rigorous − though not always lucky − flight research. NACA pilot Joseph Walker flew the X-1E’s first 21 missions between December 1955 and September 1958. His colleague John McKay completed the program with five more, from September to November 1958. The NACA’s engineers organized the project like that of any untested aircraft, taking one sequential step after another. First came a series of ground tests of the rocket engines, then several captive flights, followed by limited missions up to Mach 0.80 to ascertain the aircraft’s structural integrity in maneuvers. Beyond these preliminaries, they planned for speeds ranging from subsonic to Mach 2.2, testing longitudinal, lateral, and directional stability and control; pressure loading at the wings and tail in level flight, in turns, and in pull-ups; and aerodynamics effects such as buffeting boundaries, lift-to-drag ratios, thermodynamic heating, and wing elasticity. Among its 26 flights, it exceeded Mach 2 three times, the last and fastest of which came during flight 17 in October 1957, when it achieved Mach 2.22 (about 1,480 miles (2,382 kilometers) per hour). On the other hand, the X-1E also endured three mishaps: minor repairs after a rough landing on June 18, 1956; a heavy impact on May 15, 1957, after a Mach 2 mission, when a runway accident put the X-1E out of commission for four months; and light repairs on touching down after an aborted flight on June 10, 1958. The X-1E contributed significantly to supersonic airfoil knowledge. It flew at about twice the speed of the X-1-2 that it supplanted (although by this time the Lockheed F-104 Starfighter had achieved similar velocities with similar wings, and with far less cost and pre-flight preparation than the X-1E). In the end, the X-1E not only demonstrated the value of slender airfoils, but also offered important data that illuminated the next phase of high-speed flight: hypersonic travel, above Mach 5. Taken as a whole, the roughly ten-year lifespan of the X-1s represents the culmination of many concepts conceived or tried during the Second World War. The historic achievement of Mach speed, swept wings, and rocket power (among other advances) does not begin

16  High-Speed and High-Altitude Flight: The First X-Planes to tell the story of the X-1s, however. Rather, the essence of their combined effect can be found in the intense efforts by scientists and engineers at the NACA, the Air Force, industry, and elsewhere to discover and mitigate the idiosyncrasies and flaws inherent in them. Furthermore, in those cases where the deficiencies could only be understood rather than corrected, X-1 research served to warn aircraft designers about potential catastrophes. For these reasons, one thing can be said for certain: the X-1s profoundly influenced the world of aeronautics in ways that are still being felt. X-2 The X-2 looked like the D-558 Skyrocket, with swept wings and an elongated nose, and resembled the X-1 in its bulky fuselage and erect tail structure. But its mission set it apart from either of those two. Flown between 1952 and 1956, while these first generation supersonic aircraft still took to the skies, the X-2 Starbuster attempted a radical departure from the earlier aircraft. Its engineers designed it to fly in an unexplored frontier: above Mach 3 and at extreme altitudes, where it would investigate stability and control, as well as aerodynamic heating. The step from X-1 to X-2 seemed like a logical progression in the quest for higher Mach numbers and higher altitude, but in this single-minded pursuit, researchers may have underestimated some of the difficulties inherent in trying to treble the speed mark set by Chuck Yeager in 1947. The X-2 differed from the last great record-holder, the Mach 2 D-558 Skyrocket, in at least two obvious ways: it measured almost 38  feet (11.5 meters) long, 4  feet (1.2 meters) shorter than the Skyrocket; but it weighed 24,910 pounds (11,300 kilograms) at takeoff, exceeding that of the Skyrocket by about 9,000 pounds (4,082 kilograms). Fabricated by Bell Aircraft for the Air Force, the X-2 also relied on the NACA and its engineers and technicians. Originally, planners anticipated that the NACA would undertake its own research program once the USAF finished its work, focusing on aerodynamic heating and handling characteristics at extreme altitudes and speeds. But Air Force authorities wanted to claim the Mach 3 crown for the USAF, so they delayed

the handover for two months in order to train an Air Force pilot for the mission. In the meantime, the NACA made some decisive contributions to the project, such as wind tunnel testing at its Langley Laboratory; rocket model (pilot ejection) research at its launch pads in Wallops Island, Virginia; and stability and control instrumentation, flight simulations, and data extrapolation at the High-Speed Flight Station on Edwards Air Force Base. Some of the results raised questions about the ultimate success of the X-2. The simulations at Edwards substantiated suspicions gleaned from the NACA’s wind tunnel data, which indicated rapidly declining directional and lateral (roll) control approaching Mach 3. Researchers feared that a high velocity roll at that speed might result in inertial coupling, dooming the aircraft to a free fall. Bell hoped to deliver the X-2 to the Air Force in 1948, but the firm encountered problems in fashioning the fuselage from a heat-resistant copper and nickel alloy known as K-Monel, and in making the wings out of stainless steel, with both necessary to withstand the intense aerodynamic heating at Mach 3. Moreover, the aircraft’s two-chamber, throttleable XLR25 CurtissWright rocket − powered by a liquid oxygen engine capable of producing 15,000 pounds (6,803 kilograms) of thrust − slowed down development for months, and ultimately years, due to its propensity for explosions. Bell also needed to make other major modifications from the X-1 design, such as a crew escape system based on an ejectable nose capsule (like that on the Douglas D-558-1 Skystreak), and a skid landing gear that enabled the conservation of space for additional fuel. The USAF purchased two X-2s, but only one succeeded as hoped. The first one to fly (X-2 #2) started its career at Edwards Air Force Base on April 22, 1952, with a captive-­carry mission. After a second successful attempt it underwent three glide flights, the first suffering a rough landing, the other two satisfactory. These ended in October, when the aircraft returned to the Bell facility in Wheatfield, New  York, for captive-carry flights with the Wright-Curtiss engine installed. But during a pass over Lake Ontario on May 12, 1953, the rocket’s liquid oxygen system blew up, killing pilot Jean Ziegler and crew member Frank Wolko. The X-2 plunged into Lake Ontario, while the B-50A mother ship suffered so much collateral damage that the company decided to scrap it. Bell did not attempt a recovery mission for the X-2.

X-2  17

After more than a year’s delay at the Bell plant, the X-2 #1 left for the Mojave Desert, slung under a replacement B-50D, and arrived there on July 15, 1954. This arrival began more than two years of tumultuous flight research. On its inaugural missions on August 15, 1954, it made a captive-carry, and then a glide flight: the first uneventful, the second a hard landing that required the X-2 #1 to be sent back to New York for repair. It returned in January 1955, after which it underwent the same type of tests as before (in February, March, and April) until, on the third glide flight on April 6, it made another rough landing and had to go back to Bell again for restoration. After six more months of delay, it appeared once more in the Edwards hangars on October 25, 1955. Air Force Major Frank Everest made the X-2’s first powered flight on November 18, 1955, during which a fire in the engine bay caused minor damage. After three aborted missions, he completed a second one with the XLR25 on March 24, 1956, reaching Mach 0.985. Everest then made four successive test flights in April and May 1956, eventually climbing to Mach 2.53. Before being transferred to a new assignment, he conducted his last X-2 mission on July 23, 1956, attaining a world speed record of Mach 2.87. Captain Ivan Kincheloe succeeded him and made a total of six flights

in August and September 1956. Even though he experienced premature engine shutdowns on two of these attempts, on September 7, 1956, he pushed the X-2 to an altitude of 126,200 feet (38,466 meters), making him the first pilot to exceed 100,000  feet (30,380 meters) and subjecting him to almost complete weightlessness. Kincheloe’s last three missions had to be aborted. The final chapter of the X-2 brought achievement and loss. Captain Milburn G. Apt had been brought into the program during the two-month extension period prior to transferring the aircraft to the NACA. A seasoned pilot in a wide array of aircraft, Apt nonetheless had no experience with rocket planes, and had logged no previous X-2 flight time. Even so, on September 27, 1956, he and the USAF decided to try for the record. After being dropped from the mother ship, the X-2 climbed until it reached 72,224 feet (22,014 meters). At this point Apt lowered the nose and dived, r­eaching Mach 3.2 as he descended to approximately 65,500 feet (19,964 meters). He continued above Mach 3 for ten seconds, but then he made a fatal mistake; he took a sharp turn back to Edwards, during which the X-2 fell into a series of rapid rolls. The incident led to inertial coupling (in which centrifugal forces make the aircraft buck, pitch, and yaw in all three axes), rendering the X-2 uncontrollable. After being thrown around the

18  High-Speed and High-Altitude Flight: The First X-Planes cockpit violently as he tried to save the situation, Apt jettisoned the nose section. As he and this remnant of the X-2 fell to Earth, the drag chute deployed and he got the canopy open, but Apt ran out of time before he could parachute to safety, and he died in the subsequent crash. X-3 Despite the successes of the X-1 and X-2, senior American military officials wanted more; not just high speed, but combat capability. The initial X-planes proved that Mach 1, 2, and even 3 could be achieved, but only for short bursts. Could these breakthroughs be applied to sustained flight? The looming Cold War gave urgency to this question. Conflict in Korea, the victory of the Communist Party in China and − most concerning of all to American national interests − the threat of nuclear annihilation posed by Soviet Intercontinental Ballistic Missiles (ICBMs), presented existential challenges to U.S. security. The pursuit of high-speed flight represented just one of many efforts to counteract these and other dangers. But ICBMs, specifically the Atlas Missile, stood at the apex of American defense policy both in cost and significance. As early as 1951, Convair Aircraft got the go-ahead contract for Atlas (see the X-11 and X-12 profiles in Chapter 3), and in 1955 the Defense Department elevated the missile above all others by making its completion the highest national priority. Meanwhile, as Atlas underwent design and testing, the Air Force, Navy, and the NACA embarked on the X-3 project to demonstrate Mach 2 flight for prolonged periods of at least 30 minutes. If mastered, this capability would enable U.S. military aircraft to dominate the skies in almost any Cold War confrontation. Douglas Aircraft won the contract for two X-3s in 1949, after six years of preliminary engineering work. In the final analysis, the company abandoned the planforms of its previous high speed creations, the D-558 Skystreak and Skyrocket. Rather than recreating their comparatively bulky and conventional contours, Douglas engineer Frank Fleming and his assistants envisioned the X-3 as an exotic design, with a long, low, slender fuselage 66  feet 9 inches (20.35 meters) long; exceptionally stubby, low aspect ratio aluminum wings that measured only 22  feet 8 inches across (6.7 meters); elongated engine nacelles that ran about 40 percent of the length of the aircraft; titanium construction to conserve weight in the fuselage (a first); and a sharp, thin, needle nose that won the aircraft the nickname, “Stiletto.” As if that

level of complexity failed to promise enough, Fleming and his team designed the X-3 − weighing 23,840 pounds (10,814 kilograms) loaded − to take off and land under its own power from runways, using twin turbojets that would render the previous air launches from bomber aircraft obsolete. In fact, runway take-off proved the undoing of the project. Douglas subcontracted for the engines with Westinghouse Aviation Turbine Division, which pinned its hopes on a new 24C-10 (J46-WE-1) powerplant, a system still under development, with a hoped-for sealevel thrust of 6,600 pounds (2,993 kilograms) with afterburner. Douglas calculated that with this propulsion system, it could deliver an X-3 capable of ten minutes of Mach 2 flight to the USAF. However, as the X-3 airframe matured in the Douglas factory, progress on the engine fell behind, burdened by inadequate thrust and excessive weight. In the hunt for a temporary solution while they struggled with the 24C-10, Westinghouse’s engineers turned to the veteran J34-WE-17 jet, which produced only 4,850 pounds (2,199 kilograms) of thrust with afterburner. But as development delays stretched on and performance issues continued, it eventually became clear that the 24C-10 would not come to the rescue. To save the situation, Douglas even considered falling back on air launch − fairing over the X-3’s engine intakes, mounting two XLR-11 rocket engines (like those on the X-1), and dropping the aircraft from a bomber. NACA officials rejected this option in anticipation of the forthcoming Lockheed F-104, expected to perform in the X-3’s original flight profile. Forced to make the best of the situation, the Air Force accepted the underpowered X-3 at Edwards Air Force Base on September 11, 1952, but not before canceling the second airframe and offering its unfinished carcass to the NACA for parts. Then came the flight research program, which began on October 20 with Douglas test pilot William Bridgeman in the cockpit. Bridgeman did not have an easy time on the maiden flight. Predictably, he found the X-3 underpowered (it flew only to Mach 0.796), but he also found it hard to control and readily subject to pitch-up. “This thing doesn’t want to stay in the air,” an uneasy Bridgeman told ground control. After another contractor flight, the program halted to accommodate winter in the desert. When it resumed on April 30, 1953, Bridgeman began a string of 23 flights that lasted almost until the end of the year. The pinnacle occurred on July 28, when he put the aircraft in a 30-degree dive and pushed it to Mach 1.208 at 37,264 feet (11,358 meters), the highest altitude of the contractor flights.

X-3  19

The Air Force took the reins from Douglas on December 23, 1953. Lieutenant Frank Everest and Major Chuck Yeager alternated in the cockpit during six flights that ended in July 1954. That month, Everest set the X-3 military altitude record at 41,318 feet (12,594 meters) and reached Mach 1.075 on the 27th. Although the USAF did not test the X-3 extensively, Everest and Yeager came to the conclusion that the plane’s dangerous handling qualities and underpowered engines rendered it of limited value. The NACA then took possession of the X-3 and embarked on a series of 21 flights that ran for almost two years, from August 23, 1954, to May 23, 1956. Joseph Walker, one of the NACA’s most able pilots who had flown the X-1, both D-558s, and would later fly the X-15 24 times, sat at the controls of every NACA X-3 mission. By this time, the limitations of the aircraft had been well understood, and the flight program would be one of extracting data about its deficiencies (applicable to later aircraft designs), rather than a celebration of its successes. One worthwhile observation concerned the aircraft’s small tires, which shed big strips of rubber during the required high

take-off and landing speeds of the X-3. Future tire manufacturers took this phenomena into account when they designed tires for other high performance military aircraft. The NACA also pursued a far less prosaic and far more dangerous set of experiments on the X-3. Because the long fuselage and short wingspan of the X-3 resembled the planform of several current fighter aircraft about to enter military inventories, project engineers wanted to investigate lateral and directional stability and control during sharp rolls. Joe Walker got this assignment and, during a memorable flight on October 27, 1954, barely escaped with his life. During this mission, he attempted an abrupt roll to the left, to which the X-3 responded with a 20-degree nose pitch-up and 16-degree sideslip. The plane rolled wildly for five seconds. Not content with regaining control, Walker wanted to learn more, so he accelerated in a shallow dive past Mach 1 and then repeated the same maneuver as before. This time, inertial coupling caused all hell to break loose. The X-3 pitched downward violently, and then upward (pulling roughly 7 g in both directions), while sideslip reached 21 degrees. Walker re-established command, but only after a true white-knuckle ride.

20  High-Speed and High-Altitude Flight: The First X-Planes Wisely, the NACA decided not to push the limits of the X-3 again. Walker continued test flights geared toward understanding the X-3’s directional stability and control, but without going again to the ragged edge of safety. After ten flights following the hairraising one in October 1954, the NACA finally retired the aircraft on May 23, 1956. But the investigations proved to be highly applicable. The North American F-100A Super Sabre fighter also encountered inertial coupling, and NACA engineers on the X-3 advised the team at North American to counteract the deficiency by increasing its tail and wing surfaces. Despite being underpowered and unable to fulfill its high speed objectives, X-3 research resulted in modifications that tamed the F-100A Super Sabre. Additionally, the data realized from the NACA and Douglas X-3 flights informed Kelly Johnson during the design of the Lockheed F-104 Starfighter, a multi-role aircraft that achieved sustained Mach 2 flight and became a mainstay of the U.S. air armada during the Vietnam War. X-7 The first generation of high-speed X-planes all shared certain characteristics in common: humans in the cockpit; bomber aircraft as launch platforms (except for the X-3); and full-­scale, complete aircraft capable of testing the two main frontiers of flight: speed and altitude. Not the X-7. Like the X-1s, the X-2, and the X-3, the X-7 arose out of the competition between the U.S. and the U.S.S.R. during the Cold War. In contrast to these other aircraft, however, the X-7 had an entirely different origin and role. To begin with, its developers at Lockheed Aircraft entered the X-planes program not to verify an entire vehicle, but to provide a testbed for specific technologies. Additionally, because the X-7 served as a testbed, it flew at a wide array of speeds and altitudes. Finally, unlike the initial supersonic X-planes, the NACA participated in the X-7 program solely in a support role, with the Air Force, and later the Army and Navy, taking the lead as co-sponsors. One other factor distinguished the X-7 from its X-planes contemporaries: it had an exceptionally long service life, flying for over nine years between April

1951 and July 1960 in four baseline configurations: as the X-7 A-1, created to assess small ramjets; the X-7A-3 for larger ramjets; the X-7B, similar in design to the X-7A-3, but built to test guidance and control systems; and the X-7 XQ-5, a high-altitude, high-speed target drone. The X-7s also conducted propellant experiments, involving additives, new mixtures, and high-­ energy fuels. The X-7 came into being in December 1946, with a contract between the Air Force and the Lockheed Skunk Works to explore the use of ramjet engines for supersonic and hypersonic flight (although its role became far broader as it evolved). The concept of ramjets originated before World War I with the work of French engineer René Lorin. It gained wider credence at the end of World War II with the release of Professor Theodore von Kármán’s famous multi-volume aeronautical forecast, Toward New Horizons, drafted at the request of the Commanding General of the Army Air Forces, Hap Arnold. A portion of one of the volumes (entitled “Aircraft Power Plants”) dealt at length with the technology of ramjets. Following its publication in May 1946, and through the 1950s, ramjets gained popularity as a promising new form of propulsion. They differed radically from turbojets, the up-and-coming powerplant of the day. Turbojets required compressors to increase incoming air pressure, following which the airflow passed through to a combustor − a chamber where a hot mixture of air and fuel burned, providing thrust. Ramjets, in contrast, produced the requisite pressure by forcing air generated during flight directly into a combustor, thus eliminating the need for a compressor, with advantages in weight, simplicity, and the absence of moving parts. For it to work, however, the entire aircraft needed to be accelerated by a separate powerplant (in the case of the X-7, by a solid fuel rocket booster) to about half the speed of sound, at which point the forward motion generated enough air compression for the ramjet to ignite. From the vantage point of the late 1940s, ramjets offered a lot of promise, and for that reason the Lockheed team felt free to conceive the X-7  in ways not yet seen in the X-planes series. To fill in the void of practical ramjet knowledge, they wanted a test vehicle that recorded copious data with an engine that could be recovered after each flight. Moreover, existing airframes would not serve the purpose. Something entirely new needed to be developed in order to withstand the forces expected from hypersonic tests.

X-7  21

The X-7’s flight characteristics grew out of its testing requirements. To gain sufficient speed to engage the ramjet, the aircraft needed to be air launched. To retrieve the engine, a parachute system needed to be devised (in the end, the retrieval system most often consisted of the aircraft diving nose-first and sticking vertically out of the desert sand). To avoid potential problems with the newer delta and swept wing designs, the X-7 relied on straight, thin wings. To avoid incompatibility in the mating of the experimental aircraft with the mother ship, care had to be taken so that the X-7’s dimensions and contours allowed it to operate safely beneath the bomber’s wing. Beginning in 1947, Lockheed embarked on a series of X-7 studies that took these and other factors into account. Once the engineers arrived at some basic planforms, they fabricated 1/3 scale models and sent them to Muroc Army Air Field (later Edwards Air Force Base) for testing. The team instrumented the X-7 prototypes for drops from both a Lockheed P-38 pursuit and a B-29 bomber, which yielded aerodynamics data and gave Lockheed the opportunity to subject components of the sub-scale X-7s to simulated flight conditions. It became clear during these activities that the B-29

represented the program’s most accessible and capable launch vehicle. With that choice made, they decided to trapeze-­mount the X-7 under the left wing, between the engine nacelles on the giant aircraft − a necessary expedient because the B-29’s bomb bay lacked the space to accommodate the big vertical fin of the X-7’s booster. Not unexpectedly for an aircraft that counted on an untried method of propulsion and aimed at crossing the hypersonic threshold, the X-7 A-1 − the ramjet testbed − experienced several years of setbacks and intermittent successes. Lockheed attempted the first full-scale X-7 flight on April 26, 1951, not at the usual X-planes launch site at Edwards Air Force Base, but at the Air Force Missile Test Range in Alamogordo, New Mexico. On this mission, the X-7 dropped from the mothership and fired its booster after five seconds − prematurely − causing the pin holding the booster and the aircraft in roll alignment to uncouple. The booster rolled 90 degrees, tearing off one of the X-7’s stabilizing fins, followed by rapid pitch-up and the disintegration of the whole vehicle. The crew of the B-29 witnessed the catastrophe up close; they actually flew through the debris cloud. After a lengthy re-design, on a second

22  High-Speed and High-Altitude Flight: The First X-Planes flight in November the crew once again ignited the booster, but this time a fin failed, resulting in more debris. On the third attempt, the ramjet fired, but put out little thrust. By January 1954, the majority of the 14 launches to date had yielded uneven results, but invaluable data. In 1949, as Lockheed developed the X-7, plans for a long-­range anti-aircraft missile appeared on the drawing boards at Boeing Aircraft. U.S. defense officials sought help from industry in finding an antidote to the looming threat of nuclear-armed Soviet bombers penetrating American airspace. The Air Force sponsored the project, which came to be known as Bomarc, an amalgamation of the names of its two partners: Boeing and the Michigan Aeronautical Research Center. Air Force planners envisioned a critical role for this missile: to be able to strike an incoming air armada at altitudes up to 80,000  feet (24,384 meters) and at distances as far as 200  miles (321.9 kilometers) from Bomarc’s launch site. These ambitious goals for Bomarc proved hard to achieve. Between 1950 and 1953, the Air Force tested it as an unpiloted interceptor, but persistent propulsion system failures and control problems put the program in doubt. Because of these difficulties, Boeing and the USAF turned to the X-7 project as a testbed for Bomarc’s technologies. Boeing directed its ramjet subcontractor, the R.E. Marquardt Company, to furnish the X-7 engineers with three new engines. Flight testing began on December 17, 1952, during which time the powerplant delivered ten seconds of ignition, accompanied by a blowout with minor damage. Missions resumed in February 1953 with much the same result. But on April 8, 1953, the ramjet burned for 20 seconds and the X-7 reached almost 60,000 feet (18,288 meters). Another attempt in September roughly equaled the results of the April test, but one in December failed when the ramjet’s exit nozzle malfunctioned. Despite the spotty record (by the end of 1953 there had been no more than a few truly successful flights), the X-7 offered Marquardt and Boeing important flight data that helped improve and refine the ramjet. Boeing recognized their value by requesting more flight time and more X-7s. The ongoing trials of successive Marquardt engines continued into 1954 and beyond. Eventually, the Bomarc program came into focus, and between 1957 and 1964 it entered the U.S. inventory as a mainstay of the interceptor force, with the Air Force ultimately fielding 570 production missiles. The final Bomarc left USAF custody in 1972 as the nuclear threat from the U.S.S.R.’s bombers decreased and ICBMs stood out as the greater menace.

Meanwhile, the X-7 program soldiered on in forms other than the X-7 A-1. Starting in 1954, Lockheed announced a variant known as the X-7 A-3. The manufacturer made numerous improvements, on the original airframe, the booster design, the command response system, the test instrumentation capacity, the aerodynamics, the underwing booster mounting, and the vehicle’s camera equipment. The biggest change involved the wings, which increased in area on the X-7 A-3 and featured a greater taper ratio. Additionally, the B-50 replaced the B-29 as the launch aircraft due to its heavier carrying capacity and better performance at altitude. Next in the X-7 line came the X-7B, designed as a testbed for guidance and control systems. It underwent 12 test flights, but the program ended before it reached its peak output. Finally, the X-7 XQ-5 came into being. Called the “Kingfisher,” it served as a high-speed drone, designed to test the mettle of missiles like Bomarc and Nike. It flew with partial or full success about 80 times, often with surprising results: acting as surrogate enemy missiles, Bomarc and Nike only rarely defeated the X-7 XQ-5. Political influence finally brought the Kingfisher to an end and the overall X-7 flight program concluded with a last flight on July 20, 1960. In the end, the X-7 nurtured the development of practical ramjets, a technology that expanded the operating envelope of atmospheric flight. During 130 missions and more than nine years of research, it broke international speed and altitude records for airbreathing vehicles, flying up to 2,881 miles (4,636.5 kilometers) per hour (roughly Mach 4.31) and achieving an altitude of 106,000 feet (32,309 meters). As a result of the X-7, ramjets also powered the Bomarc and the Navaho, two of America’s frontline Cold War missiles. Finally, the ramjet prefigured the scramjet, tested successfully decades later by another X-plane, the X-43. X-9 In an odd juxtaposition, the X-9 paralleled but also differed sharply from its closest contemporaries, the X-7 and the X-10. The X-9 relied on standard rocketry, while the X-7 tested the ramjet, a new and largely untried form of propulsion, and the X-10 flew by turbojet. Also in contrast, the X-9 concentrated on air-tosurface warfare, whereas the X-7 tested air-to-air technologies, and the X-10 explored ground-­to-­ground

X-9  23 flight. Yet, in at least one decisive respect, the X-9 shared a purpose with the X-7 and the X-10: all three served as testbeds for contemporary missiles under urgent consideration for deployment by the U.S. military. The X-7 contributed to Bomarc, the X-9 foreshadowed the Rascal, and the X-10 presaged the Navaho. The X-9, also known as the Shrike, derived its name from a peculiarly aggressive, carnivorous bird common to Eurasia and Africa. The shrike kills prey (like mice) by seizing them by the neck and shaking them violently until the neck breaks. Shrikes also use their beaks to impale rodents and other victims, such as grasshoppers and lizards. Clearly, the Army Air Forces and later the Air Force wanted something highly lethal in the X-9. Like the X-7, the concept of the Shrike grew out of the events of World War II, reflected in the pages of Toward New Horizons, the science forecast prepared by the legendary Caltech professor Theodore von Kármán at the end of the Second World War. In the volume entitled “Guidance and Homing of Missiles and Pilotless Aircraft,” its co-author, Hugh L.  Dryden, applied his experiences during the war as director of the Navy’s Bat missile project. The Bat consisted of an advanced, airlaunched, radar-guided glide bomb employed successfully in attacks on Japanese shipping. Also during the war, Army Air Forces bombers encountered heavy losses due to deadly anti-aircraft fire, which the service hoped to counteract in the post-war period with stand-off missiles like Shrike and its combat version, the Rascal. Army Air Forces officials felt so strongly about the need for stand-off air-to-surface missiles that they did not wait for Toward New Horizons to legitimize them. In July 1945, only ten weeks after the German surrender, the AAF made public the desired characteristics for such a weapon: that it could be launched from a bomber at 20,000 to 40,000 feet (6,069 to 12,192 meters); attain speeds of no less than 1,200  miles (1,931 kilometers) per hour; reach targets 100 miles (160 kilometers) away or beyond; and strike within 500 feet (152.4 meters) of a target no less than 75 percent of the time. Guidance systems remained undefined at this point; either remote or onboard systems would be considered. Bell Aircraft won out over a competition with McDonnell Aircraft and the Goodyear Corporation in May 1947, but due to technical wrangling between Air Materiel Command and the War Department, the contractor did not get final program guidance until January 1948. With that milestone, Bell turned to the first order of business: to subcontract the Shrike propulsion system. Its engineers initially calculated that the project required not one, but two liquid propellant rocket motors: a large one that produced 4,000 pounds (1,814

kilograms) of thrust, and the smaller at 1,500 pounds (680.3 kilograms). Aerojet Engineering Corporation, founded in Southern California during the Second World War by none other than Theodore von Kármán, got the contract. Eventually, the X-9 relied on two Aerojet XLR-65 engines, each rated at 1,500 pounds of thrust. The Bell team envisioned an aircraft significantly smaller than Lockheed’s X-7: 22  feet 9 inches (6.93 meters) long, a wingspan of 7  feet 10 inches (2.38 meters), and a diameter of 1 foot 10 inches (0.55 meters). Fashioned out of aluminum, the X-9 weighed only 2,125 pounds (963.88 kilograms) empty and 3,495 pounds (1,585.30 kilograms) fully loaded (less than half that of the X-7). Bell ultimately fabricated 31 X-9s, designed by the manufacturer to fly between Mach 1.5 and Mach 2, and with a range of 50  miles (80.46 kilometers). Flight research began in April 1949 at Holloman Air Force Base, New Mexico, with three free-fall drops by three instrumented X-9 test dummies flown in order to obtain stability and control data. The following year, the team at the 2754th Experimental Test Wing expanded the envelope with the project’s first live firing in May 1950. An electrical short at launch doomed it to a nose-dive landing that destroyed the test vehicle. On July 26, the X-9 took a step forward when it flew 11.5  miles (18.5 kilometers) at speeds up to Mach 1.2 before it crashed, and flight three on August 30 did even better, going 14.5  miles (23.33 kilometers) as fast as Mach 1.4 prior to going out of control. It nonetheless managed to make a controlled landing by parachute. On October 26, 1950, the fourth live firing resulted in a Mach 1.5 flight, with a range of 61.5  miles (99 kilometers) − twice the expected distance − but the instrumentation package could not be retrieved. Fifteen more X-9 flights remained, launched between November 1950 and January 1953. One of them, in October 1952, tested a chemical warhead to determine how a cluster of bombs dispersed at supersonic speed would behave at launch, in flight, and in mid-course guidance. It passed these tests, but as a whole the X-9 amassed a mixed record of success and failure. The Shrike concluded its flight program before exhausting its full inventory. Impatient to field a credible weapon, the AAF directed Bell to proceed with the combat version, the Rascal, before it got data from even a single X-9 flight. Bell signed a contract for Rascal in May 1947 and put General Walter Dornberger (who directed Dr. Wernher von Braun in the development of the V-2 missile

24  High-Speed and High-Altitude Flight: The First X-Planes

program at the Peenemunde Army Research Center in Germany during World War II) in charge of the project. Dornberger seemed like the natural choice because, prior to his re-location to the U.S. after the war, he also managed the V-1 project, a terror weapon unleashed on London. Its design inspired the Rascal. Originally conceived as a stand-off missile capable of carrying nuclear, biological, or chemical payloads, its final configuration accommodated only nuclear warheads that weighed in the range of 3,000 to 5,000 pounds (1,400 to 2,300 kilograms). Rascal had a very short operational life. Bell delivered the first one to the Air Force at the end of October 1957, but after six months of flight testing that began in February 1958, the missile − launched from the B-47 bomber − showed obvious inadequacies. It flew on less than half of its 65 scheduled test flights, and had just one success. The USAF ended the program in September 1958. Despite the Rascal’s deserved retirement, the earlier Shrike − while far from perfect itself − did provide worthwhile data regarding bomber launch platforms, guidance and control technology, the pairing of liquid rocketry and nuclear warheads, and the basic aerodynamics of advanced air-to-­surface weapons.

X-10 Despite the passage of time since the attack on Pearl Harbor and the cataclysmic events of World War II that followed, post-war planning by the U.S.  Air Force5 remained deeply affected by the events of December 7, 1941. As a consequence, one of the supreme objectives of American military policy after 1945 became the prevention of surprise strikes on the U.S. homeland. For this reason, the Air Force solicited aircraft manufacturers directly after the war to bid for contracts on three vehicles that would test technologies geared toward missile defense: the X-7, the X-9 Shrike, and the X-10. Eventually, American ICBMs deployed to counterbalance the U.S.S.R.’s nuclear force diminished the need for these X-planes, but in the short term − during the 1940s and 1950s − the technologies tested in the X-7, X-9, and X-10 represented the backbone of U.S. air defense.  The X-10 story straddles the end of the Army Air Forces period and the creation of its successor, the U.S.  Air Force, on September 18, 1947. In order to avoid confusion, all references in this essay will refer to the Air Force. 5

X-10  25 Unlike the two tactical aircraft of the same generation (the ramjet-powered, air-to-air X-7 and the rocket-propelled, air-­to-­surface X-9), the X-10 represented a potential strategic weapon in itself: a cruise missile capable of guided, in-­atmosphere flight and of projecting large warheads across continents with pinpoint accuracy. The cruise missile differed radically from other systems in relying not only on blazing speed, but on a low radar signature to evade detection; turbojet engines flown at low altitudes and at a constant Mach number; and a guidance system based on the missile’s relationship to the terrain that it traversed. In short, it continued the research tradition of the ground-to-ground V-1 “Buzz Bomb” (Fieseler Fi 103/FZG 76) developed by Germany during the Second World War under the military auspices of General Walter Dornberger and his civilian deputy, Dr. Wernher von Braun. The cruise missile concept received additional validation from Theodore von Kármán’s lengthy Toward New Horizons report. The volume entitled “Guidance and Homing of Missiles and Pilotless Aircraft” helped to raise the profile and credibility of the cruise missiles among Air Force officials. This new weapon began its career on March 29, 1946, as a research program leading eventually to the Navaho strategic missile. During the previous year, no fewer than 17 contractors responded to the Air Force’s invitation to submit proposals for the ground-toground guided missile. In the end, North American Aviation submitted the favored design, and after suggesting to government officials that the project be focused on supersonic flight at a range of up to 3,000 miles (4,828 kilometers), the Los Angeles company won the contract. The agreement outlined three phases: preliminary research, specific design, and flight testing. North American did not succeed by accident. Its chief designer, the highly able Dutch Kindelberger, decided to realign his firm towards missiles and such allied fields as jet propulsion, rocketry, and automated control, due to the sharp downturn in post-World War II aircraft orders. To manage this budding initiative, Kindelberger lured William Bollay, a protégé of Theodore von Kármán, from the Navy Bureau of Aeronautics turbojet project. The post-war period of technological flux worked in North American’s favor. As military considerations wavered with time, circumstance, and developments within the U.S.S.R., the characteristics of the groundto-ground missile evolved. Finally, in August 1950 the Air Force decided on the key military requirements of

the project: an intercontinental guided missile launched from the ground, with a range of 6,329 miles (10,185 kilometers), speed of Mach 2.75, and the capacity to carry a Mark 4 nuclear payload (weighing roughly 11,000 pounds/4,990 kilograms) to its target. In light of these new objectives, North American proposed alternate steps the following month to reach the end point. It envisioned the X-10 as an interim vehicle with turbojet engines, designed to test the upcoming Navaho’s guidance, aerodynamics, structures, and stability and control. But more than that, four years after North American initially became involved, the X-10 came into being not just as a test vehicle, but as a potential intermediate range field weapon with a 4,143-mile (6,667-kilometer) range and a speed of Mach 2.75. As if to balance the years of debate and revision, the X-10 go-ahead set off a cascade of fast action, with the essential design and fabrication undertaken in a little over two years (starting with the summer 1950 decision). Conceived for long-distance strategic purposes, the X-10’s North American team envisioned a much bigger aircraft than the X-7 and X-9, its post-war stablemates. A dart-shaped vehicle with all-­ moving canards, delta wings, and outwardly canted vertical tail surfaces, it measured 66  feet 2 inches (20.1 meters) long, with a wingspan of 28 feet 2 inches (8.6 meters). Composed mainly of aluminum, supplemented by two long, heat resistant, stainless steel engine nacelles, the X-10 weighed an imposing 42,000 pounds (19,051 kilograms) gross and 25,792 pounds (11,699 kilograms) empty. Bollay’s engineers paired it with twin Westinghouse J40 turbojets, each rated at 6,500 pounds (2,948 kilograms) of thrust, with 10,900 pounds (4,944 kilograms) of thrust using full afterburners. North American fabricated 13 X-10s, and all but three of them underwent flight research. The Air Force accepted the first one on September 1, 1953, and by the end of the month the contractor had completed, or nearly completed, five of them. Meanwhile, from July to December of that year, the manufacturer dispatched 125 of its staff to Edwards to oversee the X-10’s flight test program. They witnessed the first flight on October 14, 1953, when it achieved a speed of 403 miles (648 kilometers) per hour and attained an altitude of 20,000  feet (6,096 meters). Control system data6 checked out during the 172-mile (276.8-kilometer), 32-minute event, but a narrow escape occurred when the X-10’s drag chute failed to deploy as expected, until  An unpiloted vehicle, the X-10’s control mechanism resided in two places during flight testing: a ground station, and a flying command center on board a T-33 aircraft. 6

26  High-Speed and High-Altitude Flight: The First X-Planes

the vehicle hit a bump as it crossed Rogers Dry Lake bed, which popped it open. Successes and failures followed. The second mission on December 5 went routinely, as speed increased to Mach 0.73 and altitude increased to 24,400  feet (7,437 meters), while the fourth, on April Fool’s Day 1954, surpassed Mach 1, flying to Mach 1.47. During the seventh attempt in July, the X-10 crashed on landing; on the thirteenth in February 1955, it blew up at the end of the runway when the destruct mechanism activated accidentally; and the fourteenth in March 1955 ended when the aircraft struck the ground and exploded on final approach, the product of an autopilot malfunction after an otherwise successful mission. By March 1955, the Navaho portion of the flight test program began to assume greater importance than that of the X-10. Program planners decided to test the Navaho at the Air Force Missile Test Center in Cape Canaveral, Florida, and so, in the interests of efficiency, the entire X-10 operation moved there from Edwards Air Force Base. In Florida, the X-10 continued to be tested until early 1957, and it continued to have a mixed record. After two flights in 1955, both marred by accidents caused by landing and nose gear damage, five missions occurred during the first half of 1956 to analyze aerodynamics and automatic take-off. During the

second half of the year, four X-10 flights evaluated the aircraft’s auto-navigator. After 27 total test missions conducted over roughly three and one-half years, the Air Force retired the X-10 (which had attained the maximum program speed of Mach 2.05 on its 19th flight). At the end, three X-10s remained unused, so USAF authorities re-purposed them as target drones. Unfortunately, two of them suffered bad endings in 1958: testing Bomarc, both of them overran the runway, plowed into the sand, and burst into flames. The 27th and last flight of these X-10 target drones took to the skies in January 1959. Its outcome has not been documented. Despite an uneven record, the X-10 provided valuable data to the team building its developmental successor, the Navaho missile. As a flying replica of the Navaho second stage, the X-10 proved the aerodynamics and navigation system necessary for Navaho. The X-10 also had lasting value as a testbed whose data served future remote-controlled, high-performance aircraft. But in the end, technical problems on the Navaho and the partial success of the first Atlas A ballistic missile launch in June 1957 persuaded Air Force officials that Navaho represented an asset of dubious value and a duplication of effort. They cancelled Navaho in July 1957.

2 Specific Improvements: Technology Demonstrators During the same period that the high-speed X-planes underwent development and testing − in the process capturing the public’s imagination − an entire category of X-planes progressed with much less adulation and journalistic coverage. But these lesser known vehicles contributed at least as much to our understanding of contemporary flight. Their possibilities hinged not on blinding speed, but on such varied advances as better maneuverability, nuclear propulsion, vertical flight, and high-altitude reconnaissance. This batch of flight research projects also encompassed a cluster of radical (and at the time, poorly or partially understood) aerodynamic problems, such as yaw-roll coupling, laminar flow control, and variable swept wings. Perhaps the most shocking concept of all  – forward-swept wings − also underwent trials. X-4 The X-4 aircraft falls squarely within this collection of X-planes that flew as aerodynamic testbeds. Not designed to achieve new performance benchmarks, the X-4 and a number of other X-planes instead concentrated on testing advanced theories under the realworld conditions of flight research. In the case of the X-4, the key research focus involved transonic compressibility, a subject of keen interest to the NACA going back as far as the 1920s, when Dr. Hugh

L. Dryden of the National Bureau of Standards (and the future director of the NACA) became one of the first to study the phenomenon. During the 1930s, NACA aerodynamicist John Stack continued to probe the subject, which led finally to the Bell X-1 during the 1940s. Like so many of the early X-planes, the X-4’s immediate heritage stemmed from World War II. The famed Messerschmitt Me.163 Komet anticipated the X-4’s design, with its swept, wide wings, no horizontal tail surface, single vertical stabilizer, and short-fuselage. The German government contracted with Messerschmitt in 1936, and the company gave a leading role in its development to Alexander Lippisch, one of its most able and experienced engineers. The first prototype appeared in 1941, but due to a turbulent testing period it did not see service until 1944. During the war, the Me.163 accounted for only nine kills against allied aircraft, partly because of a mismatched cannon system, but also because it only carried enough fuel for eight minutes of powered flight. The other antecedent of the X-4, the British-­made De Havilland D.H. 108 Swallow, featured a similar planform as the Me.163. Its designer, John Carver Meadows, intended it as a test vehicle for the planned swept wing De Havilland Comet, but it suffered an early and sad fate. One of the three D.H. 108 test models suffered structural failure on a flight in September 1946, killing pilot Geoffrey De Havilland, the son of the manufacturer.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 M. H. Gorn, G. De Chiara, X-Planes from the X-1 to the X-60, Springer Praxis Books, https://doi.org/10.1007/978-3-030-86398-2_2

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28  Specific Improvements: Technology Demonstrators Despite these mixed results, studies at Wright Field and at the NACA probed the work of De Havilland and concluded that tailless aircraft offered a possible antidote to many of the instability tendencies found in supersonic flight. In fact, aerodynamicists speculated that instability at transonic speeds, induced in part by the flow of supersonic shock waves between the wing and tail surfaces, might be reduced or even eliminated by removing the horizontal tail surfaces. The NACA and the Air Force sponsored the X-4, and they selected the Northrop Corporation to design and fabricate the aircraft. This decision made good sense. During the war, Jack Northrop’s company designed the successful P-61 Black Widow, the first U.S. night fighter and the first aircraft designed to use radar. But more pertinent to tailless flight, during the 1930s and 1940s he concentrated heavily on flying wings, most notably (but not exclusively) on two famous experimental bombers: the YB-35 piston engine and the YB-49 turbine engine. These pet projects of Jack Northrop gained extra credence with the frequent participation of Caltech’s eminent Professor Theodore von Kármán, director of the Guggenheim Aeronautical Laboratory at the California Institute of Technology

(GALCIT). A friend not only to Northrop, but also of innumerable corporate and Air Force research and development officials, von Kármán became an active advisor on the flying wings projects, which lent them considerable prestige. Northrop won the contract to fabricate and flight test two X-4s on June 11, 1946. Because of the company’s related flying wing experience, the Air Force and the NACA awarded the project without a competition. Northrop chose one of his trusted engineers, Arthur Lusk, to lead the X-4 team, and one early decision influenced much of the rest of the project. Rather than rely on rocket power, Lusk’s group chose relatively small, twin Westinghouse J30 engines. As a consequence, the X-4 ranks among the most diminutive and unpretentious of the X-planes, with a singleseat cockpit and standard NACA instrumentation (which took up so much of the available space that the fuselage fuel tanks had to be reduced in size, allowing for test flights of no more than 45 minutes). All but the shortest maintenance mechanics had no problems with the X-4, which could be serviced without the need for even a step stool. It earned its nickname, the “Bantam.”

X-5 29 The two models under assembly on the Northrop factory floor − both fashioned out of aluminum − measured 23  feet 3 inches (7 meters) long with a wingspan of 26 feet 9 inches (almost 8 meters). Its engineers placed the wings low on the fuselage and designed them to assume variable sweep angles of 34 and 40 degrees at the leading edge. From top to bottom, the X-4 stood 14 feet 9 inches (4.3 meters). It weighed 5,507 pounds (2,498 kilograms) empty and 7,820 pounds (3,547 kilograms) maximum. The X-4’s predicted performance envelope involved a top speed of Mach 0.92 (616  miles, or 991 kilometers, per hour), a range of 420 miles (676 kilometers), and altitude up to 42,300 feet (12,893 meters). It was not until November 1948, almost two and one-half years after the contract signing in June 1946, that crews at Muroc Air Force Base could begin preparing the X-4 for its inaugural missions. These began with an extensive contractor test program by Northrop, which assigned pilot Charles Tucker to the cockpit. He flew all 30 contractor flights, lasting for over a year between December 1948 and January 1950. The Air Force and the NACA then took delivery of both X-4s in May 1950. Significantly, the USAF and the NACA put two superior pilots on the project, who together accounted for 40 of the X-4’s 81 flights. The first mission began on August 18, 1950, with Chuck Yeager at the controls. He performed eight of the first 12 flights. Then, in December of the same year, R. Scott Crossfield flew the first of his 30 flights for the NACA, with the last in May 1953. Crossfield distinguished himself for his courage in the cockpit, as well as for his engineering acumen. As he took to the skies in the X-4, he had already contributed fundamentally (or would soon do so) to the testing of the Bell X-1-2, as well as to the Douglas D-558-1 Skystreak and the D-558-2 Skyrocket programs. Late in the 1950s, Scott Crossfield became the first to pilot the famed North American X-15. He flew the “Black Bull” 14 times in all. The USAF ended its participation in the X-4 flight test program by November 1950, at which time the NACA’s started its own. During Scott Crossfield’s many missions, he encountered acute pitch sensitivity beginning around Mach 0.88, resulting in porpoising oscillations. On one instance, in January 1951, he pushed the X-4 to a maximum of Mach 0.92 and noticed that the pitch-up oscillations increased in frequency at this top speed compared to slower speeds, but declined in amplitude. NACA’s engineers responded to this phenomenon by thickening the trailing edges of the wings using the non-technological method of inserting blocks of wood into the structure. The solution worked, and pitch-up subsequently ceased up to Mach 0.92.

While not an outstanding aircraft for firsts, the X-4 did teach researchers a great deal about transonic flight, both by what it did achieve and by its limitations. Modern (1950s era) fighter aircraft turned out to be its biggest beneficiaries, as the X-4’s data regarding stability dynamics in yaw, roll, and pitch had direct application to that of high-performance military aircraft then under development. In the end, the X-4 became a symbol for unintended consequences: not the tailless wonder that sliced through the transonic region, but a workhorse whose stability data informed many contemporary production aircraft. X-5 Like so many other early X-planes, the X-5 program grew directly out of the experiences of World War II. Just as the X-4 took its tailless cue from the Messerschmitt Me.163 Komet, the X-5 improved upon another World War II German design, the Messerschmitt P.1101. The uniqueness of the Me P.1101 lay in its wings. Both the X-4 and the X-5 pioneered swept wings, as opposed to the straight wings that prevailed up to this time. During the war, two engineers on opposite sides of the Atlantic discovered the efficacy of swept wings, which could delay spikes in drag at transonic and supersonic speeds. Robert T.  Jones, an NACA aerodynamicist, and Adolf Busemann, director of Germany’s Braunschweig Laboratory, conceived the concept independently. The progression from swept to variable swept wings happened quickly. German manufacturers became the first to produce aircraft with variable wing sweep, exemplified by the Me P.1101 whose crews locked in the wing angle prior to each mission. Like many other aircraft captured by the allies, a P.1101 found its way to Wright Field in Ohio, where it underwent several years of technical assessment. Air Force authorities then shipped it to Bell Aircraft where, under Bell’s Chief Designer Robert Woods, the contractor experimented with variable geometry at the NACA’s Langley Memorial Aeronautical Laboratory in Hampton. Meanwhile, NACA engineers led by Charles Donlon pursued the project using the agency’s 6 x 10-foot (1.83 x 3.04-meter) wind tunnel, and obtained results suggesting that swing wings might not only cure some of the problems encountered when high-­ speed aircraft traveled at low speeds, but might also control longitudinal stability. Bell hoped to augment the wind tunnel work by conducting flight research on the actual Me P.1101, but Woods and his engineers had to abandon the plan when the aircraft suffered damage in the transfer from the USAF.

30  Specific Improvements: Technology Demonstrators

Alert to these activities, Air Force officials warmed to the concept of variable sweep, but not to the P.1101 practice of setting the wing angles before missions. Instead, they wanted an aircraft that changed wing positions while in the air. Bell at first made a daring proposal to design and fabricate 24 variable sweep, combat-ready interceptors for the USAF. The Engineering Division at Wright Field declined, but when Bell countered in February 1949 with a variable wing testbed proposal (based, of course, on the hope that a production contract might follow), the Air Force responded without delay, signing an agreement with Bell for two prototypes on July 26, 1949. The parties agreed that the Me P.1101 would serve as the design template. Despite the direct help from its Me P.1101 predecessor, the X-5 led anything but a charmed life. The contract made by Bell and the Air Force set a high standard, with the testbed not only needing to demonstrate flight characteristics from 20- to 60-degree wing sweep, but also to assess this technology’s suitability for production aircraft then under consideration. Because of the wide implications inherent in these objectives, the Air Force authorities assigned to the project paid close attention. The month after USAF signatories formalized the project, engineers at the Wright Field Aircraft Laboratory expressed concerns about the design and recommended

extensive changes, arguing that the 20-degree sweep angle offered no better testing opportunities than existing straight-wing aircraft with regards to stalling characteristics. Furthermore, they felt that the data from high-sweep wings during high-speed flight could be obtained from existing aircraft, or from those about to be fabricated. In other words, in their opinion the X-5 offered nothing new or novel for swept-­wing or highspeed research. Wright Power Plant Laboratory officials added to the criticism, complaining about the vulnerability of the X-5’s fuel supply, located right above its engine. The NACA further objected to the plane’s automatic leading edge slats, preferring pilot control instead. But these worries proved to be secondary. Despite its undoubted value in testing in-flight variable wing sweep, the X-5 proved to be dangerous to fly. The positioning of its tail and vertical fin led to violent spin-stall instability, and its relatively large engine − mounted low in the fuselage − contributed to the problems. (Earlier wind tunnel tests at Langley predicted spins, but not to the extent experienced in real flight). During pitch-up, its pilots found that the X-5 yawed to the right, then rolled to the right, resulting in successive longitudinal, directional, and lateral instability. Even an old cockpit hand like Scott Crossfield, known for his steady nerves and sharp intellect, found it unnerving, describing it as “a

X-5 31 terrible airplane in a spin.” Indeed, on October 21, 1952, NACA pilot Joe Walker encountered a spin at 36,000 feet (10,972 meters), and it took him 18,000  feet (5,486 meters) to pull out. Crossfield also encountered one. The worst incident occurred on October 14, 1953, when Major Raymond Popson, an Air Force pilot, lost control of the X-5-2 as it flew in the 60-degree wing sweep position. It fell into a spin, and he failed to recover in time or to escape the ensuing crash. Despite these dangerous traits, the X-5 underwent almost four and one-half years of flight research, with 154 missions on X-5-1 alone. (Records for the X-5-2 have not been found; it flew only for Bell and the Air Force). Crews at the NACA’s High-Speed Flight Station got their first look at the X-5 in June 1951, when a team

from Bell flew with the aircraft to Edwards Air Force Base aboard a Fairchild C-119. When they uncrated it, they saw an ungainly looking aircraft fabricated overall out of aluminum, with a low belly and high tail structure. It measured 33 feet 5 inches (10.2 meters) long, 12 feet (3.6 meters) tall, with wing spans of 20 feet 9 inches (6.3 meters) swept and 33  feet 6 inches (10.2 meters) unswept. The X-5 weighed 6,350 pounds (2,880.3 kilograms) as it came out of the C-119 and 9,875 pounds (4,479.2 kilograms) fully loaded. The powerplant consisted of a single Allison J35-A-17A turbine engine without afterburner, capable of 4,900 pounds (2,222 kilograms) of thrust at sea level. Its designers built the X-5 for altitudes up to 42,000 feet (12,801.6 meters) and speeds up to Mach 0.92.

For its first mission, the X-5-1 took to the air on June 20, 1951 (X-5-2 made its maiden flight about six months later on December 10). Bell’s Jean “Skip” Ziegler piloted all 19 of the X-5-1’s contractor flights, which ended in early October. During that roughly four-month period, he steadily proved the X-5’s basic performance profile, raising the altitude level from 16,200  feet (4,937 meters) to 40,434  feet (12,324 meters), and the speed from Mach 0.638 to 0.953. But this initial period also cemented the aircraft’s

reputation as difficult and even dangerous. Even so, the USAF accepted the X-5, with the first Air Force flight by General Albert Boyd in August 1951. After a brief period of trials (six flights, from December 1951 to January 1952, flown by Frank “Pete” Everest), the Air Force released the X-5 to the NACA, which led the program for nearly four years from January 1952 to October 1955. At this point, cockpit time aboard the X-5 became dominated by one man. Like Scott Crossfield, the NACA’s Joe Walker, who held a B.A. in

32  Specific Improvements: Technology Demonstrators Physics, emerged after the war as one of the agency’s most capable pilots. After flying P-38 fighters for the Army Air Forces in World War II, Walker joined the NACA in 1945, transferred to Edwards in 1951, and flew everything from the D-558-1, to the X-15, to the B-47, to the Lunar Landing Research Vehicle. He became Chief Research Pilot at NASA’s Flight Research Center prior to his death in a crash between his F-104 chase plane and the XB-70 research aircraft in 1966. Walker flew almost two-thirds − 78 − of the X-5-1’s 128 flights for the NACA. Like Crossfield in the X-4, he brought the exceptional competence necessary to handle a plane with the X-5’s deadly quirks. By mastering them, he enabled the NACA to determine that variable sweptwing flight had merit, offering good performance in lowspeed flight with the wings fully extended and equally reliable high-speed performance at the full 60-degree sweep (although the sweep mechanism itself proved to be overly complex and cumbersome). Chronic pitch-up problems enabled the X-5 to test this phenomenon at many wing angles, adding to the pitch-­ up literature amassed by Scott Crossfield in the D-558 Skyrocket. The full-flowering of the X-5 occurred well after its retirement, when the multi-purpose, variable geometry F-111 Aardvark fighter/bomber entered the Air Force inventory during the 1960s. X-6 The concept of atomic power for long-range military aircraft circulated among the leaders of the U.S. Army Air Forces (AAF) surprisingly early, in 1944, well before the U.S. nuclear attack on the Japanese city of Hiroshima in August 1945. The concept gained ground beginning in December 1945 with the completion of Professor Theodore von Kármán’s internal AAF study Toward New Horizons. This exhaustive and highly influential report − quickly embraced by America’s air power leaders − surveyed enemy air power in the closing months of World War II, and called attention to nuclear propulsion (among many other things). One of the report’s volumes, entitled “Aircraft Fuels and Propellants”, featured a short essay by Dr. H.S. Tsien, one of von Kármán’s most able students and later the legendary founder and director of the Chinese missile and rocketry programs. In it, Tsien calculated that nuclear reaction used as a fuel source offered an energy release about one million times greater than that of conventional fuels. Tsien realized that not all air vehicles could benefit from nuclear power − rockets and ramjets did not represent good candidates − but he argued that the high rate of heat release inherent in nuclear fuel did not necessarily inhibit the development

of nuclear-­ powered turbojets. Tsien wrote that “the superiority of this system of atomic energy generation over the… ‘conventional’ system certainly warrants further investigation to determine its practicality.” Eager to have the capacity to dispatch fleets of bombers, transports, and tankers to distant parts of the world, AAF authorities recognized that atomic power could provide almost limitless range without significant losses in speed and altitude, or vulnerability to air defenses. Towards that end, the AAF formed the Nuclear Energy for Propulsion of Aircraft (NEPA) project to study the problem in 1946, eventually joined by the Navy, the Nuclear Regulatory Commission, and the NACA. Years went by and other organizations probed the concept, but not until early 1951 did the Joint Chiefs of Staff agree that the idea bore some military utility. Wasting no time, the Air Force − partnering with the Atomic Energy Commission − awarded the Convair Division of General Dynamics a contract in February 1951 to convert two B-36 Peacekeeper bombers into X-6 experimental aircraft, whose mission would be to prove the efficacy of nuclear power as a fuel for aircraft engines. It also ordered one B-36H to serve as a preliminary, flyable testbed to demonstrate the effects of radioactivity. The Air Force also considered Convair’s other gigantic bomber, the YB-60, which had a similar capacity in size and power, to carry a reactor and shield, but decided on the B-36 for reasons of cost control. Soon afterwards, the USAF signed an agreement with General Electric to produce the as yet unfinished J53 turbojet engines for the X-6. Meanwhile, the Atomic Energy Commission also contracted with General Electric to build a reactor to power the J53s. Although these actions laid the groundwork for the X-6 (even as other contractors conducted simultaneous studies on nuclear-powered aircraft), neither the Air Force nor the Department of Defense committed themselves to retrofitting B-36s − or any other aircraft − into atomic-fueled vehicles. Before funding this step, some justifiable caveats needed to be satisfied: Could the crew be protected successfully from radioactivity? Would the propulsion system work effectively? Would the reactor’s output of heat and radiation affect other on-board systems? By summer 1952, Convair and the Air Force decided on two candidate B-36s for conversion prior to the X-6. The USAF sent an XB-36 to Convair for ground tests of radiation effects and shielding, and Convair also received a B-36H (previously damaged in a tornado at Carswell Air Force Base) to be retrofitted as a flying precursor to the X-6. Designated the NB-36H, it tested radiation containment. Convair signed a contract in September of that year to modify both aircraft. Up to that point, Air Force authorities had made no decisions regarding the conversion of additional B-36 airframes to the X-6 configuration.

X-6 33

34  Specific Improvements: Technology Demonstrators The NB-36H underwent significant modifications to accommodate its mission. Designers installed a crew shield just aft of the center of gravity, about 65  feet (19.8 meters) from the rear bomb bay which housed the reactor. In all, engineers calculated a total weight of 165,000 pounds (74,843 kilograms) for the entire nuclear propulsion system, more than half of which constituted protection for the aircraft and crew against nuclear effects, namely 60,000 pounds (27,216 kilograms) of reactor shielding and 37,000 pounds (16,783 kilograms) of crew shielding. In addition, the modifications required a 10,000-pound (4,536-kilogram) reactor; 18,000 pounds (8,165 kilograms) of engine weight; and miscellaneous ducting and other equipment estimated at 40,000 pounds (18,144 kilograms). Moreover, the nose and cabin section underwent extensive redesign in order to accommodate a crew shield capsule, which alone weighed 12 tons. Viewed externally, however, the NB-36H seemed virtually identical to any other B-36H model, barring the restructured aft bomb bay area. Not surprisingly, the NB-36H looked and behaved like a massive nuclear propulsion laboratory. With a length of 162 feet (49.3 meters) and an enormous 230foot (70.1-­meter) wingspan, the giant bomber measured 46 feet 8 inches (14.2 meters) in height. It could stay aloft for 12  hours at altitudes up to 40,000  feet (12,192 meters) and fly at speeds approaching 390 miles (627.6 kilometers) per hour. Its powerplant consisted of four General Electric J47 turbojet engines capable of 5,200 pounds (2,359 kilograms) of thrust each; or six Pratt and Whitney R-4360-53 radial engines, each producing 3,800 pounds (1,724 kilograms) of thrust. The NB-36H weighed 225,000 pounds (102,058.2 kilograms) empty, and totaled an incredible 360,000 pounds (163,293.2 kilograms) fully loaded. For a program that began with expansive expectations (the USAF gave it the highest priority, equal to that of the Atlas ICBM), the X-6 ended timidly. Constrained by tight post-war defense budgets, the incoming President Dwight D.  Eisenhower decided that nuclear aircraft had no practical military worth. After lengthy internal negotiations, the White House refrained from outright cancellation but cut the project’s budget, in effect ending the X-6. Meanwhile, the NB-36H, the sister ship to the now-­ defunct X-6s, made its initial contractor flight at Carswell Air Force Base, Texas, in July 1955. There followed missions from September 1955 to March 1957 in which a live 1-megaton reactor − which provided no power to the engines but vented sufficient heat

and radiation to replicate a fully functioning nuclear system − enabled testing of shielding, nuclear effects on aircraft components, and flight operations. In total, the NB-36H made 47 flights, yielding a wealth of data regarding this much hoped for, but in the end impractical form of aircraft fuel. Even after Convair scrapped the NB-36H in the months after its last mission, the nuclear aircraft program soldiered on. In the end, the USAF expended nearly $500 million on the project, which finally reached its termination point at the start of President John F. Kennedy’s administration. X-13 In April 1947, after evaluating their experiences in World War II and assessing post-war needs, U.S. Navy officials decided to pursue a new type of flight: fixed wings, combined with vertical take-off and landing (VTOL). If the war taught anything to the Navy, it inculcated the supremacy of airpower at sea. What better, more adaptable type of aircraft than ones capable of touching down and ascending vertically, making best use of the restricted flat surfaces on naval vessels? Discussions on this project actually started privately, in 1946, between the Navy’s Bureau of Aeronautics and T. Claude Ryan, owner of the Ryan Aeronautical Company in San Diego, California. Ryan opened his original business during the early 1920s and gained fame by building Charles Lindbergh’s Spirit of St. Louis, the first aircraft and pilot to fly solo across the Atlantic. Ryan sold the company soon afterward, but re-opened it in the 1930s. As a result of the talks, Ryan’s team initiated work on an off-shoot of Ryan’s successful FR-1 Fireball fighter: the Model 38 aircraft, equipped with combined turbine and piston engines that had a promising thrust to weight ratio of 1:1. In an age before turbojets delivered massive thrust, VTOL designers found themselves with two main obstacles: the vehicle needed to be light, aerodynamic, and maneuverable; and it required jet engines of sufficient power to overcome its total mass. After considering several options, the Bureau of Aeronautics gave Ryan the broad guidance to build whatever planform it desired, powered by a turbine engine. Because the project’s ultimate success depended on the powerplant, Ryan studied seven candidates before finally selecting the Rolls Royce Avon RA.28-49, with a maximum thrust of 10,000 pounds (4,356 kilograms).

X-13 35 The airframe selection proved to be even more cumbersome, with Bureau of Aeronautics officials and Ryan engineers sending design proposals back and forth for almost six years. Ironically, the project did not begin to take shape until the Navy ran out of money. At that point, in August 1953, the Air Force signed a contract with Ryan to fabricate and flight test two aircraft, known as the Ryan Model 69 but soon designated as the X-13. The NACA and the Navy joined the USAF as co-sponsors. To make a fresh start, the contractor replaced its original engineering team and brought together a new group. From then on, the X-13 project began to jell. Beginning in January 1954, parts of the aircraft began to be assembled on the shop floor in San Diego. By mid-year, the Avon engine arrived and the X-13 passed

a mock-up inspection. Then, in August 1955, the first of the X-13s made the journey from the Ryan plant to Edwards Air Force Base for flight testing. Although the Bureau of Aeronautics gave Ryan the freedom to design an original airframe, the technicians at Edwards who uncrated it must have been surprised by what they saw. Gradually, a small, stocky fuselage took shape in the NACA hangar, topped by a massive, one-piece delta wing that covered the whole structure like a giant plate. At the rear, a big vertical fin with a single-piece rudder pointed skyward. The wings featured tip fins to increase airflow at high speeds, in addition to elevons at the trailing edge. Engineers provided the X-13’s solo pilot with a small cockpit equipped with a minimalist instrument panel.

Except for the titanium reinforcement at the highheat engine bays, the X-13 consisted of an all-aluminum structure, which helped satisfy the low weight requirement for effective VTOL. It weighed just 7,313 pounds (3,317 kilograms) gross and 5,334 pounds (2,419 kilograms) empty. Small enough in real terms, the immense wing on its back made the X-13 look even more slight.

The fuselage measured only 23  feet 5 inches (7.1 meters) long, with a height of 15  feet 2 inches (4.6 meters) and a wingspan of 21 feet (6.4 meters). Powered by its single Rolls-Royce Avon engine, it could fly as fast as 483 miles (777.3 kilometers) per hour. Ryan’s two X-13s realized brief but worthwhile flight careers. Contractor test pilot Peter Girard made the

36  Specific Improvements: Technology Demonstrators maiden flight of X-13 number 1 on December 10, 1955. During the initial phase of testing, the X-13 flew only horizontally, with a tricycle landing gear. The contractor wanted to isolate and resolve any commonplace handling problems (such as oscillation, which proved to be an issue) prior to taking on vertical flight. Finally convinced of the aircraft’s flying qualities under conventional circumstances, technicians modified the rear of the X-13 with a steel-tube truss and castering wheels, enabling the aircraft to rest on its tail for launch. (On later flights, the X-13 took-off as it hung from a hook at the end of a cable suspended vertically from a launch trailer’s elevated bed). On May 28, 1956, (the same day that X-13 number 2 completed its first horizontal flight) Girard achieved X-13-­1’s initial vertical take-off and landing, taking a low risk approach and flying no more than 50 feet (15.2 meters) high and under 30 miles (48.3 kilometers) per hour. During landings that day, he repeatedly touched down successfully within two feet of the target spot. In the VTOL role, Girard maneuvered with a vectorable exhaust nozzle connected to the flight controls, while during standard flight he guided the aircraft by means of the elevons and rudder. The test program’s penultimate high water mark occurred on November 28, 1956, when Girard made the world’s first jet-powered in-flight transition at 6,000 feet (1,829 meters). He started out horizontally, pitched the X-13 nose up and hovered for some time in the vertical position, and then returned to conventional aerodynamic mode. Next came the big test. Almost five months after the first transition flight, Girard took the controls on April 11, 1957, to make the first ever complete VTOL cycle. He took off vertically from the trailer/launcher, rose to altitude and switched to horizontal flight, conducted a series of standard maneuvers, turned nose up in a hovering position, and descended safely for a landing. The X-13 flight tests then changed venues. After a number of other vertical-horizontal-vertical flights at Edwards, Air Force authorities decided to give VTOL some publicity. They loaded X-13 number 2 on board the USS Young America, where it traveled through the Panama Canal and finally arrived at Andrews Air Force Base, Maryland. There, it flew eight more transition flights and several additional missions for defense officials. The big show came on July 30, 1957. Pete Girard flew the X-13 in conventional mode west from Andrews, crossing the Potomac River with just enough fuel for the trip and a little to spare. When he reached his destination, the Pentagon, he put the X-13 through its vertical paces as a crowd of about 3,000 looked on.

Paradoxically, this event marked both the high point and the end of the X-13. It returned to Edwards AFB, where it joined X-13 number 1 in giving Navy and Air Force pilots exposure to the vagaries of VTOL flight until the end of 1957. Out of money, Ryan stopped work on the project early in 1958, and although the NACA expressed an interest in sustaining the program, the X-13 met a quiet demise. X-14 Although the Ryan Aeronautical Company became prominent in early vertical take-off and landing (VTOL) technology with the X-13, Bell Aircraft offered intense competition. Distinguished as the manufacturer of the first U.S. jet aircraft and some of the most famous early X-planes, Bell also extended its reach to VTOL. In fact, it may actually have succeeded before Ryan. As early as January 1941, almost a year before America’s entry into World War II, Bell submitted a patent application for a VTOL aircraft with counter-rotating propellers and a single combustion engine. The company revisited the technology in 1944 when it evaluated the feasibility of a vertically-launched, twin turbine engine combat aircraft, but decided that existing powerplants did not yet have the thrust to accomplish this feat. After almost a decade of research, Bell tried again in 1949 and 1950, when the Navy’s Bureau of Aeronautics sponsored a competition for a VTOL convoy fighter. Bell’s engineers lost out to Lockheed and Convair, but in the process decided to pin their hopes on a conventional planform rather than a verticallyoriented one. The company pursued this preliminary design (known as the Model 65 Air Test Vehicle) from 1952 to 1955, funded by seed money from the Air Force. In the end, Bell’s engineers proved to themselves that turbojet engines could power vertical flight with a thrust–to-weight ratio as little as 1:1, and that pilot control could be established readily. Convinced by these findings, the USAF (with the NACA as cosponsor) awarded Bell a contract in July 1955 to develop one testbed VTOL day-fighter. It featured the daring, first-ever concept of a vectored, or deflected, jet thrust system that enabled directional control of engine exhaust. Known by Bell as the Model 68, the Air Force designated it the X-14. In order to conserve money and time, Bell’s X-14 team followed the principle of less, rather than more: less complexity, less manpower, and lower cost. They

X-14 37 developed a small, light airframe composed to the maximum extent of such off-the-shelf components as landing gears, tail structures, wings, and engines; a design capable not just of VTOL, but of short take-off and landing (STOL). The flight program consisted of two parts. First, a full demonstration of the jet-­powered VTOL role, from vertical take-off, to conventional flight, to a VTOL landing. Second, Bell planned for an extensive series of V/STOL tests of the aircraft’s variable stability and control, and the adequacy of its powerplant.

Even a momentary glance revealed the essence of the X-14. Unlike the excitement and enthusiasm generated by many, if not most of the X-planes, the X-14 turned heads because of its unpretentiousness. With a small, one-seat, open air cockpit and long, spindly landing gear legs, it appeared anything but forwardlooking and futuristic. Built mainly out of aluminum, the X-14 largely borrowed from two old stand-by, propeller driven planes: a Beechcraft Model 35 Bonanza (wings, landing gear, and ailerons); and a Beechcraft T-34A military trainer (empennage).

The X-14’s dimensions underscored its humble origins. Just 26 feet (8 meters) long, 6 feet 10 inches (1.8 meters) tall, and with a wingspan of 33 feet 9 inches (10 meters), its heaviest weight (as an X-14B) totaled just 4,269 pounds (1,936 kilograms) gross and 3,173 pounds (1,439 kilograms) empty. Its performance substantiated its unassuming look: it flew up to 18,000 feet (5,486 meters) with a range of 300 miles (483 kilometers) and a top speed of only 172 miles (277 kilometers) per hour. Bell equipped the X-14 with a pair of British-made, Armstrong-Siddeley Viper 8 turbojet engines, rated at

1,750 pounds (793.7 kilograms) of thrust each. Like the Beechcraft and T-34 components, the Viper had the advantage of being a known product, and although it barely met the 1:1 thrust-­to-­weight ratio desired by the design team, its availability and certainty made the case for the engine. The X-14 also had the distinction of engines mounted horizontally, at the far forward portion of the aircraft rather than the more logical tail of the vehicle location where the X-13 designers placed theirs. Not surprisingly, Bell found plenty of problems when it undertook preliminary tests at its facility in Wheatfield, New  York, the most vexing being the

38  Specific Improvements: Technology Demonstrators tendency for the demonstrator to be pushed downwards during flight near the runway, rendering vertical movement all but out of the question. This phenomenon resulted from engine exhaust creating a low pressure zone beneath the X-14. The obvious remedy involved lengthening the landing gear legs even more, allowing greater clearance between the belly of the aircraft and the ground. Continuing these flights in 1957, Bell tried a series handling qualities tests. The first hover flight occurred in February, followed by an initial horizontal test in June and a partial transition at 5,000 feet (1,524 meters) during the same month. The company’s pilots then put the X-14 through a set of partial take-off transitions, sometimes just above stall speeds, and got encouraging aerodynamic results. Following these successes, full vertical landings occurred from level flight. Finally, the X-14 accomplished a full VTOL cycle on May 24, 1958, going from vertical lift-off to conventional flight, to vertical touchdown. Bell then transferred the X-14 to the Air Force, which in turn transferred it immediately to NASA’s Ames Research Laboratory in Sunnyvale, California, for further research. There, pilots flew the X-14 to the limits of the Viper propulsion system. They concluded that the aircraft lacked sufficient thrust and decided to replace them with the more powerful General Electric J85-GE-5 engine. Now flying as the X-14A, it served at Ames for 11 years, testing variable stability and control in such areas as V/STOL handling, in-­flight simulations of V/STOL concept aircraft, and assessments of V/STOL control mechanisms. With the addition of a highly significant modification − the installation of a computer-­driven, fly-by-wire variable stability system that allowed the X-14 to reproduce the handling and hovering qualities of many types of VTOL aircraft − it assumed a new name, the X-14B.  It continued on at Ames until May 1981 when, after a hard landing with acute structural damage, NASA decided to retire it. The X-14 not only proved to be an uncommonly durable flying machine (lasting for almost 25 years), but also highly influential in the annals of vertical flight. More than 25 pilots flew it, including Apollo 11’s Neil Armstrong who conducted ten missions in it in 1964 to determine whether it might be an effective trainer for lunar landings (the answer turned out to be no, and NASA instead developed the Lunar Landing Research Vehicle, or LLRV). More importantly, the data gleaned from its long service informed the designers of the first combat VTOL, the famed Harrier fighter/attack aircraft.

X-16 Just as the X-15 came into being when Hugh Dryden persuaded Air Force and Navy brass to fund it as an urgent Cold War project, the X-16 also originated in the high drama of the U.S.-U.S.S.R. competition during the 1950s. But unlike the X-15, which had the backing of a national figure like Dryden, the X-16 fell victim to bureaucratic politics. In essence, it lost out because it lacked the connections necessary for it to survive. A high-altitude reconnaissance aircraft, the X-16 began life at Wright Field in Dayton, Ohio, in late 1952. There, an engineer named John D. Seaberg (a ChanceVought employee on active duty as a major during the Korean War) conceived a radically different type of reconnaissance aircraft. In essence, he combined a powerful turbine engine then under development with long, thin, low-wing-loading airfoils. Seaberg intended to create a vehicle that could fly high above enemy targets, making it hard to detect and even harder to shoot down. By March of the following year, Seaberg distributed design specifications for this concept to Fairchild Aircraft, the Glenn L.  Martin company, and Bell Aircraft, three smaller firms that he hoped would give this relatively small project a higher priority than any of the industry giants. Two proposals appealed to the USAF: Martin’s plans to modify its RB-57, and Bell’s all-new twin engine design. During spring 1954, Seaberg briefed these candidates to the commander of the Air Research and Development Command in Baltimore, and to USAF headquarters in the Pentagon. Both won Air Force approval. In order to disguise its later classified role, the Bell model received the designation of X-16. Just as the Air Force prepared to launch into the project, an outsider learned about it from sources in the Pentagon. Kelly Johnson, leader of the secret Lockheed Skunk Works and designer of the F-80 (the first American jet fighter) as well as the breakthrough F-104, found out about Seaberg’s initiative and threw his hat into the ring by sending a hurriedly prepared unsolicited proposal to Seaberg’s group. Johnson submitted plans that featured a YF-104 fuselage, high-aspect-ratio wings, and a General Electric J-73 turbine engine. Naturally, Seaberg favored the two proposals already approved by the Air Force, and defended them by arguing that the J-73 engine lacked sufficient power for the task. To no one’s surprise, the USAF turned down the Lockheed bid.

X-16 39

Still, the well-connected Johnson refused to quit. While the Air Force busied itself with the X-16, he made contact with Dr. James Killian, the future science advisor to President Eisenhower, who at the time headed a Department of Defense committee on deterring surprise attacks on the U.S. One of Killian’s subcommittees concentrated on intelligence, and Johnson managed to get his reconnaissance aircraft proposal before it. Not only did Killian like the Lockheed concept of Soviet overflight, he convinced Secretary of Defense Charles Wilson and Central Intelligence Agency (CIA) director Allen Dulles of its value. With these three endorsements, President Eisenhower gave authorization to fabricate 30 of Johnson’s aircraft in November 1954, not under the direction of the Air Force, but under the CIA. As a result of these top level commitments, Johnson eliminated the X-16 and supplanted it with the U-2. Rather than disappear immediately, however, the USAF’s X-16 continued briefly in a state of suspended animation. The project remained open during the fabrication of the highly classified U-2, which gave Johnson’s team cover and also ensured a back-up if

Lockheed failed. But the program finally ended in October 1955, two months after a U-2 took to the air for the first time. Since no X-16 ever flew, no one can say how it compared practically to the U-2. It certainly looked the part, with slender and exceptionally long swept wings (114 feet (34.7 meters) from tip to tip) and an almost painfully thin fuselage (only 4  feet (1.2 meters) in diameter and almost 61 feet (18.6 meters) long). Like the U-2, its design enabled the X-16 to take on previously impossible assignments: to fly at altitudes of over 72,000  feet (21,945.6 meters), attain speeds up to 550 miles (885.1 kilometers) per hour, and achieve an unrefueled range of 3,300 miles (5,311 kilometers). Its two cameras covered specific targets and broad vistas: close-in shots of 11 nautical miles (12.6 statute miles) wide, and landscapes of 50 nautical miles (57.5 statute miles) wide, respectively. Weight considerations played an important part in the X-16 design, as engineers sought to stretch the aircraft’s ­altitude and range to the maximum extent. The Bell team relied on aluminum for most of its construction, chose light and highly flexible high-aspect-ratio

40  Specific Improvements: Technology Demonstrators wings, and conserved weight with a narrow, single seat cockpit. Even so, the X-16’s mass still totaled about 36,000 pounds (16,329.3 kilograms) gross and 23,330 pounds (10,582.3 kilograms) empty. Not unexpectedly, the engines and fuel system constituted the heaviest components at 8,598 pounds (3,900 kilograms), especially the two wing-mounted Pratt and Whitney J57-­ PW-­37A turbojets − a variant of the standard J57 modified for the high-altitude mission. Strangely, although the X-15 had one of the longest lifespans of any X-plane in history, its X-16 successor had one of the shortest. In all, the X-16 lasted for about 17 months, with only one partially finished airframe to show for it. Paradoxically, both aircraft promised transformative changes to aeronautics, but only one succeeded. The difference was that the X-15 had Hugh Dryden behind it, a prominent leader who understood the machinery of bureaucracy. In contrast, the X-16 found itself pitted against Kelly Johnson, a man whose stature and connections led to the victory of the U-2 over the X-16. X-18 Although the X-13 and X-14 shared a common VTOL heritage with the X-18, they stemmed from different technical origins. Both the X-13 and X-14 relied on turbine engines fixed in one position. The X-18, in contrast, counted on turboprop powerplants mounted on tilt-wings, whose movement enabled the aircraft to transition from horizontal to vertical, and back again to horizontal flight. Moreover, its designers hoped to demonstrate not only VTOL, but also vertical/short take-off and landing, or V/STOL. Serious V/STOL flight research pre-dated the X-18 by a number of years, with government-sponsored work starting in the early 1950s. The Army and Air Force contracted with helicopter specialist Bell Aircraft in October 1953 to fabricate two V/STOL Model 200 convertiplanes, under the direction of company engineer Robert Lichten. Renamed the XV-3, the aircraft featured nonmoveable wings with engines that transitioned from the forward position for horizontal flight to 90 degrees for vertical. Its powerplant consisted of two Pratt and Whitney air-cooled radial piston engines. Flight testing began in 1955 and continued for 13 years, time mostly spent by Bell, Army, and Air Force engineers trying to solve persistent instability problems. The first indication

of real trouble occurred in October 1956 on a test flight, when the XV-3’s high rotor instability produced extreme cockpit vibrations, causing the pilot to lose consciousness and crash. It underwent further research at Edwards Air Force Base in 1959, and although its pilots complained about its flying qualities, they endorsed the overall feasibility of its V/STOL mission. After Edwards, the NASA Ames Laboratory in Mountain View, California, became involved, conducting wind tunnel research in its cavernous 40 x 80-foot (12.19 x 24.38-meter) full-scale facility and flying it until July 1962. By the time its career ended in 1968, the XV-3’s team had to admit its failure to cure the main weaknesses of roll instabilities in hovering, and oscillation and poor control responses at low speeds. On the other hand, its pilots compiled substantial data in 125 hours aloft, and completed 110 full conversions (transitioning from helicopter to fixed wing and back to helicopter configurations). Having seen enough of the XV-3 to know its deficiencies, the Air Force tried again to achieve workable, practical V/STOL, this time with Navy help. With their new candidate, the fundamental mechanism of conversion from horizontal to vertical flight differed. Instead of merely re-positioning the engines as the XV-3 had done, the X-18 project concentrated on raising and lowering the entire wing, along with its engine assemblies. To achieve this objective, the two services contracted with the Hiller Aircraft Corporation of Palo Alto, California, a firm started in 1942 by Stanley Hiller and known mainly for World War II helicopters. Hiller expanded its repertoire in 1947 when it launched a program of VTOL research, involving both turbine engine and rotary wing plans. The concept of the X-18 came to life in a 1955 Hiller design study sponsored by the Navy Bureau of Aeronautics. Air Force authorities saw the subsequent report, and decided to support an X-planes testbed in spring 1956. In October 1957, Hiller signed a $4 million agreement with the Air Force to fabricate one twin-engine, tilt-wing convertiplane, capable of flying like a conventional aircraft at speeds up to 398 miles (640 kilometers) per hour, in addition to vertical flight. Both services saw something of high potential value in the swing-wing design. Because Hiller envisioned the X-18 as a large, transport-style aircraft, its technology offered clear benefits to its military patrons in a number of tactical regards: take-offs and landings on long or short runways; standard logistical loading practices; high load capacity due to the aircraft’s spaciousness and

X-18 41 its conventional take-off capability; hovering or flying slowly for search, rescue, and to drop and retrieve cargo; and high-speed horizontal flight as needed. As Hiller designed and began to construct the X-18, technicians at NACA Langley’s low-speed wind tunnel mounted an 8-foot (2.4-meter) model of the original to determine its aerodynamic characteristics, a process that lasted for about six months. Later, in December 1958 − just after the NACA became NASA and Hiller finished the aircraft − the company transported the X-18 from Palo Alto to nearby NASA Ames for static tests. There, the engine power settings and the tilt-wing angles underwent extensive scrutiny, in addition to some rigorous shake testing of the whole X-18. More importantly, two Hiller pilots flew the X-18 twice at Ames: once on November 20, 1959, for a brief hop, and four days later for its first true flight. Once completed, Ames workers crated up the aircraft and loaded it aboard a caravan of trucks that traveled the California highways to Edwards Air Force Base, accompanied by 14 Hiller engineers and the two pilots. The X-18 that came together at Edwards from the disassembled pieces looked big and imposing, unlike any

other convertiplane of its time. Hiller’s decision to make a sturdy transport that really looked and acted the part − as opposed to a standard testbed − actually evolved from necessity. To save money on a relatively low payout contract, the company fashioned the X-18 from military hand-me-downs (a practice common at the NACA throughout its history). The X-18 represents a composite: a fuselage derived from a reconditioned Chase YC-122C cargo aircraft; other parts from a Convair R3Y transport; and two Allison T40-A-14 turboprops, one from a Navy Lockheed XFV-1 Salmon fighter, the second from Convair’s XFY-1 Pogo program (these two were the first VTOL aircraft developed and later abandoned). The stubby wing and wing-tilt assemblies originated with Hiller’s engineers, who installed two hydraulic actuators that lifted the wing from its hinged position at 35 percent of the chord line to vertical (90 degrees). This motion took about six seconds during flight, an event that began with the pilot pulling a lever on the control pedestal. In addition to the Allisons, a Westinghouse J34-WE-36 turbojet located inside the rear of the fuselage provided thrust for pitch control during periods of transition or hover.

42  Specific Improvements: Technology Demonstrators Before it ever flew at Edwards, the X-18 drew the attention of onlookers because of its blocky looks and transport-­like proportions. Long at 63 feet (19.2 meters) and tall at 24  feet seven inches (7.4 meters), it had a broad wingspan of 48  feet (14.6 meters). Its twin Curtiss-Wright contra-rotating propellers measured an enormous 16 feet 1 inch (4.9 meters) in diameter. The X-18 weighed 33,000 pounds (14,968.5 kilograms) gross and 27,272 pounds (12,370.3 kilograms) empty. It could fly at an impressive maximum altitude of 35,300  feet (10,759.4 meters), reach a speed of 253 miles (407.1 kilometers) per hour, and range over 224 miles (360.4 kilometers). It accommodated crews of two or three. Despite its compelling appearance, the X-18 had a brief and uneven flight record. Its research program at Edwards Air Force Base began in late 1959 and ended in July 1961. It flew on 20 missions, always flying horizontally during take-­ offs and landings unlike other V/STOLs of its day. But the final attempt almost ended in disaster. Flying at 11,000 feet (3,353 meters) with a 10-degree wing angle on November 4, 1960, the aircraft suddenly yawed left, then rolled onto its back and fell into an inverted spin. It descended to 6,000 feet (1,829 meters) before Hiller test pilot George Bright regained control. Subsequent investigation indicated that the propeller blade angle shifted wildly during the incident, caused by the operating motor being stripped of its gear teeth. This event, coupled with a trend seen throughout testing of persistent delays in propeller roll control response, forced the decision to end the flight program early. The Air Force re-purposed the X-18 as a static ground test vehicle until it ended the program in January 1964. Like so many of the X-planes, the real fruit of the X-18 rested less with its flying achievements than with the applicability of its concept and the information gleaned during its flights. In the case of the X-18, the Navy and Air Force continued to be attracted to the many missions that a swing-wing cargo aircraft could fulfill. Interested for the same reason, the Army joined them in the early 1960s in sponsoring the Vought XC-142, a second-generation V/STOL transport that derived data from the X-18. It developed successfully as a prototype and the Air Force wanted to proceed with a production model, but the Navy withdrew, in effect ending the project. The Air Force finally transferred the XC-142 to NASA in 1966, which conducted its own research until 1970.

X-19 Like the X-13, X-14, and X-18 before it, the X-19 represented a persistent effort by the U.S. military services to seize upon a new form of flight. The lure of a reliable, durable aircraft capable of conducting many missions on a single airframe proved to be irresistible. If perfected, the tilt-rotor aircraft offered a dazzling variety of tactical roles: search and rescue; pinpoint cargo drops and pick-ups; troop movement and extraction in highly remote and dangerous locales; take-­ offs and landings without runways; and, in the conventional flying mode, fast transport of soldiers and materiel (among other advantages). In effect, the X-19 stood at the fulcrum between early tilt-rotor experiments and future embodiments that ultimately resulted in production aircraft. Unlike most X-planes, which began with a government contract and a set of technical requirements, the X-19 originated as an independent project undertaken by the Curtiss-­Wright Corporation, an established firm famous for its propellers. Once the project gathered momentum, its project managers sought funding from the Army Air Mobility Command, an organization with much to gain from such an aircraft. The Army agreed to underwrite a testbed called the X-100. The process started in February 1958, when Curtiss-­ Wright launched investigations into propeller vibration and expanded them to consider rotor blade design. Building on the principle that lift increases as propellers incline from horizontal to vertical flight, the company’s engineers discovered that they could enhance this phenomenon by the use of short, broad propeller blades with a high degree of twist. They incorporated this feature into an aircraft they designated the Model 200. Soon after the Model 200 became known publicly in July 1962, the Air Force purchased two of them − after a prolonged period of negotiation − and re-­named them the X-19. To begin the program, the awkward-looking, stubby X-100 − fashioned out of various aircraft parts − underwent tethered tests in April 1959, followed by true flights in spring 1960 (one of which involved a full transition from vertical to horizontal). These missions validated the concept of truncated, wide rotors as contributors to added lift, although a number of the aircraft’s flaws also materialized and needed to be addressed in the upcoming X-19. Meantime, in October 1960 the contractor transferred the X-100 to NASA Langley, which conducted vertical-only flights to test the effects of prop-wash on a variety of surfaces.

X-19 43

As the X-19 took shape at Curtiss-Wright, onlookers took note of its novel design, one predicated on propellers in keeping with Curtiss-Wright’s specialty. Unlike the X-18, which looked like a cargo aircraft, the X-19 resembled a business jet, and not by accident. The company originally conceived the Model 200 as an executive VTOL, with four seats and ample speed. In this X-planes version, the Curtiss-­Wright team fabricated an aluminum semi-monocoque airframe, with two wings installed atop the fuselage. At each of the wingtips, technicians mounted a nacelle with a tilting propeller. Connected through a system of gearboxes and shafts, the four rotors derived power from two Lycoming T55-L-5 turboshaft engines embedded sideby-side in the aft end of the fuselage. With the propellers swung to the vertical position, they gave the aircraft sufficient lift to rise like a helicopter. Then, at a speed of over 184 miles (296 kilometers) per hour, the propellers could be lowered to the horizontal position for cruise flight, with lift provided by the twin wings. The interior of the X-19 allowed for four passengers or 1,000 pounds (454 kilograms) of cargo, a tight squeeze in a cabin with a cross-section just 4 feet (1.2 meters) high, 4.6 feet (1.4 meters) wide, and 9.1 feet

(2.8 meters) long, with a total floor space of 41.5 square feet (3.8 square meters). These slight interior dimensions indicate the X-19’s external size and its mass. It had a length of 44 feet 5 inches (13.5 meters), a height of 17  feet 5 inches (5.1 meters), a front wingspan of 19 feet 6 inches (5.8 meters), and a rear one of 21 feet 6 inches (6.4 meters). The X-19 weighed 13,660 pounds (6,196 kilograms) gross and 10,070 (4,568 kilograms) empty, with a maximum operational envelope of 25,600 feet (7,803 meters) altitude, 454 miles (731 kilometers) per hour, and range up to 325 miles (523 kilometers). In actual flight testing, these outer limits of performance proved to be elusive. The rollout of the first X-19 occurred in July 1963 and initial tests began at the Curtiss-Wright facility in Caldwell, New Jersey, with more advanced research scheduled later at Edwards Air Force Base. After a main gear assembly collapsed during the inaugural hover flight in November 1963, frustration began to build among Air Force officials as they waited for the next trial, which did not happen until June 1964. Other missions followed that summer, but none attempted more than hovers and partial transitions at low speed and low altitude. After 49 of these limited

44  Specific Improvements: Technology Demonstrators events, and perhaps reacting to growing Air Force impatience, the contractor decided to embark on a full flight test on August 25, 1964. Two Curtiss-Wright pilots, James Ryan and Bernard Hughes, made an STOL take-off and gradually tilted the nacelles downward, dropping from 77 degrees, to 65 degrees, to 55 degrees as the speed rose from 69 to 104 miles per hour (111 to 167 kilometers per hour). As they made these adjustments, lights came on in the cockpit indicating trouble with the number 2 nacelle gear box and the aft T-box. The pilots at first decided to land, but determined that they could not get back to the landing strip. Then catastrophe took hold. Ryan and Hughes moved the throttle to maximum power in the hope that they could try for the runway, but this caused the engine drive train to fail, resulting in the stoppage of the number 2 propeller at 400  feet (122 meters). The aircraft inverted but the crew ejected safely as the X-19 crashed. Even though a second, unused X-19 still awaited testing, this incident persuaded the Air Force and Curtiss-Wright to end the X-19 program in December 1965, after 50 flights and only 3.85  hours of flying time. This did not mark the end of the tilt-rotor saga, however. In 1969, the Army Aeronautical Research Laboratory and the NASA Ames Research Center agreed to collaborate with Bell Helicopter Textron on a new prototype, known as the XV-15. Established in the early 1970s, the Tilt Rotor Project Office at Ames managed the program until its termination in August 1993. In the end, the technologies demonstrated in this long, fruitful partnership manifested themselves in the BellBoeing V-22 Osprey aircraft, first flown in 1989 and delivered as a production vehicle to the Marine Corps in 1999. X-21A Like the X-19 before it, the X-21A originated not from one of the NACA centers, nor from the military services, but instead as a research project pursued by one of the major aircraft industries. Also like the X-19, if the concept proved viable in actual flight, it offered to transform the landscape of aeronautics, in this case by reducing overall aircraft drag by about 25 percent, decreasing fuel consumption, and increasing range to an unprecedented extent. Additionally, the X-21A shares a kinship with many of the other X-planes that owe a debt to Theodore von Kármán. The work of this Hungarian-American

professor − who arrived in America in 1930 to teach at Caltech − remains alive not only because of its fundamental importance, but also because at the close of World War II he and a group of American scientists crystallized the existing body of aeronautical knowledge in the study entitled Toward New Horizons. This work included an entire volume entitled “Aerodynamics and Aircraft Design”, co-authored by H.S. Tsien, one of von Kármán’s most accomplished students, which foresaw the basis of the X-21A project: the pursuit of laminar flow control (LFC). Aware of the inherent limitations of LFC through aerodynamics alone, Tsien and the others wrote that, “The most effective method of maintaining the laminar boundary layer is… by partial removal of the flow by suction through slots cut in the wing surface.” As it turned out, the leader of the company that wanted to pursue LFC enjoyed a close relationship with von Kármán. Jack Northrop often sought his advice on new designs (like the flying wing), even sending a company car and driver to Caltech to take him to the Northrop headquarters in nearby Hawthorne. Whether this relationship led directly to the X-21A or not, in 1949 Northrop decided to put a team of engineers on the laminar flow control project. After more than a decade of research (during which Northrop invited the originator of suction laminar flow control, the Swiss Dr. Werner Pfenninger, to Hawthorne), the company felt confident enough about it to begin flight research. Northrop somewhat extravagantly predicted that laminar flow control had the potential to increase aircraft range by at least 50 percent, improve performance regardless of the weight of the aircraft, and increase endurance between two and four times. Impressed by these sweeping claims, the Air Research and Development Command signed a $34 million agreement with Northrop in August 1960 for two test aircraft, both designated X-21A. (The Federal Aviation Administration added limited funding with the understanding that LFC had the potential to slash the operating costs of the airlines). The USAF’s Wright Air Development Division considered two testbeds, both deactivated Douglas WB-66Ds (an Air Force tactical bomber in the B-66 configuration; and an electronic weather-reconnaissance vehicle with the WB designation). Government officials believed that the WB-66Ds made sense because they offered safety, the potential for quick adaptation, and ready availability. But with convenience came trade-offs. The long-standing tradition dating from the NACA days of using hand-me-down military aircraft for

X-21A 45 flight research always made for compromises and workarounds, and this proved to be the case once again for the X-21A. Not only did the engines have to be detached from the wings and re-mounted at the aft end of the fuselage (to lessen turbulence and noise), but Northrop technicians also found it necessary to replace the original twin Allison J71s with General Electric J79 powerplants. The company made one

more major LFC-related modification to the WB-66Ds. In order to prevent (or at least minimize) wing root airflow from disturbing the laminar motion on the wing itself, Northrop engineers added a large hump/fairing to the top of the fuselage. In general, these adjustments proved to be workable, but not optimal for the achievement of the X-21A research objectives.

More refined and specific transformations followed these sweeping changes to the X-21A planform. Success for the X-21A depended, of course, on the installation of LFC slots and holes on the wings. These modifications required Northop technicians to fabricate a custom, 1,250-foot-­square, 30 percent sweptwing made especially for the X-21A, with uniformly smooth external surfaces free of waves. The team developed unique tooling and manufacturing processes that enabled them to engrave the wing with roughly 120 very small, closely spaced suction slots on the upper wing, and 120 on the lower. The company also had to devise a unique, automated system on tracks to drill the LFC suction holes.

Beyond that, the mechanics of suction needed to be worked out. The matrix of slots and holes came to life from two underwing pods containing two pumps developed by the Airesearch Company, designed to remove boundary layer air from the wing surfaces. In one pod, a low pressure pump compressed suction air from the leading end of the upper surface wing slots, mixed it with air from additional wing slots, and sent it to pod number 2, a high-pressure apparatus that expelled the air through exhaust nozzles located at the end of the pods. With the completion of these alterations and additions, the X-21A flight research program got underway. After 18 months of preparation, the initial X-21A made

46  Specific Improvements: Technology Demonstrators its first flight in April 1963, a puddle-jump from Hawthorne to Edwards Air Force Base. The second arrived at Edwards in August. These two aircraft made an impressive sight on the runway: 75  feet 2 inches (22.9 meters) long with an immense, drooping wingspan of 93  feet 6 inches (23.5 meters), they stood 25 feet 6 inches (7.7 meters) tall. The X-21As weighed 83,000 pounds (37,648 kilograms) full and 45,828 pounds (20,787 kilograms) empty. According to Northrop’s specifications, the crew of five could carry out missions that ranged up to 4,780  miles (7,692.6 kilometers), fly as fast as 560  miles (901 kilometers) per hour, and attain altitudes over 42,500 feet (12,954 meters). In actuality, the program’s 38 test flights covered a wide span of operating environments, at altitudes from 25,000 to 45,000 feet (7,620 to 13,716 meters) and at speeds up to Mach 0.80. A breakthrough came on May 14, 1963, when instrumentation recorded the first ever suction LFC, which reduced friction drag aboard X-21A number 1. But with this achievement came an important qualification, in that the outer third of the wing consistently showed laminar action, while the inboard two-thirds only rarely registered the same, even with the later installation of leading edge fences and other supplementary fixtures. Still, the group routinely measured LFC on 73 percent of the upper wing surfaces and 75 percent on the lower, a landmark result. Once the engineers established the fundamental worth of the LFC system, the X-21A pilots received assignments to fly the aircraft through a variety of adverse conditions, including snow, sleet, rain, turbulent air, humidity, clouds, and − worst of all − insects, dust, and dirt, to determine the impact on the slots. The results spoke for themselves because the slots clogged, and the high cost of maintenance became obvious. Government officials decided to terminate the X-21A in 1964. But the shutdown did not end the search for workable LFC. Not unlike the promise of V/STOL aircraft, LFC proved too alluring to abandon. At the NASA Dryden Flight Research Center, investigators pursued four main candidate systems during the 1980s and 1990s. The JetStar Leading-­Edge Flight Test and the F-16XL supersonic LFC achieved laminar flow by mechanical means. On the other hand, the F-111 Natural Laminar Flow research project and the F-14 Variable Sweep Transition Flight Experiment tackled passive, or non-mechanical LFC.  As a group, these four efforts encountered problems and successes. JetStar’s heavy pumping machinery added too much

weight for efficient handling, but the F-16XL’s 10-million-hole wing glove proved remarkably adept at inducing LFC.  On the passive front, the F-111 Transonic Aircraft Technology (TACT) demonstrator showed − with limited success − the value of laminar flow gloves to induce LFC on swept wings. Two years later, the F-14 program refined the TACT database with more advanced gloves. In the end, the two projects discovered that natural LFC techniques worked effectively on wings at sweep angles up to about 18 degrees. X-22A Like most of the X-planes, the X-22A came into being to test a specific aerospace system, in this case a novel V/STOL configuration based on a system of dual, tandem-ducted propellers. But the X-22A differed from nearly all of the other X-planes in that its designers envisioned a second and even more important role, as a general V/STOL platform that could be reconfigured to prove a wide variety of vertical/short take-off and landing technologies. In this sense, it served as a universal V/STOL testbed, one that offered project managers a host of uses. Because of its universality, the X-22A also differed from most of its X-planes predecessors and successors in its longevity (22 years in the skies) and in the number of missions (more than 500). Also unlike the majority of the X-planes, all three military services sponsored the X-22A, with the Navy assuming the lead. In fact, the Navy and Bell Aircraft collaborated on the project well before the X-22A came into being in 1962. During the early 1950s, Bell’s engineers decided to try the ducted fan configuration as a V/ STOL alternative to conventional propellers and to turbine engines. The system worked by encasing short propeller blades inside circular housings, which reduced the speed and intensity of tip vortices, thus cutting noise. More importantly, this innovation resulted in significantly greater thrust than in standard propellers. Around 1957, the Navy authorized Bell to conduct design studies applying ducted fan/vertical flight technology to utility, transport, observation, and rescue aircraft. Meanwhile, the contractor and the University of Wichita conducted a test program that resulted in a database of ducting fan thrust, pitch, lift, and drag. Air Force researchers took notice of the Navy’s work and paid particular attention to the design of Bell’s D-190 ducted fan assembly, thinking that it might be mated with a conventional aircraft like the C-130 and adapted for search-and-­rescue missions. The concept

X-22A 47 also piqued the interest of Army authorities, for obvious reasons. In early 1961, the three services opened a competition between Bell and Douglas Aircraft for a ducted fan V/STOL configuration. Bell designated

their candidate as Model D-2064, the basic design of which had been tested in NASA wind tunnels for over a decade. In November the following year, the Navy gave the go-ahead to Bell for fabrication.

After full-scale wind tunnel tests at NASA Ames in 1963, Bell engineers set up a propulsion stand to test the X-22A’s engines (four turbine General Electric YT58-GE-8Ds) and associated dynamic components. The following May, the first of two X-22As rolled off the Bell factory floor in Wheatfield, New York. Number two followed suit in October 1965. St. Patrick’s Day in 1966 proved to be lucky for the X-22A. On that date in March the aircraft made its first flight, consisting of four take-offs and landings and a 10-minute hover flown below 25 feet. But the good fortune soon ran out. In August, the first of the X-22As crashed. Investigators attributed the loss to a failure of the dual hydraulic system, and the extent of the damage prevented its future use. The second X-22A flew for the first time in January 1967 and continued the contractor research phase under Bell (assisted by NASA) until January 1971. It amassed 228 missions and 125 hours

aloft, including 443 vertical take-offs and 257 conversions (flying from vertical, to horizontal, to vertical). Once the contractor program ended, the Navy took possession of the X-22A and signed an agreement with the Calspan Corporation for additional test flights. These lasted an incredible 11 years from August 1971 until September 1982, accounting for 259 missions. Calspan fulfilled five task orders for the Navy: 1) longitudinal STOL flying qualities in instrument and visual landings; 2) the same, but related to lateral directional dynamics; 3) control, display, and guidance during VTOL transitions from vertical to horizontal flight; 4) elaboration of task 3; 5) sundry additional flights. Bell’s design concept for the X-22A proved to be functional, durable, and unique. Made of aluminum skin and structural components, the X-22A used shafts to connect its four GE turbine engines to the four ducted fan propellers, which rotated up to five degrees per

48  Specific Improvements: Technology Demonstrators second by hydraulic power. The fuselage measured 39.5 feet (12 meters) long with a roughly square crosssection. Its twin wingspans varied greatly in length: 23 feet (7 meters) for the front, and more than half that again − 39.3 feet (11.9 meters) − for the rear. Despite a substantial gross weight of 18,016 pounds (8,171.9 kilograms), the X-22A could range up to 445  miles (716.1 kilometers), fly at 255 miles (410.3 kilometers) per hour, and reach an altitude of 27,800 feet (8,473.4 meters). The fact that the V/STOL ducted fan configuration is yet to be adopted by any production aircraft does not diminish the X-22A’s achievement. Many years of X-22A flights proved the viability of the ducted fan, in addition to demonstrating a great number of technologies pertinent to the whole family of V/STOL vehicles. X-25 Unlike virtually every other X-plane, the X-25 arose out of the specific needs of an ongoing war, During the Vietnam conflict, the frequency of American pilots being shot down deep over enemy territory prompted U.S. defense officials to consider measures to save them from parachuting into hostile hands. Among the solutions considered was the unlikely sounding idea of packing small autogyros into cockpits as a means of escape. In theory at least, these ultralights would enable aircrews to evade capture and offer the hope of reuniting them with friendly forces. As the war escalated, the idea gained traction, and in late 1967 Air Force officials issued a call to industry for ultralights able to perform the this novel mission. Called the Discretionary Descent Vehicle (DDV) program, it envisioned an airframe equipped with a small engine, capable of transporting the endangered crew to safety. Bensen Aircraft Corporation of Raleigh, North Carolina, proved to be the leading candidate when it submitted plans for a modified version of its popular B-8 homebuilt autogyro. The Air Force Flight Dynamics Laboratory awarded Benson a contract to fabricate three ultralights, one a simple, unpowered gyro-­chute, and the other two as slightly modified Bensen products: the B-8M gyrocopter and the B-8 gyroglider. In due course the Air Force named them, respectively, the X-25, the X-25A, and the X-25B. These three airframes consisted of an all-aluminum ­tubular structure, with one seat (for the X-25) or two (for the X-25A and B); a twin-bladed rotor; a rudder;

and (in the cases of the X-25A and B) a single McCulloch 4318E single-­ stroke piston engine. The three aircraft mainly differed in bulk and in height. The X-25 and X-25B each weighed 325 pounds (147.4 kilograms) gross (without the 77-pound (34.9-kilogram) engine) and 125 pounds (56.7 kilograms) empty; the X-25A weighed 500 pounds (227 kilograms) gross and 247 pounds (112 kilograms) empty. At 8  feet (2.4 meters), the X-25 had considerably greater height than the 25A and B, which measured 6  feet 8 inches (2 meters) and 6 feet 3 inches (1.9 meters), respectively. Other than that, they had about the same length (11 feet (3.35 meters) for the X-25, versus 11  feet 3 inches (3.429 meters) for the X-25 A and B), and the very same width for all three (5 feet 8 inches, or 1.7 meters). After delivery of the X-25A and B in February 1968, flight research on both them and the unpowered X-25 began in three stages. First came ground tow tests of the X-25B, designed to give fighter pilots − the ultimate consumers for these aircraft − a say about their handling qualities. Second, pilots flew approach and landing tests, taking the X-25A to 2,000  feet (609.6 meters) and then shutting off the engine to check the aircraft’s descent characteristics. Finally, anthropomorphic dummies stood in for the live pilots, to determine the physiological effects of autogyro landings. A total of 136 towed flights occurred between February 1968 and April 1969. Then the powered phase began in May 1969, when the X-25A pilots flew it to the predetermined height, cut the engine, and landed using autorotation. Perhaps the most interesting tests happened when more than 20 fixed-wing pilots (none with helicopter experience) took the controls of the X-25B and soloed for about 20  minutes each. The resulting data told project engineers how long fighter pilots needed to learn the peculiarities of these airborne lifeboats. In the end, the X-25 project came to naught. It tested the Discretionary Descent Vehicle concept and found that it had the potential to succeed in combat, but more pressing necessities of the war prevailed and the program ended in 1968. X-26 When most people think of the X-planes, they imagine some of the most advanced experimental aircraft ever conceived, such as the X-1, the X-15, and the X-20 Dyna-Soar. But in spite of this inclination, some of the X-planes stand out for their simplicity rather than for their complexity. The diminutive X-25 autogyro,

X-26 49

50  Specific Improvements: Technology Demonstrators designed to carry endangered aircrews escaping capture behind enemy lines, represents one of these uncomplicated projects. Two others, pursued at about the same time, also offered practical, low cost solutions to pressing problems. The X-26A and B not only defied the X-planes tradition of high-stakes, high pay-off aircraft, but surprisingly still attracted three powerful government sponsors: the Defense Advanced Research Projects Agency (DARPA), the U.S. Army, and the U.S. Navy. Lockheed Missiles and Space Company of Sunnyvale, California, joined them in the project. Like the X-25, the X-26A and B grew out of the contingency of the Vietnam War. The process began circuitously in early 1965, when John Foster, the Defense Department’s Director of Research and Development, asked Lockheed for an assessment of the main combat deficiencies faced by the U.S. in the Vietnam War. The contractor’s Advanced Concepts Group studied the proposition and pinpointed two areas in which it could help: night reconnaissance, and the infiltration of U.S. ground positions by enemy forces. The team studied the spectrum of available sensor technology and found that the difficulty lay not so much in identifying effective ones − acoustic or otherwise − but in finding aircraft quiet enough to offer little or no interference with the collection of data. As a second reason for silence, or near silence, these platforms needed to fly over hostile terrain without calling attention to themselves. After testing many off-the-shelf alternatives, the Lockheed engineers finally came to the conclusion that the sailplane offered the best option. The team selected the popular Schweitzer SGS 2-32 as its airframe and acquired two of them. Lockheed chose the Schweitzer for several reasons. It had been in production for many years, with a fine safety record, and its high performance and all-metal design suggested suitability and durability. Because of its origins as a trainer, its cockpit offered two seats, roominess, and adaptability for Lockheed’s purposes. In keeping with its commitment to complete the project in minimal time and with low cost, Lockheed made relatively minor modifications on the two Schweitzers. The engineers added a 100-horsepower Continental 0-200-A air-cooled engine and drive train, and increased the height of the Schweitzer’s vertical fin in order to counteract some of the vehicle’s well-known proclivity for yaw-roll coupling. They used off-the-shelf components wherever possible. Lockheed chose Tracy Airport near San Francisco for the initial test flights due its proximity to the company’s Sunnyvale location, later moving the project to nearby Crow’s Landing, a Navy auxiliary field that offered more

privacy and more open airspace. Lockheed called the modified Schweitzers the QT-2 (Quiet Thruster, twoseat); the government designated them as the X-26B. Flight research began on both aircraft in August 1967. The results brought good news, after a series of sensitive sonic trials. The researchers determined that at 1,000  feet (305 meters), in quarter moonlight, and with 50 decibels of ambient sound (considered to be average night time conditions over a small country town), the X-26B proved to be almost undetectable. Impressed so far, the Department of Defense (DoD) directed Lockheed to prepare the QT-2s for combat by adding sophisticated avionics and navigation systems. In mid-1968 they made the journey to Vietnam and, about one week prior to the first Viet Cong attacks of the Tet Offensive, the X-26Bs took to the skies. Flown by Navy, Air Force, and Army crews, they conducted five-hour, night time missions for two months, flying a total of 600  hours and providing on-the-spot data on enemy movements. Before their return to Lockheed, and ultimately to DARPA, the QT-2s proved their efficacy on the battlefield and offered insights about how their design could be improved for future platforms. Based on these lessons learned, Lockheed acquired a third Schweitzer for further modification. Called the Q-Star, it featured a strengthened wing center section, additional fuel capacity at the wings’ leading edges, sprung steel landing gear, and a more powerful engine than that of the QT-2s. Further revised by Lockheed under the name YO-3A, nine of them saw service in Vietnam for 18 months, beginning in 1970. By now a fully operational combat aircraft, they contributed significantly to the U.S. war effort. Once the QT-2s ended their Vietnam tour of duty, DARPA handed them off to the Navy. The timing proved to be fortuitous. During the late 1960s, Navy trainers found themselves struggling to teach inexperienced pilots the dangers of roll coupling, in part because conventional trainer aircraft simply flew too quickly to demonstrate the appropriate countermeasures. Then someone thought about the two QT-2s, with their slow roll rate and easy recovery characteristics. It seemed like a workable and imaginative answer to the problem. Navy officials also bought two stock Schweitzers to join the QT-2s, and added them to the inventory at the U.S.  Naval Test Pilot School at Patuxent Naval Air Station (NAS) in St. Mary’s County, Maryland, in August 1968. Even though the intended program lacked any experimental ambitions, the Navy designated these four aircraft as X-26As, indicating their kinship with the X-26B airframes.

X-26 51

Indeed, because both relied on the Schweitzer basic airframe, the X-26A and B shared some dimensions: 57 feet 1.5 inches (17.4 meters) in wingspan, with 180 square feet (16.7 square meters) of wing area, and a height of 9 feet 3 inches (2.7 meters). But

the X-26B’s propulsion system made for some obvious distinctions. The X-26B measured 4  feet (1.2 meters) longer at 30  feet 99 inches (9.1 meters), compared to the X-26A’s 26  feet 9 inches (7.9 meters). Also, the X-26B weighed 2,182 pounds

52  Specific Improvements: Technology Demonstrators (990 kilograms) gross and 1,432 pounds (649.5 kilograms) empty, in comparison to the X-26A’s 1,430 pounds (648.6 kilograms) and 857 pounds (388.7 kilograms), respectively. The X-26B also flew higher and faster than the X-26A: an altitude of 18,500 feet (5,638.6 meters) versus 13,000 feet (3,962 meters); and a speed of 158 miles (254 kilometers) per hour versus 115  miles (185 kilometers) per hour, respectively. During its early operational life, the X-26A experienced some deadly setbacks. Two of them went down in fatal accidents in March 1971 and May 1972, while a third did the same in September 1980. Their lack of propulsion systems and their unusual flight characteristics (especially for inexperienced pilots) made them problematic, but Navy officials persisted, replacing two of the damaged ones and keeping them in the inventory. Ironically, what may have been the least experimental of all of the X-planes has proven to be the most durable. Initiated in 1968, two X-26As continued to serve pilot trainees at Patuxent NAS in 2020. X-27 In the annals of the X-vehicles, the X-27 embodies something unique as an X-plane with no governmental sponsor. But it did have a connection, although a weak one, with the U.S. military. The X-27 represented an attempt by Lockheed Aircraft to extend the success of its famous F-104 Starfighter, a lightweight combat aircraft not only highly regarded by the U.S. armed forces, but an immensely popular model in the DoD’s foreign military sales program. During its 50  years of active service, around 2,500 F-104s rolled off the assembly lines and it became the primary NATO fighter aircraft, entering the inventories of about 15 countries. So the objective of lengthening the service of the F-104 made good corporate sense. To achieve the goal, Lockheed’s Clarence L. “Kelly” Johnson, the director of the company’s famed Skunk Works, envisioned a follow-on Starfighter that modified the original design and improved its performance, but at the same time stayed faithful to the original. Kelly wanted to ensure that this revised model, known in-house as the CL-1200 Lancer, borrowed heavily from the Starfighter, which he hoped would reassure his old customers that what they liked in the F-104 − including familiar training protocols,

logistics support, and maintenance − would all be carried forward in the Lancer. The updated F-104, which the Air Force referred to as the X-27, faced a steep and rock-strewn path from its inception. The project began in 1961, with Lockheed determined from the start to retain the F-104 airframe, retain but reposition its distinctive wings, and provide a new and more powerful engine. After about 700 studies by the Skunk Works team, Lockheed settled on the CL-1200 Lancer configuration in spring 1971. Their design certainly fitted the bill in terms of ­good-­looks and a modern profile. It also made genuine improvements over the F-104, with wing area increased by 59 percent, internal fuel storage up 56 percent, and greater payload capacity. Lockheed’s engineers also changed the F-104 planform by mounting the wings on top of the rear fuselage (rather than on either side of it, as in the F-104) and placed the tail low on the fuselage (rather than at the top of the vertical fin, as before). Overall, the X-27/Lancer measured 56 feet 4 inches (17.16 meters) long and stood 16 feet 4 inches (4.98 meters) tall. With a wingspan of 28 feet 8 inches (8.7 meters), it had a wing area of 311 square feet (29 square meters). According to plans, the Lancer would weigh a good deal more than the F-104, at 32,500 pounds (14,741.7 kilograms) gross and 17,250 pounds (7,824 kilograms) empty versus about 19,800 pounds (8,981 kilograms) gross and 14,000 pounds (6,350 kilograms) empty, respectively. The two also differed in speed and altitude, but not by much: 60,000  feet (18,288 meters) and 1,450 miles (2,333.5 kilometers) per hour for the Lancer; 58,000 feet (17,678 meters) and 1,320  miles (2,124 kilometers) per hour for the F-104. The Lancer did show a significant improvement in range at 2,100  miles (3,379.6 kilometers), compared with 1,250  miles (2,011.6 kilometers) for the F-104. Similarly, the Pratt and Whitney candidate engine for the Lancer (the TF30-PW-100) developed 15,000 pounds (6,804 kilograms) at military thrust, whereas the General Electric J79 on the F-104 delivered 10,000 pounds (4,535 kilograms) of thrust at military power. In trying to sell the improved F-104 to DoD officials, even the persuasive and driven Johnson faced an almost insurmountable task. Indeed, even though the Air Force committed no funding to the X-27, he sought to apply pressure by lining up European customers in anticipation of future developments, and

X-27 53

54  Specific Improvements: Technology Demonstrators succeeded in getting two European governments to contribute major components and systems to the project, at low or no cost. But this external campaign failed to persuade the DoD, and more specifically the U.S. Air Force and Navy. By the early 1970s, officials within these services had committed themselves to a new generation of fighter aircraft: respectively, the McDonnell-Douglas F-15 and the Grumman F-14. With little warning (or, for that matter, regret), the DoD turned its back on the Lancer/X-27 during this timeframe. X-28 Like the X-25 and the X-26, the X-28 grew directly out of a particular military need made evident during the Vietnam War. Specifically, the U.S. Navy found itself deficient in conducting civil police patrols in Southeast Asia. After an internal study, officials determined that the service could resolve the problem with a small, single-engine sea-plane, which the service designated the Air Skimmer.

In a highly atypical acquisition process, the Navy turned to a private citizen named George Pereira to solve its problem. Pereira used his experience of fabricating and racing power boats to construct a vehicle at his own home in Sacramento, California; a short takeoff-and landing (STOL) flying boat that he finished in 1969 and named the Osprey 1. Somehow, a Navy representative familiar with the Air Skimmer program found out about the Osprey and contacted Pereira in August 1971. Tests of the aircraft, designated the X-28A by the Navy, began the following month at the Naval Air Engineering Center, located on the Philadelphia Naval Base. In just a few weeks (September 16 to October 22, 1971), Navy pilots familiarized themselves with its characteristics and practiced mock low-altitude patrol and surveillance missions. The Osprey did well. It took just eight seconds to take-off, needed only 300  feet of runway, cruised at 65 to 100 miles (104.6 to 160.9 kilometers) per hour and proved adept at tight turns at bank angles up to 45 degrees. Pilots noticed a few deficiencies, including a high proclivity for pitch-up, left wing heaviness, and the lack of an engine starter.

X-29 55 In keeping with the federal government’s strict acquisition regulations, the Osprey did not win immediate approval. It had to compete against six other airframe candidates, with all of them needing to fulfill some exacting requirements: a single-place cockpit; low horsepower; under 1,000 pounds (453.5 kilograms) gross weight; instrumentation necessary for VFR flight; a simple airframe; good handling on narrow waterways; good flying qualities; low maintenance; and a bulk price under $5,000 apiece. The winner also needed to lend itself to manufacture in Southeast Asia. The Osprey won the battle but lost the war. No victor could have been less pretentious. Pereira built the Osprey’s fuselage from wood and the other parts out of marine plywood. The powerplant consisted of one Continental C90-12 four-cylinder, air-cooled engine, which ran on automobile gasoline. Its propulsion system and construction materials suggested its dimensions and bulk. At just 17  feet 3 inches (5.2 meters) long, it measured only 5 feet 3 inches (1.6 meters) tall, with a wingspan of 23 feet (7 meters) and a wing area of only 97 square feet (9 square meters). It weighed a mere 522 pounds (236.7 kilograms) empty and 900 pounds (408.2 kilograms) gross. Yet it could fly up to 18,000  feet (5,486.4 meters), had a range of over 375 miles (603.5 kilometers), and could attain a speed of up to 135 miles (217.2 kilometers) per hour. Despite its potential, the Navy decided to terminate the Air Skimmer project following the flight research program that ended in October 1971. Like the other aborted Vietnam-­era X-plane − the X-25 ultralight − the X-28 became a casualty of the shifting requirements of an ongoing war. X-29 In its audacity and divergence from the norm, the X-29 occupies a prominent place in X-planes history. While it might not rank with such icons as the X-1, the X-15, and the X-20 Dyna-Soar, it still deserves special notice. More than even the most famous X-planes projects, the X-29 stands alone in challenging an almost inviolate canon of aircraft design: that planforms conform at least loosely to the anatomy of birds. As early as the late eighteenth century, English inventor Sir George Cayley (1773–1857) envisioned and fabricated gliders based on an avian pattern, with a lengthwise body, straight or swept wings, and a tail structure. Those involved with the X-29 forward-swept-wing (FSW)

aircraft upended this foundational principle of heavierthan-air flight. Others tried before them. During World War II, Hans Wocke and his team at the Junkers Aircraft Company in Germany fabricated a prototype bomber with FSW, designated the Junkers-287. Wocke believed that the design offered significant performance advantages, and held flight tests in August 1944 that confirmed the value of the concept, though only at low speeds. The idea traveled to Russia when Soviet forces captured the Junkers-287, along with Wocke himself. In America, the Cornelius Aircraft Company constructed a small, forward-swept-wing tailless aircraft named the Mallard. It flew 18 times in 1943 and 1944, but the subsequent crash of a scaled-up version cast doubt on the validity of the concept. Researchers long suspected that by reversing the standard wing, aircraft would experience more agile maneuvering, less compressibility in the transonic range, and higher lift at subsonic speeds. But two technological hurdles needed to be overcome before a vehicle with FSW even had a hope of flying with full success. First, engineers needed to resolve the issue of mass. Metal wings with sufficient strength to withstand the structural loading of this new design weighed far too much to be practical. Second, due to its inherent instability a full-functioning forward-swept-wing aircraft could not remain airborne by conventional, pilot-actuated, stick and rudder control methods. The answer to both conundrums occurred during the 1970s. Newly conceived lightweight laminates and composites solved the strength versus bulk problem, and digital fly-by-wire offered the moment-to-­moment inputs necessary to sustain a FSW aircraft in flight. Decades after the Junkers-287 and the Mallard, these twin aeronautical breakthroughs made the X-29 a possibility. The prospect attracted keen interest at the Defense Advanced Research Projects Agency (DARPA) and the Air Force Flight Dynamics Laboratory (AFDL), which joined forces to issue study contracts to industry in 1977. The responses proved to be encouraging, and with that the DARPA/AFDL team invited the manufacturers to submit plans for an actual FSW testbed. Three aerospace companies responded: Grumman Aerospace, Rockwell International, and General Dynamics. Following three years of design work, the award went to Grumman in December 1981 with a $71.3 million contract to fabricate two full-sized airframes. To manage the overall project, the Air Force organized an X-29 advanced program office at Wright Patterson Air Force Base in Ohio. NASA, meanwhile, made important

56  Specific Improvements: Technology Demonstrators

contributions by offering its facilities at the Dryden Flight Research Center in California, in addition to the technical support of its pilots and engineers. The X-29 represented an opportunity for DARPA and AFDL to answer some open questions: Could composites resolve the weight penalties associated previously with FSW? What overall advantages did FSW offer to aeronautics? In what ways could the existing technology base of FSW be expanded? Could FSW find its way into new aircraft designs? Although Air Force officials hoped to pursue flight research on the X-29 expeditiously, an expectation reflected in Grumman’s decision to rely on major off-theshelf components for the project, the process of development took longer than expected. Wind tunnel testing consumed several thousand hours in 1981, and although Grumman started fabrication in January 1982, software and associated problems took almost two years to sort out. This delay should not have been surprising since the X-29 relied totally on computer guidance. Its 35 percent instability in pitch (compared to ten percent for the F-16 and the Shuttle Orbiter) rendered it utterly dependent on the skill of its control law designers for its safe flight. Grumman finally rolled out the second of the X-29s from its plant at Calverton, New York, in August 1984.

The next day, its twin started a series of taxi tests to determine ground handling. Once completed, technicians disassembled X-29 number 1 and sent it on a long, indirect journey west by barge to Bayonne, New York, and from there on a container ship through the Panama Canal to the port of San Pedro, California. A trip by truck across the Southern California sprawl ended at the Dryden Flight Research Center on Edwards Air Force Base. The vehicle that they uncrated looked surprisingly like the Northrop F-5 fighter. The X-29’s designers copied the famous military aircraft’s forward fuselage and cockpit canopy profiles, and also borrowed some of its components. The F-5 even shared similar overall dimensions with the X-29. From tip to tail, the X-29 totaled 48 feet 1 inch (14.6 meters), just 1 inch (0.02 meters) shorter than the F-5E Tiger II. Its wingspan of 27 feet 3 inches (8.3 meters) measured only 7 inches (0.17 meters) less than that of the F-5, while its height of 14  feet 3 inches (4.2 meters) made it about 12 inches (0.30 meters) taller than the F-5E.  Moreover, despite their very different wing compositions and orientations, their wing areas did not differ by much, at 188.4 square feet (17.5 square meters) for the X-29, and 186 square feet (17.28 square meters) for the Tiger II. Even the gross weight did not vary radically,

X-29 57 at 17,303 pounds (7,848.5 kilograms) gross for the X-29, and 15,745 pounds (7,141.8 kilograms) gross for the F-5E. The performance specifications for the

X-29 allowed for a maximum altitude of 55,000 feet (16,764 meters), a one-hour range, and a speed of up to Mach 1.8.

Once reassembled, X-29 number one − and later the number two ship − began a long and exceptionally active flight research career. The initial engagement occurred on December 14, 1984, when Grumman pilot Chuck Sewell flew it across the blazing Mojave skies. During about an hour aloft, he took the X-29 to a speed of Mach 0.43 and an altitude of 15,000 feet, tested the flight control system, simulated landings and stalls, and finally touched down. He reported no difficulties. Three more contractor flights followed, on February 4 and 22, 1985, and again on March 1, covering functional checks, handling qualities, and calibrations related to angle of attack (AoA) and simulated flame-­out. Project engineers deemed these flights successful, although Sewell earned the displeasure of his Air Force colleagues when he added unscheduled roll maneuvers to the flight program at the end of flight number three. With the turnover of X-29-1 from Grumman to the Air Force on March 12, 1985, a long succession of flights got underway. Beginning with the first government test by NASA pilot Steve Ishmael on April 2, 1984, a total of

22 pilots flew the two X-29s a monumental 436 times over the course of almost eight years (December 1984 to October 1992). That quantity represents a schedule of more than 4.5 events per month, a truly exceptional output. To better understand the intense workload, 122 of those missions flew with Ishmael in the cockpit, with another NASA pilot, Rogers Smith, taking the controls for 99 more. Together, their total of 221 accounted for more than half of all X-29 flying activity. During the outings aboard X-29 number 1, the pilots concentrated on six key maneuvers: wind-up turns to evaluate divergence, buffet, loads, and performance; push-over/pull-­ups; stick raps to determine flutter and aeroelasticity; formation flying to simulate refueling; accelerations and decelerations in level flight in order to calibrate airspeed and performance; and stability and control maneuvers to find aerodynamic derivatives. Ishmael, Smith, and their colleagues also discovered a great deal about the practicalities of flying the X-29 − its handling qualities, AoA characteristics, buffeting, and loads. Overall, the flight research yielded a vast

58  Specific Improvements: Technology Demonstrators library of data that confirmed the durability of the X-29’s aeroelastic wings, and proved the airworthiness of an inherently unstable aircraft equipped with digital-fly-by-wire. The X-29 number 2 achieved a far different objective, in determining whether FSW contributed significantly to the level of agility desired in future fighter aircraft. It began its career in May 1989 and concentrated on high AoA flight, with project engineers hoping to reach angles as high as 70 degrees. Pilots tested the aircraft’s military applicability, flying qualities, control system, and nimbleness in this extreme regime. They found that the maneuverability far exceeded the mathematical models up to 45 degrees, but the X-29 offered limited control at 67 degrees. During the final phase of X-29 flight research, the team undertook vortex flow control experiments by outfitting X-29 number 2 with two small-­nozzle nitrogen jets mounted on the upper part of the nose. Sixty of these test flights occurred between May and August 1992, and the jets enabled the pilots to achieve high AoA while at the same time successfully steering the aircraft from side-to-side. In addition to their achievements aloft, the X-29s made some important contributions on the ground. To be fair, the airframe itself offered little new: aluminum construction overall, with steel in high load areas; the forward fuselage, cockpit canopy, and nose landing gear all borrowed from two scrap Northrop F-5As; main landing gear, an emergency power unit, and servo-actuators supplied by the General Dynamics

F-16A; and a single General Electric F404-GE-400 turbofan engine, a Navy derivative of the powerplant used for Northrop’s YF-17 prototype. Nor did the digital-fly-by-wire software applied to the X-29 actually break new ground. Instead, its developers adapted it to the peculiar needs of the X-29s. For instance, at one time in the test program the aircraft could achieve no high than 21 degrees AoA, until engineers at the Dryden Flight Research Center and the NASA Langley Research Center, along with help from Grumman, modified the software and increased the control limits. The most significant technological achievement of the X-29 involved the composition of its airfoils, which consisted primarily of aeroelastically-tailored composites (that is, composites arranged in specific patterns that protected the wings from structural divergence). In addition, the aircraft’s team designed the wings to achieve bend/twist coupling, necessary to counteract the FSW tendency to wash in, or twist, during loading. Like many other X-planes, the X-29’s main technical contribution − its advanced composite wing structures − enjoyed far-reaching applications, but its overall concept did not translate into adoption by military or commercial decision makers. Composites more than proved themselves in the X-29 and in other settings, but no clear victor emerged in the contest for maneuverability between the forward-swept wings of the X-29 and the conventional airfoils on modern fighter aircraft. For that reason, FSW failed to make an impact on the design of production aircraft.

3 Prelude to the High Frontier: Early Space Vehicles

X-8 In the closing months of World War II, Henry “Hap” Arnold, Commanding General of the Army Air Forces (AAF), persuaded Caltech’s eminent aeronautics Professor Theodore von Kármán to lead a team of American scientists to Europe and Asia. Their mission: to collect documentation and artifacts pertaining to advanced aeronautical research undertaken during the war by Germany, Japan, Russia, and other nations. They subsequently published the survey in the multivolume work entitled Toward New Horizons. Quickly embraced by the U.S. air power establishment, Toward New Horizons shaped American aeronautics research for decades to come (and receives mention in several sections of this book, including the X-4, X-6, X-7, X-9, and X-10, among others). Within the broad front covered by Toward New Horizons, the “Technical Intelligence Supplement” volume included a section devoted exclusively to World War II German rocketry. This report helped to propagate and even to popularize this subject as it then existed, but the allies took it a step further by managing to seize many functional V-2 ballistic missiles during and after the war. Indeed, the U.S.  Army Ordnance Department, which took charge of the V-2s, scheduled

about 25 V-2 flights from White Sands Proving Ground in New Mexico, using them for high-altitude atmospheric launches. As a medium to conduct research, however, the V-2s had a major flaw in that only a finite number existed. While it supervised the V-2 program, Army Ordnance also sponsored the development and testing of the Jet Propulsion Laboratory’s WAC Corporal sounding rocket, primarily a military and meteorological vehicle capable of carrying small payloads of up to 10 pounds (4.5 kilograms) and attaining altitudes over 200,000 feet (61 kilometers). Inspired by these early but limited forays into the upper atmosphere, the NACA, the Navy, the Army, and the Air Force (after its founding in 1947) collectively supported a post-war program for indigenous, relatively simple research rockets able to carry packages of roughly 150 pounds (68 kilograms) to altitudes as high as 300,000  feet (91.4 kilometers), a much heavier cargo and a much higher altitude than the limitations of the WAC Corporal. The internationally accepted Kármán line differentiated the Earth’s atmosphere from space at 62 miles (327,360 feet, or 99.77 kilometers); the U.S. reckoned it at 50  miles (264,000 feet, or 80.4 kilometers). By the American standard, the proposed rocket would be capable of spaceflight.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 M. H. Gorn, G. De Chiara, X-Planes from the X-1 to the X-60, Springer Praxis Books, https://doi.org/10.1007/978-3-030-86398-2_3

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60  Prelude to the High Frontier: Early Space Vehicles

The origins of the X-8 project ultimately lead back to Theodore von Kármán, as did so much of post-war aeronautics. In addition to directing the Guggenheim Aeronautical Laboratory at Caltech, von Kármán also founded the Jet Propulsion Laboratory (JPL) and the Aerojet Company, both Southern California institutions as is Caltech. When the time came to press forward with this new generation of research rocketry, the same combination of participants assembled by von Kármán for the WAC Corporal − Army Ordnance, the Army Air Forces, Caltech, JPL, and Aerojet (this time abetted by the NACA and the Navy) − again took the lead. Combining a variant of the Aerojet booster that powered the WAC Corporal with the second stage of the Navy’s Bumblebee missile, this alliance created the Aerobee missile, a vehicle flown by many organizations but designated as the X-8 for AAF and USAF flight research. A contract between the AAF’s Air Materiel Command and Aerojet for the Aerobee liquid-propellant sounding rocket came into effect in July 1946. Built along the lines of the WAC Corporal, it included a parachute recovery system for the nose cone and main body. Under subcontract to Aerojet, Douglas Aircraft designed the aerodynamics of the rocket. Separately from the X-8 program, the Navy contracted for 40

Aerobees, which it first flew in 1947 at the White Sands Proving Ground in New Mexico. The USAF followed suit with its inaugural X-8 mission in December 1949 at Holloman Air Force Base, New Mexico, the home of the majority of X-8 launches. Although many aeronautical research projects that start with simple designs end up with lots of adornment, program managers on the X-8 resisted this temptation. The standard model (replaced by the more powerful Aerobee-Hi around August 1954) measured 20.2  feet (6.1 meters) long and just 15 inches (0.4 meters) in diameter, with its three fins having a span of only 5  feet 3 inches (1.6 meters). Its weight made it seem even more diminutive: empty, the missile weighed only 135 pounds (61.2 kilograms), and the booster 315 pounds (142.8 kilograms); loaded, the missile weighed 1,097 pounds (497.6 kilograms) and the booster 575 pounds (250.8 kilograms). It carried a payload of between 150 and 300 pounds (68 to 136 kilograms). Its performance belied its modest appearance, capable of flying at Mach 6 (4,020 miles or 6,469.5 kilometers per hour) and reaching altitudes of 800,000  feet (244 kilometers). Aerojet technicians fashioned the X-8 cylinder out of aluminum, with a nose cone consisting of stainless

X-11 61 steel. As a single-stage, unguided rocket, it used a twin launch system, starting from a 143-foot (43.5-meter) vertical tower. Its expendable solid-propellant booster burned for 2.5 seconds and lifted the rocket to roughly 1,000 feet (304.8 meters), and then a liquid propellant rocket took over for about 45 seconds, propelling the X-8 upwards to about 19.5 miles (31.3 kilometers), at which point it proceeded on its own momentum. Despite its slight appearance and structure, the X-8 withstood the test of time. It made its very first contractor flight in November 1947, and once delivered to the Air Force, 66 missiles undertook 74 missions during nearly seven years between December 1949 and November 1956. Its varied payloads investigated many phenomena of interest to scientists. Military researchers benefited from data regarding warhead trajectories and the influence of high altitudes on warhead designs, while civilians collected information about the atmosphere, high-altitude winds and temperature, solar and cosmic radiation, and the Earth’s magnetic field. In the end, the X-8 represents part of a bigger story. Aerojet fabricated more than 800 Aerobees, which contributed to the developing U.S. presence in space. In addition to these, the American space program drew on a number of sources: ongoing ballistic missile research; the NACA’s Wallops Island missile launches and its famed X-15; the Navy’s Viking rocket; the longstanding JPL missile projects; and, of course, the left-over German V-2s, first tested and improved at White Sands, and later in Huntsville, Alabama. X-11 Unlike the first ten X-planes − most of which directly or indirectly pursued technological advances initiated during World War II − the X-11 represented something apart and different: a creature of the post-war world. The German V-2 ballistic missile may have provided its general inspiration, but the concept underlying the X-11 actually embodied a uniquely American design and structure; although, paradoxically, the person who devised it did not himself originate in America. Initially, weapons designers assumed that the enormous atomic payloads of the day necessitated massive machines to carry them to their targets. They therefore conceived a giant nuclear missile, 160  feet (48.76 meters) tall, 12 feet (3.65 meters) in diameter, with an estimated weight of 440,000 pounds (199,580 kilograms), and propelled by four booster engines and a lone sustainer engine. But subsequent research by the U.S. national laboratories succeeded in sharply

reducing the size of the nuclear cargo, enabling a scaling back of the missile itself as a result, down to 92 feet (28 meters) high, 10  feet (3 meters) in diameter, and 180,000 pounds (81,647 kilograms) gross weight. Accordingly, its powerplant declined from five engines to three (two boosters and a sustainer). These diminished proportions describe the Atlas A missile, the ultimate product of a request for proposals from the Army Air Technical Service, which invited aerospace industries to offer bids on research and development concepts for long range missiles in October 1945. (The X-10 also germinated from this call). By an accident of history, the winner of the Atlas contract − Consolidated-Vultee Aircraft of San Diego, California − counted a former Belgian national named Karel Jan Bossart among its employees. Born in Antwerp in February 1904, Bossart graduated from the University of Brussels in 1925, but rather than pursue a career in aeronautics at home, he won a post-graduate fellowship at the Massachusetts Institute of Technology (M.I.T.), which set him on a different course. Bossart graduated with a master’s degree in 1927 and returned to Belgium to fulfill his military service commitment. Having done so, however, he decided that his future lay in the U.S. and returned to America, becoming a citizen in 1936. During World War II, Bossart worked on several allied missile programs. His abilities led to his promotion to construction manager at Consolidated, which in turn positioned him to participate in the acquisition, dissection, and flight testing of German V-2 missiles after the war. This experience led him to a breakthrough insight that transformed the development of the Atlas, as well as every other U.S. missile that succeeded it. Rather than depending on extensive and heavy internal bracing as key fabrication elements, as the German engineers had done, Bossart found a revolutionary new way to make rockets. Embedded in a monocoque framework, his concept relied on fuel tanks with walls so thin that they would collapse without pressurized propellants. Yet when inflated by these fuels, the tanks provided strength and stiffness for the entire structure. In addition, he worked out another brilliant concept: a nose cone that separated from the rocket itself and carried the warhead to its destination. These twin innovations eliminated weight and complexity from the missile itself without sacrificing reliability or integrity, and because the nose cone detached, it became unnecessary for the entire vehicle to be hardened to survive reentry into the atmosphere. Meanwhile, pressed by intelligence reports that the Soviets had begun their own ballistic missile program with their own German émigrés (as well as with the contributions of the peerless Sergei Korolev and his

62  Prelude to the High Frontier: Early Space Vehicles Russian design team), Air Force officials decided that Atlas needed to be accelerated. In response, Convair (Consolidated-Vultee’s new name after it was purchased by General Dynamics in 1953) suggested that a testbed ought to be fabricated to prove the essential effectiveness of the Atlas − its propulsion system,

airframe, and systems − to get faster results. Convair claimed that such a program would take a year off the rocket’s development, and Air Force officials, under heavy pressure to surpass the Russians, agreed without hesitation. They enrolled the project in the X-planes series, and named it the X-11.

X-12 63 The Department of Defense (DoD) initiated the X-11 during May 1953, the same month that Convair began to lay out the design details of the Atlas A. As envisioned, the X-11’s basic structure consisted of a single-stage stainless steel c­ ylinder with no aerodynamic control surfaces. Its ablative nose, sheathed in copper, included a retrievable compartment that could hold either instrumentation or experimental warheads. The X-11 had three types of guidance: an on-board, programmed inertial system; an airborne inertial autopilot; and a radar tracker on the ground. When the missile reached the desired velocity, the nose cone would separate and follow a predetermined free-fall trajectory with no guidance. The missile’s powerplant consisted of one North American XLR43-NA-5 rocket engine, rated at 120,000 pounds (55,338 kilograms) of thrust and fueled by alcohol and liquid oxygen. Bigger but lighter than the Atlas A, the X-11 stood 96 feet (29.2 meters) high, with a diameter of 12 feet (3.6 meters). It weighed 80,000 pounds (36,287 kilograms) gross and only 12,490 pounds (5,665.3 kilograms) empty. The X-11 could carry up to 1,200 pounds (544.3 kilograms) of payload and could range up to 600 nautical miles (690.4 standard miles, or 1,111 kilometers). Despite the urgency that originally prompted the X-11, it soon became clear that progress on the Atlas A rendered it unnecessary. The Air Force withdrew its designation in 1954 without fabricating or flying a single one. In the end, Convair built 16 Atlas As, and although only half of them proved to be reliable in eight flight tests (the first on June 11, 1957), the extraordinary design achievement of Karel Bossart proved itself again and again in the first Atlases deployed to operational status in 1959, and in each of the generations of ICBMs that followed. X-12 Because Karel Bossart’s Atlas deviated so sharply from any previous rocket design, the Air Force not only planned to test it with the X-11, but also with a second vehicle, designated the X-12. USAF officials wanted to confirm three key constituents of the design: the ground-monitored inertial guidance; the nose cone reentry system; and Bossart’s radical use of inflatable tankage to give strength and stiffness to the missile’s overall structure. Convair began work on the X-12 in May 1953, during the same month that it initiated the X-11. History

repeated itself in this case: just as Convair concentrated mainly on design studies for the X-11, the same held true for the X-12. And just as the Air Force decided to go directly from the X-11 to the Atlas A, the same officials leaped right from the X-12 to the Atlas B, placing an order with Convair for 13 of them. The one and onehalf stage Atlas B did not differ appreciably from the Atlas A. This time, Convair’s contract called for a sustainer engine combined with two boosters. Atlas B’s test plans focused on the delivery of full-sized dummy warheads to targets; actual, functioning guidance systems; and a re-­configuration of the pressurized fuel tanks to withstand full loads. The Atlas B’s physical characteristics diverged moderately from those of the Atlas A, being shorter at 85 feet (26 meters), retaining the same diameter at 10  feet (3.05 meters), and with a heavier gross weight of 244,130 pounds (110,740 kilograms). Beginning on July 19, 1958, the Air Force attempted the first of ten Atlas B launches. The inaugural flight broke up 60 seconds after lift-off, while the last, fully successful mission happened on February 4, 1959. In between, two flights (on September 18, 1958 and January 16, 1959) did not live up to expectations, both due to stage 1 failures. A major program milestone occurred on November 28, 1958, when an Atlas B traveled 6,325 statute miles (10,179.1 kilometers), representing America’s first ballistic missile test to cover intercontinental range. In its planning for the X-12, the Convair team envisioned a missile somewhat bigger than the Atlas B, and much like the X-11  in overall construction and size. The main difference between the two X-vehicles involved weight, at 80,000 pounds (36,287 kilograms) gross weight for the X-11, and 240,000 pounds (108,862 kilograms) gross for the X-12. Not surprisingly, the difference concerned propulsion. Instead of the single Rockwell engine for the X-11, the X-12 housed three Rockwell XLR43-NA-5 powerplants, one a gimbaled sustainer. Both the X-11 and X-12 systems relied on a mixture of alcohol and liquid oxygen for their propellants. Rated at 120,000 pounds (54,431 kilograms) of thrust per engine, the planned X-12 trebled the thrust of the X-11, giving it a range of 6,000 nautical miles (6,905 statute miles, or 11,112 kilometers). In addition to their overall dimensions, internal structure, and propulsion systems, the X-11 and X-12 shared another feature: both served only as paper testbeds, for the Atlas A and B, respectively.

64  Prelude to the High Frontier: Early Space Vehicles

Like the X-11, Air Force authorities cancelled the X-12 before any real fabrication occurred. Under intense pressure to deprive the Soviets of a missile victory, they chose to skip the construction and testing phases of the X-12 and instead apply its design concepts directly to the Atlas B.

X-15 While many of the early X-planes had been ground-­ breaking, none of them foreshadowed the X-15. It differed radically from what had come before, almost as if pulled from a magician’s hat. Despite appearances,

X-15 65 however, it had a solid but unexpected grounding, though not in the series itself, or among military leaders, nor even among captains of the aerospace industry. Its main source of support stemmed from a modest, professorial, and brilliant civil servant named Hugh L.  Dryden. The son of a Baltimore streetcar conductor, Dryden earned a Ph.D. in Physics from Johns Hopkins University at age 19. He rose from obscurity to become one of the most

influential aerospace figures of the twentieth century, climbing to the top ranks of the National Bureau of Standards, leading the Navy’s breakthrough Bat guided missile program in World War II, and becoming the Director of the National Advisory Committee for Aeronautics (NACA) from 1947 until its closure in 1958. Finally, he served as the first Deputy Administrator of NASA.

In these varied roles, Dryden excelled equally as both a science administrator and a scientist. He spent much of his time at the NACA championing high-speed flight, a research specialty that he began pursuing in the 1920s at the Bureau of Standards. Although as NACA director he threw his full weight behind the X-1, D-558, and other early supersonic aircraft, the X-15 received his personal attention and direction. Dryden not only participated in drafting its technical specifications, he found patrons to support it, negotiated the terms of its development and flight testing, and assumed direct command of its overall management.1

Dryden’s enthusiasm for this project arose in part from the climate of the times. Even before the D-558 Skyrocket made the world’s first Mach 2 flight in November 1953, he felt that hypersonic (Mach 5+) flight could be achieved. (See the section on the X-1A in Chapter 1 for more about the D-558 aircraft.) In fact, others in the NACA, in aerospace manufacturing, and in the military thought the same way, foreseeing no

 The strong-willed Walter Williams, one of Dryden’s subordinates and Director of the NACA’s High-Speed Flight Station on 1

Edwards Air Force Base, California, where the new X-plane’s test program took place, made the mistake of pressing for greater personal authority in this project. Williams paid for his mistake when the normally mild-­mannered Dryden warned him (in writing, and in no uncertain terms) that if he wanted to keep his job he needed to leave the big decisions to headquarters; that is, to Dryden himself.

66  Prelude to the High Frontier: Early Space Vehicles major technical obstacles to a rapid increase in Mach numbers, in part because of the availability of powerful new rocket engines being fabricated for the nation’s guided and ballistic missiles. But this expectant atmosphere also drew from other sources. During 1950 and 1951, engineers Hubert Drake and Robert Carmen of the NACA’s High-Speed Flight Research Station proposed modifications to the as yet untested Bell X-2 for sustained Mach 3 flight. They chose this aircraft because of its heat-resistant K-Monel nickel alloy fuselage and stainless steel wings. (See the section on the X-2 in Chapter 1.) At about the same time, Dr. H. Julian Allen of the NACA Ames Laboratory in Northern California made a discovery of keen interest to those eager to pursue hypersonic speeds and more. Allen found the answer to one of the most vexing problems of early spaceflight: how to design spacecraft to withstand the intense heating of re-­entry. His research predicted, counterintuitively, that because of a strong bow-shaped shock wave that deflected the heat, blunt-nosed objects plunging through the atmosphere generated lower temperatures than those with pointed contours. Meanwhile, Robert J.  Woods of Bell Aircraft − a contributing designer of the X-1, X-2, and X-5 airplanes − urged NACA officials to raise the priority of its hypersonic and nascent space research. The NACA’s advisory Committee on Aerodynamics responded in spring 1952 by asking Dr. Jerome C. Hunsaker − the titular leader of the NACA, who chaired its Main and Executive Committees − to initiate a hypersonics program. Hunsaker agreed, and instructed all NACA labs and stations to begin to focus on flight not just to Mach 5, but up to Mach 10 and beyond. (Hunsaker, it should be added, had been a close friend and admirer of Hugh Dryden for many years. He almost certainly ordered this momentous new line of research with the prior approval of Dryden, who made all practical decisions as the NACA’s director). Engineers and scientists at the NACA’s Langley Laboratory played a lead role in fulfilling Hunsaker’s directive. Discussions there gave rise to a bedrock concept: a piloted, rocket-powered aircraft, capable of reaching the limits of the Earth’s atmosphere and flying home by controlled glide to a runway landing. In 1953, the Langley team − in cooperation with colleagues at the NACA’s missile launch site at Wallops Island, Virginia, and its testing grounds at the High-Speed Flight Station − took the Drake-Carmen proposal for a Mach 3 aircraft and broadened it, envisioning a Mach 4.5 vehicle fitted with reaction controls and launched by two expendable solid rockets.

But NACA headquarters saw things differently, certainly reflecting the wishes of director Dryden. Instead of transforming an existing research airplane, Dryden asked his field organizations to establish requirements for an entirely new hypersonic airframe. Once again, Langley stepped forward. Starting with a clean slate, a hypersonics panel convened in Hampton, Virginia, led by the chief of the Compressibility Research Division, John V.  Becker, and assisted by Langley rocket propulsion expert Maxime Faget. By April 1954, they had arrived at the design of the most celebrated of all the X-planes: a vehicle of long, bold lines, with a cruciform tail and wedge-shaped vertical fin; powered by either three or four rocket motors, capable of achieving Mach 7 speed and an altitude of several hundred thousand feet (in excess of 90,000 meters); launched from a mother ship, as with the X-1; and covered with a heat-shedding skin made of Inconel X chrome-nickel alloy. NACA headquarters embraced the concept and set the project in motion by assigning a distinct role to each NACA field office: flight planning to the High-­Speed Flight Station; propulsion work to the Lewis Laboratory; aerodynamics analysis to Ames; and hypersonic wind tunnel research to Langley. Hugh Dryden had already played a pivotal role in hypersonic flight, but with the unveiling of the Langley design he assumed direct leadership of the X-15. Superficially, he pursued the task with his customary modest and understated approach, but in reality it constituted something of a passion project that he pursued tenaciously, motivated in part by his lifelong intellectual interest in high-speed flight. Even more motivating was his ongoing drive to re-focus the NACA’s energies on hypersonics, and eventually on the distant goal of space travel. To that end, Dryden organized a meeting of the Research Airplane Committee − the same body that inaugurated the X-1, the D-558, and many of the initial X-planes − in his office in October 1954. Just like the conference he had attended at Langley ten years earlier for the X-1, this gathering also included prominent members of the Defense Department. Much as his predecessor at the NACA, George Lewis, had known a decade earlier, Dryden was well aware that the NACA lacked the budget to fund long-term, costly undertakings like the X-15. Without the expansive resources of the armed services, the new X-plane under discussion simply could not materialize. Therefore, on this fall day he welcomed four officials from the military’s research and development

X-15 67 establishment to NACA’s Washington, D.C. headquarters: Rear Admirals Lloyd Harrison and Robert Hatcher, famed Air Force test pilot Brigadier General Benjamin Kelsey, and USAF chief scientist Albert Lombard. As on other occasions, two things also proved decisive here: Dryden’s close connection to military leaders that stemmed from his leadership of the Navy’s Bat Missile project during World War II; and his reputation for scientific acumen and personal integrity. During the meeting, Dryden made no overt effort to persuade his guests about the project. Instead he explained the technological obstacles and, most importantly, framed his case on the basis of American security or, as he shrewdly called it, “national urgency.” In the tense climate of the Cold War these words resonated, and his argument succeeded. Although the program offered no direct military utility, at least in the short term, the proposal won the support of the Navy and Air Force representatives, who agreed to provide the funding despite the high cost and risk of failure − a risk apparent in the bold design released earlier by the NACA’s Aerodynamics Committee. The subsequent Memorandum of Understanding (MOU) signed by the NACA, the Navy, and the USAF at Christmas time 1954 recognized Dryden’s predominant role, designating him as the project’s technical chair. Moreover, in an even greater concession to Dryden − and contrary to the standard X-planes practice of the armed services conducting their own flight testing programs − this document not only gave the NACA complete control over, and unchallenged possession of, all test vehicles, but also named the NACA’s High-­ Speed Flight Station as the sole site for all research flights. The contract to fabricate the X-15 attracted keen interest from the aerospace industry. At first, nine airframe manufacturers came forward, but five had withdrawn by May 1955 leaving Douglas Aircraft, Bell Aircraft, Republic Aviation and North American Aircraft in the competition. After a summer of evaluations, the NACA, Air Force and Navy Bureau of Aeronautics agreed in August 1955 that North American of Inglewood, California, best met the design criteria set out by the Langley research team. However, in a strange turn of events, and despite the prestige inherent in the selection, North American’s executives initially turned their backs on the award. The firm officially attributed its r­eticence to a backlog of other orders, although profit-and-loss probably had more to do with their hesitancy. The fact that the entire project consisted of only three prototypes (with no follow-on production

contract expected) rendered it a less than attractive investment. However, after some brokering by Hugh Dryden and General Howell Estes of Air Force Research and Development Command, North American president J.L. Atwood agreed to accept a revised contact that offered a longer production period and extra start-up money. Yet even with an initial disbursement of over $40 million for the three airframes, North American’s top leaders continued to express private concerns about the X-15’s thin margin. They also had another worry, fearing that it might divert the company’s best talent away from more lucrative work. Luckily, Harrison A. “Stormy” Storms, the firm’s seasoned manager of research and development, protected the X-15 from internal predation, assembling a team of 35 select designers, engineers, and technicians and taking the program under his wing. His group got good news as they prepared the first prototype, when North American selected Reaction Motors of New Jersey, the manufacturer of the X-1’s highly successful XLR11 rocket engine, to develop the X-15’s historic XLR99 powerplant. As the X-15s underwent fabrication and assembly at the North American factory, the NACA acted almost as a business partner rather than a detached observer. This unusually close relationship partly reflected the Becker Committee’s detailed design objectives for the X-15, but it also hinged on the presence of a prominent former NACA employee on the factory floor. Pilot Scott Crossfield, who had been intimately involved with the X-1, the D-558, and other X-planes, left the NACA in 1955 to join North American as a test pilot and consultant. Almost a resident on the X-15 site, Crossfield made indispensable practical contributions to the aircraft. Not only that, the NACA High-Speed Flight Station director Walt Williams sent some of his top engineers and technicians directly to Inglewood to collaborate with their counterparts at North American. Other NACA organizations also got involved: aerodynamicists enlisted Langley’s 9-inch (23-cm) blowdown wind tunnel for further design work, while engineers at the Lewis Propulsion Laboratory in Cleveland advised Reaction Motors on fuels for the XLR99 engine. At long last, the public caught its first glimpse of the slender, black rocket plane at an unveiling at the North American plant in October 1958. It breathed speed and modernity: 49 feet (15 meters) long; 14 feet (4.3 meters) tall at the vertical tail; 31,275 pounds (14,186 kilograms) launch weight, about twice that of the heaviest D-558; and capable of 57,000 pounds (25,854 kilograms) of thrust from the XLR99.

68  Prelude to the High Frontier: Early Space Vehicles Yet even before being rolled out for the cameras, the X-15 and its team came under scrutiny. The rise in interest began a year before, after Russia’s Sputnik 1 satellite became the first man-made object to orbit the Earth in October 1957. In the wake of the Soviet achievement, the lagging U.S. space program invited unfavorable comparison that raised a storm of criticism, with many wanting to assign blame for America’s inferior position. For better or worse, the X-15 came to prominence in a period of national self-doubt, but at the same time it offered a chance for redemption, with the U.S. trailing at the starting gate in the race with the Soviets. In fact, the relatively smooth construction of the three X-15 airframes gave some substance to this hope, but its propulsion system failed to cooperate. In 1958, Reaction Motors conceded that the XLR99 system needed further work before being mated to the X-15. In the interim period, the government and its contractors relied on two XLR11 engines similar to those on the X-1. As it turned out, the twin XLR11s stayed in service for the entire first year of the X-15’s flight research, while Reaction fought to overcome the XLR99’s serious developmental problems. Meanwhile, expectations had grown even greater by late 1958. Once Sputnik 2 (in November 1957) and 3 (in May 1958) widened the Soviet lead − and proved that Sputnik 1 did not happen by accident − the importance of the X-15 to American pride and to its future in space became even clearer. Walt Williams expressed this sentiment when he told a group at North American that the X-15 represented much more than a placeholder until future U.S. space achievements. He expected it to answer one of the most fundamental questions of the time: Could human beings survive and operate safely outside of the Earth’s atmosphere? The answer to Williams’ question rested with testing, which differed starkly with the X-15 to that of most of the other X-planes. Typically, contractors initiated brief flight programs on new X-planes, after which government authorities declared the aircraft fit (or unfit) for transfer to the sponsoring agency. In contrast to this practice, North American flew the X-15 for nearly two years, interweaving its own missions with those of the NACA. The manufacturer had little choice in this case, since its contract bound it to demonstrate airworthiness above Mach 2 and to verify the efficacy of the XLR99 engine. Neither could be achieved without a long series of trials. North American began the X-15 program at Edwards Air Force Base on March 10, 1959. The maiden lift-off involved a captive-carry mission in which NASA’s newly acquired B-52 mother ship and the X-15 itself

underwent an assessment of their combined aerodynamics. Fortunately, the man who flew this initial attempt − and who would pilot all of the other contractor flights − came with ample bona fides. Scott Crossfield, who served as the midwife of the rocket plane during its development, now assumed the biggest role of his career, as the first X-15 test pilot. But even with such preparatory work as breaking Mach 2 and acting as the lead pilot on the treacherous D-558 Skyrocket pitch-up tests, Crossfield could not have been prepared for the X-15. His past assignments did give him a nose for survival, however. As he sat in the X-15 cockpit preparing for the inaugural free flight on June 8, 1959, Crossfield noticed something disturbing, namely that the pitch damper failed to operate properly. In this situation, the standard rules instructed the ground crew to abort the mission, but as pilot, Crossfield could overrule them. He did so, thinking that the immense expanse of Rogers Dry Lake and the uncomplicated flight path offered some safety. However, as with so many previous inaugural missions, the unexpected reigned supreme. After dropping from the B-52 mother ship and approaching for a landing − without power, gliding all the way − Crossfield felt the beginnings of dangerous, longitudinal (nose to tail) oscillations. Unable to stop them as they increased, he saved the day by dropping the X-15 down on its belly as it dipped closest to the ground. Although he suffered no injuries, the impact caused extensive damage to the plane’s landing gear, with the repairs taking six months to complete. Project engineers combated the problem by increasing the rate on the horizontal stabilizer actuators, which prevented this incident from recurring on any other X-15. Crossfield piloted the “Black Bull” (as NASA research pilot Milt Thompson called the powerful but capricious machine) 13 more times, including three of pivotal importance: the first powered flight (using the XLR11 engines) in September 1959; the first use of the new XLR99 in November 1960; and in the same month, the first attempt to re-start and throttle the XLR99 as it flew. Although subsequent X-15 pilots attained much higher speeds (up to Mach 6.33) and altitudes (as high as 354,200  feet, or 107,960 meters), Crossfield overcame the biggest obstacles as he tested the X-15’s fundamental capacity to fly, which enabled the vehicle’s later research milestones. He also protected subsequent pilots in another, decisive way. During his famed Mach 2 mission in the D-558-2, Crossfield wore the David Clark Company Model 12 full-pressure suit, and he insisted  – with back up from North American  – that only a suit of this kind would suffice for the much

X-15 69

70  Prelude to the High Frontier: Early Space Vehicles

X-15 71 higher-performing X-15. Air Force Aeromedical Laboratory researchers argued instead for a partialpressure suit, but at a meeting at the North American plant in June 1956, Crossfield and the manufacturer stood their ground. The Air Force eventually conceded the point, and all X-15 pilots took to the skies wearing the specially designed, groundbreaking David Clark MC-2 full-­pressure garment. Of course, the X-15 gave the American people much more than a comforting interlude while the U.S. space program matured. It actually served a number of immediate and practical objectives. To begin with, it inducted a group of eight Americans into the astronaut corps, defined by the international standard of those who travel at least 50 statute miles (264,000  feet; 80,467 meters) above the Earth. Due to multiple spaceflights by some of the X-15 pilots, the program achieved this feat 13 times. It also served as a trainer for America’s most celebrated astronaut. Neil Armstrong, then a test pilot at the NASA Flight Research Center, flew the X-15 seven times between November 1960 and July 1962, achieving hypersonic speed (Mach 5.74) and high altitude (over 207,500 feet or 63,250 meters; ironically, too low to join the X-15 astronaut corps). Even so, when he reached the 207,500-foot (63,250-meter) mark during his fourth flight on April 20, 1962, Armstrong got a strong taste of the idiosyncrasies inherent in spaceflight. As he glided back to Earth from the apogee, the X-15 at first failed to penetrate the atmosphere, instead bouncing back out into space. When he finally did reenter and regain control of his flight surfaces, Armstrong found himself far to the south of Edwards Air Force Base and high over the Rose Bowl in Pasadena. He turned and headed northward, but because X-15s returned from space without power, he faced the stark challenge of conserving energy as he tried to cover the extra distance. It ended in a nerve-jangling approach and landing, with a touch down on Rosamond Dry Lake about 10  miles (16 kilometers) from the designated spot. Armstrong showed great skill, but the outcome could have been catastrophic for him and for the program. It turned out to be the longest X-15 flight ever made, lasting roughly 12 and one-­half minutes. Not surprisingly, Armstrong’s brush with mortality did not constitute the only X-15 misadventure. In fact, it epitomized an aircraft that, like a high-strung thoroughbred, suffered from many eccentricities and frailties. Delays and flight cancellations abounded. The X-15’s stability augmentation system and inertial guidance equipment caused many early headaches, while mechanics also struggled with a propellant system prone to malfunctions in the pressure regulators, as well as in the pneumatic vent and relief valves. A spare

parts rejection rate of up to 30 percent plagued repair work, and the combination of maintenance glitches and unpredictable weather along the vast X-15 flight trajectory caused the postponement of many missions. Despite these problems, pilots and ground crew taught themselves over time to compensate for the aircraft’s weaknesses and keep it in the air. In addition to the X-15’s sensitivities, it should also be remembered that it had the power to wound and kill. On November 5, 1959, just five months after Crossfield’s perilous landing during the first glide flight, he had another bad episode when a minor engine fire forced him to make an emergency landing on Rosamond Dry Lake. Unfortunately, he had almost a full load of fuel as he flew X-15 number 2 that day, despite a prior attempt to dump some of it, and as the aircraft made its approach and landing carrying too much weight, the nose wheel dug into the dry lakebed on impact, causing the fuselage to buckle just behind the cockpit. Crossfield’s luck held again and he walked away from the accident, but the damage put the aircraft out of commission for three months. Crossfield sacrificed another of his nine lives the following year, on June 8, 1960. During the second XLR99 ground test at Edwards, he strapped himself into the cockpit of X-15 number 3 so that he could experience the feel of the new engine first hand. He got a terrible surprise. A stuck regulator triggered an explosion in the aircraft’s hydrogen peroxide tank and the blast tore off the front 30  feet of the X-15, hurling it 20  feet with Crossfield still sitting in the cockpit. Meanwhile, fire erupted in the aft portion of the aircraft, leaving it in cinders. Somehow, Crossfield survived unhurt, while technicians at Edwards subsequently shipped the whole burned carcass of the X-15 back to the North American factory, where it took a year to rebuild. More serious incidents followed. After veteran NASA pilot Jack McKay took off in X-15 number 2 on November 9, 1962, he encountered an engine malfunction that eliminated all but 30 percent of its thrust. He headed for Mud Lake, Nevada, for an emergency landing, but at the point of touchdown the landing gear failed and the wing and horizontal stabilizer buried themselves in the lakebed, causing the plane to flip over and come to a stop on its back. McKay got medical attention, and at first his injuries seemed limited, allowing him to return to the X-15 cockpit about five months after the crash. Despite an ultimate diagnosis of cracked vertebrae, he continued to fly the Black Bull for nearly four more years until the condition forced him to retire. X-15 number 2 fared poorly, too, remaining out of commission for about 19 months while North American technicians repaired and restored it.

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74  Prelude to the High Frontier: Early Space Vehicles The worst event of all effectively ended the X-15 program. Air Force Major Mike Adams took the controls of X-15 number 3 on November 15, 1967, and as he climbed after the launch all seemed well. But above 90,000 feet (27,432 meters) he became distracted by a marginal electrical disturbance, which impacted his control of the aircraft. However, he continued on to the day’s objective of 266,000  feet (81,077 meters), at which point he conducted a scheduled wing-rocking maneuver (so that a camera on board could survey the horizon). This simple act set off a disastrous chain of events. Once it stopped, the aircraft drifted 15 degrees off heading, and as it started its descent into the atmosphere, it fell into a spin. At 120,000  feet (36,576 meters), the spin became an inverted dive at an angle of between 40 and 45 degrees. Adams lost control of the aircraft and died in the subsequent crash. An accident board determined that in the future, the mission control room at the Flight Research Center needed to add a telemetered heading indicator to its instrument panel. Had it been in operation during the Adams emergency, the ground crew would have understood the gravity of the situation much earlier. The board also suggested that pilots should undergo vertigo screenings. Adams became disoriented with dizziness on a previous flight and investigators suspected that this also probably contributed to his death in this instance. Despite these disturbing occurrences, the X-15 flight envelope expanded steadily over its almost nine and one-half year flying career. Air Force Major Pete Knight reached the ultimate X-15 speed of Mach 6.7 (4,520 miles/7,274 kilometers per hour) on October 3, 1967, although this achievement also came with a nasty surprise because aerodynamicists underestimated the impact of the 3,000-degree F (1,650-degree C) temperatures generated by such speeds. As Knight flew, the scorching heat burned a seven- by three-inch (17.8 x 7.6 cm) gash on the leading edge of the plane’s ventral fin. NASA pilot Joe Walker reached the top X-15 altitude of 354,200  feet (107,960 meters) on August 22, 1963. The extremes endured by Knight, Walker, and the other X-15 pilots attracted intense interest from aeromedical researchers, offering a chance to learn the biological effects of spaceflight prior to the long duration missions planned for the future. To measure physiological changes, scientists outfitted X-15 crews with monitoring devices to check their vital signs, measure vertigo, and record other factors. Taken as a whole, this information constituted an initial catalog of human

responses to space travel, essential data for those preparing for the bigger missions to come. Scientists and engineers also used the X-15 as a testbed to discover the potential stresses that later spacecraft might endure as they escaped Earth’s gravity. During the envelope expansion phase of the program, they instrumented the X-15 to determine the subtleties of hypersonic aerodynamics, as well as the effects of thermodynamic heating on the vehicle’s skin and structure. They uncovered several surprising phenomena. Despite extensive wind tunnel tests that failed to reveal a problem, in actual flight the aft end of the X-15 experienced 15 percent more drag than predicted. Moreover, doubling the speed of the X-15 vastly increased the temperature effects. Researchers calculated the heating load at Mach 6 to be eight times that at Mach 3. In addition, some parts of the airframe − the front and the lower portions especially − experienced a greater susceptibility to damage from high temperature than others. The plane’s raised cockpit also generated turbulence and heating problems, resulting in damage to the X-15’s windshield on two occasions. Aside from these specific anomalies, the X-15 served as an open air laboratory for a wide variety of spacecraft systems, enabling program managers to assess features such as handling qualities, the stability augmentation system, and reaction controls, among others. If the X-15 yielded just one lesson from its long flight history, then it enabled aerodynamicists to discover that a hypersonic aircraft could fly through every speed regime below Mach 5 without major adverse effects, but over that threshold researchers needed to expect the unexpected. During its final years, the X-15 contributed to the U.S. space program as a platform for hundreds of science experiments, many of which anticipated later human spaceflight. Among many other projects, it collected micrometeorites to assess their possible impact on space vehicles. It also carried an infrared scanning radiometer that recorded the Earth’s radiation levels from 70,000 to 100,000 feet (21,336 to 30,480 meters), and on four of the X-15’s forays into space, NASA tested a zero-gravity heat exchanger designed to cool future spacecraft. Other space-related activities included observations of high-altitude sky brightness, atmospheric density measurements, ultraviolet stellar photography, and trials of advanced integrated data systems.

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NASA research pilot Bill Dana made the 199th and last X-15 flight on October 24, 1968. During the project’s long tenure, the Flight Research Center crew launched more than 1.7 missions per month over almost ten years. Considering all of the uncertainty and complexity in getting the Black Bull into the air, this alone constituted a remarkable achievement. To that can be added what the X-15 actually accomplished: higher altitude and higher speed than any aircraft to that time; immensely useful science experiments that proved invaluable to the early U.S. manned space program; flight data of supreme importance to the designers of the Space Shuttle; about 700 technical papers derived from X-15 flight data; and 13 missions in space. Ultimately, despite its high risks and high maintenance, the X-15 accomplished the bulk of its objectives, and embodied not just another X-plane, but something far more: in essence, a technological phenomenon. During the long gap of nearly three and onehalf years between Sputnik 1 and the first manned Mercury mission, when the U.S. fell further and further

behind the Soviets, the X-15 offered hope and expectations. Over that period, it flew on 36 occasions, and in the same timeframe, NASA’s public affairs offices won over the world’s mass media with stories featuring this photogenic machine. On television, in magazines, and in newspapers, its achievements and its pilots garnered enthusiastic coverage. In this role alone − as a bridge to Mercury, Gemini, and Apollo − the X-15 paid dividends that more than justified Hugh Dryden’s early confidence. X-17 The story of the X-17 and many other X-planes proves that the U.S. military lost no time in broadening the scope of the series soon after the X-planes began, not only with piloted aircraft that flew to Mach 1 and above, but also to a wide array of aerospace vehicles that underwent testing. Rather than expand supersonic flight, most of the X-planes from the X-4 to the X-14

76  Prelude to the High Frontier: Early Space Vehicles instead tried out guided and ballistic missiles, or addressed specific new aircraft configurations such as VTOL, variable geometry wings, or tailless aircraft and, in the case of the X-6, propulsion. Just as the X-11 and X-12 preceded the Atlas A and B ballistic missiles, the X-17 concentrated on the cargo that would one day be carried by these and other launch vehicles. Prior to and during the X-17 program, the NACA’s Langley and Ames laboratories conducted studies on the aerodynamics of re-entry designs between 1952 and 1956. Their work, along with that of the military and industry, suggested that the ablation theory of Dr. H. Julian Allen of Ames appeared to be correct, in that blunt bodies returning from space offered the best contour to shed, rather than absorb the intense temperatures of re-entry. In late 1954, Air Force and Navy officials decided that, with the ballistic missile program galloping at full speed, the time had come to put the theories and wind tunnel research up against the cold reality of actual flight. It resulted in a sole-source contract in January 1955 between the services and Lockheed Aircraft of Sunnyvale, California, to design, fabricate, and flight test the X-17. (Perhaps coincidentally, Julian Allen also worked in Sunnyvale, the location of NASA Ames.) Lockheed’s contract required it to satisfy several key criteria: the missile had to achieve very high altitude and speed; it had to be easy to transport; and it had to be inexpensive. The Thiokol Corporation (the purchasers of Reaction Motors, the maker of the X-1’s XLR11 and the X-15’s XLR99) collaborated with Lockheed on the missile’s design and contributed its engines. The project received the X-17 designation shortly after construction began. Fashioned from steel and aluminum, the X-17 consisted of three stages, all fed by solid-fuel propellants: a primary booster powered by one Thiokol XM-20 Sergeant engine, with four fins at its base; a second stage made up of three Thiokol XM-19 Recruit engines; and a third stage consisting of a single Recruit and a nose cone instrumented for reentry. A specially-designed transport trailer carried the X-17 to the pad, raised it vertically using an erector unit, and then launched it. Its relatively small size reflected its tight-budget status: 40  feet 6 inches (12.3 meters) long, 2.6 feet (0.8 meters) in diameter, with a span across the fins of 8  feet 6 inches (2.4 meters). It weighed just 7,400 pounds (3,356.5 kilograms) gross.

In practical operation, the X-17 lifted off with intense acceleration once its booster ignited. Rising at an angle of 85 degrees, the missile consumed all of its first stage propellant by about 90,000  feet (27,432 meters) but continued to climb until about 500,000 feet (152,400 meters). At that point, it relied on its fins to reverse its trajectory, turn nose downwards, and fall back towards the atmosphere. When it reached between 90,000 and 70,000 feet (27,432 to 21,336 meters) during its descent (depending on payload), an explosive charge separated the booster from the upper stages. Stage two ignited less than a second later, and stage three followed suit almost instantaneously. During the trip back to Earth the X-17 reached speeds up to Mach 15, testing the aerodynamics of its cargo. A destruct system ended the mission roughly six minutes after launch. Not only did the X-17 accelerate quickly from the launch pad, it also did so during its development. About three months after Lockheed signed on, its X-17 team arrived at the Air Force Missile Test Center at Cape Canaveral to set up for the launches. These began with a series of scale-model flights designed to test the components and systems prior to the full-scale X-17s. The first came in May 1955, with three trials of a quarter-scale prototype. The first two fared well but the last one exploded. In late June, the half-scale versions attempted to verify the quarter-scale data. These threestage missiles (which carried nose telemetry) flew successfully, but failed to yield adequate information regarding aerodynamic stability or achieve the desired velocity. Despite this disappointment, the X-17’s program managers − who had no time to spare due the pace of ballistic missile progress − proceeded straight to the full-scale development tests. Six of them took to the skies between August 1955 and June 1956, and their results proved to be mixed. The first ended in a terminated flight, the second in an explosion. The final two lost telemetry data, but significantly the X-17 reached an altitude of 460,000 feet (140,208 meters) on the last of these developmental missions on June 26, 1956, coming close to the desired maximum. Then the final phase began, a series of research flights in breakneck succession, with 24 attempts packed into less than one year (from April 1956 to March 1957). Not unexpectedly, adhering to such a tight timeline took a toll. In two of the lift-offs, the missiles’ stages failed to ignite; in one instance, the missile exploded; in a third, range safety had to destroy an

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78  Prelude to the High Frontier: Early Space Vehicles X-17; and in another, telemetry quit before the missile reached its apogee. But most of the other missions succeeded to a greater or lesser extent. Researchers launched blunt-body payloads 15 times and tried cubic paraboloids and hemisphere dummies nine times. The highest flight achieved an altitude of 507,000  feet (154,533 meters) and the fastest reached a speed of Mach 14.4. Despite a less than flawless flight record, the X-17 succeeded in one pivotally important respect, providing essential flight data for the optimal contours of ballistic missile warheads, and perhaps more importantly, for the design of future space capsules. Remarkably, it satisfied these objectives by launching 37 flights in less than two years, and by completing the total program − from contract signing to the final test flight − in only 26 months. X-20 Dyna-Soar During the first decade of X-planes research, the NACA and its military partners concentrated on high-speed and high-­ altitude aircraft. But even during this intense period of envelope expansion, the NACA began to formulate the next steps in an ambitious program of future research. In particular, Assistant Director of the Langley Laboratory, Hartley A. Soulé, played a leading role in defining these upcoming objectives, with the advice and consent of the NACA’s activist director, Hugh L. Dryden. (Dryden had consistently guided the NACA towards space research since his appointment in 1947, allocating increasing portions of the agency’s limited budget for that purpose). Dryden, Soulé, and other NACA figures saw the progression from travel within to travel outside the atmosphere in three distinct phases. In round one, as Soulé described it, the early X-planes such as the X-1s, the X-2, the X-3 (and the D-558 Skyrocket) would test technologies and push the limits of atmospheric flight. Round two consisted of hypersonics, as embodied by the X-15, a spaceplane capable of short-duration escapes from the atmosphere − like a fish leaping out of water, as Dryden described it. Finally, in round three, a boostglide winged orbital spacecraft would open the new age of spaceflight. This blueprint became accepted NACA doctrine by the mid-1950s. Of course, neither these steps, nor anyone at that time, could foretell the eventual direction of the

American manned space program as represented by Mercury, Gemini, and Apollo. The alternative path plotted out by the NACA considered of an entirely different, parallel route to spaceflight, a path undertaken, but in the end abandoned. The NACA’s “three rounds” route depended for its final stage on an old engineering concept. Rather than attempt heavy launch access to space, of the kind represented by Wernher von Braun and the V-2 rocket, the NACA approach instead relied on an idea also pursued during World War II, but conceived long before. In 1933, Austrian aeronautical engineer Dr. Eugen Sanger and mathematician Dr. Irene Bredt (later to become husband and wife) announced an entirely original way for humans to travel into space. In his graduate thesis, Sanger proposed a manned rocket plane with lifting body contours, which would be launched from a horizontal sled and ascend to the edge of space. It would then skip along the upper atmosphere (but not orbit), and descend to Earth as a hypersonic glider, making a runway landing on skids. Sanger developed the idea  – called Silbervogel or Silverbird − over the next few years until the German Air Force asked him to perfect his concept in 1936, as the director of a new research institute at Trauen. The parties agreed that his work would be undertaken separately from that of von Braun at Peenemunde. His superiors hoped that Sanger could serve the war effort by converting his glider into an operational, intercontinental weapon known as the Antipodal Bomber. The Trauen team envisioned a big vehicle: almost 92 feet (28 meters) long, with a wingspan just over 42  feet (12.8 meters), and a full weight of 200,000 pounds (90,718.4 kilograms), with the capacity to carry 660 pounds (299.3 kilograms) of high explosives. But a machine as complex as this one required a timeline far in excess of the limits of World War II, and Sanger’s group disbanded without any weapon to show for it. Although his ambitious spacecraft may have exceeded the timetable of the war, Sanger’s boost-glide system lived on in at least two places in the post-war world. Hugh Dryden, Hartley Soulé, and their NACA (soon to be NASA) colleagues adopted it for round three of their space initiative, as a hypersonic vehicle to carry on the X-15 tradition of research and provide a platform for short-duration spaceflight, offering eventual access to orbital missions.



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The Air Force also valued the Sanger approach, but saw things differently than the NACA. Not surprisingly, they regarded it as a potential military spacecraft, capable − with an astronaut in the cockpit − of dropping live warheads anywhere on Earth with exceptional accuracy. Additionally, they also envisioned reconnaissance and anti-satellite roles. Although the two organizations seemed poles apart in their applications of the Sanger-Bredt model, the Air Force and NASA joined forces to realize this tantalizing means of space access. They sustained their partnership even as it became increasingly likely that, in the aftermath of Sputnik 1, their spaceplane would lose out to big boosters tipped with small capsules. Just around the time that the NACA embraced the “three rounds” formula, the Air Force took steps on its own to revive the Sanger-Bredt concept. After several years of dormancy, boost-glide reappeared in a study by Bell Aircraft in April 1952 which proposed BoMi (“Bomber Missile”), a manned bomber/sub-orbital glide aircraft that abandoned the sled launch proposed by Sanger and replaced it with a two-­stage rocket. At first, USAF officials hesitated about a bomber version, and instead awarded Bell a contract to study a reconnaissance BoMi in 1955. During this key year, Bell

program managers − abetted by the in-house presence of Walter Dornberger, who served as the military director of the Peenemunde rocket works during the war − unveiled a three-­stage approach to development. They proposed a vehicle with a range of over 5,700  miles (over 9,173 kilometers) in phase one; about 11,500  miles (18,507 kilometers) in phase two; and finally an orbital spacecraft in phase three. At this point, the NACA began to contribute to the project, and Bell completed design work on phase one by the end of 1956. As the contractor continued on this track, Air Force authorities reconsidered their reluctance about a BoMi bombardment role. Strategic thinkers in the service reasoned that America’s adversaries might succeed in hardening their targets by the 1970s, to such an extent that incoming ballistic missiles would be rendered less than effective, and mobile targets would be able to elude attack completely. In contrast, the space bomber would be able to destroy opponents’ positions from any direction, attack with just a few minutes notice (compared to a 20-minute delay after an ICBM launch), and be able to re-calibrate missions on the fly. So the USAF contacted the Boeing, North American, Douglas, Convair, McDonnell, and Republic

 companies to submit proposals for a manned hypersonic aircraft that incorporated bombardment plus reconnaissance, by flying orbital missions at altitudes as low as 100,000 feet (30,480 meters). Each received a study contract in June 1956 for this weapon system, known as RoBo (“Rocket Bomber”). The following year, each of the contractors, including Bell, appeared for three days before an Air Force acquisitions panel. As a result, Air Research and Development Command made a pivotal decision in October 1957 to combine the existing design work into three sequential parts: a hypersonic glider that s­ ignificantly exceeded the capabilities of the X-15; a reconnaissance aircraft launched from a two-stage rocket and capable of covering over 5,754  miles (9,260.1 kilometers); and a bomber that could orbit the Earth. The command issued the birth certificate of the stage one glider in December 1957, which by now had acquired the name Dynamic Soaring, reduced (with a sense of humor) to Dyna-Soar. This decision inaugurated a possible but improbable pathway for Americans to reach space by means other than Mercury, Gemini, and Apollo. Dyna-Soar phase 1 one concentrated on three research tasks (as opposed to later military objectives): to demonstrate lifting body re-entry and collect data for future efforts; to assess the ability of pilots to accomplish tasks in space; and to determine whether pilots could land successfully on standard runways. To succeed in these goals required intensive research in high-speed aerodynamics, high-temperature structural materials, and thermal protection systems. In order to overcome these hurdles, the USAF and NASA agreed to forge a formal partnership in November 1958, one in which the space agency offered technical help and advice, while the Air Force controlled funding and overall direction. By this time, Dyna-Soar had already assumed its characteristic design contours: a compact glider with a thin fuselage, delta wings, and a flat underside. It proved to be far less imposing than the World War II model envisioned by Sanger: about 60 percent shorter (at 35 feet 4 inches; 10.8 meters), less than half the wingspan (at 20  feet 5 inches; 6.2 meters), and 18 times lighter (at 11,390 pounds − 5,166.4 kilograms − gross weight) than the original. Despite its reduced proportions and mass, Dyna-Soar’s creators still pushed for performance, anticipating that it would fly as high as 550,000 feet (167,640 meters), range over 19,000  miles (30,577.5 kilometers) in the orbital

X-20 Dyna-Soar 81 model, and attain a top speed of 16,670  miles (26,827.7 kilometers) per hour. To resist the high heat and high loading of re-entry, plans called for the external shell and some internal structures to be fashioned from exotic metal alloys. These included columbium (possessing a hardness like pure titanium), molybdenum (with one of the highest melting points of any pure element), and René 41 steel (a nickel-chromium alloy often used in missile and jet engine components). In a stiff competition that lasted almost two years (January 1958 to November 1959), 13 companies vied for the phase 1 Dyna-Soar glider contract. The contestants included Bell (which initiated the project in the first place), and industry leaders such as Martin, Convair, Douglas, Lockheed, North American, McDonnell, and Republic, among others. To the surprise of many, Boeing won the race over the much smaller Bell, a company that the USAF associated with prototypes rather than with production work. Martin, selected to lead booster development, scored points on its proposal by recommending the Titan I for the glider missions and the Titan C for orbital flights. Boeing and Martin received development contracts in April 1960. Martin struggled until the end of 1961 to make a final rocket choice, eventually recommending the Titan III for all Dyna-Soar launches. Meantime, Boeing made straightforward progress on the glider, passing all sub-­system critical design reviews by the close of 1962 and making progress on high-temperature materials and airframe fabrication. In September of that year, perhaps to counteract the progress finally being achieved by the competing Mercury project (John Glenn and Scott Carpenter flew successful, multi-orbit missions in February and May 1962), Boeing’s public affairs office rolled out a mock-up Dyna-Soar display at the annual Air Force Association convention in Las Vegas. It received excited reviews across the country. In fact, the company seemed on track to deliver the initial Dyna-Soar to the Air Force in October 1964 and planned for the first orbital mission in late 1965. As construction went ahead, the project completed the selection of the Dyna-Soar astronauts, leaving time for adequate training. In April 1960, the Air Force and NASA announced seven crewmembers: three NASA X-15 research pilots (Neil Armstrong, William Dana, and Milton Thompson); and four graduates of the Air Force Test Pilot School (Major Henry Gordon, Captain William “Pete” Knight (also of X-15 fame), Major Russell Rogers, and Major James Wood).

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In the end, Dyna-Soar fell victim to politics. With the change in administrations from Eisenhower to Kennedy, presidential and congressional support for it began to wane. Moreover, the rise of NASA’s successful manned space program and the high cost of U.S. intervention in the Vietnam War put a squeeze on all unrelated defense spending. Even though many recognized the technological and operational superiority of a reusable spaceplane − in stark contrast to the functional limitations of space capsules − the tough-minded Secretary of Defense, Robert McNamara, cancelled Dyna-­Soar in December 1963, despite some estimates that about 90 percent of the project had been completed. Ultimately, in the midst of an intense, global contest with the Soviet Union for dominance in space, the simpler alternative won out. Still, the concept of DynaSoar, and of the Silbervogel before it, did not vanish. Instead, it continued in transmuted form into the 1970s, when the Space Shuttle, comprising many of the technical components of Dyna-Soar, took shape and eventually flew in April 1981.

X-23A Being, for the most part, small, bulbous aircraft designed to test runway landings from space, the lifting bodies originated separately in the Air Force and at NASA.  Eventually, the two agencies merged their programs. The Air Force began its research during the mid1950s, in pursuit of the X-20A Dyna-Soar inspired by the work of Austrian engineer Eugen Sanger, who first described the lifting body design in 1933. The NACA entered the field in 1957 (and continued after becoming NASA the following year), building and flying a series of candidates of increasing complexity and capability until 1972. The space agency first got involved through the work of Dr. Alfred J.  Eggers at its Ames Research Center. In 1957, Eggers propagated a new aircraft design that he called the M-1: a wingless cone cut in half lengthwise, and incorporating rear vertical fins, elevons for directional control, and a cockpit canopy. In wind tunnel tests, Eggers’ model offered the promise of a hypersonic vehicle capable of runway landings.

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This concept came to the attention of R. Dale Reed, an engineer at the NASA’s Flight Research Center in Southern California. Reed persuaded NASA management to test Eggers’ design with an actual full-scale lifting body aircraft, which Reed called the M2-F1 (Modification 2, Flight Version 1). An inexpensive, gumdrop-shaped vehicle (or “flying bathtub,” as some called it) fashioned from plywood, it began a series of glide flights in 1963, towed at first behind a speeding Pontiac Catalina, and later by a C-47 aircraft. After improvements, it performed well enough to be succeeded by two heavyweight, XLR11 rocket-powered successors: the Northrop M2-F2 and M2-F3. (These two aircraft actually shared an airframe; after the M2-F2 suffered a major crash, the team at Northrop rebuilt it as the M2-F3.) Although plagued by lateral roll control problems, the M2s contributed substantial data to the project. Meanwhile, a group of engineers at NASA Langley conceived a lifting body with different contours: the flat-bottomed HL-10 (Horizontal Lander Model 10). It, too, suffered from lateral roll control, but like the M2s did better with the addition of a computerized stability augmentation system. The HL-10 and M2-F3 further benefited after technicians mounted a third, vertical fin between the two original outboard ones. As NASA made these strides, the Air Force continued with its own lifting bodies. Representatives of Air Force

Systems Command made contact with the Martin Marietta Company of Baltimore, Maryland, in the early 1960s, to design and fabricate a lifting body, mount it on a ballistic missile, and gather data as it re-entered the atmosphere from space. They chose a seasoned partner in Martin, a competitor for the Dyna-Soar prime contract and a firm that had already invested heavily in lifting body research. During its early studies Martin concentrated on refining Eggers’ M1 design, but by April 1962 their engineers departed from this platform in order to satisfy two Air Force requirements: a vehicle capable of maneuver during re-entry, and of testing the effectiveness of an ablative heat shield. Their approach yielded a contract with the Air Force, announced in August 1964, to construct four unmanned vehicles known as the SV-5D (Space Vehicle Number 5D) PRIME (Precision Recovery Including Maneuvering Re-entry). Air Force officials soon designated it as the X-23A. The X-23A staked out new design territory. Shaped like a blunt-nosed triangle when viewed from above, its head-on profile formed a gently rounded V that tapered to a flat bottom. With a length of only 6 feet 8 inches (2 meters), a height of 2  feet 10 inches (0.86 meters), a wingspan of just 4 feet (1.21 meters), and weighing a mere 894 pounds (405.5 kilograms), it ranks as one of the smaller X-planes.

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 The X-23A flight plan called for it to be launched atop an SLV-3 Atlas booster, imposing the full stresses of re-entry on its main structures which consisted of beryllium, titanium, stainless steel, and aluminum. Project engineers formulated an ablative silicone heat shield that covered all of the exterior surfaces. Two of the three X-23A launches from Vandenberg Air Force Base in California delivered worthwhile data, although with one major flaw: despite their recovery system, neither made it back for examination. The first, on December 21, 1966, underwent pitch maneuvers as it re-entered the atmosphere and fell into the Pacific Ocean before it could be retrieved. The next, on March 5, 1967, succeeded in banking the vehicle using the lower surface flaps, enabling it to fly (intentionally) about 500  miles from its predetermined glide path. With the loss of its flotation apparatus, however, it also sank into the ocean. Finally, on April 18, 1967, the third flight resulted in a capture off the coast of Alaska, near Kwajalein Island, enabling researchers to study the efficacy of the ablative shield in addition to examining its internal structure. Satisfied with the project’s data, Air Force officials decided to cancel the X-23A’s scheduled fourth flight.

X-24A, B, C 87 The Air Force and NASA finally combined their efforts, and in the early 1970s flew a lifting body that made the concept a viable option for return from space, the X-24A, B, and C. X-24A, B, C With the decision by NASA and the Air Force to build and test the X-24A lifting body together, the two parties forged a close partnership, much like the one that prevailed in the still-active X-15 program. In contrast, up until this point the relationship between the USAF and the space agency regarding lifting bodies had been cordial but competitive. Their local representatives − NASA’s Flight Research Center (FRC) and the Air Force Flight Test Center (AFFTC), both located on Edwards Air Force Base − coordinated with each other on such projects as the M2-F2 and the HL-10 through the Joint FRC-AFFTC Lifting Body Flight Test Committee. The committee kept the two participants informed of each other’s activities, but the Air Force and NASA pursued their lifting body work independently of one another.

88  Prelude to the High Frontier: Early Space Vehicles The two sides drew closer with the X-24A.  In October 1966, they decided to add this new project to the Joint Committee’s existing Memorandum of Understanding, and made the FRC center director Paul Bikle the chair of the committee. Six months later, they took their alliance a step further, agreeing to share program management of the X-24A through a Comprehensive Lifting Body Joint Operations Plan, by which they consented not just to coordinate with each other, but to manage the X-24A flight test program in full collaboration. By this time, the X-24A had evolved into a program of national significance, and leaders of NASA and the Air Force determined that the lifting bodies deserved broader attention than the limited confines of Edwards Air Force Base. In October 1967, NASA Deputy Administrator Robert Seamans signed an agreement with John Foster, the DoD’s Director of Defense Research and Engineering, merging the M2-F2, the HL-10 and the X-24A into one organically unified initiative. It further committed the parties to a close, bilateral affiliation, in which the Air Force placed the X-24A and supporting equipment on loan to NASA and enabled the space agency to avail itself of the Edwards Base services, chase aircraft, fuel, B-52 operations, and XLR11 maintenance. Seamans pledged NASA to provide full-scale wind tunnel access, instrumentation of the aircraft, overall vehicle maintenance, and ground support. They also joined forces on test piloting, mission planning, data reduction, test operations, and range support activities. Meantime, reflecting their satisfaction with the company that had fabricated the X-23A, Air Force officials signed with Martin Aircraft to fabricate a single X-24A in March 1966. As its baseline, the contractor used a design conceived by the Air Force Flight Dynamics Laboratory known as Space Vehicle Number 5P (SV5P). Martin delivered the completed X-24A to Edwards on August 27, 1967, less than a year and a half after the project began. But once at its destination, it took two years to get it into the skies. It first underwent full-scale wind tunnel tests at NASA Ames, followed by systems checkout and extensive modifications at the NASA Flight Research Center to cure persistent control problems. The delay also reflected an abundance of caution, after distinguished NASA pilot Bruce Peterson

suffered severe injuries in the crash landing of the M2-F2 on May 10, 1967. The vehicle that technicians finally rolled out of the hangar at Edwards on April 17, 1969, did not look like the epitome of success. Homely, bulbous, and flat-bottomed, it may have set the record as the most unprepossessing of the lifting bodies. In the initial glide flight on that day, Air Force pilot Captain Gerry Gentry had to fend off disaster. Like the M2-F2 before it, the X-24A fell victim to lateral instability during approach and landing. Gentry discovered roll oscillation at 1,800  feet (548 meters), and he responded by raising the angle of attack, cutting speed to 270 knots (310 miles, or 500 kilometers, per hour), and firing the landing rockets. He made a satisfactory touch-down. The eight glide flights that followed gave engineers time to mitigate this critical problem. Then, with the preliminaries finished, Gentry and the team started the powered flight program on March 19, 1970. Propelled by the X-24A’s single XLR11 rocket engine, Gentry flew the aircraft to Mach 0.87, and with the exception of a sharp roll to the right as he shut down the engine prior to the glide landing, he praised the handling, saying that it, “went like clockwork.” NASA pilot John Manke crossed the next big X-24A hurdle on October 13, 1970, by flying over Mach 1. After separating from the B-52 mothership at 52,000 feet (15,850 meters), he pushed the aircraft downwards to gain speed, but at Mach 0.9 he felt simultaneous roll sensitivity and pilot induced oscillations (PIO). He raised the angle of attack and the danger receded. Roll control remained good as he reached the goal of Mach 1.05, and later the glide down to the lakebed occurred without incident. Manke told the ground crew that it flew “as stable as a rock.” Eighteen additional X-24A missions followed quickly, the last one in June 1971. These subsequent flights, which eventually achieved Mach 1.6 and an altitude of 71,400  feet (21,763 meters), confirmed Manke’s early discovery that serious longitudinal instability occurred in the transonic range at angles of attack below four and above 12 degrees. So while the X-24A marked a notable advance in the behavior of lifting bodies, there remained further room for improvement.



X-24A, B, C 89

90  Prelude to the High Frontier: Early Space Vehicles

The last chapter of the lifting bodies saga entailed a radical new design approach, first pursued in 1969 by NASA and the Air Force on separate tracks. NASA’s candidate originated with aerodynamicists at the Langley Research Center. Known as the Hyper III, it differed radically from the rounded and bathtubshaped lifting bodies that came before. Flat-­bottomed and with a long, slender nose cone, it produced a hypersonic lift-to-drag ratio of 2.5, nearly twice that of past lifting bodies. With this capability, the Hyper III represented a breakthrough for the prospect of a spacecraft capable of landing on any suitable runway. Why did it inspire such confidence? Thanks to its high liftto-drag ratio, it could deviate as much as 1,500 miles (2,414 kilometers) from the established orbital reentry path and still get to its targeted touch down point safely. Dale Reed, of earlier lifting body fame, decided to test the Hyper III in actual flight conditions. To do so, he asked technicians at NASA’s Flight Research Center to build a 35-foot (10.66-meter) remotely piloted model. Flown from the ground by Milt Thompson, one of NASA’s most admired pilots, it

delivered an astonishing lift-to-drag ratio of 4.0 in subsonic flight. But further Hyper III testing stalled when NASA Headquarters turned down FRC director Paul Bikle’s request to fund a full-scale Hyper III with a pilot in the cockpit. Meanwhile, during the same year that Reed experimented with the Hyper III, the Air Force Flight Dynamics Laboratory culminated years of wind tunnel studies with a new design, known as the FDL-8. Its engineers considered converting a spare Martin SV-5J prototype into an FDL-8, until someone saw a more direct path to an advanced and possibly revolutionary lifting body. Rather than make an all-new test aircraft, the Air Force engaged Martin to encase the pudgylooking X-24A in a stiletto-like fuselage much like the Hyper III, with a flat underside and a 78-degree double-­delta planform, tapering to a sharply pointed nose. The FDL team made one major improvement on the Hyper III: project engineers eliminated the pivoting wings of the original Langley design while still managing to preserve the aircraft’s lift-to-drag ratio of at least 4.0.



X-24A, B, C 91

92  Prelude to the High Frontier: Early Space Vehicles In the end, the Air Force and NASA merged their efforts on the project − henceforth known as the X-24B − because of cost. The Air Force balked at Martin’s estimate of over $1,000,000 to transform the X-24A into the X-24B, but after intense discussions at NASA headquarters involving Bikle and lifting body program manager John McTigue, the space agency at last agreed to transfer $550,000 to the Air Force, which the USAF agreed to match. Martin got the contract in February 1972, and in the same month the two government agencies signed an MOU to conduct X-24B testing as a joint venture. Secured in the belly of a C-5 transport, the X-24B arrived at Edwards Air Force Base in October 1972. No one who recognized the bulky X-24A could have anticipated the swan that rolled down the ramp of the mighty Galaxy. Gleaming in the desert sun, its elongated aluminum body and greater dimensions made the diminutive X-24A look small by comparison. The new aircraft measured 37 feet 6 inches (11.4 meters) in length, and 19 feet 2 inches (5.8 meters) from wing tip to wing tip, with 330 square feet (30.6 square meters) of wing area (compared to its predecessor’s 24  feet 6 inches (7.4 meters), 13 feet 8 inches (3.9 meters), and 162 square feet (15 square meters), respectively). Like the X-24A, the X-24B used a Thiokol XLR11 engine, but modified so that its chambers could withstand higher pressures, raising its thrust rating to 9,800 pounds (in contrast to 8,480 for the X-24A). Not surprisingly, the X-24B weighed more than the X-24A: 13,800 pounds (6,260 kilograms) gross and 7,800 pounds (3,538 kilograms) empty, compared to 11,450 pounds (5,194 kilograms) and 6,300 pounds (2,858 kilograms), respectively, for the X-24A.  Performance between the two differed by only a little: maximum altitude and speed for the X-24B reached 74,130 feet (22,595 meters) and Mach 1.74; for the X-24A, 71,400 feet (21,763 meters) and Mach 1.55. While their raw performance limits did not diverge by much, flight research revealed that the characteristics of these two X-planes varied immensely. Before being asked to show its stuff in the air, however, the X-24B underwent a lot of preliminary work. Between February and August 1973, technicians at the FRC subjected the aircraft to an exhaustive set of ground and B-52 captive experiments to test its fundamental aerodynamics. Additionally, fearing that the elongated nose and farforward center of gravity of the X-24B might impose a hazardous load on the main landing gears and result in its collapse during touch downs, engineers reconstructed the main gears’ locking system to forestall this eventuality. They assessed the strength of the nose gear in a

time-honored way; by raising and then dropping the aircraft’s front end from increasing heights. NASA research pilot John Manke, who had flown the X-24A a dozen times, got the assignment to make the first of six X-24B glide flights, scheduled for August 1, 1973. All of the previous lifting bodies, without exception, had exhibited worrying, and in some cases life-threatening behavior. Now he would discover what the X-24B held for him. Manke and his colleagues almost could not believe what happened during the maiden flight. From the separation from the B-52 at 40,000 feet (12,192 meters) to the touch down, the X-24B handled like no previous lifting body. Although he encountered some buffeting, a mild tendency to pitch up, and the need for some aileron trim to maintain level wings, Manke discovered that it exhibited far better handling qualities than any other aircraft of its kind. “I just had this beautiful control of the airplane above the runway, no PIO tendency either in pitch or roll. It was just one of the most pleasant flying [experiences] right above the runway that I’ve ever flown.” Fifteen weeks later (on November 15, 1973), he made the first of three subsonic powered flights, none of which offered surprises. Then came the biggest tests: a series of 21 supersonic missions. During the first one, on March 5, 1974, Manke achieved Mach 1.086, and although he felt a little buffeting in the transonic region, he declared it a “super-­duper flight” aboard an aircraft “which sure does handle nicely. It was good or better than I had hoped all the way [through].” Manke’s initial impressions remained unchanged during the remainder of the program. Nor was it just Manke himself, who reached the highest altitude (74,130  feet (22,595 meters)) in May 1975; or Lieutenant Colonel Mike Love, who flew the X-24B to its ultimate speed (Mach 1.76 in October 1974); all six of the pilots who flew the X-24B (36 times in total) marveled at its steady handling and its reliable lateral motion at all speeds, comparing it to nothing less than the F-104 fighter for ease of control. The timing of these flights gave them deeper meaning. During the period in which the X-24B underwent testing at Edwards (August 1973 to November 1975), engineers at Rockwell International and at NASA’s Johnson Space Center (JSC) labored over the contours of the Space Shuttle Orbiter. As they made their calculations, data from the X-24B offered compelling evidence that aircraft (and spacecraft) with high lift-to-drag ratio designs could return from space safely and make precise landings on appointed runways.



X-24A, B, C 93

94  Prelude to the High Frontier: Early Space Vehicles Well before the termination of X-24B flight research, NASA and Air Force planners held meetings to consider new hypersonic aircraft candidates capable of carrying forward the legacy of the X-15, whose last flight occurred in 1968. During the mid-1960s, NASA Langley initiated two such programs: one for a Mach 8, and a second for a Mach 12 vehicle. At the same time, the Air Force prepared for its own hypersonic candidates, in the range of Mach 5 and Mach 6. The two sides met in July 1974 to review the research and discuss a possible joint project. As they did so, the USAF’s FDL-8 configuration − the one that contributed to the X-24B − seemed to be the most likely candidate. Using it as a baseline, they formed an X-24C Joint Steering Committee, which in turn conceived the National Hypersonic Research Facility aircraft. NASA proposed a budget of roughly $200 million for two Mach 8

aircraft, with a flight program much like that of the X-15: ten years long, consisting of about 200 flights. The space agency signed a study contract with the Lockheed Skunk Works, which favored two propulsion alternatives: a scramjet, and rocket power. Lockheed pursued the project until 1977, envisioning a big vehicle: 74 feet (22.5 meters) long, 24 feet 2 inches (7.3 meters) wide, and 20  feet 7 inches (3.6 meters) tall. It even included a 12-foot (3.6-meter) cargo bay just behind the cockpit. But NASA lost interest in the X-24C as costs mounted and the agency’s budget tightened. It withdrew in September 1977 and the X-24C came to an end. Despite that, after years of experiments with a succession of unstable and often dangerous lifting bodies that threatened the lives and limbs of pilots, the subsequent X-37B validated the design concept and proved its practical value.

Part II X-Planes Since the Cold War, 1990−2021

4 Flight Testing for Combat: Military Vehicles

X-31A What the X-29 forward-swept-wing project sought to achieve − maneuverability far in excess of contemporary fighter aircraft − the X-31 actually realized, although many obstacles lay in the path to success. When it seemed destined to require exorbitant design and fabrication costs, its originators in the German Defense Ministry sought partners. They looked first to their European counterparts, but when none stepped forward, they tried the U.S. Because of this necessary outreach, in the end the X-31 attracted a longer list of collaborators than any X-plane before it; and because of that, it represented the most complex international X-plane effort to date. The Defense Advanced Research Projects Agency (DARPA), the German Federal Ministry of Defense (MOD), the U.S.  Naval Air Systems Command, Rockwell International Corporation, and the Daimler-Benz Aerospace Corporation all signed on initially. The U.S. Air Force and NASA joined the others in order to orchestrate the flight research portion of the project. In fact, to assure full coordination, the group set up a body called the International Test Organization (ITO) to map out the X-31A’s flying activities. Negotiation became a key ingredient in this multilayered landscape. First, the ITO members asked

themselves whether the X-31A concept really offered exceptional maneuverability for close-in aerial combat. Then came a review of the demonstrator itself. The X-31A was the creation of a Daimler Benz aerodynamicist named Dr. Wolfgang Herbst, whose team had devoted almost 10  years to high angle of attack (AoA) research. Finally, fabrication needed to be synchronized across an ocean, linking Rockwell International (manufacturer of the fuselage, vertical tail, and canards) with Daimler-Benz (which conceived and built the carbon fiber/metal substructure wings and the thrust-deflecting paddles, also made from carbon fiber). Aside from the bureaucratic hurdles, the X-31A also incorporated a package of technological uncertainties. High angle of attack (or high alpha) maneuvering had many skeptics, and for good reason. Questions abounded regarding the X-31A’s actual flight envelope, the consequences of its disturbed airflows, and the impact on aerodynamics and structures by vectored thrust and the air intake design. The new concept of greatest concern tied thrust vectoring directly to the flight control system, necessitated by an inherent instability in the X-31A’s flight regime that required computer commands to fly the aircraft. Adding to the burden, engineers involved in the project agreed that the demands on the pilots needed to be kept simple, in order to avoid workload exhaustion in the cockpit.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 M. H. Gorn, G. De Chiara, X-Planes from the X-1 to the X-60, Springer Praxis Books, https://doi.org/10.1007/978-3-030-86398-2_4

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Once authorities at DARPA − by now the overall program manager − felt that these and other problems could be resolved, the X-31A (designated as such in February 1987) received funding in August 1988 for two testbeds, to be completed in 22 months at an initial cost of $47.3 million. In the end, the total expenditure, which included complete development and flight testing, reached approximately $270 million. Prior to the X-31A going aloft, NASA, the Air Force, and the Navy verified the computerized flight control system developed by their German partners. At the same time, a scale model of the X-31A underwent about 25 aerodynamics drop tests at NASA Langley. Then, on October 11, 1990, Rockwell pilot Ken Dyson took off from the company’s factory at Plant 42  in Palmdale, California, on the X-31A’s maiden mission, which lasted 38 minutes. This modest first step inaugurated a period of high activity that continued for nearly five years and involved a combined 580 test flights in the two X-31As. For the first 108 of them, the Palmdale Airport continued to be the test venue. There, between November and December 1991, the first post-stall flight testing occurred and the

project tested high angles of attack up to 52 degrees. Once the government agencies represented by the International Test Organization took command of the X-31A from Rockwell in January 1992, the project moved from Plant 42 to the NASA Dryden Flight Research Center on Edwards Air Force Base. The flights resumed in April and for the following three and one-half years (until June 1995), 14 pilots tested the X-31A. Three of them carried the bulk of the load: Fred Knox of Rockwell flew 120 times; Karl Heinz-Lang of Germany flew 113; and Rogers Smith of NASA piloted 77. Together, their 310 missions accounted for well over half of all the X-31A flights. For the most part, the X-31A system proved its mettle as a potential combat aircraft technology. Pitted in mock competition against a Navy McDonnell Douglas F/A-18 fighter in September 1993, the X-31A won in 78 of 94 engagements, with eight ties and eight successes for the F/A-18. On the other hand, the X-31A scored worse than an Air Force General Dynamics F-16, although the advantage may have been due to the latter’s newer Pratt and Whitney F-100-PW-229 engine, which gave the F-16 a better thrust-­to-­weight ratio.

98  Flight Testing for Combat: Military Vehicles The X-31A also showed its value as a technology demonstrator. The aircraft underwent a series of AoA trials beginning in June 1992, including envelope expansion, flight safety tests, and technical and tactical demonstrations. In September of that year the aircraft flew at 70 degrees AoA with 45-degree bank angles, and followed that feat on November 6 with 360-degree rolls at 70 degrees AoA, the first such maneuver ever flown. Then, in April 1993, the X-31A chalked up another achievement never before ­ accomplished by any combat vehicle, by making a 180-degree turn in a post-stall condition, with a minimum radius. Overall, the X-31A proved its performance in a fully controlled and flyable manner past the point of stalling. Paradoxically, the design and construction of this exceptional aircraft proved to be quite ordinary in one sense, but revolutionary in others. Its fuselage followed a conventional design, comprised mostly of aluminum with 11 bulkheads, and many of the X-31A’s major components borrowed from a wide cast of characters, including the F-16, the Bell V-22, the Vought A-7, and the McDonnell Douglas F/A-18, among others. But the cranked-delta wings, fashioned from composites, looked to the future, as did the all-moving canards covered in graphite epoxy skins. Aside from the crucial computer-­operated flight control system, perhaps the most novel part of the X-31A involved the three thrustvectoring paddles fabricated from graphite epoxy and attached to the rear fuselage. These devices interacted with the engine exhaust plume, channeling and manipulating the intense stream of air in such a way as to give the X-31A’s pilots pinpoint control over the aircraft’s pitch and yaw. In fabricating the X-31A, Rockwell made a small, relatively lightweight aircraft. At 43 feet 4 inches (13.2 meters) long and with a wingspan of 23 feet 10 inches (7.2 meters), it measured significantly less than the F-16 fighter, itself regarded as a compact weapon system. The X-31A’s wing area (226.3 square feet, 21 square meters), height (14  feet 7 inches, 4.4 meters), and gross weight (15,935 pounds, 7,228 kilograms) also undershot the F-16 by a wide margin. Its single General Electric F404-GE-400 turbofan engine, fed by an under-fuselage air intake, enabled a maximum altitude of 40,000 feet (12,192 meters) and a speed of up to Mach 1.4. Toward the end of its service, the X-31A project experienced a setback, followed by a resurrection. The last flight of ship number 1 occurred on January 19, 1995, when the X-31A went out of control due to an undetected freezing in the pitot system. Before the

aircraft crashed, pilot Karl-Heinz Lang ejected to safety at 18,000 feet (5,486.4 meters). Aircraft number 2 continued flying until June 1995, at the end of which it went into a long retirement, finally going into storage in February 1999 at a Boeing facility in Palmdale. But the X-31A sprang back to life in April 2000 when it arrived at Naval Air Station Patuxent River, Maryland, for a new role. There, technicians rebuilt it to serve in a program called VECTOR, an acronym for a long and awkward title: Vectoring Extremely Short Take-Off and Landing Control Tailless Operation Research. Just as its name suggests, the VECTOR team (a combined U.S. Navy and German group) hoped to demonstrate highly restricted take-offs and landings using thrust vectoring. The aircraft made its first VECTOR flight in February 2001, followed by 14 months of improvements and ground testing. Then, between May 2002 and April 2003, it underwent a set of automated runway landings in which the X-31A approached touchdown points at 24 degrees AoA, flying at only 139 miles per hour (30 percent slower than the standard 202 miles per hour descent). After demonstrating its abilities in VECTOR, the X-31A returned to permanent storage at Patuxent River. X-32 and X-35 At the beginning of the twenty-first century, the X-planes returned to their origins. Rather than test a specific technology (like nuclear propulsion on the X-6) or build a demonstrator to enhance a facet of performance (like the X-29 forward-swept-wing aircraft), two new candidates offered high-stakes design concepts, with an eventual eye towards military adoption. In this sense, they more closely resembled the X-1, the X-2, and even the X-3, than most of the X-planes tested during the almost 50 years since this famous trio took flight in the 1940s and 1950s. This return to the past originated with an emerging combat aircraft known as the Joint Strike Fighter (JSF), meant − as the name implies − to be used commonly by the U.S. Air Force, Marine Corps, and Navy, as well as many overseas buyers. The risks could not have been higher. Beyond the testing of these two concept X-planes lay perhaps the richest defense payout ever realized − estimated at the time at over $200 billion − for the production of 5,000 to 8,000 total units, including vast foreign military sales. In time, the actual costs dwarfed even these early estimates.

X-32 and X-35  99

This ambitious and historic program originated with the Defense Advanced Research Projects Agency (DARPA), a subordinate office of the Department of Defense (DoD). Planners at DARPA, interested in frugality and commonality, wondered whether the existing generation of front line, but aging combat equipment − the USAF’s F-16 fighter and the A-10 close air support aircraft; the Marine Corps’ AV-8B Harrier; and the Navy-Marine Corps’ F/A-18 − could be succeeded by a single vehicle capable of fulfilling all of their roles. In other words, could one aircraft embody a ground-­based fighter, a carrier–based fighter, close air support, and vertical flight? DARPA drew confidence from its earlier Joint Advanced Strike Technology (JAST) program, but many skeptics doubted it. During the early 1960s, Defense Secretary Robert S.  McNamara attempted to impose a common fighter airframe as a cost-saving measure on the Air Force and Navy, both of which preferred to embark on their own separate programs. McNamara instead pushed for the Tactical

Fighter-Experimental, or TFX, later known as the F-111 for the Air Force and the F-111B for the Navy. Ultimately, the F-111 satisfied neither service because it failed to be fully competent in either of the original missions required by the two services: as a Navy carrierbased interceptor or as an Air Force low-altitude penetrator. Despite this black mark against commonality, which may have been partially forgotten in the passage of time, DARPA pressed forward with its even more ambitious modern equivalent. Like the contemporary X-31 high-alpha aircraft, the Joint Strike Fighter attracted intense government and private sector interest, only more so. DARPA found partners in the U.S.  Air Force, Navy, and Marine Corps, as well as Great Britain’s Royal Air Force. Among three finalists for JSF, McDonnell Douglas dropped out, leaving the DoD to choose between Boeing and Lockheed Martin in November 1996. The two contractors each received instructions to make a pair of Concept Demonstrator

100  Flight Testing for Combat: Military Vehicles Aircraft (CDA): one a CTOL, or conventional take-off and landing variant; the other a STOVL (short take-off and vertical landing) system. The Air Force named these CDAs the (Boeing) X-32 and the (Lockheed Martin) X-35. The X-32 received the further designations of X-32A (the CTOL model) and B (the STOVL) because these vehicles would be fabricated on two entirely separate airframes. Lockheed’s candidate also offered two airframes, but in three models: the X-35A, B, and C (achieved by converting the A to the B during the flight research phase). The engineering teams at Boeing and Lockheed Martin approached the conundrum of the JSF entirely differently. At first, Boeing offered an aircraft with a blended delta wing, twin vertical fins, and a prominent chin-type air intake. But after further study that considered reduced weight for STOVL and high-alpha capability, the company unveiled an updated design in January 1999, including a swept version of the initial delta wing, a huge air intake with a forward leading edge, and horizontal stabilators mounted at the base of the vertical fins. The new plans also proposed removing 18 inches (45.7  cm) of fuselage behind the wing trailing edge in the STOVL model, but this only represented a part of the compromises necessary to achieve short take-off and vertical landing. In its mode of propulsion, the designers decided to pattern the X-32B after the British Harrier, the very aircraft that it proposed to replace. To do so, two lift nozzles needed to be mounted at the center of gravity, and as a result, the engine − whose thrust powered the nozzles − had to be placed far forward in the fuselage, giving the aircraft a snout-like nose. The vertical flight capability also required a moveable intake cowl to allow increased air into the engine during hover. In contrast, the X-32A required none of the STOVL modifications, but it did include a stationary air intake and a wider wing span with prominent tip extensions. Lockheed’s designers also built two airframes, but ­produced three distinct demonstrators: an Air Force CTOL X-35A variant; an X-35B, constructed on the X-35A airframe but modified for Marine Corps and U.K.  STOVL purposes; and a Navy/aircraft carriercompatible STOVL, designated X-35C.  The X-35A’s design adhered to the fundamental Air Force requirements, but in a key difference from the X-32, the X-35B’s engineers decided to break from the Boeing decision to adopt the Harrier design. Instead, Lockheed chose to follow in the path of the Russian Yakovlev-141, a vertical take-off and landing fighter aircraft. Unlike the direct lift of the Harrier, the X-35B relied on a

second, smaller engine that augmented the thrust from the aircraft’s main powerplant. Lockheed’s system produced significantly more thrust than Boeing’s, but it also increased the aircraft’s weight due to the added engine. It likewise added complexity. As for the X-35C, it differed from the other Lockheed JSF airframe in at least two respects in order to satisfy Navy requirements: a bigger wing with a wider span to accommodate more fuel, and expanded horizontal tails and flaperons to give greater control on low-speed landings at sea. The two competitors faced off in 2000 and again in 2001. The X-32A’s first flight occurred on September 18, 2000, with a very short route from the Boeing factory in Palmdale, California, to nearby Edwards Air Force Base in Lancaster. After 20  minutes in the air, pilots in both F/A-18 chase planes noticed a hydraulic leak on the X-32A’s fuselage, later determined to be caused by a loose O-ring. Small hydraulic leaks persisted in other flights, resulting in testing delays. At the same time, project engineers faced issues that prevented the landing gear from being retracted until the tenth mission. By two months into the program, the X-32A had completed 18 flights, or about 24 percent of the planned workload. Then, from November 15, 2000 to mid-December, the aircraft began a series of mock carrier landings, conducting up to five flights per day with no incidents. It also reached two milestones that December, accomplishing its first aerial refueling, and flying to Mach 1 at 30,000 feet. Eventually, the X-32A underwent four months of testing and 66 total flights. Meanwhile, the completed X-32B went on a cross-­ country ferry flight from California to Patuxent Naval Air Station in Maryland, where it underwent simulated carrier tests. It made its initial flight on March 29, 2001, and when the program ended in July it had flown a total of 78 times. The X-35A began its flight research on October 24, 2000, about a month after the X-32A. The program as a whole attempted to determine three things: the degree of commonality among the A, B, and C; the flying qualities encountered in carrier approaches; and the viability of Lockheed’s STOVL system. Like the X-32A, the initial foray of the X-35A involved a brief run from Palmdale (at the Lockheed facility), to Edwards Air Force Base (during which the pilot ended the flight when a door on the aircraft’s nose failed to close). At Edwards, the six pilots assigned to the X-35A completed the initial test phase − mostly involving performance benchmarks − with 27 missions in 30 days. The penultimate of these events (on November 21,

X-32 and X-35  101

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X-32 and X-35  103

2000) proved to be a milestone, when the X-35A not only made its first supersonic flight, but also conducted a carrier landing demonstration, doing six touch-andgos. After November 22, the X-35A flew back to Lockheed’s Palmdale plant for a two-month re-fitting as the X-35B, and in that guise it underwent a brief testing program from June 23 to August 6, 2001. Despite the identical reasons for their existence, the two competitors for the JSF contract differed remarkably in design and concept. The profile of the X-32 had a novel, if ungainly look. At the front of the aircraft, three unusual features prevailed: a short nose, a high cockpit, and an expansive air intake, which together gave it the appearance of an openmouthed shark. Its broad delta wing − a single-piece structure fashioned from composites − sat atop a wide fuselage configured to channel the onrushing airflow into the intake. Parked on a runway near the X-32, the X-35 almost looked like it vied for a different contract. Designers of both aircraft worked with the challenge of producing the smallest possible radar signature, but the X-32’s broad, rounded contours tried to accomplish the goal through surface blending, enabled by composite structures. The X-35 addressed the problem head-on, with a compact fuselage that cast a miniscule presence on radar. The X-35’s overall appearance made a distinct

contrast to the X-32, as a long, sleek structure with small wings mounted far aft, average-sized air intakes on either side of the fuselage, and two tall, canted tail structures. Despite the yawning gulf in the looks of the JSF candidates, the competitors shared similar fabrication materials: lightweight metal alloys (including titanium) blended with composites. They also used the same Pratt and Whitney F119 afterburning turbofan powerplant (the -614 variant for the X-32, the -611A for the X-35). Even their dimensions did not diverge as radically as might be expected. At more than 50 feet (15.2 meters) in length, the X-35A did measure about three feet (0.9 meters) longer than the X-32A, but its 35-foot (10.7-meter) wingspan turned out to be just 12 inches (0.7 meters) shorter than that of the X-32A.  The X-35A’s wing area of 458 square feet (42.5 square meters) did measure significantly less than the 554 square feet (51.4 square meters) of the X-32A, but their respective heights − 13.3 feet (4 meters) for the X-32A versus 13.8 feet (4.2 meters) for the X-35A − differed negligibly. The mass of each aircraft also did not vary by much, with empty weights of 24,030 pounds (10,900 kilograms) for the X-32A and 26,500 pounds (12,020 kilograms) for the X-35A. The take-off weights for the two aircraft each approximated 50,000 pounds (22,680 kilograms).

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After the fly-offs between the two aircraft, the Department of Defense made the much anticipated JSF contract announcement − valued at almost $19 billion, the most costly ever awarded by the DoD − on October 26, 2001. Among other reasons, the Defense authorities chose the X-35 because of its superior stealth capability; its reliance on the more modern (although admittedly more complex) shaft-­ driven (as opposed to direct) lift fan system; and the fact that even though the Navy and Air Force insisted that Boeing use separate airframes to test the conventional and carrier missions, the manufacturer actually used the same demonstrator − the X-32A − for both CTOL and CV testing. Why the X-35 prevailed over the X-32 may be debated, but in the years since the decision it has become clear that the JSF has been massively expensive, at times disappointing technically, and uneven in its ability to serve the fighter needs of three U.S. military services.

X-44A Aside from its technical achievements, the X-44A stands apart as an X-plane that shape-shifted: from a well-­articulated aircraft in the beginning, into something elusive at the end. In 1999, word began to circulate in aeronautical circles that the Air Force had reserved the designation X-44A for a full-sized, piloted demonstrator, one that combined several advanced features: flight control by means of thrust vectoring nozzles; the absence of any tail structure; a light airframe; and a simple structure. The aircraft promised major advantages, including supersonic cruise, a low radar signature (no moveable control surfaces and no empennage) and the capacity to hold more fuel than comparable vehicles. Beyond that, any specifics remained cloaked in secrecy. Then, in 2003, rumors suggested that a feasibility study of the X-44A had been set in motion, with participation by DARPA, NASA, the Air Force Research

X-44A  105 Laboratory, Lockheed Martin, and Pratt and Whitney. Rather than begin with an all-new vehicle, it appeared that Lockheed Martin would make an F-22 Raptor prototype with a large trapezoidal wing available for the project. It even bore an acronym: the Multi-Axis No-Tail Aircraft, or MANTA. Fifteen years after these speculations, the long silence ended. Published reports indicated that the MANTA program expired in 1999, to be supplanted by a shockingly different X-44A. Instead of a nextgeneration fighter pilot’s dream, the Lockheed Martin Skunk Works dreamed up a small, autonomous, potato-shaped flying wing, positioned between two other Lockheed Unoccupied Aircraft

Systems, or UASs: the RQ-3 Darkstar (an unmanned, long duration spyplane sponsored by DARPA), and the RQ-170 Sentinel (a medium-altitude surveillance vehicle that overflew Osama Bin Ladin’s compound, among other accomplishments). In fact, the Skunk Works actually revealed the existence of the new, unimposing X-44A in a highly unorthodox way in 2018: at the Los Angeles County Air Show in Lancaster, California, where it served as an exhibit near the entrance. Patented in 1996 and first flown in 2001, it had been veiled from public view until Lockheed displayed it in Lancaster. The Skunk Works evidently fabricated the latter-day X-44A in the same year that the MANTA disappeared.

Why two aircraft should occupy one designation at almost the same time remains unclear. Some speculated that the MANTA may have been propagated intentionally to draw attention away from the UAS, the kind of aircraft that gained ascendancy after the attacks on the Pentagon, New  York’s Twin Towers, and United Airlines Flight 93 on September 11, 2001. Drones rose in importance after that date, due to their

ability to pinpoint and target concealed or inaccessible enemy forces without risking American lives. Perhaps the rise of the humble, clamshell-contoured X-44A over the large, graceful MANTA may be explained by the urgent need for new, more nimble counterinsurgency tools, capable of quick manufacture and the ability to cover long distances over hostile terrain.

106  Flight Testing for Combat: Military Vehicles Still under wraps, not much has been revealed about the X-44A, other than its wingspan (about 30 feet, or 9.1 meters), its construction (probably fashioned from nano-carbon fiber), and its engine (a Williams F122 turbofan, a powerplant common to cruise missiles). It also has control surfaces on its trailing edge, squared wingtips, and most importantly, serves as a platform for a wide range of sensors. Recent reporting suggests that the aircraft has been retrofitted in order for it to assess the visual cueing capabilities of the Navy’s Carrier Based Aerial Refueling System, a tanker drone program. X-45A Despite having invested so much money and energy in the proposition that combat pilots and their machines represented the ultimate in aerial warfare, the U.S. armed forces’ acceptance of, and conversion to, unoccupied aircraft systems (UASs) has been surprisingly

swift and certain. The proposition that UASs should take the warfighting risks while human pilots stay safely on solid ground took a big step forward with the X-45A, which embodied the first modern drone designed specifically for military strikes. But it did not emerge without antecedents. As far back as the early 1980s, defense studies considered the concept of unmanned combat air vehicles (UCAVs), based on the experiences of drones during the Vietnam War and Middle East conflicts. By the end of that decade, advances in remote control, propulsion, lightweight materials, radar-evading airframes, and autonomous navigation, elevated these systems from ancillary to front-line status. By the early 1990s, UCAVs began to come into focus as a prominent weapon. Following the attacks of September 11, 2001, UCAV development accelerated to meet the terrorist threat. Capable of flying almost anywhere with a low chance of detection, they have proven to be an effective antidote to the shadowy forces that comprised Al-Qaeda and other such groups.

X-46 (X-45N)  107 Boeing Aircraft became involved in unmanned combat aircraft well before 9/11, initiating research as early as 1998, making it one of the pioneers in the field. Recognizing this capability, the Defense Advanced Research Projects Agency (DARPA), which initiated the X-45A project, awarded Boeing $110 million in March 1999 to build and test two X-45As over the course of three and one-half years. With the incentive of future work if this X-plane succeeded (that is, actual production contracts), the contractor added $21 million of its own resources to the funding pool. Boeing rolled out a Block I configuration from its St. Louis factory in September 2000, which arrived at Edwards Air Force Base for assembly and static testing in October. Once flight research began in 2002, DARPA handed off management of the X-45A to the Navy and Air Force the following year. These services then created the Joint Unmanned Combat Air System (J-UCAS) Office to guide the X-45A (and later the X-47) under one team. The X-45A’s novel shape, akin to a flattened dolphin, with two prominent, swept wings, set it apart in the skies. It measured 26 feet 5 inches (8 meters) long, far surpassed by its wingspan of 33 feet 8 inches (10.3 meters). It stood just 3  feet 7 inches (1.1 meters) tall and weighed about 8,000 pounds (3,629 kilograms) empty, but its uncomplicated, sleek lines belied its complexity. Designed for stealth, the X-45A lacked either a vertical or canted tail and featured a lowmounted wing and blended fuselage. Boeing fabricated the aircraft from a foam matrix core and composite reinforced epoxy skin. Its internally-housed weapon bays and its Honeywell F124-GA-100 turbofan engine could handle a payload of 1,499 pounds (680 kilograms). The aircraft possessed an impressive flight envelope, with a maximum speed of Mach 0.75 and altitude of up to 35,000 feet (10,670 meters). Adaptable and portable, its detachable wings enabled it to be crated and shipped by air freight to forward areas of operation, while its tricycle landing gear permitted conventional take-offs and touch downs. The X-45A premiered a number of baseline UCAV capabilities during its three years in the skies. It became the first high-performance, autonomous, combat-capable vehicle of its kind to fly; the first UCAV to release a weapon; and, flying with its twin stablemate, the first to conduct an autonomous, coordinated flight with another unmanned aircraft. It made its maiden voyage, of 14 minutes, at NASA’s Dryden Flight Research Center on May 22, 2002, during which time it demonstrated its flight characteristics as well as the communications link between the aircraft and mission control. After

many more tests, including an intense ten-day period in March 2004, a key mission occurred in April 2004. On that occasion, the X-45A struck a ground target with a 250-pound (113-kilogram) inert Small Smart Bomb, a guided weapon released from the aircraft’s internal weapons bay at 35,000  feet. In August that year, the two X-45As made the first coordinated, multi-UCAV flight under the control of a single pilot. By the time the flight program ended in August 2005, after a series of decisive tests that transferred flight-­decision making from the ground-based station to the aircraft itself, the X-45A had flown 64 missions in all and demonstrated the feasibility of UCAVs as combat weapons. X-46 (X-45N) As the X-45A program proceeded along its own course, DARPA ordered the X-45B from Boeing, a system able to deliver bigger payloads and with a wider operating range than its predecessor. But DARPA authorities rescinded the contract in early 2003, instead deciding to concentrate on the X-45C, which surpassed the X-45B in fuel capacity and trebled its range. Boeing set to work fabricating three of these demonstrators after DARPA made an award of $767 million in October 2004. But this project also came to a halt when the Air Force pulled out of the X-45 in March 2006. This development did not end the X-45, however. Rather, the U.S. Navy launched its own UCAV undertaking, known as the Naval Unmanned Combat Air Vehicle (N-UCAV) Advanced Technology Program (ATP), which ran parallel to the USAF’s X-45 activities. To begin the Navy’s work, DARPA and the Navy awarded two contracts for preliminary research in June 2000: one to Boeing for an X-46A; and the other to Northrop Grumman for an X-47. Either of these two might have achieved for the Navy what the X-45s accomplished for the Air Force. However, when the Air Force and Navy joined forces in 2003 with the Joint Unmanned Combat Air Systems (J-UCAS) project, it became clear that the services did not need the Boeing and the Northrop Grumman candidates, since both concentrated on fulfilling the same J-UCAS requirements. As a result, the two parties decided to cancel the X-46A. Boeing did not give up despite this disappointing set back, and undeterred, it continued on with the X-45N, a Navy-­based UCAV that it had been pursuing in concert with the Air Force’s X-45A, B, and C. Unlike these USAF models, the Navy had far different requirements, with naval aviation officials looking for an unmanned,

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X-47  109 low-observable aircraft capable of operating from aircraft carrier decks, suppressing enemy defenses, conducting surveillance, engaging in electronic attack, and launching strike missions. Boeing sought to satisfy these requirements with the X-45N. Boeing’s executives had an expansive vision for the X-45N. Although the aircraft owed a debt to the other X-45s, it really embodied a break from them. The engineering team designed it to be 25 to 30 percent bigger than the ambitious but cancelled X-45C,

making it a 36,000-pound (16,300-­kilogram) heavyweight with a wingspan of about 70 feet (21.3 meters). It could hold a lot of cargo, with a weapons bay capable of lifting 5,997 pounds (2,720 kilograms) of armament and other materiel. Indeed, its payload compartment had been configured specifically to carry Raytheon’s AGM-154 air-to-surface missile. The designers also added a bulge that ran the length of the bottom of the aircraft to hold special sensors required by the Navy.

Plans called for the first flight of the X-45N to be held in November 2008, but in August 2007 Northrop Grumman’s X-47B won out as the Navy’s final choice. The X-47 might have succeeded because the contractor designed it from the ground up, unlike Boeing having to adapt the X-45N to the Navy’s unique sea-based requirements. It might also reflect the close and decades-old partnership between Grumman and naval aviation. Regardless, the X-45 program ended with this decision.

X-47 Pursued down a long and winding path, the X-47A and B ultimately served the Navy’s need for a combat drone demonstrator, much as the X-45A satisfied the same objective for the Air Force. The process began in 2000, when DARPA and the Navy offered preliminary contracts to two manufacturers: Boeing, for an X-46 (giving the manufacturer a chance to re-cast its X-45A for Navy needs); and

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X-49  111 Northrop Grumman, to pursue an X-47A and B.  The X-46 project dropped out of the running shortly afterwards in 2003, when Navy and Air Force representatives came together on the Joint Unmanned Combat Air Systems (J-UCAS) committee. With the Navy’s blessing, the two parties decided to cancel the X-46 as redundant and continue with the X-47A and B. Despite this choice, however, Boeing did not step aside and chose to develop the X-45N, a variant of the Air Force X-45A combat drone heavily modified for Navy use. When the J-UCAS team closed down in 2006, the Navy made its own final selection the next year, picking the X-47B. Northrop Grumman took a more radical design approach than Boeing’s more conventionally contoured X-45A (with its appearance of a dolphin with swept wings). The X-47 took a page from the diamond-shaped concept that originated during the making of the F-117 stealth fighter, resembling a kite with swept wings apart from a bulge on its upper deck that ran forward to aft. It also had no vertical control surfaces. Constructed mostly of all-carbon composites, the larger of the Northrop Grumman demonstrators (the X-47B) measured almost 38.2  feet (11.6 meters) long, with a wingspan (the wings could be folded for carrier storage) of 30.9 feet (9.4 meters). It stood 10.4 feet (3.1 meters) tall with an empty mass of 14,000 pounds (6,350 kilograms). Fully loaded, it weighed an incredible 44,567 pounds (20,215 kilograms). Scaled Composites of Mojave, California, a small company made famous by the SpaceShipOne suborbital spacecraft, fabricated the X-47 for Northrop Grumman. Despite its lightweight construction, it needed to be durable in order to withstand the forces generated by carrier arrestor hooks during take-offs and landings. In addition to this necessity, the X-47 also required a complex suite of avionics, and a vehicle management computer to govern the bulk of the aircraft’s functions: navigation, command and control, engine operations, autopilot systems, and flight control processing. One Pratt and Whitney Canada JT15D-5C turbofan engine powered the early X-47B variant, enabling it to travel long distances by carrying up to nearly 1,581 pounds (717 kilograms) of fuel. Stealthy but slow, it flew subsonically. The X-47 (also referred to as Pegasus in reference to the winged horse of Greek mythology) evolved over a long gestation period of 15  years, during which time the Navy dealt with the inherent difficulties of both carrier landings and operations on the open seas. The

aircraft also underwent extensive enlargements and improvements. After a proof of concept phase sponsored by DARPA and the Navy that began in 2000, the X-47A rolled out in July 2001 and made a first, successful flight in February 2003. Encouraged by its performance, the Navy awarded Northrop Grumman a second contract for three X-47B demonstrators in August 2004. The new Pegasus would be bigger than the X-47A and geared more toward operational flying. Construction got underway in June 2005, following which flight testing took place from 2007 to 2009. Meanwhile, the Navy opened a new chapter in the program. In August 2007, the service called on Northrop Grumman again, this time for an even more advanced X-47B equipped with a Pratt and Whitney F100-PW-229 engine. Called demonstrator air vehicle one and two (AV-1 and 2) for the purposes of this phase, both still flew as X-47Bs. AV-1 left the Northrop factory in October 2009 and went to Edwards Air Force base for flight testing in 2010, followed by AV-2  in 2011. AV-1 conducted its first flight in 2011 and underwent carrier landings in July of that year. Flight research proceeded into 2013, by the end of which AV-1 and 2 had recorded 100 total missions. The following year, sea trials began on the carrier USS Theodore Roosevelt. The long history of the X-47B ended at last in April 2015, when it made the world’s first fully autonomous aerial refueling. During its lengthy career, the X-47 persuaded Navy officials that it could not only work effectively from aircraft carriers, but that it could be integrated into standard carrier operations with manned aircraft. The service therefore made plans for a carrier-based fleet of UCAVs to enter the inventory, beginning in the early 2020s. X-49 Thrust vectoring flight control and the U.S. Navy have had a long and positive relationship, both in the pursuit of carrier-­based operations and in the support of Marine combat engagements. The Harrier Jump Jet, produced by Hawker-­Siddeley from 1967 to 2003, found a cherished place in the service’s inventory. But recognizing in the 1990s that the Harrier production line must shut down at some point, naval authorities collaborated with DARPA, the Air Force, NASA, and the German Ministry of Defense (MOD) in the X-31 thrust vectoring project, which took to the air in 1990 and closed down in 1995.

112  Flight Testing for Combat: Military Vehicles Interestingly, the Navy did not let the X-31 die a quiet death. After remaining in storage for five years, the X-31 took on a new role when the Navy and its German partners launched the Vectoring Extremely Short-Take-Off and Landing Control Tailless Operation Research, known as VECTOR.  Rebuilt expressly to serve the Navy’s needs, the X-31 VECTOR experiments flew at Patuxent River Naval Air Station between 2001 and 2003, demonstrating highly restricted, automated take-offs and landings at slow speed and high alpha. When VECTOR ended, the X-31 disappeared again behind lock and key. Given the relationship between the Navy, the Harrier, and the X-31, it did not seem surprising that another thrust vectoring opportunity presented itself around the same time as the Navy pursued the supplemental X-31 research. But this one appeared in a surprising guise: as thrust vectoring propeller blades on a helicopter. The concept sprang from the compound helicopter, an idea first proposed in the 1930s. It represented an effort to increase the speed of helicopters flying in conventional (non-hovering) format, accomplished by augmenting the overhead rotors with a separate propulsion system to accelerate horizontal flight. In doing so, designers hoped to combine the best

qualities of helicopters and fixed wing aircraft on one airframe. Modern military leaders saw high value in compound helicopters that could fly rapidly (around 230 miles (370 kilometers) per hour), yet still achieve good range, reliability, and durability in combat conditions. Most decisively, their added speed made them less vulnerable to ground attack. The search for speedier rotorcraft came at the start of the twenty-first century, at a time when planners realized that a replacement for the highly regarded, but aging Sikorsky H-60 Army Blackhawk transport needed to be considered. Piasecki Aircraft of Essington, Pennsylvania, brought a new wrinkle to this dilemma. Rather than buy a whole new fleet, Piasecki proposed modifying the existing Blackhawks, utilizing the company’s proprietary innovation called the Vector ThrustDucted Propeller (VTDP). Encased in a circular cowling, the VTDP acted as a rear propulsion unit, giving the helicopter added speed and a more horizontal level of attack in forward flight. Additionally, this source of power at the tail also took much of the load from the main rotor. Piasecki undertook the challenge with a sense of familiarity: their Pathfinder 16H-1A compound helicopter flew faster than 225  miles (362 kilometers) per hour during the early 1960s.

X-50A  113 Naval Air Systems Command authorities had sufficient interest in this approach to offer Piasecki a demonstration contract in October 2000. A team at the firm then transformed a Sikorsky SH-60 Seahawk (the Navy version of the Blackhawk) that had been used for a previous technology demonstration, by removing the tail section and supplanting it with the large, round VTDP. In addition, they mounted two small, straight wings low on the fuselage (roughly in line with the rotor mechanism) to provide added lift. Bearing the name “Speedhawk” in tribute to its origins, it received the designation X-49A in May 2003. Piasecki transferred it to the Army’s Aviation Applied Technology Division the following year, where it made its first flight in June 2007 and afterwards recorded nearly 100  hours in the air, using the New Castle County Airport in Wilmington, Delaware, as its base. During that time, the X-49A flew as fast as 207 miles (333 kilometers) per hour (well in excess of its listed maximum rate of 167 miles (268.7 kilometers) per hour); achieved a range of up to 440 miles (708 kilometers) and an altitude of 19,000 feet (5,791.2 meters); and reached a rate-of-climb of 700 feet (213.3 meters) per minute. The X-49A’s three-­person crew flew it with the assistance of digital fly-by-wire. With the realization of these achievements, the X-49A concluded phase 1 testing in February 2008, during which time the Army recognized some worthwhile prospective improvements in helicopter speed, stability, range, survivability, and reliability. The helicopter flown during the Army tests drew its propulsion from three sources: two General Electric T700-GE-­401C turbo shaft engines that drove the fourbladed main rotor, augmented by the VTDP thrust vectoring propeller in pusher configuration. The X-49A measured 64.8 feet (19.7 meters) long, 53.6 feet (16.3 meters) wide, and 17.1 feet (5.2 meters) tall. Its empty mass totaled 13,669 pounds (6,200 kilograms). Piasecki made plans to modify the X-49A prior to the phase 2 tests, upgrading the flight controls, improving the aerodynamics (by such means as adding a retractable landing gear), and adding a supplementary power unit, all designed to increase its forward speed. But it became clear at this stage that the Army only intended the X-49A as a demonstrator of particular technologies, not as the model for a production vehicle. The service continued to test the X-49A as late as 2015, with the understanding that, at best, some of its systems might find their way onto other rotorcraft. Meanwhile, by 2020, Army officials made a further decision to make a clean break from past rotorcraft. At this juncture, the almost 40-year-old Blackhawk urgently required a successor, and two candidate

vehicles won out. The Army narrowed the choices down to Bell Textron’s V-280 Valor Tilt-Rotor − not really a helicopter at all, but a cousin to the V-22 Osprey − and the Boeing/Sikorsky SB-1 Defiant, a significantly different helicopter: instead of a five-bladed main rotor it has two that contra-rotate, as well as an aft-facing push rotor. Observers expect the Army to make its choice in 2022 and to have it in service in 2030. X-50A Even though the Army decided not to choose a compound helicopter to replace its venerable Blackhawk in 2020, a new generation of all-in-one aircraft/rotorcraft vehicles continued to be studied actively and to be funded. Indeed, in an indirect way the thrust vectoring/ compound airframe expectations for the X-49A emerged in a parallel, but unrelated project. In June 1998, the Defense Advanced Research Projects Agency (DARPA) awarded the Boeing Phantom Works $24 million to build two technology demonstrators designated the X-50A. The project represented a new and daring attempt to merge the qualities of an aircraft and a rotorcraft in one seamless machine, built for that purpose from the ground up. Known functionally as a Canard Rotor/Wing (CRW) vehicle, it grew out of NASA research into high-speed rotorcraft and from Hughes Aircraft studies of rotor wings. McDonnell-­ Douglas engineers had picked up these two strands and began a VTOL reconnaissance and surveillance UAV project in 1992. When Boeing bought out McDonnell in 1997, the idea migrated with the corporate take-over. Like so many of the X-planes of its era, the X-50A required no human pilot, and also offered the advantages of compact size and relatively low cost. Flying as an aircraft, it looked unlike anything else, with three wing surfaces spanning its fuselage: a twin-boom tail structure, a rotor that doubled as a wing during conventional flight, and a low-­mounted canard at the front. As a rotorcraft, its Dragonfly nickname served it well; once the rotor stood up, turning forcefully as three long, spindly landing legs dangled from the vehicle’s underbelly, it did look like an energetic insect. Just 17.6 feet (5.4 meters) long and 6.4  feet (2 meters) tall, its rotor measured 12  feet (3.6 meters) across. It weighed a maximum of 1,422 pounds (645 kilograms) at take-off and could carry up to 200.6 pounds (91 kilograms) of payload. Team engineers expected the X-50A to fly at 172.7 miles (278 kilometers) per hour at cruise and up to 430 miles (700 kilometers) per hour at its highest speed.

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One of the surprising features of the X-50A involved its powerplant. The standard Williams Research F112 turbofan engine provided all of the necessary power through the clever use of thrust control. In helicopter/ vertical mode, diverter valves brought exhaust from the engine to the rotor tips, thus making such bulky parts as a mechanical drive train, anti-­torque system, and transmission unnecessary. In horizontal flight, the rotor remained fixed and served as a regular wing, with exhaust emitted from a nozzle at the rear, enabling high speed forward flight. The transition from rotorcraft, to fixed wing, to rotorcraft occurred as the Dragonfly plied the skies. Once airborne as a helicopter, the pilot accelerated it to around 140  miles (225 kilometers) per hour, after which the X-50A began the metamorphosis to horizontal flight. This began by extending the flap surfaces of the aft mounted wings and the foreplane, which together offered enough lift so that the rotor could be stopped and then locked into a perpendicular position to the fuselage. Between these three lifting surfaces − the canards at the front, the tailplane at the rear, and the

rotor − the X-50A transformed into a conventional aircraft capable of flying at relatively high speed. To land, the procedure needed to be reversed, enabling the X-50A to touch down on helicopter platforms or on carrier decks. As with so many promising advances in aeronautics, the anticipation of the X-50A exceeded its realization. The X-50A’s maiden flight occurred in December 2003 at the Army’s Proving Ground in Yuma, Arizona, where it hovered for 80 seconds at around 12 feet. But during the third flight in March 2004, cross-coupling in the controls forced X-50A number 1 to make a crash landing. Boeing, meanwhile, fabricated the second and improved X-50A, which embarked on a planned program of 11 test flights. Disaster struck again, however, on April 12, 2006, when the number 2 airframe went down in wreckage at Yuma, the result of extreme sensitivity to nose pitch-up brought on by a flaw in the fuselage design. In the end, neither X-50A number 1 nor 2 could demonstrate full forward flight. As a result of this deficiency, DARPA cancelled the project in September 2006.

X-51A  115 X-51A Just before NASA concluded its X-43 program in late 2004, Air Force authorities laid plans for their own airbreathing, hypersonic scramjet project. The USAF saw two immense opportunities in this technology: the potential for a future hypersonic weapon system able to strike targets anywhere on Earth within an hour; and a potential new avenue for access to space. (See the section on the X-43 in Chapter 5.) Three government agencies − DARPA, the Air Force, and NASA − joined forces on the X-51A in early 2003, with the bulk of the work centered at the Air Force Research Laboratory’s (AFRL) Aerospace Systems Directorate. Because of the interconnectedness of the X-51A’s airframe and its scramjet powerplant, officials at the AFRL bypassed the normal prime contractor/subcontractor format, completing a consortium agreement among the Boeing Phantom Works and Pratt and Whitney Rocketdyne to pursue this project in September 2003. In January of the following year the two firms signed a contract, valued at around $140 million, to fabricate and flight test four autonomous

X-51As (also referred to as the WaveRider) around the 2007 to 2008 time period. Unsurprisingly for such an advanced undertaking, the calendar for the first flight stretched out to 2010. In the meantime, the two industry partners fabricated the component systems, with Pratt and Whitney supplying the SJY61 scramjet engine and Boeing integrating it into the expendable WaveRider airframe. Like the X-43 before it, the X-51A required an external source of acceleration to push it to the point when the scramjet could take over the propulsion (about Mach 4.5). For this booster role, the designers enlisted the solid propellant MGM-140 Army Tactical Missile System (ATacMS). The full stack measured roughly 25  feet (7.6 meters), with the cruiser itself about 14  feet (4.2 meters) long. The X-51A looked like a narrow wedge, with a fine, straight edge at the front end that gradually tapered to a four-­sided fuselage towards the rear. The combined vehicle weighed roughly 4,000 pounds (1,814.3 kilograms) and would be released from a B-52 mothership at elevations as high as 70,000 feet (21,336 meters).

116  Flight Testing for Combat: Military Vehicles The first flight of the X-51A proved its mettle, showing how far scramjet technology had advanced in the few short years since the final X-43 mission. Mounted aboard the B-52H mothership, the test on May 26, 2010 occurred over the Pacific Ocean, as it had with the X-43. But this time, rather than the 12-second burn as before, the Pratt and Whitney scramjet fired for 200  seconds, accelerating the WaveRider to Mach 5 at 70,000  feet (21,336 meters). Unfortunately, the telemetry shut down at around 140 seconds, and the thrust declined during the final 30  seconds. Two other decisive differences with the X-43 pertained to the speed (the X-43 flew much faster at Mach 9.68), and the fuel used (the X-43 used hydrogen fuel, a more volatile propellant than the hydrocarbon mixture (JP-7) aboard the X-51A). The next two attempts ended less satisfactorily. One ended prematurely when the switch-over from rocket to scramjet power failed, while the other involved a cruiser control fin failure that plunged the third X-51A into the ocean after separation from the B-52. But the fourth and final flight of X-51A brought vindication. On May 1, 2013, the B-52 carried the WaveRider over the Point Mugu Naval Air Warfare Center Sea Range, where it dropped the stack. The Army missile boosted it to Mach 4.8, where the scramjet then separated at 60,000  feet (18,288 meters) and lit, propelling the WaveRider to Mach 5.1. Full forward thrust lasted for 240  seconds, until the fuel supply became exhausted. Even after that point, the telemetry continued to function, sending back data until the moment the WaveRider sank into the ocean, 370 seconds after the X-51A left the ATacMS missile. With this fourth mission, the X-51A program disbanded after having accumulated over nine minutes of flight data and having proven its key technologies. Like many early estimates, the final costs ran higher than anticipated; in this case, to about $300 million. Even so, and encouraged by the outcome, the USAF moved quickly to apply the knowledge gleaned during the project. The Air Force Research Laboratory (which guided the X-51A program) has followed it up with the High Speed Strike Weapon Program, a projected Mach 5+ scramjet missile that might enter service as early as the mid-2020s. X-55A Just as the U.S. Army turned to the X-planes to demonstrate technologies for a successor to its beloved but aged Sikorsky H-60 Blackhawk helicopter, so did the Air Force with the venerable C-130 Hercules transport. First flown in 1954, the Hercules remains the U.S. military aircraft with the longest record of continuous

production. As its prospective replacement, the USAF chose a vehicle distinguished by being made mostly of composites. Known originally as the Advanced Composite Cargo Aircraft (ACCA), it later received the X-55A designation. A long time in the making, the ACCA originated in the mid-1990s and underwent ten years of study at the Air Force Research Laboratory (AFRL) in a project called the Composite Affordability Initiative. NASA, other government labs, and private companies contributed with parallel work on advanced materials and manufacturing techniques. These efforts came into practical focus in 2006, when Secretary of the Air Force Michael Wynn asked the AFRL team to issue a proposal for a military transport made almost wholly from composites. Wynn set the fundamentals of the project: keep the budget below $100 million and fly the aircraft within 18 months of awarding the contract. Boeing, Lockheed Martin, and Northrop Grumman each participated in feasibility studies with AFRL, following which the Air Force asked industries to submit requests for proposals (RFPs) in January 2007. Saying that the Air Force asked the manufacturers for a lot understates the RFP’s demands, but with the future prospect of a production contract to replace the ubiquitous C-130 in the offing, few could refuse. Nine proposals came in for ACCA, each of which sought to comply with the following requirements: a funding cap of no more than $50 million; a first flight by October 2008; and the fabrication of a composite demonstrator at 50 to 75 percent the size of the C-130, capable of landing and take-off from unprepared fields, enduring extreme temperatures, and cruising at speeds of 460 miles (740 kilometers) per hour at a minimum. Lockheed Martin’s Skunk Works − whose reputation rested on quick turnarounds of exceptionally advanced projects, as well as on the C-130 itself − signed an agreement with the Air Force in 2007 to fabricate the ACCA in a little more than two years: five months for a finished design, 20 months for construction, and a first flight in June 2009. Pressed hard by the steep demands of the contract it signed with the USAF, Lockheed made a daring decision. To save time and money, it decided to cannibalize a Dornier Do-328J high wing regional jet. Technicians at the Palmdale Skunk Works stripped down the Do-328J, saving the cockpit, flight controls, engines, subsystems, wings, and tail section, and eventually mating them with a composite fuselage. Exotic materials also made up the floor supports, cargo door, pressure bulkheads, fairings, frames, and the vertical tail. At final assembly, the X-55 fuselage and nose measured 55 feet (16.8 meters) long and 9 feet (2.7 meters) in diameter.

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The manufacturing techniques and materials used for the X-55A might have been familiar to plastic aircraft modelers, or to science fiction lovers, but they seemed revolutionary on a full-scale, practical vehicle. Lockheed’s engineers designed large, unitized, bonded structural components, cured at low temperature in low-temperature ovens. The resulting fuselage consisted of lengthwise upper and lower half sections made of a Nomex core and a carbon fiber skin, held together at a seam by adhesive and ply overlays. In effect, the entire ACCA fuselage constituted a single, massive component. These practices had a tremendous impact on both the speed of construction and the weight of the final product. ACCA needed only 300 structural parts and 4,000 mechanical fasteners, whereas the original Do-328J required 3,000 metallic pieces and 40,000 mechanical fasteners. This staggering reduction in components implied a potentially drastic cut in future labor costs by eliminating fastening and hole drilling tasks, as well as the time taken for fabrication, scheduling, sourcing, purchasing, and calibration, among other work. Such economies, if instituted widely after the X-55A, would have profound effects on aircraft construction the world over. Taking advantage of these extraordinary time-saving measures, the fuselage and vertical tail of the X-55A

came together on the Lockheed factory floor in a mere ten months. But problems with composite applications and with bonding large-scale structures postponed the first flight of the ACCA by nine months. It seems that a second fuselage needed to be constructed after the failure of the skin to adhere to the original lower half. Once remedied, the initial flight happened on June 2, 2009. It took off from the Lockheed facility at Plant 42, located in Palmdale, California, flew for about 1.5 hours, conducted all planned test points, and landed without incident. The aircraft went on to fly about 20 additional missions, with roughly 600 sensors onboard to record flight data, in addition to pilot observations regarding the performance characteristics of the X-55A. By that point, it became clear that the aircraft had proven its worth, making a strong case not only for large-scale bonded and integrated composites, but for cost-effectiveness; the project fell within the $50 million budget. Rather than mothball the X-55A outright, AFRL officials decided to employ it as a test vehicle for other advanced technologies, such as the Joint Future Theater Lift program and the Long Range Strike Cargo aircraft. Lockheed, meanwhile, scheduled additional flight and structural research, until it retired the aircraft to the Joe Davies Heritage Airpark at Plant 42 in Palmdale.

118  Flight Testing for Combat: Military Vehicles X-56A In parallel with the success of the composite X-55A transport demonstrator, the Air Force also explored a far different method of increasing the economy and the range of long-­distance vehicles. To stretch the fuel of airliners, transports, and reconnaissance aircraft, the Air Force Research Laboratory (AFRL) − also the spawning ground of the X-55A − tried a whole new approach with the X-56A Multi-­Technology Testbed, or MUTT. Unlike the piloted X-55A, the MUTT embraced the trend for small, remotely piloted machines. Also unlike its immediate predecessor, the X-56A pursued efficiencies not through its overall construction, but just by its wing design. In their mutual low cost objectives, however, the X-55A and X-56A complemented one another. Aerodynamics theory and practice recognized that one way to improve fuel efficiency in long-range flight lay in long, thin, flexible airfoils, as distinct from the stiff, short wings common on most aircraft. But the high-aspect-ratio design had a flaw, falling prey to flutter, or uncontrollable vibrations, in addition to atmospheric forces and wind gusts. MUTT offered the opportunity to test the efficacy of flexible wings (without risk to the safety of pilots), using a combination of onboard instrumentation that predicted the occurrence

of wing flutter, linked to a special flight control system that suppressed aeroelastic anomalies. AFRL awarded the X-56A project to the Lockheed Skunk Works, just as it had with the vastly different X-55A. The resulting aircraft looked like a dragonfly, with a rounded little fuselage, three wheels dangling from spindly legs, and immensely long, thin, swept wings, graced by winglets at their ends. With a weight of only 480 pounds (217.7 kilograms) and a wingspan of 28 feet (8.5 meters) − nearly four times that of its 7.5-foot (2.3-meter) length − it required just two 90-pound-thrust (40.8-kilogram) JetCat P400 turbojet engines for power. The MUTT had a novel construction, almost like that of an Erector set. Built up from parts according to its researcher’s needs, the X-56A came in a kit ­provided by Lockheed that allowed for maximum adaptability: two center bodies, three pairs of flexible wings, two stiff airfoils, a transportation trailer, and a ground control station. Flight testing of the MUTT started on July 13, 2013, at the Dryden (later Armstrong) Flight Research Center on Edwards Air Force Base, under the supervision of AFRL and Lockheed. Operating over the Edwards dry lakebed, the aircraft flew 16 times using the stiff wings, but the X-56A number 1 crashed during the first flexible airfoil flight in November 2015, causing severe damage.

X-60A  119 Earlier that year, in April, MUTT number 2 flew under NASA auspices for the first time. Funded for five years by two NASA sources − the Headquarters Advanced Air Transport Technology and the Flight Demonstration Capabilities directorates − the X-56A underwent eight missions with the stiff wings. Project officials then took the big leap, making the initial flexible wing flight in August 2017. This event launched a series of NASA MUTT missions that ran from 2017 to 2019. Following the August 2017 milestone, the team made a breakthrough in September 2018 by demonstrating that flutter could indeed be controlled using a standard controller at speeds up to roughly 127 miles per hour. Having taken this step, they turned to an even bigger challenge: conducting tests with a cutting-edge controller. Flying the X-56A with limited weight on board, the NASA team gradually increased the speed of the MUTT in 10-mile-per-hour (16-kilometer-­per-hour) increments, starting at about 81 miles (130 kilometers) per hour. During late 2018, the engineers experienced another historic first when they succeeded in suppressing flutter using both the classic and the modern controller. These missions continued in 2019. By their conclusion, 39 flights had been made and about 1,000 research maneuvers completed. With the five-year terminus of the X-56A having been reached in 2020, NASA authorities closed down the active portion of the program. But the X-56A left researchers at the space agency and elsewhere with a deep well of flight data with which to assess the future prospects of long, slender, high-aspect-ratio wings on the world’s largest aircraft. Some of the participants even expressed the hope that a retrofitted MUTT might re-emerge, with a fiber-optic sensing system. X-60A During the year after the historic 2013 flight of the X-51A WaveRider − a program in which a scramjet showed real promise for practical application − its sponsor, the Air Force Research Laboratory (AFRL), took another leap into modern hypersonics by initiating a second unmanned program to test the limits of Mach 5+, this time on a drastically diminished budget. Two factors other than low cost may have motivated the Air Force to invest in another scramjet project so soon after the first: to remain competitive with the hypersonic

missiles then under development in Russia; and more speculatively, to assess it as a potential weapon system. In pursuing its second scramjet project, the USAF abandoned tradition by selecting a contractor through a mechanism reserved exclusively for start-up or small companies, by which they received limited funding to research promising technical concepts. In this case, the AFRL’s Aerospace Systems Directorate awarded Generation Orbit Launch Services of Atlanta, Georgia, a Phase 1 Small Business Innovative Research (SBIR) contract in July 2014 to design a one-stage, air-launched liquid-propellant rocket. (Generation Orbit certainly qualified as a modest-sized business, with only 25 fulltime and about an equal number of part-time employees). As of 2020, AFRL invested about $25 million for this seminal research, a small outlay indeed by DoD standards. The Air Force hoped that the new system, called GoLauncher 1, or GO1, would prove capable of flying hypersonic experiments involving microgravity, astrophysics, and avionics − among many other payloads − on a routine basis. The project seemed especially appealing to defense officials because it offered flexibility; the front end and the flight profile of the rocket could be adjusted with each mission. Generation Orbit representatives described the proposed suborbital rocket not only as adaptable but also inexpensive, able to accomplish each mission for less than $5 million. At the same time, the GO1 had definite limitations: it could not send spacecraft into orbit; it could not serve as a space launch platform; and it could not be reused. The initial SBIR research period lasted only nine months. Paid just $150,000 as a first installment, Generation Orbit conducted studies focused on requirements, configuration, and trajectory. Additionally, during the same month as the signing of the SBIR agreement, the vehicle made its first captive-carry flight aboard a Learjet 35. As the project progressed beyond these early stages, the Air Force gave the GO1 a sign of confidence by designating it as the X-60A in October 2018. Much happened to the X-60A in a relatively short time. It completed three “dummy” captive-carry flights in December 2017 at the NASA Armstrong Flight Research Center, and underwent a hot-fire test of a fullsized operational prototype in June 2018 (which validated the X-60A’s propellant feed system, pressurization system, and flight controls, as well as the rocket’s throttling effectiveness). As it passed these milestones, the

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GO1 assumed its final design as a vehicle powered by a Hadley liquid oxygen/kerosene engine, prospectively capable of Mach 5 to 8 in dash mode with a payload of up to 701 pounds (318 kilograms). Its potential altitude ranged from 70,000 to 130,000 feet (21,336 to 39,624 meters), and its designers outfitted it with a small delta wing for increased maneuverability, as well as an onboard telemetry system to record flight data. Taking these and many other factors related to the X-60A program into consideration, an Air Force team conducted a critical design review in March 2019 and approved the X-60A, which advanced it to the fabrication stage. It also passed muster during a verification ground test of

the integrated propulsion system, which occurred in January 2020. Project engineers planned for the X-60A to undergo flight research at Cecil Spaceport in Jacksonville Florida − an unorthodox location given the Defense Department’s many in-house test ranges. But in this case, AFRL officials took the opportunity of the X-60A to diversify launch sites and take a more commercial approach. Initially, AFRL planned for two test flights, one in March 2020 at sustained Mach 5 speed, and a second in September exceeding Mach 6. However, neither the Air Force, Orbital Generation, nor the media confirmed the achievement or failure of these milestones.

5 Aerodynamics and Engines: NASA’s Research Agenda

X-36 Before the Air Force and the Navy began work with Boeing and Lockheed on the X-32 and X-35 (the demonstrators for the eventual Joint Strike Fighter), several defense contractors looked ahead to a new and radical combat aircraft configuration. In one go − perhaps in a case of exuberant expectations – its designers hoped for high maneuverability and high angle of attack (AoA), while reducing the radar cross section, weight, and drag commonly associated with big war-­ making machines. NASA played a limited role in the actual pursuit of the X-36 project, by offering just its flight research services. In historic terms, however, the space agency’s contribution to this X-plane loomed far larger than testing alone. Beginning in the mid-1980s, NASA’s Ames Research Center undertook experiments related to lowobservable designs for military aircraft. Engineers at Ames considered a variety of aircraft planforms, propulsion systems, and materials capable of evading radar detection. As the research progressed, several contractors stepped forward to contribute to the project. Among them, McDonnell-Douglas assumed a lead role, and by the end of the 1980s NASA and this manufacturer had forged a close working partnership. After about five years of concept studies, McDonnell and Ames officials decided that a scale model demonstrator offered the best option for exploring this would-be fighter of the future. NASA funded half of the $20 million program cost, with McDonnell making the same

commitment in a cooperative pact. Like the X-33 and X-34, the X-36 bore the imprint of NASA Administrator Daniel Goldin’s acquisition formula of “faster, better, cheaper.” It also bore a long but descriptive formal name: the X-36 Tailless Fighter Agility Research Aircraft. Fabrication started in mid-1994 on a pair of 28 percent scale models with absolutely no horizontal or vertical tail surfaces. McDonnell-Douglas (which merged into Boeing in 1997) delivered the first one to NASA’s Dryden Flight Research Center in 1996, where highspeed taxi tests began in October. Initial flights started in May 1997 over Rogers Dry Lakebed, and lasted into late July. Project engineers investigated data system operations, ground station efficacy, and control software, in addition to handling qualities and functional checkout. A second round of flights in August ramped up the expectations, putting the X-36 through 360-degree rolls at up to 15 degrees AoA, in addition to fast turning and rolling at AoA as high as 35 degrees. Then, a series of low-speed/high-alpha flights occurred, ending in November 1997. In all, the X-36 amassed an impressive data base, logging 31 missions in six months, amounting to more than 15 hours of total time aloft. In the end, it attained an angle of attack as high as 40 degrees. Without any tail structures and equipped with twin canards, cranked arrow wings, and split ailerons, the X-36 confounded viewers with its strange, pancakelike profile. Its skin consisted of graphite composites that covered an airframe made mostly of aluminum.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 M. H. Gorn, G. De Chiara, X-Planes from the X-1 to the X-60, Springer Praxis Books, https://doi.org/10.1007/978-3-030-86398-2_5

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Aerodynamicists who worked on the project and analyzed the flight data realized at least two historic objectives: ten percent less drag and five percent lower weight compared to conventional aircraft. The dimensions of the X-36 suggest some of its design intention. Even at 28 percent scale it still measured a lengthy 28 feet 3 inches (8.6 meters), with a wingspan of 10 feet 4 inches (3 meters). It also had an exceptionally low stance, with a height of just 3 feet 2 inches (0.9 meters). The X-36 weighed a mere 1,083 pounds (491.2 kilograms) empty and it flew slowly, at no more than Mach 0.35, powered by one Williams Research F112 turbofan engine rated at 700 pounds (317.5 kilograms) of thrust. The X-36 represented a much-needed victory for the Dan Goldin approach to the acquisition of experimental flight vehicles, which had found mixed results in the space-­oriented X-33 and X-34 programs (see Chapter 6). In the case of the X-36, its low cost and daring break from planform norms resulted in an aircraft that performed very well in flight and produced valuable data about pilotless and low radar signature design.

X-43A The scramjet engine might be considered the mirage of aeronautics and spaceflight. For decades, researchers felt that it could be harnessed as a practical form of propulsion; an air-­ breathing means of escaping the atmosphere without the bulk, blast, and cost of bruteforce rocketry. But like a phantom, it kept vanishing when approached. Engineers who worked on the National Aerospace Plane (NASP) learned the elusiveness of the scramjet as a realistic, high-energy powerplant. But rather than give up on it, many persisted because of its almost too-good-to-be-true promise. Scramjets trace their lineage to the ramjet, first conceived just before World War I.  This technology required a cone-­shaped spike mounted in an engine inlet, which slowed the onrushing air to subsonic velocity. Forced into a combustion chamber, the incoming airflow became pressurized, at which point a spray of fuel sent into the chamber mixed with the compressed air. By igniting these ingredients with flame, a stable combustion occurred, producing hot gasses expelled through a nozzle. Using this mechanism, the unlikely

X-43A became real; thrust produced by this process required no turbine blades or any other moving parts. But the system had flaws. No ramjet could be started without air already flowing through it, so it required a rocket booster to push it up to the necessary speed. This fact rendered ramjets of limited value due to the weight of the ancillary rocket, which counterbalanced gains in performance. Additionally, the ramjet could propel an aircraft no faster than Mach 6 due to the potential for overheating in the combustion chamber. As a result, ramjets found use only at the periphery of flight, powering surface-to-air-missiles and target drones. Among its many research initiatives, NASA’s predecessor organization, the National Advisory Committee for Aeronautics (or NACA), investigated ways to improve the ramjet. Two researchers at the NACA’s Lewis Flight Propulsion Laboratory in Cleveland, Ohio, found a possible solution during the mid-1950s. Richard Weber and John McKay theorized that using the same techniques as the ramjet, but subjecting the engine to supersonic airflow and supersonic combustion, would improve engine performance and reduce internal heating. Their findings described the design requirements for a scramjet and predicted that it would be more efficient than the ramjet above Mach 5. Encouraged by these and other researchers, practical engineers tried for decades to make it work, in part because scramjets had so much appeal as an alternative to standard rocketry. The USAF tried first, with the Aerospaceplane of the late 1950s to 1964. Then NASA’s Hypersonic Research Engine (HRE) project proposed using the X-15 as a carrier aircraft for the scramjet, and conducted ground testing in 1971 and 1973. NASA’s Langley Research Center explored an airframe-integrated scramjet during the 1970s. The planned X-24C lifting body also got some attention, renamed as the National Hypersonic Flight Research Facility; it ended in 1977. Finally, the NASP took the lead from 1986 to 1994. (See the section on the X-30 in Chapter 6 for the saga of the NASP.) Despite these aborted attempts spread out over four decades, the vision of a workable scramjet persisted. During the mid-1990s, several engineers at NASA Langley, familiar with the massive database collected during NASP, pressed to open a new line of scramjet research. They conducted preliminary design work and submitted it to NASA headquarters, which approved their proposal in July 1996. This initial go-ahead represented the starting point for the X-43A (also called the Hyper-X), enabling design, fabrication and (ultimately) testing to begin.

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Surprisingly, MicroCraft, a small company located in Tullahoma, Tennessee, won the airframe contract over McDonnell-Douglas. Orbital Sciences Corporation also participated, agreeing to provide a modified Pegasus rocket as the X-43A’s second stage launch vehicle. (See the section on the X-34, also in Chapter 6, for more on OSC and the Pegasus.) NASA Langley assumed overall responsibility for the program and conducted extensive wind tunnel research. Meanwhile, NASA Dryden prepared for the upcoming flight tests, among other things by preparing the B-52B mothership to be used as a first-stage launcher. All of these activities came to life in March 1997 with the awarding of the MicroCraft contract. NASA budgeted about $170 million for the whole endeavor. The vehicle that technicians eventually mounted on the B-52B looked like a small black wedge. It measured about 12 feet (3.6 meters) long, stood just 2 feet (0.6 meters) tall with a wingspan of 5 feet (1.5 meters), and weighed roughly 3,000 pounds (1,400 kilograms). Engineers at MicroCraft chose conventional materials for the airframe, using aluminum and steel. But because plans called for flights ranging from Mach 7 to Mach 10, they relied on an alumina-enhanced thermal protection system to cover the fuselage, and carbon-­carbon composites to protect the nose, wings, and vertical stabilizer leading edges against high heat. MicroCraft produced three X-43A airframes, but the scramjet engine − fabricated by the General Applied Science Laboratory of New York, which specialized in advanced propulsion − arrived at NASA Dryden first, in August 1998. While technicians there integrated incoming systems on the three vehicles and conducted ground tests, mission planners finished their work. They plotted a flight path that took the B-52B bomber west from Edwards Air Force Base to the Pacific Ocean. Once there, the Pegasus/X-43A stack would drop from the mothership at about 25,000 feet (7,600 meters) and accelerate to between Mach 7 and 10, at which point the X-43A positioned at the front of the Pegasus would separate and fly under its own power. Guided by preprogrammed controls, it would plunge into the water at the end of its flight, after which the big bomber would fly home. NASA scheduled what all hoped would be the world’s first successful scramjet flight for June 2, 2001. The NB-52B lifted off with the Pegasus/X-43A under its wing and, after an approximately 75-minute ride westward, it released the payload. But the combined stack went out of control after eight seconds, unable to achieve its Mach 7 objective. Both the Pegasus and the

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X-43A fell into the sea. Later investigations determined the cause to be an error in calculating maximum aerodynamic loads as the two vehicles accelerated to the speed required for the ignition of the scramjet. There followed a long hiatus of nearly three years before X-43A number two made a second attempt to prove the worth of scramjet propulsion. On March 27, 2004, the Dryden flight crew followed the same route to the Pacific and once again released the twin cargo at the appointed moment. Once launched by the Pegasus, the X-43A flew for about 10 seconds and slightly accelerated on its own at about Mach 7, producing the first positive net thrust of any scramjet in history. Almost eight months later, the X-43A team at NASA Dryden readied themselves for the defining mission. The NB-52B left Edwards at 1:08 pm on November 16, 2004, (along with two F-18 chase aircraft) and reached the launch point above San Nicholas Island, the westernmost of Southern California’s Channel Islands, at about 2:30 pm. There, it jettisoned the X-43A/Pegasus combination. The vehicle fell for five seconds then ignited, and for 75 seconds it roared through the Mach numbers until just exceeding Mach 10, at which point the Pegasus rocket burned out and the whole stack glided forward. The instant of consequence came at

Mach 9.74, at an altitude of 109,440  feet (33,357 meters), when the X-43A separated from the rocket. The Dryden control room held its breath for three seconds as the X-43A re-­positioned itself to assume the right angle of attack. Then hydrogen fuel began to spray into the scramjet and ignited with silane, a colorless gas used for paint and glass manufacture. The engine lit and then burned for 11 seconds, propelling the X-43A for about 20  miles, during which time it attained a speed of Mach 9.689 (nearly 7,000 miles per hour), the fastest of any air-breathing aircraft in history. The mission did not quite end there. As the X-43A cruised unpowered, Dryden researchers collected significant hypersonic flight data. By the time X-43A number 3 finally dropped into the sea, it had traveled about 978  miles (1,574 kilometers) from the initial launch over St. Nicholas to the termination point. All three of the X-43As remain at the bottom of the ocean. In the end, the X-43A met its objective. The scramjet system produced thrust equal to the drag on the aircraft, enabling cruise flight. But crucially at this sensitive juncture, just when scramjet technology had at last made a real step towards proving itself in actual flight, the project ended abruptly as funding ran out. Only

 data analysis continued, but the pursuit of the scramjet did not end there. The USAF laid the groundwork for the X-51A WaveRider in the year before the last X-43 flight. (See the section on the X-51A in Chapter 4.)

X-48B and C

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2002 timeframe) forced Boeing to fall back on the smaller X-48B, for which Boeing left the actual construction to a subcontractor: Cranfield Aerospace in the United Kingdom. Cranfield fabricated the X-48B with a wingspan of 20.4 feet (6.2 meters), and made it mostly of composite materials. It weighed a scant 523 pounds X-48B and C (237.2 kilograms) gross and derived power from three model turbofan engines, together rated at just 150 Over the course of 75 years, the X-planes have explored pounds (68 kilograms) thrust and mounted far aft on a large proportion of the emerging technologies perti- the aircraft’s deck. Its long, slender, swept back wings nent to aeronautics and spaceflight. But one type of air- (with vertical fins and rudders at the wingtips and elecraft − the biggest in the skies − has gone almost vons on the trailing edges), blended seamlessly into a unnoticed, despite their enormity. These include airlin- thin, wide, diamond-shaped fuselage. ers, as well as military vehicles used for cargo and Known formally as the Blended Wing Body (BWB) troop transport, strategic bombing, and aerial refueling. aircraft, its contours won it the nickname “Manta Ray”. One exception is the X-6, which used an NB-36H Cranfield built two of them for Boeing, and in May bomber to test nuclear propulsion. 2006 X-48B number 1 finished 250 hours of tests in the The X-48B and C offers another exception to this Langley full-scale wind tunnel. Meanwhile, number 2 rule. The project originated with an engineering group aircraft underwent preliminary checks at the Dryden at McDonnell-­Douglas, who developed a new aerody- Flight Research Center prior to its flying debut, which namic model. Boeing acquired McDonnell-Douglas in occurred with five flights in early 2007. Engineers 1997 and at first showed little interest, but at the start of learned that the design had real promise from the 92 the twenty-first century, the Boeing Phantom Works – a total X-48B missions accomplished between July 2007 subsidiary of a company that built its reputation on and March 2010. Its ground controllers flew it at low military and civil jumbos, after all − assembled a team speeds, up to 138 miles (222 kilometers) per hour; at to study this somewhat speculative endeavor. A ques- low altitudes, under 10,000 feet (3,048 meters); and for tion hung over the inquiry: in combining the aerody- short durations (no more than 40 minutes). They paid namics of the flying wing with a conventional planform, particular attention to the flight behavior of the Blended could an aircraft be manufactured that would offer Wing Body in areas such as stall characteristics, hangood flying qualities and control, while at the same dling qualities, and control with engines off. time offering radically improved range and fuel In light of these positive results, Boeing produced economy? the X-48C, a modified version of the X-48B. It had an Several government agencies helped Boeing arrive entirely different purpose than its predecessor, in that at the answer. Even before the company made a formal instead of testing for enhanced fuel efficiency and proposal, the NASA Langley Research Center offered range, the X-48C concentrated on noise generation its wind tunnels for aerodynamics studies. Once the during low-­speed flight (also conducted under the ausproject got underway, NASA’s Dryden (now Armstrong) pices of the ERA program at NASA Headquarters). But Flight Research Center stepped forward with its flight it did share with the X-48B the intention of one day testing experience. NASA headquarters also expressed seeing BWB advances on airliners and other large an interest, as it pertained to the agency’s aircraft. Environmentally Responsible Aviation (ERA) program The X-48C required significant modifications to designed to foster cleaner, quieter, and more fuel-­ assume this new role. First, its designers added power efficient aircraft. The Air Force Research Laboratory by supplanting the three 50-pound-thrust (22.67-kilo(AFRL) also signed on as a sponsor. Funding for the gram) engines on the X-48B with two in the 89-pound X-48 project came from Boeing, as well as from (40.4-kilogram) range, positioned on top of the fuseNASA’s Aeronautics Research Mission Directorate. lage, but not so far to the rear as on the X-48B. They The X-48B − a remotely piloted, 8.5 percent scale extended the end of the deck back by about 2 feet (0.6 model − served as the initial test vehicle. There had meters) and modified the aft profile by turning the been an X-48A on the drafting table prior to it; a 14 X-48B’s wingtips into two vertical tails (by canting the percent scale vehicle that measured 35  feet (10.7 winglets towards the engine exhaust ducts). To commeters) wide. But its cancellation (around the early pensate for the altered handling qualities resulting from

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X-53 the changes, the team decided to modify the control system software, mindful that future aircraft may adopt the BWB contours. Despite these alterations, the basics did not change appreciably: the X-48C had a wingspan slightly longer than 20  feet (6 meters), a mass of roughly 500 pounds (226.7 kilograms), and a top speed of about 140 miles (225.3 kilometers) per hour at up to 10,000 feet (3,048 meters) altitude. In the end, the X-48C flew 30 times between August 2012 and April 2013 (bringing the total number of X-48 missions to 122). Those involved at NASA Headquarters, NASA Dryden, and Boeing all expressed optimism about the concept, praising its ability to fly with solid control at low speed throughout the flight envelope, yet still meet targets for quiet flight. Boeing’s Blended Wing Body program manager used superlatives to describe what had been learned: “We have shown a BWB aircraft, which offers the tremendous promise of significantly greater fuel efficiency and reduced noise, can be controlled as effectively as a conventional tube-and-wing aircraft during takeoffs, landings and other low-speed segments of the flight regime.” It remains to be seen whether tests of the BWB concept at higher speeds and under piloted control will substantiate these early results. X-53 Wing warping is a time-honored practice in the history of aeronautics. In their quest for mastery of the first flying machine, Wilbur and Orville Wright experimented with and perfected wing twisting as a primary means of controlling the direction and motion of their aircraft. Lacking ailerons or flaps, which came much later, they devised a system by which the pilot − lying prone − moved his hips to one side or the other, a motion transferred by cables to the wingtips which then moved up or down accordingly. A NASA/Air Force Flight Dynamics Laboratory (AFFDL) project of the 1980s also attempted to reshape wings in flight. Known as the Advanced Fighter Technology ­ Integration (AFTI) aircraft, it used a General Dynamics F-111A to demonstrate the efficacy of the Mission Adaptive Wing (MAW). Designed by Boeing Aircraft, the MAW consisted of a digital flight system that controlled a mechanism inside the AFTI wing. It flexed the airfoil’s outer skin depending on the immediate aerodynamic need: high camber for subsonic speed; supercritical wing for transonics; and symmetrical proportions for supersonic flight. But the

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objective did not consist merely of stepped, incremental changes in wing contours. Instead, it aimed for automatic adaptation; that is, an uninterrupted flow of adjustment, similar to the way in which birds re-position their wings during each moment of flight. AFTI-F-111 flew 59 times between November 1985 and 1988 and achieved satisfactory results, reducing drag by around seven to 20 percent, depending on flight conditions. On the other hand, the 16 power drive units that drove the system actuators had to be repaired or replaced 37 times. Eight years after AFTI-F-111 ended, the same three participants − NASA, Boeing, and the Air Force (this time represented by the Air Force Research Lab rather than the earlier Flight Dynamics Lab) − joined forces to update the technologies involved in AFTI-F-111 in a new program known by two names: the Active Aeroelastic Wing (AAW) and the X-53. The X-53 program paralleled the AFTI-F-111  in several significant ways. In addition to assembling the same institutional partners, the leaders of the AAW decided to borrow a military aircraft and modify it as a testbed, rather than build an all-new vehicle. The modifications also confined themselves mainly to the wings. But the differences also reflected the increasing domination of computer software in contemporary aeronautics. If the research proved out, AAW’s computer-directed control surfaces would automatically twist the wings into contours most conducive to the conditions encountered by aircraft at any one moment of flight, seamlessly improving handling qualities (especially roll) from the subsonic to the supersonic range. Two sponsors − NASA’s Headquarters Aeronautics Research Mission Directorate and the Air Force Research Laboratory (AFRL) − offered Boeing a budget of $45 million for the fabrication of a single X-53. The money got stretched when project managers won permission from the U.S Navy to borrow an F/A-18 Hornet fighter for the X-53’s flight research, which would be conducted at the Dryden (now Armstrong) Flight Research Center. The team at Boeing did not need to fabricate all-new airfoils; instead, NASA recovered a pair from its retired F/A-18 #840, once used on its High-Alpha Research Vehicle (HARV). The dimensions of the AAW brought no surprises; this version of the Hornet measured 56.1  feet (17.1 meters) long, 38.3 feet (11.7 meters) wide, and 15.2 feet (4.6 meters) tall. In spite of these programmatic advantages, progress came slowly due to some complexities unique to this

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ambitious project. Boeing got to work on AAW in 1996, but five years went by before the team finished the design work and reassembled the wings. Then the X-53 underwent a series of preliminary flights and ground activities for a year, beginning in early 2001: vibration, wing stiffness, and structural load tests; installation of control software; systems checkout; and flight simulations. The aircraft also acquired a new flight control computer during this period, and instrumentation engineers designed and then installed about 350 strain gauges on each wing. Using the data gleaned during this initial phase, a program of 50 parameter identification flights started in late 2002 and ended in April 2003. These missions gauged the forces acting on the aircraft’s surfaces and assessed their relationship to wing twisting and aircraft control. Then another year went by, during which software engineers analyzed the parameter identification results and modified the initial software to improve the performance of the airfoils. At long last, the full flight research program began. Between late 2004 and March 2005, NASA pilots flew the X-53 about 25 times to test its software control laws and to determine its performance and handling qualities in missions ranging between Mach 0.85 and Mach

1.3, and at altitudes between 5,000 and 25,000  feet (1,524 to 7,620 meters). In assessing the entirety of the data from the X-53, researchers drew a number of useful conclusions. While this project might not result in a new generation of front-line fighters with wings capable of adjusting themselves to every flight condition, it did suggest that active flexible wing control could achieve adequate roll rates (stable lateral flight) at transonic and supersonic speeds, even without a tail surface. Combining AAW wing structures with control law techniques could also result in thinner, more efficient airfoils, about ten to 20 percent lighter than standard wings, with a beneficial effect on fuel economy, range, payload size, and even radar evasiveness. These advantages might be applied not only to high-performance aircraft, but also to future transports, airliners, and to high-altitude, long-range drones. X-54A Since the initiation of the X-planes nearly 75 years ago, sonic booms have been present in high-speed aeronautics, but beyond solution. Although no explosive noises

X-54A could be heard in the Antelope Valley or surrounding areas when Chuck Yeager and the X-1 flew above the speed of sound on October 14, 1947, his success ushered in an age in which ground-shaking sound effects became all too common. Double blasts rolled across the Mojave Desert during the 1950s as the Century Series fighters − the YF-100 Super Sabre, the YF-102 Delta Dagger, the YF-104 Starfighter, the F-101 Voodoo, the YF-105 Thunderchief, and the F-106 Delta Dart − underwent testing at Edwards Air Force Base. Eventually, as they (and even bombers such as the B-58 Hustler) entered the inventories at military bases around the country, the nation as a whole became sensitized to this, the most disturbing consequence of supersonic flight. As a consequence, the National Advisory Committee for Aeronautics (NACA) − a fundamental contributor to most of the early high-speed X-planes − became sensitive to the issue of quieter flight. The NACA’s successor, NASA, intensified this interest, especially as it applied to supersonic transports (SSTs). The space agency conducted flying experiments with two XB-70 Valkyrie demonstrators during the 1960s. These behemoths flew up to Mach 3 and simulated the sonic signatures of commercial airliners, but despite this XB-70 research, the booms persisted. Frayed nerves and cracked windows continued for decades, as two large vehicles that crisscrossed the world’s airspace − the SR-71 Blackbird reconnaissance aircraft that routinely broke Mach 3, and the world famous Concorde airliner that achieved Mach 2 – added to the mayhem caused by military jets. NASA did not roll back its attempts to dampen or control sonic booms, testing dozens of aerodynamic designs in its Supersonic Commercial Air Transport (SCAT) program. But in this and many other projects, it failed to find a solution, or even a mitigation strategy for sonic booms. Only during the late 1990s did researchers at NASA and in industry begin to find a critical mass of promising concepts, by concentrating less on the complex aerodynamics of transports, jetliners, and military aircraft, and instead looking to improvements in smaller airframes. One of the first of these initiatives became the Shaped Sonic Boom Demonstrator (SSBD). Started in February 2001 by a team at Northrop Grumman, it delved into the efficacy of replacing the standard nose of an F-5E fighter with a long, narrow prosthesis. Some called the resulting aircraft “The Pelican” because of its front end contours. DARPA sponsored the research, which aimed to reduce the strength of booms rather than eliminate them. Flight research conducted by

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NASA on the F-5E ended in early 2004, and an encouraging conclusion followed the analysis of data from ground stations under the path of the test vehicle: the F-5 SSBD registered consistently lower pressure than the signatures of the standard F-5E. The conclusions from SSBD helped persuade NASA officials to pursue another X-plane to expand the horizons of the earlier work, a decision bolstered by efforts such as NASA’s own Quiet Spike project, begun in 2006. Quiet Spike consisted of an F-15B research aircraft outfitted with a 24-foot-­long (7.3-meter) needle nose, and it paid dividends. Instead of propagating the strong shock waves that typically ­accumulated at the front end of supersonic aircraft as they traveled at high speeds, Quiet Spike produced only three small waves that fell to the ground parallel to one another. Encouraged by these further results, as well as by other research at DARPA and in industry, NASA took the next step in 2008 when it signaled a pending collaboration with Gulfstream Aerospace to produce the X-54A.  The undertaking, as presented, had an ambitious charter: to realize in one project several decades of NASA research on subduing sonic booms. To do so, Gulfstream would fabricate a low-boom demonstrator built expressly for the purpose, which NASA would flight test and ultimately fly over populations to amass data on its effectiveness. Strangely, however, no such project ever emerged. Gulfstream remained largely silent about it, and in 2008, Aviation Week and Space Technology reported that while the Air Force (which handles all X-planes designations) did approve the X-54A designation on the strength of NASA’s support for Gulfstream’s application, space agency officials regarded it merely as a placeholder. NASA sources told the magazine that it and Gulfstream had not, in fact, actively worked together on the project, nor even discussed collaboration. It seems more than likely that the X-54A never graduated to reality because NASA and some of its industry partners saw truly promising technologies just on the horizon. As a consequence, they decided to abort the X-54A, defer action, and let things develop. By 2011 and 2012, the hoped-for improvements began to take shape. New computational fluid dynamics techniques (substantiated by wind tunnel research) suggested advanced designs that could offer low noise potential, not just for small aircraft like the F-5, but for 35- to 70-passenger supersonic jetliners in the near term, expanding to 100–200 on board by about 2030. NASA referred to this concept as the Low-Boom Experimental Vehicle (LBEV) and presented a detailed, fully

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articulated briefing about its technical and administrative development in March 2011. The audience heard clear, ambitious, and even startling objectives that had eluded NASA for decades, including the reduction of sonic boom emissions to allow supersonic overland flight; the achievement of noise levels acceptable for airport communities; and the minimization of highaltitude cruise emissions. With these public statements, NASA returned to an objective that had circulated within aeronautics since the time of the X-1 but had never been attained: the conquest, or at least the dampening, of sonic booms, the single biggest factor that inhibited commercial supersonic travel. Rather than take an intermediate step toward this goal with the X-54A, and equipped with new and convincing data, the agency decided to launch a more daring X-plane − the Lockheed Martin X-59 Quiet SuperSonic Technology (QUESST) aircraft. (See the section on the X-59 in this chapter). X-57 Although electrical propulsion represented a new venture for the X-planes, NASA had a distinguished recent history in this field. During the 1990s and into the

twenty-first century, the NASA Dryden (now Armstrong) Flight Research Center flight tested a series of advanced uninhabited aerial vehicles (UAVs), flown in collaboration with several of the space agency’s other centers as well as private industry. Conducted under the Environmental Research Aircraft and Sensor Technology (ERAST) program, the project fielded a number of aircraft, some powered by the Sun and others by standard engines. The ones that ran on electrical power attempted a series of high-altitude, long-duration missions. ERAST began in 1993 and tested several flying wing aircraft built by AeroVironment, Inc. by the end of the decade, all equipped with solar arrays embedded on their immense airfoils: Pathfinder, with a 98-foot (29.8meter) wingspan; Centurion, which measured 206 feet (62.7 meters) from wingtip to wingtip; and finally the 247-foot (75.2-meter) expanse of Helios Prototype. Engineers on the Helios Prototype team had big plans. During its flights, between 1999 and 2003, they used from eight to 14 solar-powered electric motors (depending on the mission) to prove that it could stay aloft for long periods and achieve extremely high altitudes. Flying over Hawaii in August 2001, Helios reached 96,863 feet (29,523.8 meters), the highest ever recorded for any non-rocket powered aircraft. After that

X-57 milestone, the Helios group counted on rapid advances in fuel cell technology to enable the aircraft to fly uninterruptedly for a week, perhaps even indefinitely. The lead up to this objective came in a series of tests in June 2003, as warm-ups to a 40-hour endurance flight. During the second try, however, Helios broke apart in flight. ERAST as a whole ended in the same year. The seeds of electricity-based flight planted in Pathfinder, Centurion, and Helios lay dormant at NASA for a number of years, until revived in an entirely different format. NASA Administrator Charles Bolden put electricity center-stage of NASA aeronautics when he announced the initiation of the X-57 project in June 2016, 13  years after the crash of Helios: “With the return of piloted X-planes to NASA’s research capabilities − which is a key part of our ten-year-long New Aviation Horizons initiative − the… X-57 will take the first step in opening a new era of aviation.” In returning to the theme of electrical power after a long hiatus, NASA’s first X-plane in roughly ten years (and its first one with a human pilot in about 20), differed from the earlier efforts. In the case of Helios and the others, researchers pursued electrical power to sustain flight over long distances and great heights, with only minimal reference to the environmental rewards. In the new program, reflecting the increasing alarm over climate change, the main objectives concentrated on cutting aeronautical emissions, noise and fuel consumption, and on providing federal regulators with data useful to future flight certification of electric aircraft. The month after Bolden’s announcement, the fuselage for the X-57 arrived in America from Italy, though not at a NASA facility. A small airport at Pismo Beach, California, near the site of the X-57 prime contractor Empirical Systems Aerospace (ESAero) of San Luis Obispo, received a Tecnam P2006T commuter aircraft sent from Naples (the popular, lightweight P2006T began service in 2010 and is the product of Professor Luigi Pascale, the founder of the Tecnam company). Researchers decided to use an off-the-shelf vehicle simply because data from the baseline combustion engine model could be compared directly to the electrified version. As the uncrating occurred, NASA engineers from the Langley Research Center and the Armstrong Flight Research Center, who had been working on the space agency’s Scalable Convergent Electric Propulsion Technology and Operations Research (SCEPTOR) project for several years, got a look at the carcass of the future X-57. But many of them had seen it before. In 2015, NASA pilots at Edwards Air Force Base flew an instrumented,

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production Tecnam P2006T to determine the unmodified aircraft’s lift, drag, cruise efficiency, energy consumption, and flying qualities, all useful data to contrast with the forthcoming electric Tecnam. In addition, the dry lakebed at Edwards saw the unusual site of a big rig truck kicking up massive amounts of dust as it roared across the sand at 80 miles (128.7 kilometers) per hour during the same year. Far above the cab, on a derricklike structure, X-57 researchers had mounted an experimental airfoil with 18 electric motors and propellers. They generated a combined 300 horsepower, and doubled the lift of a stationary airfoil. Many hands at a number of small California companies contributed to the X-57, more commonly referred to as the X-57 Maxwell (in honor of nineteenth century Scottish physicist James Clerk Maxwell, who made the earliest observation that electricity, light, and magnetism together comprised one phenomenon: electromagnetic radiation). After the fuselage underwent an inspection at Empirical Systems, it went south by truck to Scaled Composites in Mojave, the home of the famous SpaceShipTwo suborbital aircraft. At Mojave, technicians began the initial X-57 work, known as the Modification I phase, by installing an all-­ electric power system and two large electric motors onto the stock P2006T.  These parts would enable researchers to make preliminary determinations about the safety and efficacy of this combination. At some future time, the ultimate step of mounting two new, high-aspect-ratio wings on the Tecnam fuselage would be taken, arrayed with a total of 12 small motors and propellers at their leading edges. Designed by engineers at NASA Langley, these airfoils would be fabricated by Xperimental LLC, a firm located in San Luis Obispo, California. Electric Power Systems, of the City of Industry, California, signed on to supply the batteries that powered the motors and rotors. Activity intensified during the Modification II phase of the X-57 Maxwell. The aircraft arrived at NASA Armstrong in October 2019, after which preliminaries (such as structural ground tests) got underway. At the same time, the aircraft itself underwent more changes as technicians installed two new inboard electric motors. Flight tests followed to validate the modified vehicle, its battery system, and the X-57’s instrumentation. Additionally, a set of lithium ion batteries underwent an in-house review to discover whether they could sustain a full flight profile without emitting dangerous heat. Engineers redesigned the batteries in 2017, enabling their contractor to proceed with fabrication. Meanwhile, pilots and others at Armstrong developed a

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flight simulator to anticipate the cockpit experience well in advance of the actual flights. Finally, the X-57 team assessed the compatibility of the proposed electrical system with the Maxwell, including a high-voltage ground test in February 2021. The Modification II phase continued during that year. Events in the Modification III phase will be happening concurrently with those of Modification II. During this period, integration of the high-aspect-ratio wings with the fuselage will begin, while technicians will trade the two comparatively heavy inboard electrical engines with new ones of about half their weight and install them at the wingtips. In the last phase, or Modification IV, the new airfoils will be mounted on the Tecnam. During flight tests, the six small motors on each wing will lift the aircraft at take-off. They will shut off once in cruise mode and fold into their nacelles to reduce drag, leaving the bigger engines at the wingtips to take over for standard flight. The ambitious objectives for the Maxwell revealed themselves in the Modification IV aircraft. Engineers hoped that the fully modified Tecnam would exceed the performance of the standard model in three ways: with a 500 percent increase in high-speed cruise efficiency;

with no carbon emissions; and with a far quieter overflight of communities below its flight path. Powered by a lithium ion battery weighing 860 pounds (390 kilograms), the final X-57 will have a mass of about 3,000 pounds (1,360.7 kilograms), fly at an operational altitude of 14,000  feet (4,267.2 meters), cruise at up to 172  miles (276.8 kilometers) per hour, and require a take-off speed of 67 miles (107.8 kilometers) per hour. The 12 small motors each require 10.5 kilowatts of power, while the two cruise motors and propellers need 60 kilowatts each. By March 2021, expectations that the first flight of the X-57 Maxwell would happen soon were growing in the media. X-59 What failed to materialize in the X-54A may at last find fruition in the X-59. In 2008, NASA appeared ready to move ahead with an industry partner (Gulfstream Aerospace) to produce the X-54A, a relatively small, low-sonic-boom demonstrator designed to apply existing knowledge to dampen, but not eradicate, the noise

X-59 133 associated with supersonic flight. But budgetary constraints, recent developments, and technical prospects just over the horizon prompted space agency officials to withdraw from the X-54A and wait for research that would lead to a more capable X-plane. The decision to hold off paid a big dividend. By 2011 and 2012, advances in computational fluid dynamics, supported by wind tunnel work, suggested that noise reduction techniques could be applied successfully not only to small aircraft (like they had been in the F-5E Shaped Sonic Boom Demonstrator program) but to mid-sized, and eventually full-sized jetliners. The X-59 Quiet SuperSonic Technology (QueSST) aircraft came into being to test this ambitious objective, a goal that had tantalized aeronautical engineers since the 1950s but continued to prove elusive. The tide of this historic trend turned in March 2011, when NASA’s supersonics project manager Peter Coen delivered a detailed, lengthy briefing that presented an artist’s rendering of a new supersonic transport. Viewers saw an aircraft with an exceptionally long nose and canards set far forward; a slender fuselage with many passenger windows; thin, swept wings; and an empennage with twin canted tails and two engines

mounted inboard of the tails at the far rear of the aircraft. This initial concept took some years to translate into reality. In February 2016, the Lockheed Martin Skunk Works signed an agreement with NASA for a preliminary design of the X-59. Twenty-six months later, in April 2018, the space agency awarded a $247.5 million cost-plus-incentive-fee contract to Lockheed Martin (the sole bidder) to design, fabricate, and flight test a single X-59 demonstrator. Initial milling of parts began in November 2018 at Lockheed’s facility in Palmdale, California. Meanwhile, researchers at NASA Langley conducted wind tunnel tests in fall 2018 to determine the X-59’s suitability in high angle of attack (AoA) flight. In the same year, NASA deployed an F/A-18 fighter to Galveston, Texas, and nearby waters, to simulate up to eight thump-type sonic booms a day by diving from 50,000  feet and briefly attaining supersonic speed. Feedback from 400 residents and data from 20 sensors on the ground measured the impacts. While these activities progressed, assembly of the aircraft’s major structural components got underway in 2019, and in September 2020 NASA and Lockheed conducted a successful critical design review.

134  Aerodynamics and Engines: NASA’s Research Agenda Overall, the Skunk Works’ portrait of QueSST did not differ radically from the picture shown by Peter Coen nearly ten years earlier. Lockheed conceived an unusually low-slung, thin profile demonstrator, with an uncommonly long forebody and canards just in front of the almost centrally-­ positioned cockpit. The design also incorporated imposing swept delta wings that joined the fuselage from the front of the cockpit back almost to the vertical tail − a single tail in this case. Faired in below the tail, a long nacelle housed the single X-59 powerplant. Unlike the Peter Coen model, the Lockheed fuselage looked more like a backbone that held the aircraft’s structures together, rather than a credible compartment for passengers. NASA and the Skunk Works both thought big for the X-59. Their work harkened back to some of the bestknown piloted research aircraft of the past, such as the X-15 and the XB-70 Valkyrie. The X-59 fell somewhere between these two legendary vehicles in size, measuring 96 feet 6 inches (29.4 meters) long, with a 29-foot 6-inch (8.9-meter) wingspan and a height of 14 feet (4.2 meters) at the tail. It had a maximum gross weight of roughly 24,300 pounds (11,022.2 kilograms) and could carry up to 600 pounds (272.1 kilograms) of payload. General Electric supplied the engine: an F414-GE-100 powerplant, commonly used in fighter jets. Capable of cruising at 55,000 feet (16,764 meters), the X-59 could fly at up to Mach 1.4. QueSST’s designers made bold predictions about its performance and legacy: to those on the ground, its sonic boom would sound like a car door closing, or like a thump, or even be inaudible, while future derivatives of the X-59 could reduce the time taken to get from point to point by air by 50 percent compared to the existing standards. Although sweeping, these claims had some validity based on the years of research NASA devoted to the supersonic airliner conundrum. The design features that culminated in the X-59 reflected

these discoveries. For instance, and perhaps most tellingly, the X-59’s overall shape had been conceived to spread out supersonic shock waves as they fell to Earth, rather than letting them concentrate at any one point. The swept-back wings helped reduce drag and contribute to speed and quieter flight, while the long fuselage has been shown to dampen noise on smaller test vehicles. The engine would cause less disruption to the communities below because it had been mounted above the airframe; and to enable the pelican-like nose so essential to the X-59’s quieter performance, NASA developed the eXternal Vision System (XVS), a combination of HD displays, computers, and sensors that, if successful, would overcome pilot sight limitations imposed by the aircraft’s long front body. Despite the ravages of the Coronavirus Pandemic that gripped the world in 2020 and early 2021, the X-59 project carried on, but at a deliberate pace reflecting the difficulty of the tasks and the potentially historic nature of QueSST. Overall fabrication reached approximately mid-point in January 2021, as Lockheed completed the wings and turned its attention to the nose section and the empennage. The Skunk Works team hope to integrate these three main components by the end of 2021. If they achieve this milestone, NASA plans for the first flight of the X-59 in mid-2022. Not until the year 2024, however, and only with the assurance that QueSST can fly safely, will NASA begin to overfly select cities to determine whether people in the flight path can tolerate the supersonic activity overhead. If all goes well, NASA plans to transfer the project’s data set to U.S. federal − as well as international − regulatory bodies in order for them to draft guidelines for the forthcoming era of supersonic flight over populated areas. If the X-59 fulfills the hopes of its creators, it will join a short and select list of X-planes that have made a profound impact on global aeronautics and spaceflight.

6 Beyond the Horizon: Access to Space

X-30 When they first lent their support to the National AeroSpace Plane (NASP), hard-headed engineers and hopeful politicians succumbed to a beguiling concept fed by technological and political exuberance: an aircraft that took off horizontally, ascended into space, and returned to Earth to land on a runway. Like the story of the Greek hero Icarus who rose on wax wings only to fall with the heat of the Sun, the story of NASP involved a vision so enchanting that its supporters suspended disbelief in order to pursue it. NASP began at the start of President Ronald Reagan’s first term, coincident with his administration’s arms build-up against the Soviet Union. As part of this rearmament, it became clear that the Space Shuttle had fallen short of its low cost, space truck allure since its first launch in April 1981. Military leaders in particular pressed for a more capable hypersonic spaceplane. The Defense Advanced Research Projects Agency (DARPA) took a first step by converting a proposal for a hypersonic scramjet into a classified program codenamed “Copper Canyon”, a

feasibility study for a single-­stage-to-orbit vehicle. DARPA underwrote the project from 1983 to 1985. As Copper Canyon germinated, NASA planted its own hypersonic seed. The space agency took notice of the work of Tony Du Pont, the owner of a small computer firm conducting research on a multi-cycle engine that combined jet and rocket power. This study piqued NASA’s interest, and the space agency awarded Du Pont a $30,000 contract for a computer analysis of engine cycles. His findings riveted the attention of DARPA, the Air Force, the Navy and NASA in their common quest for hypersonic access to space. NASA gave him a second contract to elaborate on the first. It yielded something fantastic (or perhaps, in hindsight, fantastical): his computer model suggested that a scramjet engine could develop the thrust-over-drag necessary to launch a 50,000-pound (22,679 kilogram) aircraft into orbit without a boost from a rocket. Stunned by this result, the partners in the project accepted Du Pont’s research as the baseline data for an aerospace plane (despite the fact that almost no one but Du Pont himself seemed able to replicate his results using his model).

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 M. H. Gorn, G. De Chiara, X-Planes from the X-1 to the X-60, Springer Praxis Books, https://doi.org/10.1007/978-3-030-86398-2_6

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Once Du Pont’s research became known, the project swept forward on the wings of exuberance. In 1983, he unveiled an aircraft design that followed on from his propulsion assumptions, envisioning a long, sleek vehicle with an arrow-like profile. Copper Canyon, combined with Du Pont’s revolutionary theory, roared into the nation’s consciousness when President Ronald Reagan mentioned it obliquely in his State of the Union Address in February 1986 − postponed for a week, it should be noted, due to the Shuttle Challenger accident. The Shuttle tragedy may have prompted the president to add this optimistic sentence to his remarks: “And we are going forward with research on a new Orient Express that could, by the end of the next decade, take off from Dulles Airport and accelerate up to 25 times the speed of sound, attaining low-Earth orbit or flying to Tokyo within two hours.” As a result of Reagan’s statement, the National Aerospace Plane leapt into the headlines. Despite the fact that it had yet to be proven in any sense, NASP benefitted from Reagan’s endorsement, gaining attention, funding, and political support. The concept also gained momentum because of its antecedents; the idea

of a transatmospheric vehicle had been discussed since the 1940s, but without progress due to its technical barriers. Now, with the president’s apparent endorsement of NASP, the Air Force moved to designate the X-30A as a hardware testbed, designed to assess the systems and technologies required to bring a family of hypersonic spacecraft into being. Unfortunately, this reasonable bureaucratic decision began a journey to nowhere, though it began with hope. The X-30A attracted widespread interest and excitement from government and industry alike. Leaders of the Strategic Defense Initiative (SDI) − a massive Reagan-era defense project designed to field a space-based system capable of shielding the U.S. from nuclear attack − saw NASP as a potential supply ship for its space assets. NASA also jumped aboard the NASP train, having tried for years to extend hypersonic flight research beyond the X-15 (ineffectually pinning its hopes on an advanced X-15, on the X-20A Dyna-­Soar, and on the X-24C lifting body). In addition, the most prominent aircraft manufacturers stepped in for a piece of the action during the first week of April 1986. The Department of Defense (DoD)

X-30  137

138  Beyond the Horizon: Access to Space and NASA awarded about $450 million (of a total $3.3 billion budget), to Boeing, General Dynamics, Lockheed, McDonnell-Douglas, and Rockwell International for airframe analyses. Powerplant manufacturers General Electric, Rocketdyne, and Pratt and Whitney received contracts to work on the engines. Air Force, Navy, and NASA representatives opened a joint program office at Wright-­Patterson Air Force Base in Ohio. About 64 percent of the X-30A budget came from the DoD, with NASA supplying the remaining third. Despite the warm reception given by the Reagan announcement and the high hopes of finally overcoming the expense and waste of brute-force rocketry, many factors rendered the X-30A not just challenging, but technically out of reach. The means of propulsion seemed problematic at best, relying on as yet unproven scramjet technology. Moreover, scramjets require hypersonic speed in order to ignite, which in the case of the X-30A meant the need for booster rockets. So even if the aircraft achieved the necessary Mach number for the scramjets to operate, it would be uncertain that they would be able to supply the further thrust needed for spaceflight. Additional high hurdles also had to be overcome: building a light, strong, reusable airframe that could withstand very high and very low temperatures; integrating aero-­ propulsion, thermal management, and control systems; and devising advanced computational testing methods, backed by actual testing where possible. Most discouraging of all, attempts by no less experienced propulsion authorities than those at General Electric and Pratt and Whitney failed to confirm Du Pont’s theory that his scramjet could achieve orbital velocity. By the time of the George H.W.  Bush and Bill Clinton administrations, all of these concessions to reality had obliterated the original Du Pont design, converting his picturesque arrow configuration into a consensus-based X-30A platform: a long, wedgeshaped blended wing/body that looked a little like a deep-sea fish. The dimensions varied from contractor to contractor, but the NASA generic, or baseline configuration consisted of a vehicle about 141  feet 3 inches (43 meters) long, with a wingspan of 34 feet 1 inch (10.3 meters) and a wing area measuring 1,360 square feet (126.3 square meters). It would weigh roughly 325,000 pounds (147,417.5 kilograms) gross and, if it ever materialized, aimed for a top speed of Mach 25.

It never did materialize, of course. Initially, program managers hoped for a first flight in 1992, but after missing many milestones the timeline receded into the future, to either 2000 or 2001. However, patience ran out long before then. Budget cutting in the early Clinton administration resulted in the demise of the X-30A in 1993. The withdrawal occurred in part because the original allotment of $3.3 billion showed signs of ballooning to as high as $17 billion. Despite the extravagant budgetary problems, the missed deadlines, and the technological hurdles that proved too high to surmount, the X-30A cannot be dismissed wholly as a failure. Researchers in materials science arrived at titanium aluminide metal-coated matrix composites and coated carbon-carbon composites as a means of counteracting the extremes of heat and cold. In addition, the work on scramjets pursued during the seven years of the NASP helped usher in the X-43A, which achieved Mach 6.8 in March 2004 and Mach 9.6 eight months later, propelled by scramjet engines. X-33 Among the many prominent X-planes, the X-30 and X-33 are two with particularly high profiles. They not only sought to demonstrate new spaceflight technologies, but actually became symbols of American space pre-eminence. Both stemmed from the aftermath of the Challenger accident in 1986, as well as from the growing consensus that, for all of its virtues, the Space Shuttle failed to fulfill its early promise as the embodiment of reliable, inexpensive access to space. This understanding coincided with Daniel Goldin, a new NASA leader who directed the space agency between the start of the Clinton presidency and early in the first term of George W.  Bush (April 1992 to November 2001, the longest tenure of any NASA Administrator). In charge just after the Cold War ended, Goldin wanted a vigorous American presence in space, while pressing for economies under the acquisition banner, “faster, better, cheaper.” Clearly, the Shuttle did not fit well into Goldin’s formula: it took years to develop, it failed the Challenger astronauts, and it consumed a significant part of the space agency’s budget. Accordingly, to clarify NASA’s space launch posture, Goldin appointed a

X-33  139 commission to examine the U.S. space program in January 1993 (the month of President Clinton’s inauguration). The commission produced a report entitled Access to Space. This document led to a complete reassessment of American space travel, concentrating (in part) on new, heavy lift spacecraft capable of carrying up to 25,000 pounds of payload for the massive task of constructing the planned International Space Station, then under development. For this assignment, the authors envisioned a fully reusable launch vehicle (RLV) propelled by neither jettisoned boosters nor by an expendable fuel tank. In short, they proposed a leap over traditional rocketry by supplanting it with a single-­stage-­to-orbit (SSTO) spacecraft. One practical outcome of Access to Space materialized in a half-scale, suborbital technology demonstrator. Designated the X-33, it offered the opportunity to assess the SSTO concept through rigorous flight research. Three manufacturers vied for the X-33 prime contract: Rockwell International (builder of the Space Shuttle), Lockheed Martin, and McDonnell-Douglas. Under Dan Goldin’s rule of faster development, the three industry giants had only two months  – beginning in March 1995 − to draft and submit technical designs. If this did not present a sufficient challenge, NASA also insisted that the winner complete the entire project quickly and at relatively low cost. Despite this last requirement, a big payoff awaited the winner of the X-33 prize: a contract of almost one billion dollars ($942 million to be exact). Beyond that, the three competitors realized that success with the X-33 would lead to even greater riches when NASA ordered the expected follow-on, consisting of a fleet of fullscale, operational SSTOs. After subjecting the manufacturers’ proposals to wind tunnel tests and computational analysis, NASA authorities made their choice in July 1996, selecting Lockheed Martin, which subsequently subcontracted with Boeing/Rocketdyne for the X-33’s propulsion system. The agreement signed by the space agency and Lockheed included an unusual stipulation: while it committed NASA to allocate the $942 million by 1999, it also obligated Lockheed to contribute $200 million of its own money − Dan Goldin’s way of controlling costs and of getting more “skin in the game” from Lockheed. NASA chose the Marshall Space Flight

Center, the agency’s expert on rocket engine development in Huntsville, Alabama, to lead the effort for the government. Despite the technological advances inherent in SSTO, Lockheed based its proposal (in two of its four main parts) on tradition. For the external design, the project engineers envisioned a reinterpretation of the classic lifting body contours, this time proposing a clean-lined, wedge-shaped vehicle that built on NASA’s research of the 1960s and 1970s. This experimental spacecraft (again, a half-scale demonstrator) measured about 67 feet (20.4 meters) long, and 68 feet (20.7 meters) across, with canted wings and two vertical stabilizers mounted at the rear. It featured a 5 x 10-foot (1.5 x 3-meter) cargo bay and would weigh about 64,000 pounds (29,030 kilograms). For the powerplant, Lockheed relied on Rocketdyne, which proposed the linear aerospike engine devised for the Air Force in the 1960s but never flown. Lockheed embarked on a new, metallic protection system to shield the spacecraft against the heat of launch and re-entry, while the engineering team decided to fashion the hydrogen tanks from composites rather than standard metals. If all of these components formed a successful X-33, Lockheed had a full-scale follow-on spacecraft in its corporate mind that it had already named the VentureStar. Ultimately, the success of the X-33 rested on its propulsion. Without an adequate system (contractually required to achieve Mach 15, later reduced to Mach 12), all else would be irrelevant. Unlike conventional rocketry, the linear aerospike expelled exhaust from two high-performance propellants: liquid hydrogen and liquid oxygen. The X-33 engine consisted of eight small combustors (compared to 20 on the fullscale VentureStar) arranged in parallel rows and positioned to fire on curved, rectangular plates. The consequent angle of exhaust flow, unconstrained on the outside, expanded and contracted with the conditions of atmospheric density encountered during ascent. Because the thrust increased and decreased according to necessity − rather than the fixed flow produced on the Space Shuttle’s bell-shaped nozzles, for instance − this means of propulsion offered both better fuel economy and reduced overall weight. Ground testing had already demonstrated durability, simplicity of design, high thrust, and appropriate expansion ratios at all altitudes.

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X-34  141 But inconclusive flight tests on the linear aerospike cast a shadow over the whole project. At the Dryden Flight Research Center, technicians mated the untested engine to NASA SR-71 number 844 in August 1997, to begin a series of research flights with the acronym LASRE: Linear Aerospike SR-71 Experiment. The first cold-flow flying tests (cycling gasses through the engine during flight) happened on March 4, 1998, but disappointment soon followed. During the spring, oxygen leaks materialized during flights two to four. Concerned about this possibility in proximity to hydrogen, project engineers cancelled plans for a hot-firing aboard the SR-71. Instead, they evaluated information gained during earlier static tests and the four cold-flow tests, and made extrapolations about the engine’s probable characteristics as it burned hot gasses in flight. LASRE ended in November 1998 (but aerospike research continued at NASA’s Stennis Space Center in Mississippi, where one fired for 290 seconds during a ground experiment in May 2000). In the end, something as prosaic as tankage proved to be the downfall of the X-33. Lockheed fashioned the hydrogen tanks from composites in order to economize on weight, and a key test of this subsystem in November at Marshall at first seemed to affirm the decision to do so. Technicians filled the structure with liquid hydrogen and it appeared to be a success, until they emptied the contents. When they did, the tank warmed, causing the outer and inner skins to pull away from one another. NASA investigated and found construction and design inadequacies. The company then faced an impossible choice: abandon the composite concept (essential to the success of any future reusable launch vehicle), or fabricate the tank from more conventional (and heavier) aluminum lithium. They scrapped the composite version, but it took a heavy toll on the project. In 2000, estimates predicted at least a two-year delay for the first flight, in order for the new tanks to be built and tested. Lockheed and NASA now found themselves in an irreconcilable dilemma. On the one hand, the aerospike engine looked promising but had not proven itself, and the tankage issue hung over everything; on the other hand, 95 percent of the X-33’s parts and 75 percent of its assembly respectively had been fabricated and completed by summer 2000. That September, the parties decided to proceed, but massive cost overruns that became public in late 2000 and early 2001 finally doomed the X-33. NASA withdrew on March 1, 2001, admitting that the technology did not yet exist to develop an SSTO reusable launch vehicle. (New priorities from the incoming Bush administration may also have played a part in

the demise of the X-33.) Still, the program pushed progress in several areas: on the linear aerospike; on advanced thermal protection systems; on self-healing avionics; and on a twenty-first century update to the lifting body design. Like many of the more daring X-planes, the X-33 itself failed to meet expectations, but it demonstrated some of the essential ingredients required for future projects. X-34 The same impulses that led Dan Goldin to apply “faster, better, cheaper” to the pursuit of the X-33 also informed his decisions on the X-34. Once again, in an era of American reliance on the Space Shuttle − a costly launch vehicle that had already failed catastrophically in 1986 − Goldin continued to look for alternatives. In the X-33 he hoped for a system that would send astronauts and heavy cargo into space. In the X-34, he anticipated the technologies necessary for a fully autonomous (unpiloted), suborbital, reusable launch vehicle capable of flying with great frequency (and eventually serving as a gateway to space for small satellites). For Goldin, the issue remained cost: the Space Shuttle consumed $10,000 to deliver each pound (0.45 kilograms) of payload into low Earth orbit; he hoped to lower that figure to $1,000 or less. The launch platform for the X-34 did not exist at Kennedy Space Center, nor at Vandenberg Air Force Base or any other fixed point, but instead, on a jumbo jet. This method followed a trail paved by the Orbital Sciences Corporation (OSC), which developed the Pegasus spacecraft in concert with NASA, launched from an L-1011 mother ship. As early as February 1993, Pegasus placed a commercial satellite into orbit, marking the first time any airliner had acted as a launch pad. Based on the same concept, the X-34 therefore had a solid grounding in reality and possibility, with the main variation being that the X-34 would itself escape the atmosphere in a suborbital arc, rather than discharge a satellite. Not surprisingly, NASA turned to Orbital Sciences for the X-34 and signed a contract with the Virginiabased company on March 30, 1995. A relatively small firm, OSC brought in Rockwell International as a partner. As with the X-33, Goldin wanted the private sector to have a big stake in the outcome, so for the X-34, OSC and Rockwell each agreed to contribute $50 million to its development; NASA added another $70 million. Together, this budget covered the procurement of three X-34s.

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NASA expected a lot for this comparatively small outlay. In terms of mission capability, the space agency wanted the X-34 to demonstrate quick launch capability (about 25 flights per year), low cost operation, safe flight through bad weather, and reliable runway landings in cross winds up to 20 knots (23  miles, or 37 kilometers, per hour). On the technical side, the contract required an airframe fashioned entirely from composites (including its RP-1 fuel tank), fully autonomous flight operations, inexpensive avionics, and a state-of-the-art thermal protection system. Once flight worthy, the space agency expected the X-34 to travel at Mach 8, reach 250,000 feet (76,200 meters), and land horizontally. NASA Marshall developed the X-34’s engine, a single-stage rocket called Fastrac powered by kerosene and liquid oxygen, and the first new liquid propellant powerplant produced in America since the Shuttle’s main engines. Built for economy, Fastrac offered performance similar to that of the X-15’s XLR99. Eventually, OSC and Rockwell unveiled a long, slender vehicle, with relatively small swept wings mounted far aft on the fuselage, and a single vertical tail structure. About 58 feet (17.6 meters) long, it stood 11 feet 6 inches (3.5 meters) at the tail. The wingspan

measured almost 28 feet (8.5 meters) with a wing area of 357.5 square feet (33.2 square meters). It weighed roughly 47,000 pounds (21,319 kilograms) gross and 17,000 pounds (7,711 kilograms) empty. Orbital Sciences trucked in one of the three airframes to the Dryden Flight Research Center (located on Edwards Air Force Base, California) in spring 1999. Three captive-carry flights (with the X-34 aboard an OSC L-1011 airliner) occurred on June 29, and September 3 and 14. The gap in testing occurred because the L-1011 developed cracks in its skin after the June mission requiring extensive repairs at Dryden, but in the end, none of it mattered. These few missions marked the beginning and the end of the X-34 program. Facing steep cost increases due to changing requirements, NASA officials decided to end the X-34  in March 2001, the same month and year as the demise of the X-33. New spaceflight initiatives planned by the incoming George W.  Bush administration may have played a part in the almost simultaneous cancellation of these two space pioneers. However, the X-33 and X-34 shared something more than a termination date: both contributed to the science and literature of reusable launch vehicle technology, leaving a pathway for future researchers.

 X-37A and B During Daniel Goldin’s nine and one-half year tenure as NASA Administrator (the longest on record) he pursued many transformative goals. While one of his main ambitions involved the adoption of his “faster, better, cheaper” acquisition strategy, the other embraced the

X-37A and B  143 development of a replacement vehicle for the costly and hazard-prone Space Shuttle. Goldin’s Space Shuttle successor began as an autonomous orbital vehicle (distinct from the unmanned suborbital X-34 and the manned orbital X-33). The Air Force joined the project, and the two partners designated NASA’s Marshall Space Flight

144  Beyond the Horizon: Access to Space Center to be the lead government agency. A request for proposals in late 1996 attracted submissions from the Lockheed Martin Skunk Works, and from the Boeing Phantom Works. The space agency selected Boeing’s design and signed the contractor to a fouryear, $173 million cooperative agreement in July 1999. Boeing won out partly because Rockwell (bought out by Boeing in 1996) had designed the Shuttle Orbiter, and its new proposal borrowed heavily from the aerodynamics of the earlier vehicle. NASA and Boeing split the costs roughly in half − once again, the NASA of Dan Goldin structured projects so that industry shared some of the risks alongside the government. Named the X-37, NASA had ambitious plans for the project right from the start. To begin with, it would be deployed on atmospheric and orbital flights to test its thermal protection apparatus, as well as its engines, airframe, and ­ operational systems. It would then be launched from the cargo bay of the Space Shuttle as a secondary payload, circle the Earth independently for several days, and touch down at either Edwards or Vandenberg Air Force Base. Designers of the X-37 built it for at least 20 missions and landings. Tellingly,

even during its early stages, Air Force Space Command officials envisioned the X-37 as a potential testbed for its own unmanned Space Maneuver Vehicle (SMV), a proposed spacecraft that the military hoped to send on orbital missions for a year at a time. Thanks to its comparatively small size and capacity to change orbital positions readily, the SMV appealed to the Air Force, which also saw advantages in its geographic sensor coverage, tactical flexibility, interchangeable payloads, low-risk subsystem components, and on-orbit satellite repair. To investigate the full capabilities of the X-37, Boeing and NASA agreed to two separate variants: the X-37A, known as the Approach and Landing Test Vehicle (ALTV), and the X-37B, otherwise called the Orbital Test Vehicle (OTV). NASA planned to flight test the ALTV at its Dryden Flight Research Center to assess the X-37A’s autonomous approach and landing systems, its integrated control center, range operations, structural worthiness, aerodynamic qualities, and the turnaround time between missions. To determine some of these variables, engineers prepared the ALTV for five unpowered flights in 2004, conducted by dropping it from a B-52H mothership at altitudes up to 40,000 feet.

 Plans called for the OTV to be tested in 2006, concentrating on ascent, orbital flight, re-entry and landing, all in an autonomous environment. In doing so, project engineers wanted to study thermal protection systems (such as conforming, reusable insulation); avionics; high-temperature structures; advanced guidance, navigation, and control; and high-temperature seals. Despite these carefully laid out milestones, in reality the X-37 did not follow a predictable pattern of development, but instead took many surprising turns. After proving itself in fabricating the X-37’s complex composite wings, in November 2002 Boeing announced the award of a $301 million contract by NASA to proceed with development and flight testing of the X-37A and B. The new agreement called for the contractor to complete one airframe and conduct flight testing of X-37A in April 2004. But this deadline came and went, and a bolt from the blue changed the entire project shortly afterward. Quite suddenly, NASA officials transferred the whole X-37 project to the Defense Advanced Research Projects Agency (DARPA) which took control of it in September 2004, turning it into a classified project. DARPA began its participation with glide tests of the X-37A, conducted not at Edwards Air Force Base but at the nearby Mojave Airport, where the Virgin Galactic White Knight carrier aircraft lifted the X-37A on its first captive-­ carry mission in June 2005. It incurred minor damage on landing when it overshot the runway. After repairs, an initial glide flight occurred at Mojave in April 2006, followed by five more unpowered missions at Plant 42  in Palmdale (during which time the X-37A still remained based in Mojave). On two of these instances, in August and September 2006, the X-37A performed free flights and successful landings. These tests validated the flight dynamics of the X-37 and opened the flight envelope to higher speed and altitude regimes. Once the Plant 42 activities ended, Air Force officials examined the flight test data, and announced their plan on November 17, 2006, to develop NASA’s Orbital Test Vehicle concept into an actual

X-37A and B  145 spacecraft, capable of remaining aloft for up to 270 days per mission. To boost the OTV into orbit, and in light of the Shuttle Columbia accident in 2003, the USAF decided to abandon the concept of the Shuttle as an OTV first stage and instead use the Atlas V rocket. Just as it had for NASA, Boeing remained the X-37B prime contractor, committed to building two OTVs. Not intended as a Space Shuttle replacement, the X-37B had its own unique objectives, defined by the Air Force as “an experimental test program to demonstrate technologies for a reliable, reusable, unmanned space test platform… The primary objectives of the X-37B are twofold: reusable spacecraft technologies for America’s future in space, and o­ perating experiments that can be returned to, and examined, on Earth.” Generalized statements such as these reflect the X-37B’s classified status, and Air Force statements only tell the broad outlines of individual missions. But despite the veil of secrecy, even outsiders could see the program generating plenty of activity. Launched six times up to the time of writing this book (mid-2021), the X-37B’s first mission began on April 22, 2010, with all OTVs since then launching from Kennedy Space Center (KSC), Florida. OTVs 1, 2, and 3 landed at Vandenberg Air Force Base, while OTVs 4 and 5 touched down at KSC. All used the Atlas V, except for OTV-5 which went into orbit on a SpaceX Falcon 9 rocket. The first five spent a combined 2,865  days on orbit averaging 573 days each, a tremendous record of long-duration spaceflight. The most recent mission, OTV-6, lifted-off from Kennedy in May 2020, and carried more experiments into space than the previous X-37Bs, including a pair of NASA investigations: one on the effects of space radiation on seeds; the other testing the reaction of a select group of materials in the space environment. Additional research sponsored by the U.S. Naval Research Laboratory concerned the conversion of solar power into radio frequency microwave energy, and explored the transmission of that energy to Earth. OTV-6 is understood to be still in orbit at the time of writing.

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X-37A and B  147

148  Beyond the Horizon: Access to Space Compared to many other complex, autonomous spacecraft, the compact size of the X-37B belies its big impact. It measures 29 feet 3 inches (8.9 meters) long and 9 feet 6 inches (2.9 meters) in height, with a wingspan of 14 feet 11 inches (4.5 meters). It weighs about 11,000 pounds (4,990 kilograms). The X-37B travels at approximately Mach 25, powered by a single Rocketdyne AR2-3 rocket engine, using hydrogen peroxide/JP-8 propellants and rated at 7,000 pounds (3,175 kilograms) thrust. (Some sources describe a hypergolic system consisting of nitrogen-tetroxide/hydrazine propellants). In the end, Daniel Goldin’s vision of a “faster, better, cheaper” method of spacecraft acquisition has fallen into disuse at NASA.  But, remarkably, his goal of a Shuttle successor lives on, not in the civilian space agency that he led but in the Air Force, embodied by the X-37B. X-38 During the 1980s and 1990s, five X-planes projects − the X-30, X-33, X-34, X-37, and the X-38 − attempted in one way or another to redress perceived deficiencies in the Space Shuttle. Planners at NASA pursued alternate means of access to space, as the high cost of Shuttle operations became clear and its capacity for sudden failure manifested itself in the Challenger accident in January 1986. NASA Administrator Daniel Goldin became the chief architect of the new thinking. During his term, the X-33 explored single stage-toorbit technologies that might result in a vehicle to replace the Shuttle, while the X-34 and X-37 represented smaller, fully autonomous suborbital and orbital vehicles that could do some of the Shuttle’s work at lower cost and with no threat to human life. The X-38 also emerged from a growing realization of the Shuttle’s inadequacies. Specifically, as the first

modules of the International Space Station (ISS) went into orbit in the late 1990s, officials at NASA feared that an emergency on the ISS could doom its crew before a Shuttle could scramble to retrieve them. With that in mind, engineers at the NASA Johnson Space Center (JSC) in Houston1 drew up plans in early 1995 for a Crew Return Vehicle, or CRV, a small, on-­board “lifeboat” on which up to seven astronauts could fly back to Earth in case of a crisis. The X-38 served as a demonstrator to test the technologies needed to make such a vehicle possible. The Johnson design closely followed that of the bulbous X-24A lifting body that last flew in 1971. In early 1996, NASA contracted with a small but familiar manufacturer called Scaled Composites, of Mojave, California, located close to the Dryden Flight Research Center, to build two 80-percent-sized X-38 airframes, designated the V-131 (later modified as V-131R), and the V-132. (Scaled Composites, along with its founder Burt Rutan, later became famous for SpaceShipOne, a supersonic aircraft that eclipsed the sound barrier in December 2003.) As the X-38s came together in the Scaled Composites workshops, the JSC team prepared their avionics and computer software, which it installed in the first of the two airframes in September 1996. That airframe arrived at Dryden in June 1997 for flight testing. Like many of the Goldin-era spacecraft, the X-38 aimed for quick development based upon simplicity and low cost. According to the plan, the ISS crew would enter the vehicle (docked permanently to an ISS port) in an emergency, detach it from the station, and fire the de-orbit engine (which the astronauts would jettison after use). Then, acting as a lifting body, the CRV would glide home from orbit without power (as with the Space Shuttle). As it approached for a landing, an autonomous, steerable parafoil parachute (developed by the U.S.  Army, with pilot back-up) would open and carry the crew to their destination.

1  JSC had logic on its side in taking the lead on the X-38. It represented the institutional home of the astronauts, the place of their selection, training, and assignments. Crew safety aboard the ISS especially impacted JSC, hence its direct involvement in CRV development.

X-38  149

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X-40A  151 The X-38 flight research program at Dryden had four objectives: the transition from lifting body to parafoil flight; an assessment of the flight controls of the X-38 and the parafoil; a demonstration of the subsystems; and an evaluation of the flight dynamics of the two systems. During these tests, V-131 and V-132 would be dropped from the venerable Dryden NB-52B mothership at altitudes of up to 39,000 feet (11,887 meters). Against the massive scale of the bomber, the X-38 looked insignificant. The V-131 and V-132 measured just 24.5  feet (7.4 meters) long and 11.6  feet (3.5 meters) wide, with a height of only 8.4 feet (2.5 meters), a wingspan of 12.5 feet (3.8 meters), and a wing area of 162 square feet (15 square meters). They weighed roughly 14,900 pounds (6,758.5 kilograms) apiece. Scaled Composites, as the name might suggest, actually fashioned the X-38s out of fiberglass and graphite epoxy. Both demonstrators landed on skids − like the famed X-15 − rather than on wheels. Flight research on the X-38 began slowly and cautiously. Before the arrival of the two X-38s, Dryden engineers conducted 13 drop tests on a 1/6th scale model. Then a series of captive-carry flights began in July 1997, with the X-38s mounted on NASA’s NB-52B (famously called “Balls 8” due to its 008 serial number). These trials lasted through that year and into 1998. During March 1998, the first actual drop test of the X-38 revealed some problems with the parafoil. After investigations at the Army Proving Grounds in Yuma, Arizona, free flights resumed at Dryden in February 1999 and continued in March and July (on the V-132 equipped with flight control surfaces, and on the V-131 without). These proved to be successful, and in September and November 1999, V-132 underwent additional captive-carry missions. The drop tests of the X-38 (once again using the V-132) culminated in March 2000, when it broke project records for altitude, speed, and length of time aloft (respectively, 39,000  feet (11,887.2 meters), 500  miles (804.67 kilometers) per hour, and 45  seconds). It deployed its parachute and landed safely on Rogers Dry Lake bed. The X-38 flew for the final time in December 2001, and to the surprise of many the program ended in April 2002. It fell victim to lavish cost overruns on the International Space Station, its ultimate funding source. American space officials chose to rectify the budget crisis by eliminating the Crew Return Vehicle (and implicitly, its testbed X-38), electing to use the venerable but reliable Russian Soyuz capsule for crew rescue purposes.

X-40A An uncommon convergence happened in the case of the X-40A. During its brief lifespan, NASA and the Air Force found themselves running along parallel lines of development for an autonomous orbital spacecraft. Their two endeavors eventually merged, not through their own efforts but by the intervention of an unexpected third party, leaving only one of the two original sponsors with an operational space vehicle. It represented at least a partial victory for NASA Administrator Daniel Goldin. Formal work on the X-40A started in October 1996, when the Air Force Wright Laboratory contracted with Boeing to study the concept of a proposed Space Maneuver Vehicle (SMV). Boeing signed a subsequent agreement to fabricate a single X-40A − an 80 percent scale demonstrator version of the SMV − with the Air Force Research Laboratory. The agreement included fiscal contributions by NASA, the USAF, and Boeing (to the tune of $174 million in total). It also symbolized an important Air Force ambition: because of the high cost and uneven safety record of the Space Shuttle, the Air Force made it a priority to acquire an unmanned, highly maneuverable, long-duration orbiter of its own, with the capacity to launch satellites. Construction of the X-40A occurred in late 1997 and early 1998. White Sands Missile Range in New Mexico witnessed the first X-40A approach and landing glide flight, which occurred in August 1998. Dropped from a Sikorsky UH-60 Blackhawk helicopter, the X-40A fell about 9,000 feet (2,743 meters), guided by an integrated inertial navigation/global positioning system. It made a near-perfect, hard runway landing after a 90-second journey to Earth. Surprisingly, even at this very early stage in its development, the X-40A came to a crossroads. Air Force officials realized that Boeing had another, similar program under contract to NASA called the X-37A and B; respectively, an in-­ atmosphere test vehicle and an orbiter. Rather than proceed with the X-40A, the USAF decided to loan it to NASA to continue the low-speed flight dynamics tests initiated at White Sands, with the intention of it augmenting the research being conducted by NASA on the similar (and similarly contoured) X-37A. In overall size, the X-40A and the X-37A did not differ significantly. At only 22  feet (6.7 meters) long and 7 feet 3 inches (2.1 meters) in height, the X-40A measured about 5  feet (1.5 meters) shorter and stood 2  feet (0.6 meters) lower than the X-37A.  The

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X-40A  153 wingspans also diverged by very little, at 11  feet 6 inches (3.3 meters) for the X-40A, and another 3 feet 6 inches (0.9 meters) wider for the X-37A. In two areas, however, the X-40A did stand apart: its wing area measured 41.6 square feet (3.9 square meters), while the

X-37A’s covered nearly twice that; and most at odds, the X-40A weighed a mere 2,646 pounds (1,200 kilograms) gross, while the X-37A came in at more than four times that, at roughly 12,000 pounds (5,443 kilograms).

The X-40A arrived at NASA Dryden Flight Research Center in May 2000 and underwent seven missions, each involving release from a CH-47 Chinook helicopter at approximately 15,000 feet. In all cases, the vehicle autonomously located the runway and touched down like an aircraft. In doing so, it proved the algorithms for guidance, navigation, and control; tested the software and the integrated GPS/INS systems; and assessed the aerodynamics of the X-40A. The last mission occurred on May 19, 2001. Once this milestone had been reached, however, everything changed. After it assigned the X-40A to NASA, Air Force authorities watched the parallel development of the X-37 with growing interest. They paid particular attention in September 2004, when the

Defense Advanced Research Projects Agency (DARPA) took over the X-37 from the space agency, making it into a classified program. DARPA then conducted a series of drop tests on the X-37A in 2006. After reviewing these flights, the Air Force acted; in November of that year it adopted NASA’s X-37B (also referred to as the Orbital Test Vehicle, or OTV) as its own, and perfected it over the next four years. These efforts bore fruit in 2010, when the USAF deployed the OTV as a fully operational spacecraft. During six long-­duration, classified missions, it has conducted experiments and completed other defenserelated work. Although cloaked in secrecy, the X-37B has proven to be a major X-plane success; the X-40A, consigned to history.

About the Authors

Michael H. Gorn

Giuseppe De Chiara

Michael H. Gorn is an author and historian who specializes in aeronautics and spaceflight. He earned a doctorate in history from the University of Southern California and served as a federal historian in the U.S. Air Force, the EPA, and NASA for nearly 30 years. In addition to X-Planes, Gorn and Giuseppe De Chiara have collaborated previously on Spacecraft: 100 Iconic Rockets, Shuttles, and Satellites That Put Us in Space (Quarto Publishing Group, 2018). Gorn’s other books include NASA: The Complete Illustrated History (Merrell Publishers, 2005). Michael Gorn is the recipient of the AIAA’s Gardner-­ Lasser Aerospace History Literature Award for his book Expanding the Envelope: Flight Research at NACA and NASA (The University Press of Kentucky, 2001). Gorn and his wife Christine live in Southern California.

Professional aerospace illustrator Giuseppe De Chiara was born in Naples, Italy, in 1968, and later graduated from the University of Naples “Federico II” having earned a degree in Architecture. Giuseppe is an accomplished aerospace professional who began his career as a designer of experimental facilities for Parabolic Flights. He experienced microgravity himself on six such flights with ESA.  He was later assigned to the ISS program as a Training Manager and Operations Leader, and is currently (2021) employed as System Engineer and Technical Manager. In 2003, Giuseppe De Chiara began a second career as a professional aerospace illustrator, with his work appearing in several books and publications. He currently lives with his family in the city of Caserta, in the Campania region of Italy.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 M. H. Gorn, G. De Chiara, X-Planes from the X-1 to the X-60, Springer Praxis Books, https://doi.org/10.1007/978-3-030-86398-2

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Bibliography Selected Sources and Further Reading I. Books The books listed below that are marked with an asterisk (*) concentrate on the X-planes themselves; the others provide context and historical background. Anon: Edwards AFB Then and Now, [Air Force Flight Test Center, 2001]. Benson, Lawrence: Quieting the Boom: The Shaped Sonic Boom Demonstrator and the Quest for Quiet Supersonic Flight, [NASA, 2013]. Dana, William: X-38: Flight Testing the Prototype Crew Return Vehicle*, [NASA, 2005]. De Chiara, Giuseppe and Gorn, Michael: Spacecraft, 100 Iconic Rockets, Shuttles, and Satellites that Put Us in Space, [Quarto, 2018]. Evans, Michelle: The X-15 Rocket Plane: Flying the First Wings into Space*, [University of Nebraska Press, 2013]. Ferguson, Robert: NASA’s First A: Aeronautics from 1958 to 2008, [NASA, 2012]. Gatland, Kenneth and Bono, Phil: Frontiers of Space, [Blandford Press Ltd, 1969].

Gorn, Michael: Expanding the Envelope: Flight Research at the NACA and NASA*, [University Press of Kentucky, 2001]. Hugh Dryden’s Career in Aviation and Space, [NASA, 1996]. NASA: The Complete Illustrated History, [Merrell Publishers, 2005, 2008]. The Universal Man: Theodore von Kármán’s Life in Aeronautics, [Smithsonian Institution Press, 1992]. Hagerty, Jack, and Rogers, Jon: Spaceship Handbook, [ARA Press, 2006]. Hallion, Richard, and Gorn, Michael: On the Frontier: Experimental Flight at NASA Dryden*, [Smithsonian Books, 2002]. Hansen, James: Engineer in Charge: A History of the Langley Aeronautical Laboratory, 1917−1958, [NASA, 1987]. Hunley, J.D.: Prelude to U.S. Space-Launch Vehicle Technology: Goddard Rockets to Minuteman III, [University Press of Florida, 2008]. Toward Mach 2: The Douglas D-558 Program, (ed.), [NASA, 1999]. U.S.  Space-Launch Vehicle Technology, [University Press of Florida, 2008]. Jenkins, Dennis: American X-Vehicles: An Inventory, X-1 to X-50*, [NASA, 2003].

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 M. H. Gorn, G. De Chiara, X-Planes from the X-1 to the X-60, Springer Praxis Books, https://doi.org/10.1007/978-3-030-86398-2

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156 Bibliography

Dressing for Altitude: U.S. Aviation Pressure Suits − Wiley Post to Space Shuttle, [NASA, 2011]. Space Shuttle: The History of the National Space Transportation, The First 100 Missions, [Specialty Press, 2010]. X-planes Photo Scrapbook*, [Specialty Press, 2004].

Hypersonic Technology), [Air Force History and Museums Program, 1998].

Jenkins, Dennis, and Landis, Tony: Hypersonic: The Story of the North American X-15*, [Specialty Press, 2003].

Thompson, Milton, and Peebles, Curtis: Flying Without Wings: NASA Lifting Bodies and the Birth of the Space Shuttle, [Smithsonian Institution Press, 1999].

Libis, Scott: Skystreak, Skyrocket, and Stiletto: Douglas HighSpeed X-Planes*, [Specialty Press, 2005]. Loftin, Laurence: Quest for Performance: The Evolution of Modern Aircraft, [NASA, 1985]. Maisel, Martin, Giulianetti, Demo, and Dugan, Daniel: From Concept to Flight: The History of the XV-15 Tilt Rotor Research Aircraft, [NASA, 2000] Miller, Jay: The X-Planes: X-1 to X-45*, [(Midland Publishing, 2001]. Miller, Ron: Spaceships: An Illustrated History of the Real and Imagined, [Smithsonian Books, 2016]. Dream Machines: An Illustrated History of the Spaceship in Art, Science and Literature, [Krieger, 1993]. Peebles, Curtis: Road to Mach 10: Lessons Learned from the X-43A Flight Research Program*, [NASA, 2008]. Reed, R. Dale: Wingless Flight: The Lifting Body Story, [NASA, 1997]. Rotondo, Louis: Into the Unknown: The X-1 Story*, [Smithsonian Institution Press, 1994] Schweikart, Larry: The Quest for the Orbital Jet: The National AeroSpace Plane Program (1983−1995) (Vol. 3 of The Hypersonic Revolution: Case Studies in the History of

Sloop, John: Liquid Hydrogen as a Propulsion Fuel, 1945−1959, [NASA, 1978].

Van Pelt, Michel: Rocketing into the Future: the History and Technology of Rocket Planes, [Springer-Praxis, 2012]. II. Articles The following are links to articles that informed this book: X-4: X-35: X-35: X-37A: X-44A: X-45A: X-46: X-47: X-49A: X-49A:

h t t p s : / / w w w . a i r s p a c e m a g . c o m / military-­a viation/98-­p ound-­w eakling-­ research-­airplanes-­180951413/ https://www.nytimes.com/2019/08/21/magazine/f35-­joint-­strike-­fighter-­program.html h t t p s : / / w w w . a i r s p a c e m a g . c o m / military-­aviation/13_sep2018-­c ancelled-­ f111b-­1-­180969916/ h ttps://www.globalsecurity.org/space/ systems/x-­37a.htm h ttps://www.flightglobal.com/civil-­u avs/ lockheeds-­s kunk-­works-­r eveals-­m issing-­ link-­in-­secret-­uav-­history/127509.article h ttps://www.airforce-­t echnology.com/ projects/x-­45-­ucav/ https://www.flightglobal.com/boeing-­pins-­ airframe-­hopes-­on-­ucas-­d-­success/75288. article https://www.airforce-­technology.com/projects/x47/ h t t p s : / / d e f e n s e r e v i e w . c o m / piasecki-­x -­4 9a-­s peeedhawk-­c ompound-­ helicopter-­250-­mph-­black-­hawk/ h ttps://verticalmag.com/press-­r eleases/ p i a s e c k i -­s -­x -­4 9 a -­s p e e d h a w k -­v t d p -­ compound-­h elicopter-­c ompletes-­i nitial-­ phase-­one-­flight-­testing-­html/

Bibliography 157 X-49A: https://www.flightglobal.com/boeing-­eyes-­ x-­4 9a-­t echnology-­f or-­a pache-­a ttack-­ helicopter/78215.article X-49A: https://www.airspacemag.com/flight-­today/ h o w -­t h i n g s -­w o r k -­t h r u s t ­vectoring-­45338677/ X-49A: h t t p s : / / w w w . b u s i n e s s i n s i d e r . com/2-­o fficial-­c andidates-­t o-­r eplace-­t he-­ army-­black-­hawk-­helicopters-­2020-­3 X-49A: https://www.popularmechanics.com/military/aviation/a31706694/ blackhawk-­replacement-­helicopter/ X-49A: https://www.globalsecurity.org/military/systems/aircraft/x-­49.htm X-50: https://www.globalsecurity.org/military/systems/aircraft/x-­50.htm X-51A: https://www.space.com/20967-­air-­force-­x-­ 51a-­hypersonic-­scramjet.html X-51A: https://www.space.com/20967-­air-­force-­x-­ 51a-­hypersonic-­scramjet.html X-51A: https://www.globalsecurity.org/military/systems/aircraft/x-­51.htm X-53: https://www.globalsecurity.org/military/systems/aircraft/x-­53.htm X-53: h t t p s : / / w w w . a i r f o r c e m a g . c o m / article/0788afti/ X-55: h ttps://www.compositesworld.com/colu m n s / a d v a n c e d -­c o m p o s i t e -­c a r g o -­ aircraft-­proves-­large-­structure-­practicality X-55: https://www.globalsecurity.org/military/systems/aircraft/acca.htm X-55: https://www.globalsecurity.org/military/systems/aircraft/acca-­specs.htm X-55: https://www.globalsecurity.org/military/systems/aircraft/acca-­program.htm X-56A: h ttps://www.flightglobal.com/lockheed-­ b u i l t -­x -­5 6 a -­c r a s h e s -­a t -­u s a f -­t e s t -­ range/118984.article X-56A: http://www.aero-­news.net/getmorefromann. cfm?do=main.textpost&id=686a3e1c­43c4-­4ed7-­ae66-­20644e2c22c8 X-56A: h ttps://www.aerotechnews.com/blog/ 2018/12/07/x-­56a-­suppresses-­flutter-­with-­ two-­controllers/ X-57: h ttps://robbreport.com/motors/aviation/ nasa-­first-­all-­electric-­airplane-­almost-­ready-­ for-­lift-­off-­1234600417/

X-60A: h ttps://www.airforce-­t echnology.com/ projects/x-­60a-­hypersonic-­flight-­research-­ vehicle/ X-60A: https://www.flightglobal.com/fixed-­wing/x-­60a-­h ypersonic-­t est-­v ehicle-­c ompletes-­ rocket-­hot-­firings/136181.article X-60A: https://www.popularmechanics.com/military/research/a30549935/hypersonic­rocket-­booster/ X-60A: h t t p s : / / w w w . a i r f o r c e m a g . c o m / afrls-­e xperimental-­h ypersonic-­f light-­ testing-­rocket-­passes-­cdr/ X-60A: h ttps://aviationweek.com/defense-­s pace/ hypersonic-­x -­6 0a-­t estbed-­p asses-­g round-­ test-­milestone X-60A: https://www.janes.com/defence-­news/news-­ d e t a i l / u s -­a i r-­f o r c e -­p r ov i d e s -­x -­6 0 a -­ hypersonic-­flight-­test-­details III. Internet Websites A. Federal government websites 1. NASA Armstrong Flight Research Center The Armstrong website features surprisingly detailed capsule descriptions that accompany photographs of 21 different X-planes (as well as dozens of other experimental aircraft). They are in the Dryden Historical Aircraft Photo Collection portion of the Armstrong website, at this link: https://www.dfrc.nasa.gov/Gallery/Photo/index. html Also on the Armstrong website are many informative and reliable press releases on the X-planes. Here, for instance, is a link to one on the X-45A: https://www.nasa.gov/centers/dryden/research/ X45A/index.html Finally, the Armstrong website features the X-Press, an in-house publication produced at Armstrong and its predecessor organizations, going back almost to the beginning of the X-planes. Many of the issues are available online. There is a sample at this link: https://www.nasa.gov/centers/dryden/pdf/147648 main_crossfieldxtra.pdf

158 Bibliography

2. Other NASA websites In addition to Armstrong’s internet media activities, press releases from NASA Headquarters and the other NASA field centers cover many of the X-planes. For example, here is a link to a NASA Headquarters press release on the X-48C. https://www.nasa.gov/home/hqnews/2013/apr/ HQ_13-­105_X-­48C_Final_Flight.html 3. Other federal agencies The U.S. Air Force has published many online fact sheets about the X-planes that it sponsored, an example of which is this one on the X-51A WaveRider: https://www.af.mil/About-­Us/Fact-­Sheets/Display/ Article/104467/x-­51a-­waverider/ B. Corporate websites

curatorial descriptions of the X-planes in their collections. For instance, National Air and Space Museum posts an article and photographs of the X-15: https://airandspace.si.edu/collection-­objects/north-­ american-­x-­15/nasm_A19690360000 D. Independent online sources 1. Encyclopedia Astronautica Although it specializes in spaceflight, this massive and authoritative online presence includes articles related to the space-oriented X-planes. For example, here is an article about the Navaho missile, which grew directly out of the X-10: http://www.astronautix.com/n/navaho.html 2. Government Attic

The websites of major aerospace corporations like Boeing, Northrop Grumman, and Lockheed Martin post press releases on the X-planes that they helped develop. Here is a link to a story on the X-50 on the Boeing website:

This website offers Toward New Horizons in its entirety, the seminal, multi-volume report assembled by Theodore von Kármán and presented to Gen. Henry H. Arnold on World War II aeronautical advancements.

https://boeing.com/news/frontiers/archive/2002/ may/ts_pw.html

https://www.governmentattic.org/TwardNew Horizons.html

Smaller companies often do the same. For instance, Bell-­Textron provides the following link to an article on the X-22A. https://news.bellflight.com/ en-­US/188762-­vintage-­bell-­the-­bell-­x-­22-­aircraft C. Museums Two U.S. aerospace museum websites in particular − the Smithsonian National Air and Space Museum, and the National Museum of the Air Force − provide

3. American Institute of Aeronautics and Astronautics 1. The AIAA publishes technical papers, in some cases on aspects of the X-planes, such as this one on the X-21A: https://arc.aiaa.org/doi/10.2514/3.43672 4. NASA Spaceflight.com Spaceflight.com covers a wide range of NASA activities and history, such as this article on the X-20 Dyna-Soar: https://www.nasaspaceflight.com/2006/01/ the-­story-­of-­the-­dyna-­soar/

Index

A Aerojet Company Air Force X-8, 60 Navy Aerobee Missile, 60 Air Force/Army Air Forces facilities Air Force Flight Dynamics Laboratory, 48, 55, 88, 90, 127 Air Force Research Laboratory, 115, 116, 118, 119, 125, 127, 151 Air Research and Development Command, 38, 81 Edwards Air Force Base, 8, 11, 15, 16, 18, 21, 26, 31, 35, 40–43, 46, 56, 65, 68, 71, 87, 88, 92, 97, 100, 107, 110, 118, 123, 129, 131, 142, 145 Muroc Army Air Base, 2, 21 Wright Field Engineering Division, 2, 30 Armstrong, Neil, NACA/NASA research pilot and astronaut X-1B reaction control flights, 12 X-14 (potential lunar lander), 38 X-15 missions, 19, 68, 71, 75, 78 B Bell Aircraft Bell’s arrival on Muroc Army Air Base, 2 Stanley, Robert (versus Walter Williams), 2 X-1-1, 2 X-1-2, 2 X-1-3, 2 X-1A, 8, 10, 11 X-1B, 11 X-1C, 11, 12 X-1D, 12 X-1E, 15 X-2, 16, 66 X-5, 29, 66 X-9, 23 X-14, 36 X-16, 38 Bensen Aircraft X-25, 48 Boeing Aircraft/Aerospace/Phantom Works X-20 Dyna-Soar Boost/glide versus capsule/rocket, 78, 80, 81 McNamara cancellation, 84 Origins: Eugen Sanger, Irene Bredt, 78 Titan III booster, 81 X-32, 99, 100, 104 X-36, 122

X-37A, B, 144 X-45A Air Force UCAV, 106, 107 X-45B (cancelled), 107 X-45C (cancelled), 107, 109 X-46, 107, 109, 110 X-48B and C Blended Wing Body, 125, 127 X-50A, 113, 114 X-51A, 115 C Convair Division, General Dynamics X-6 nuclear powered aircraft, 32 X-11(Atlas A), 18, 62, 63 X-12 (Atlas B), 18, 63 Crossfield, Scott, NACA/North American research pilot D-558 Skyrocket Mach 2 flight Skyrocket pitch-up research, 68 X-4, 29, 32 X-5, 30–32 X-15 Consultant at North American, 67, 68, 71 Pilot of the first X-15 test flights, 68 D Douglas Aircraft D-558 Skyrocket, 12, 16, 32, 65, 68, 78 D-558 Skystreak, 18 X-3, 18–20 Dryden, Hugh L. Compressibility research in the 1920s, 66 NACA director, 15, 65 Role in converting the NACA’s X-1-2 to the X-1E, 15 Toward New Horizons, 23 E Electric Propulsion X-57 Maxwell Arrival at Armstrong 2019, 131 Flight test objectives, 131 NASA HQ announces initiation in 2016, 131 Empirical Systems Aerospace X-57 prime contractor, 131

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 M. H. Gorn, G. De Chiara, X-Planes from the X-1 to the X-60, Springer Praxis Books, https://doi.org/10.1007/978-3-030-86398-2

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160 F Fuel Efficiency X-48B Blended Wing Body Composites, 125 Merger of flying wing/conventional planform, 125 Remotely piloted, 8.5 percent scale, 125 X-53 Active Aeroelastic Wing NASA, USAF, Boeing partnership, 127 Successor to AFTI F-111 adjustable wing, 127 Software-controlled wing adaptations, 127 X-55A Advanced Composite Cargo Aircraft Demonstrator Air Force Research Laboratory, 116 Dornier Do-328J testbed Bonded structural components replace metal, 117 Faster assembly, 117 Radical weight reduction, 137 Lockheed Skunk Works, 116 X-56A Multi-Technology Testbed (MUTT) Demonstrator Air Force Research Laboratory, 118 Long, high aspect ratio wings, 118, 119 Small RPV, 118 Suppression of wing flutter, 118 G Generation Orbit Launch Services X-60A, 119 Grumman Aircraft X-29, 72–75 X-47A, B, 110 H High Angle of Attack Aircraft X-31A Herbst, Wolfgang, 96 International Test Organization DARPA, USAF, NASA, Navy, Germany, 96, 97 Mock combat versus F/A-18 fighter and F-16, 97 Rockwell International, 96 VECTOR Navy tests, 98 J Joint Strike Fighter Competition (X-32 v. X-35) Commonality and F-111, 99 Differences between X-32 and X-35, 100 Fly-off, 104 Winner of JSF, 98–100, 103, 104 K Kotcher, Ezra, Air Force engineer Air Force objectives for the Bell X-1, 3 Versus the NACA’s X-1 objectives, 7 L Laminar Flow Control Natural laminar flow control, 46 X-21A Suction laminar flow control, 44 Lifting bodies Early NASA lifting bodies HL-10, 85, 87, 88 M2-F1, 2, 3, 85

Index X-20A Dyna-Soar (see “Boeing”) X-23A Air Force SV-5D PRIME, 85 Atlas booster, 87 Lifting body re-entry from space Maneuver tests, 85 Heat shield tests, 85, 87 X-24 A,B,C Joint USAF/NASA project, 87, 88, 90, 92, 94 Martin Marietta, 85 X-24A control problems Air Force SV-5P, 88 X-24B radical redesign NASA Hyper III, 90 Prince of the lifting bodies, 92, 94 Termination of X-24C, 94 X-33 (see “Spacecraft”) Lockheed Aircraft/Lockheed Skunk Works/Lockheed Martin Johnson, Kelly, 38 X-7, 20 X-16 (v. U-2), 38 X-24C lifting body, 94 X-27 (and F-104), 52 X-33, 139 X-35, 99, 100 X-44A (autonomous flying wing), 105 X-55A, 116, 118 X-56A, 118 X-59 Quiet SuperSonic Technology (QUESST) Demonstrator, 130, 133 Lockheed Space and Missiles Company X-17 Warhead and space capsule aerodynamics, 78 X-26, 50 M Martin Marietta X-20 Dyna-Soar Booster, 81 X-23A, 84–87 X-24 A, B, 87–94 Missiles/Rockets X-7 Bomarc testbed, 22, 23 Lockheed, 20–23 Multiple test roles, 20–24 Ramjet, 20–22, 25 X-8 Aerojet, 60, 61 Air Force X-8, 60, 61 Navy Aerobee, 60, 61 Sounding rocket, 60 X-9 Bell aircraft, 23 Shrike, 23, 24 Stand-off air-to-surface missile, 23 Testbed for Rascal, 23 X-10 Cruise missile, 25 North American Aviation, 25 Testbed for Navajo, 23, 26 X-11 Bossart, Karel Jan Tankage as structural support, 63 Convair, 18, 62, 63 Paper testbed for Atlas A, 63

Index X-12 Bossart, Karel Jan, 63 Convair, 18, 63 Paper testbed for Atlas B, 63 X-17 Aerodynamics of re-entry vehicles, 76 High-acceleration launch, 76 High Mach number re-entry Flight tests for capsules and warheads, 78 Lockheed, 76 NACA Langley and Ames re-entry designs, 76 X-60A Demonstrator Low cost suborbital launcher for scramjets, 119 Start-up company Generation Orbit (GO1) Launcher, 119 Mojave Desert Climate/terrain at Muroc Army Air Base/Edwards AFB, 2 Isolation for classified work, 2 N National Advisory Committee for Aeronautics (NACA) facilities Ames Aeronautical Laboratory, 38, 40, 44, 66, 76, 84, 88, 121 High-Speed Flight Research Station, 66 High-Speed Flight Station, 11, 12, 15, 16, 32, 65–67 Langley Memorial Aeronautical Laboratory, 29 Muroc Flight Test Unit, 2 Wallops Island Virginia Station, 16, 61, 66 National Aeronautics and Space Administration (NASA) facilities Ames Research Center, 44, 84, 121 Armstrong Flight Research Cente, 118, 119, 125, 127, 130, 131 Dryden Flight Research Center, 46, 56, 58, 97, 107, 121, 125, 141, 142, 144, 148, 153 Johnson Space Center, 92, 148 Langley Research Center, 58, 90, 123, 125, 131 North American Aviation X-10 cruise missile, 25 X-15 Accidents and incidents, 71 Astronauts, 71, 81 Crossfield, Scott, 29, 32, 67, 68, 71 Origins, 20 Research platform, 81 Testbed for future U. S. spacecraft, 74 Northrop Corp. Flying wing aircraft, 130 HL-10 lifting body, 85, 87, 88 M2-F2, M2-F3 lifting bodies, 85, 87, 88 Northrop, Jack and Theodore von Kármán, 28, 44 X-4, 27–29, 32, 59, 75 X-21A, 44–46 Northrop Grumman Shaped Sonic Boom Demonstrator, 129–134 X-47A, B, 109, 110 O Orbital Sciences Corporation (with Rockwell International) X-34, 121, 141–143, 148 P Piasecki Aircraft X-49A, 113

161 R Rocket Engines XLR11 (X-1 and early X-15 flights), 3, 5, 7, 8, 10, 12, 67, 68, 85, 88, 92 XLR25 (X-2 flights), 16, 17 XLR43 (X-11 and X-12; Atlas A and B), 63 XLR65 (X-9), 23 XLR99 (X-15 flights), 67, 68, 71, 142 Rockwell International X-31A, 96–98 X-34, 121, 123, 141–143, 148 S Scaled Composites X-38, 148–151 X-57, 130–132 Scramjets X-43A Mach 9.7, 124 Microcraft airframe, Pegasus booster, 110, 123, 124, 141 Ramjet to scramjet, 122–124, 138 Scramjet v. standard rocketry, 22, 123 X-51A WaveRider Air Force Research Laboratory, 115, 116, 118, 120, 125, 127, 151 Potential High Speed Strike Weapon, 116 Pratt and Whitney scramjet engine, 34, 40, 52, 97, 103, 105, 110, 115, 116, 138 Surpassed X-43A in flight duration Mach 5.1, 116 Sonic boom mitigation X-54A Grumman F-5 Shaped Sonic Boom Demonstrator, 128–130, 132–134 NASA/Gulfstream collaboration on X-54A, 129, 132 NASA Quick Spike, 129 NASA shifts to X-59, 129, 130, 132 Technology breakthrough, 129, 130 X-59 Quiet SuperSonic Technology (QueSST) Demonstrator LBEV breakthrough, 129 Lockheed Skunk Works, 20, 38, 94, 118 Long nose, slender airframe, 133 Minimal sonic boom predicted, 128–130, 132–134 Spacecraft X-15 (see North American Aviation) X-20 (see Boeing Aircraft/Aerospace/Phantom Works) X-30 Copper Canyon, 16, 63, 135, 136 du Pont, Anthony (Tony), 135, 136, 138 Eight contractors, 138 Reagan’s announcement, 136–138 Redesign, 123, 137, 138 X-33 Challenger accident, 138, 148 Goldin, Daniel, 121, 122, 138, 139, 141, 148 Half-scale demonstrator, 139 Lifting body design, 139 Reusable launch vehicle Single-stage-to-orbit, 139 Termination, 142 Unproven powerplant, 138 X-34 Autonomous, suborbital flights Reusable satellite launcher, 141, 142 “Faster, Better, Cheaper”, 121, 141, 143, 148

162 Spacecraft  (Cont) Initial flight tests: Termination, 121 Orbital Sciences Pegasus booster, 123, 141, 142 X-37 Autonomous, orbital spacecraft, 143–145, 148, 151, 153 DARPA takes X-37 from NASA, 145, 153 Low cost access to space, 148 OTV missions, 144, 145, 153 USAF converts NASA’s X-37B OTV Classified, long duration missions, 145, 153 X-38 Crew return vehicle from ISS, 148, 151 Drop tests/captive carry missions, 151 Johnson Space Center design Like the X-24A lifting body, 148 Terminated (ISS cost overruns), 151 X-40 DARPA takes X-37 from NASA, 153 USAF adopts X-37B rather than the X-40, 153 USAF flies X-37B on classified orbital missions, 153 USAF pursues X-40 Space Maneuver Vehicle Like NASA’s original X-37, 151 Stack, John, NACA engineer D-558 Skystreak transonic research, 8 Langley Compressibility Research Division, 7 X-1-2 transonic research, 7, 8 Stanley, Robert, Bell Aircraft test pilot X-1 flight testing, 7, 8, 27 Supersonic/Transonic flight D-558 Skyrocket Crossfield’s Mach 2 flight Extensive pitch-up research, 10 Sponsor: Navy Bureau of Aeronautics, 10 D-558 Skystreak Control problems Death of pilot Howard Lilly, 8 NACA transonic aircraft, 8 Sponsor: Navy Bureau of Aeronautics, 8 X-1-1 (USAF) Flight program, 3 Mach 1, 5 NACA instrumentation, 3, 5, 7 Yeager, Chuck and Walter Williams, 2, 3, 5, 7 X-1-2 (NACA) Mach 1, 3, 7 Transonic research Stability and control, 5, 7 X-1-3 Destroyed in explosion, 15 X-1A X-1 design modified for high speed and altitude, 8 Yeager’s perilous mission, 8, 10 X-1B Aerodynamic heating research, 11 Reaction control experiments Neil Armstrong pilot, 12 X-1C Cancelled by USAF, 12 Testbed for armament and munitions, 12 X-1D Destroyed in explosion, 15 Mach 2 candidate, 12 X-1E Dryden, Hugh, as financier, 15 Modified X-1-2, 15 Slender airfoil testbed Reduced buffeting, 15

Index X-2 Death of pilot Milburn Apt in Mach 3 attempt, 17 Destruction of X-2-2, death of two Bell pilots, 16 Hypersonic implications Aerodynamic heating and handling, 16 Mach 2.87 flight, 17 X-3 Candidate for sustained Mach 2 flight Dangerous handling qualities, 19 Underpowered turbine engine, 18 Testbed for aerodynamics deficiencies Inertial coupling, 19, 20 Swept wings X-5 variable swept wings Bell Aircraft, 29 Busemann, Adolph, 29 Crossfield, Scott, 30–32 Jones, Robert T., 29 Messerschmitt P.1101, 29, 30 Spin and pitch-up, 30, 32 X-29 forward-swept wings Air Force Flight Dynamics Laboratory, 55 Composite aeroelastic wings, 58 Grumman, 55–58 Junkers Ju.287, 55 Maneuverability, 58 Software control, 58 Wocke, Hans, 55 T Tailless aircraft X-4 Crossfield, Scott, 29, 32 DeHavilland Swallow, 27 Lippisch, Alexander, 27 Messerschmitt Me.163, 27, 29 Porpoising/pitch-up, 29 Transonic lessons, 27–29 X-36 Ames Research Center origins, 121 Fighter demonstrator, 121 High alpha, 121 No tail, lower weight, 122 Pilotless, 28 percent scale, 122 Radar evasion, 121 U Unmanned Combat Air Vehicles (UCAVs) X-45A Demonstrator Boeing design for Air Force, 107 First to demonstrate autonomous weaponry, 106, 107 X-45N Demonstrator Boeing design for Navy, 107 Navy rejection, 107 X-47A, B Demonstrator Northrop Grumman design Accepted by Navy, 109, 110 Suitability for carrier operations, 110 V Vertical/Short Take-Off and Landing (V/STOL) NASA Ames Research Center, 44 V-22 Osprey, 44 X-13

Index Jet-powered VTOL First jet-powered transition from horizontal to vertical, 36 Navy Bureau of Aeronautics program, 34, 35 X-14 Air Force/NASA program, 36, 38 All-purpose VTOL testbed, 36–38 Vectored jet thrust, 36 X-18 Air Force/Navy program, 42 Transport applications, 40–42 Turboprop tilt-wing, 40 Variant: Army XC-142, 42 X-19 Air Force program, 44 Curtiss-Wright concept, 42, 43 Follow-on: XV-15, 44 Turbo shaft tilt-rotor, 43 X-22A Joint military sponsorship, 46 Tilting ducted fan propellers Enclosed propeller housing, 46, 47 Turbine engines, 46, 47 X-49 Army/Navy program, 110 Blackhawk helicopter retrofit, 113 Compound helicopter, 112 Turbo shaft engines, 113 Vector Thrust Ducted Propeller, 112 X-50A DARPA program, 113 Turbofan engine, 114 Rotorcraft/aircraft in one airframe, 113, 114 Unpiloted Canard Rotor/Wing Vehicle, 113 Vietnam War X-25, 48, 50, 54, 55 X-26, 48, 49, 54 X-28, 54, 55

163 von Kármán, Prof. Theodore Aerojet Company, 60 Arnold, General Henry H., 20, 59 Feasibility of Mach 1 flight, 23 Guggenheim Aeronautical Laboratory/Caltech (GALCIT), 28, 60 Jet Propulsion Laboratory, 59, 60 Toward New Horizons, 20, 23, 25, 32, 44, 59 Tsien, H.S., 32, 44 X-8 sounding rocket, 59 W Walker, Joseph, NACA/NASA research pilot Career, 74 X-1E, 15 X-3, 19, 20 X-5, 31, 32 X-15, 19, 74 Williams, Walter, NACA/NASA engineer Versus Chuck Yeager, 3 Housing for the Muroc Flight Test Unit’s employees, 2 Instrumentation on the X-1s and D-558s, 3, 5 Muroc Flight Test Unit, 2 Pinecastle Army Air Field, 3 Versus Robert Stanley, 2, 3 Y Yeager, Chuck, Air Force test pilot Ridley, Captain Jack, 3 Versus Walter Williams and the NACA, 3 World War II service, 3 X-1 Mach 1 flight, 8 X-1A Mach 2+ flight, 10 X-3, 19 X-4, 29