Hypersonics Before the Shuttle: A Concise History of the X-15 Research Airplane - History of the Design, Development, Operations, and Lessons Learned
National Aeronautics and Space Administration (NASA), World Spaceflight News, Dennis R. Jenkins
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CHAPTER 1 - The Genesis of a Research Airplane
CHAPTER 2 - X-15 Design and Development
CHAPTER 3 - The Flight Research Program
CHAPTER 4 - The Legacy of the X-15
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Hypersonics Before the Shuttle: A Concise History of the X-15 Research Airplane
Dennis R. Jenkins
Monographs in Aerospace History Number 18
June 2000
NASA Publication SP-2000-4518
National Aeronautics and Space Administration * NASA Office of Policy and Plans * NASA History Office * NASA Headquarters Washington, D.C. 20546
Preface
Introduction
It is a beginning. Over forty-five years have elapsed since the X-15 was conceived; 40 since it first flew. And 31 since the program ended. Although it is usually heralded as the most productive flight research program ever undertaken, no serious history has been assembled to capture its design, development, operations, and lessons. This monograph is the first step towards that history.
Not that a great deal has not previously been written about the X-15, because it has. But most of it has been limited to specific aspects of the program; pilot's stories, experiments, lessons-learned, etc. But with the exception of Robert S. Houston's history published by the Wright Air Development Center in 1958, and later included in the Air Force History Office's Hypersonic Revolution, no one has attempted to tell the entire story. And the WADC history is taken entirely from the Air Force perspective, with small mention of the other contributors.
In 1954 the X-1 series had just broken Mach 2.5. The aircraft that would become the X-15 was being designed to attain Mach 6, and to fly at the edges of space. It would be accomplished without the use of digital computers, video teleconferencing, the internet, or email. It would, however, come at a terrible financial cost—over 30 times the original estimate.
The X-15 would ultimately exceed all of its original performance goals. Instead of Mach 6 and 250,000 feet, the program would record Mach 6.7 and 354,200 feet. And compared against other research (and even operational) aircraft of the era, the X-15 was remarkably safe. Several pilots would get banged up; Jack McKay seriously so, although he would return from his injuries to fly 22 more X-15 flights. Tragically, Major Michael J. Adams would be killed on Flight 191, the only fatality of the program.
Unfortunately due to the absence of a subsequent hypersonic mission, aeronautical applications of X-15 technology have been few. Given the major advances in materials and computer technology in the 30 years since the end of the flight research program, it is unlikely that many of the actual hardware lessons are still applicable. That being said, the lessons learned from hypersonic modeling, simulation, and the insight gained by being able to evaluate actual X-15 flight research against wind tunnel and predicted results, greatly expanded the confidence of researchers. This allowed the development of Space Shuttle to proceed much smoother than would otherwise have been possible.
In space, however, the X-15 contributed to both Apollo and Space Shuttle. It is interesting to note that when the X-15 was conceived, there were many that believed its space-oriented aspects should be removed from the program since human space travel was postulated to be many decades in the future. Perhaps the major contribution was the final elimination of a spray-on ablator as a possible thermal protection system for Space Shuttle. This would likely have happened in any case as the ceramic tiles and metal shingles were further developed, but the operational problems encountered with the (admittedly brief) experience on X-15A-2 hastened the departure of the ablators.
Many people assisted in the preparation of this monograph. First and foremost are Betty Love, Dill Hunley, and Pete Merlin at the DFRC History Office. Part of this project was assembling a detailed flight log (not part of this monograph), and Betty spent many long hours checking my data and researching to fill holes. I am terribly indebted to her. Correspondence continues with several of the program principals—John V. Becker, Scott Crossfield, Pete Knight, and William Dana. Dr. Roger Launius and Steve Garber at the NASA History Office, and Dr. Richard Hallion, Fred Johnsen, Diana Cornelisse, and Jack Weber all provided excellent support for the project. A. J. Lutz and Ray Wagner at the San Diego Aerospace Museum archives, Tony Landis, Brian Lockett, Jay Miller, and Terry Panopalis also provided tremendous assistance to the project.
Dennis R. Jenkins
Cape Canaveral, Florida
February 2000
The Genesis of a Research Airplane
It was not until the mid-1940s that it became apparent to aerodynamic researchers in the United States that it might be possible to build a flight vehicle capable of hypersonic speeds. Until that time, propulsion systems capable of generating the thrust required for such vehicles had simply not been considered technically feasible. The large rocket engines that had been developed in Germany during World War II allowed concept studies to be initiated with some hope of success.
Nevertheless, in the immediate post-war period, most researchers believed that hypersonic flight was a domain for unmanned missiles. When an English translation of a technical paper by German scientists Eugen Sanger and Irene Bredt was provided by the U.S. Navy's Bureau of Aeronautics (BuAer) in 1946, this preconception began to change. Expanding upon ideas conceived as early as 1928, Sanger and Bredt had concluded during 1944 that a rocket-powered hypersonic aircraft could be built with only minor advances in technology. The concept of manned aircraft flying at hypersonic speeds was highly stimulating to researchers at the National Advisory Committee for Aeronautics (NACA).1 But although there were numerous paper studies exploring variations of the Sanger and Bredt proposal in the late 1940s, none bore fruit and no hardware construction was undertaken at that time. It was from this background, however, that the concept for a hypersonic research airplane would emerge.2
At the time, there was no established need for a hypersonic aircraft, and it was assumed by many that no operational military3 or civil requirement for hypersonic vehicles would be forthcoming in the foreseeable future. The need for hypersonic research was not overwhelming, but there was a growing body of opinion that it should be undertaken.
The first substantial official support for hypersonic research came on 24 June 1952 when the NACA Committee on Aerodynamics passed a resolution to "... increase its program dealing with the problems of unmanned and manned flight in the upper stratosphere at altitudes between 12 and 50 miles,4 and at Mach numbers between 4 and 10." This resolution was ratified by the NACA Executive Committee when it met the following month. A study group consisting of Clinton E. Brown (chairman), William J. O'Sullivan, Jr., and Charles H. Zimmerman was formed on 8 September 1952 at the Langley5 Aeronautical Laboratory. This group endorsed the feasibility of hypersonic flight and identified structural heating as the single most important technological problem remaining to be solved.
An October 1953 meeting of the Air Force's Scientific Advisory Board (SAB) Aircraft Panel provided additional support for hypersonic research. Chairman Clarke Millikan released a statement declaring that the feasibility of an advanced manned research aircraft "should be looked into." The panel member from Langley, Robert R. Gilruth, played an important role in coordinating a consensus of opinion between the SAB and the NACA.
Contrary to Sanger's conclusions, by 1954 it was generally agreed within the NACA and industry that the potential of hypersonic flight could not be realized without major advances in technology. In particular, the unprecedented problems of aerodynamic heating and high-temperature structures appeared to be so formidable that they were viewed as "barriers" to sustained hypersonic flight.
Fortunately, the successes enjoyed by the second generation X-1s and other high-speed research programs had increased political and philosophical support for a more advanced research aircraft program. The large rocket engines being developed by the long-range missile (ICBM) programs were seen as a way to provide power for a hypersonic research vehicle. It was now agreed that manned hypersonic flight was feasible. Fortunately, at the time there was less emphasis than now on establishing operational requirements prior to conducting basic research, and perhaps even more fortunately, there were no large manned space programs with which to compete for funding. The time was finally right for launching a hypersonic flight research program.6
The specific origins of the hypersonic research program occurred during a meeting of the NACA inter-laboratory Research Airplane Panel held in Washington, DC, on 45 February 1954. The panel chairman, Hartley A. Soule, had directed NACA research aircraft activities in the cooperative USAF-NACA program since 1946 and was well versed in the politics and personalities involved. The panel concluded that a wholly new manned research vehicle was needed, and recommended that NACA Headquarters request detailed goals and requirements for such a vehicle from the research laboratories.
In responding to the NACA Headquarters, all of the NACA laboratories set up small ad hoc study groups during March 1954. Langley had been an island of hypersonic study since the end of the war and chose to deal with the problem in more depth than the other laboratories. After the new 11-inch hypersonic wind tunnel at Langley became operational in 1947, a research group headed by Charles H. McLellan was formed to conduct limited hypersonic research.7 This group, which reported to the Chief of the Langley Aero-Physics Division, John V. Becker, provided verification of newly developed hypersonic theories while investigating such important phenomena as hypersonic shock-boundary-layer interaction. The 11-inch tunnel later served to test preliminary design configurations that led to the final hypersonic aircraft configuration. Langley also organized a parallel exploratory program into materials and structures optimized for hypersonic flight.
Given this, it was not surprising that a team at Langley was largely responsible for defining the early requirements for the new research airplane. The members of the Langley team included Maxim A. Faget in propulsion; Thomas A. Toll in configuration, stability, and control; Norris F. Dow in structures and materials; and James B. Whitten in piloting. All four fell under the direction of Becker. Besides the almost mandatory elements of stability, control, and piloting, a fourth objective was outlined that would come to dominate virtually every other aspect of the aircraft's design— it would be optimized for research into the related fields of high-temperature aerodynamics and high-temperature structures. Thus it would become the first aircraft in which aero-thermo-structural considerations constituted the primary research problem, as well as the primary research objective.
The preliminary specifications for the research aircraft were surprisingly brief: only four pages of requirements, plus six additional pages of supporting data. A new sense of urgency was present: "As the need for the exploratory data is acute because of the rapid advance of the performance of service aircraft, the minimum practical and reliable airplane is required in order that the development and construction time be kept to a mini-mum."8 In other versions of the requirements this was made even more specific: "It shall be possible to design and construct the airplane within 3 years."9 As John Becker subsequently observed, ". it was obviously impossible that the proposed aircraft be in any sense an optimum hypersonic configuration."
In developing the general requirements, the team developed a conceptual research aircraft that served as a model for the eventual X-15. The aircraft they conceived was ". not proposed as a prototype of any of the particular concepts in vogue in 1954 ... [but] rather as a general tool for manned hypersonic flight research, able to penetrate the new regime briefly, safely, and without the burdens, restrictions, and delays imposed by operational requirements other than research." The merits of this approach had been convincingly demonstrated by the successes of the X-1 and other dedicated research aircraft of the late 1940s and early 1950s.10
Assuming that the new vehicle would be air launched like the X-1 and X-2, Langley established an aircraft size that could conveniently be carried by a Convair B-36, the largest suitable aircraft available in the inventory. This translated to a gross weight of approximately 30,000 pounds, including 18,000 pounds of fuel and instrumentation.11 A maximum speed of 4,600 mph and an altitude potential of 400,000 feet were envisioned, with the pilot subjected to approximately 4.5g (an acceleration equal to 4.5 times the force of gravity) at engine burnout.12
The proposed maximum speed was more than double that achieved by the X-2, and placed the aircraft in a region where heating was the primary problem associated with structural design, and where very little background information existed. Hypersonic aerodynamics was in its infancy in 1954. The few small hypersonic wind tunnels then in existence had been used almost exclusively for fluid mechanics studies, and they were unable to simulate either the high temperatures or the high Reynolds numbers of actual flight. It was generally believed that these wind tunnels did not produce valid results when applied to a full-scale aircraft. The proposed hypersonic research airplane, it was assumed, would provide a bridge over the huge technological gap that appeared to exist between laboratory experimentation and actual flight.13