At the Edge of Space

by Milton O. Thompson

  The program had been an extremely productive and successful one up until Mike’s accident. One hundred and ninety flights had been made without a fatality. Admittedly, there had been several accidents and Jack McKay had been injured, but considering the experimental nature of the program, the overall success record was outstanding. For every hazardous flight that I have described, there were ten good, routine ones. Even the hazardous flights produced some good data. The program and its personnel deserved an upbeat ending to the program.

  In the year after Mike’s death, eight more flights were made. Bill Dana and Pete Knight alternated in flying the single remaining flightworthy aircraft. All of these flights except the first were flights to high altitude to carry piggyback experiments into the space environment. There were no significant problems or incidents on any of these flights. The last attempt to fly was made on December 12, 1968. The flight was aborted after takeoff due to bad weather up-range. It was snowing on several of the planned emergency lakebeds. The program ended with a white Christmas.

  The program has been described as the most successful flight research program ever conducted. It produced over 750 research papers and reports. John Becker, one of the early advocates of the program, summarized the program in a paper written in 1968. A list of the accomplishments of the X-15 program, compiled by Becker, included the following:

  • Development of the first large restartable man-rated throttleable rocket engine, the LR-99.

  • First application of hypersonic theory and wind tunnel work to an actual flight vehicle.

  • Development of the wedge tail as a solution to hypersonic directional stability problems.

  • First use of reaction controls for attitude control in space.

  • First reusable superalloy structure capable of withstanding the temperatures and thermal gradients of hypersonic reentry.

  • Development of new techniques for the machining, forming, welding, and heat treating of Inconel-X and titanium.

  • Development of improved high temperature seals and lubricants.

  • Development of the NACA Q-ball hot nose flow direction sensor for operation over an extreme range of dynamic pressure and a stagnation air temperature of 1,900° F.

  • Development of the first practical full pressure suit for pilot protection in space.

  • Development of nitrogen cabin conditioning.

  • Development of inertial flight data systems capable of functioning in a high dynamic pressure and space environment.

  • Discovery that hypersonic boundary layer flow is turbulent and not laminar.

  • Discovery that turbulent heating rates are significantly lower than had been predicted by theory.

  • First direct measurement of hypersonic skin friction and discovery that skin friction is lower than had been predicted.

  • Discovery of hot spots generated by surface irregularities.

  • Discovery of methods to correlate base drag measurements with tunnel test results so as to correct wind tunnel data.

  • Development of practical boost guidance pilot displays.

  • Demonstration of a pilot’s ability to control a rocket boosted aerospace vehicle through atmospheric exit.

  • Development of large supersonic drop tanks.

  • Successful transition from aerodynamic controls to reaction controls and back again.

  • Demonstration of a pilot’s ability to function in a weightless environment.

  • First demonstration of piloted lifting atmospheric entry.

  • First application of energy management techniques.

  • Studies of hypersonic acoustic measurements used to define insulation and structural design requirements for the Mercury spacecraft.

  • Use of the three X-15 aircraft as testbeds carrying a wide variety of experimental packages.

  There were several attractive experiments that were not accomplished in the X-15 program. One, of course, was the ramjet or scramjet demonstration program. The experimental flight engine fell way behind in its development program. It was to have been flown on the modified X-15A-2 aircraft. When that aircraft was damaged on its maximum speed flight, the momentum behind the scramjet program seemed to dissipate. Wind tunnel testing of that engine continued after the X-15 program terminated and it was successfully operated at speeds near Mach 8 in the wind tunnel. It still has not flown, however, and probably never will.

  Another very challenging proposed experiment was a delta wing for the X-15. This wing was to be constructed like a hypersonic cruise vehicle wing. It was to be radiatively cooled and capable of sustained flight at speeds of Mach 8. This experiment never got beyond the conceptual design stage, but it generated a lot of interest. It would have been a very expensive project. It is too bad that it was not proposed early in the program when money and political support were available.

  The X-15 flight program received very little attention or publicity. Both NASA and the USAF were simultaneously involved in much more glamorous space programs. NASA was launching Mercury and Gemini missions and the USAF was trying to launch Dyna-Soar and later the MOL (Manned Orbital Laboratory). The X-15 program and the pilots were, as a result, completely overshadowed in the news media. The program did receive recognition from the aerospace community. Among the many awards received were the Collier Trophy, the Harmon Trophy, the Schilling Award, the John J. Montgomery Award, the Ivan C. Kinchloe Award, the James H. Wyld Award, three Distinguished Flying Crosses, three NASA Distinguished Service Medals, two Octave Chanute Awards, and numerous other NASA and USAF medals and awards. However, few people outside of the aviation community knew anything about the program.

  In some respects, the X-15 was ahead of its time. Data and technology from the X-15 have still not been applied to a subsequent operational aircraft. As far as I know, there are currently no operational hypersonic aircraft. The X-15 is still the fastest aircraft in the world. The space shuttle has flown at higher speeds, but it is a spacecraft, not an aircraft. The proposed National Aerospace Plane will be the first aircraft to fly faster than the X-15 if it survives the current budget cutting exercises.

  This status quo was not foreseen when the X-15 was originally conceived. The country had a much grander vision of the future of aviation. The Dyna-Soar X-20 was to immediately follow the X-15 to expand the usable flight envelope to orbital speeds. A Mach 3 bomber, the B-70, and a Mach 3 fighter, the F-108, were to follow the Mach 2 B-58 and the Mach 2 Century series fighters. In civil aviation, the airlines would soon be flying a Mach 2.7 Supersonic Transport. If the X-15 program faltered, it would be quickly overtaken by more grandiose follow-on programs. We felt we had to expedite the X-15 program to make the data available to all those who would need it to design the next generation of aircraft. We worked three shifts during the X-15 program to stay ahead of the planned advances in aviation.

  Then it all stopped. Progress in aviation did not just slow down, it stopped. The X-20 Dyna-Soar program was cancelled in the spring of 1963. The F-108 was cancelled in 1965. The B-70 was cancelled in 1967. The SST hung on a little longer, but was finally terminated in 1968 by the overzealous environmentalists who prophesied the extinction of the human race if the SST took to the air.

  Progress in aviation then began moving in a different direction. Instead of faster fighters, we developed more maneuverable fighters. These fighters were actually slower in top speed than the century series fighters. We believed a proposed theory that all aerial combat in the future would occur at subsonic speeds and, as a result, we designed and built new transonic fighters. We currently do not have a fighter that can catch the Russian Foxbat.

  The military still has no firm plans to build an operational hypersonic combat aircraft. It is currently working with NASA to develop the National Aerospace Plane, but it is not putting a lot of priority on the program, nor is NASA. Both agencies are willing to spend the money that Congress authorizes for the program, but neither agency is willing to contribute from its own budget.
With this type of support, the program appears to be questionable.

  The military has been unable to define a mission for a hypersonic airplane. Until it does, there will be no operational hypersonic aircraft. The huge cost of developing an airbreathing hypersonic propulsion system will stifle the development of any research or prototype aircraft. There must be a military mission to justify that cost since only the military has sufficient funding to develop a new class of propulsion system.

  An even more discouraging outlook is evident regarding a manned military space mission. There are no plans for a military presence in space, other than the military astronauts assigned to NASA. Again, the military has not defined a manned space mission. They are satisfied that they can do without a military man in space. This is awesome. Only the Russians will know what a military man can contribute in space. We have given up the high ground both in the air and in space.

  Civil aviation has progressed in a similar manner. Transport aircraft have become much more efficient and generally much bigger to capitalize on the passenger miles per pound of fuel, but transports still fly at subsonic speeds and in fact fly 40 to 50 knots slower than they did in the 1960s and 1970s to conserve fuel. The Concorde is still flying but only as a status symbol. There will be no supersonic transport until the military develops an efficient supersonic airbreathing propulsion system.

  Although X-15 data were not applied to follow-on aircraft, they were used in the design and operation of manned spacecraft. Alan Shepard made the nation’s first suborbital space flight on May 5, 1961. During his flight, he was boosted to a maximum speed of 5,180 MPH and reached an altitude of 116 miles above the earth. His flight duration was 15 minutes and 22 seconds from launch to splashdown. He covered a distance of 302 miles during that 15- minute flight. Prior to that flight, the X-15 had flown thirty-six times. It had achieved a maximum speed of 3,074 MPH and a maximum altitude of 169,600 feet or approximately 34 miles above the earth. The X-15 flights were roughly 10 minutes long from launch to landing to cover distances up to 130 miles.

  Some X-15 advocates felt that the X-15 program paved the way for the first Mercury flight and even the follow-on orbital flights. The X-15 did demonstrate a lot of technology that was critical to Mercury, such as the use of reaction controls outside the sensible atmosphere, the use of an inertial guidance system, the use of a side arm controller, the use of a full pressure suit, and the ability of a pilot to control a vehicle under high g accelerations and extended 0 g flight. The X-15 program also verified that we could fly at hypersonic speeds, that actual heating rates were lower than predicted at hypersonic speeds, and that wind tunnel predictions for flight at hypersonic speeds were accurate.

  All of this X-15 experience did not really pave the way for the Mercury program, however. The Mercury spacecraft was being designed and developed before the X-15 reached hypersonic speeds or exoatmospheric flight. Thus the X-15 did not contribute from its experience to the Mercury design. There was obviously some benefit of the X-15 design to the Mercury design since the X-15 designers had to address the problems of space flight and come up with a solution before the Mercury program came into existence.

  The X-15 did, however, provide the Mercury program personnel with a lot of confidence since the X-15 had successfully flown at hypersonic speeds and had also flown outside the sensible atmosphere. The X-15 proved that a spacecraft could successfully fly out of the atmosphere and then execute a controlled reentry into the atmosphere without losing control or burning up during entry.

  The Mercury program quickly surpassed the X-15 flight envelope as Gus Grissom predicted it would to Joe Walker. The X-15 program was eclipsed by the Mercury program and almost totally ignored by the news media. This was a serious blow to the ego of some X-15 personnel, but it was a blessing in another sense. We were free to conduct our research program without a lot of oversight or second guessing. We produced research data instead of headlines. We were doing research 15 years ahead of its application—a good position to be in and one we are not in now.

  Many of the scientific experiments flown on the X-15 were prototype systems for Apollo. Much of the X-15 experience was used in the design and operation of the space shuttle. Indeed, the shuttle is presently the prime benefactor of the X-15 program. I hope someday that there will be a conventional airplane that will cruise at hypersonic speed as a result of X-15 experience. As of now, I am one of the fastest airplane pilots in the world. I am too old for that. Someone younger should have that honor.

  Surprisingly, the United States is the only country that flew a series of rocket research aircraft. The Germans flew some operational rocket aircraft near the end of World War II, but no other nation took advantage of rocket power to expand the flight envelope of manned aircraft. It was a risky and expensive operation. Several aircraft and pilots were lost, but there were some significant advances made in aeronautics as a result of this effort. I believe it was worth the gamble.

  As it turned out, the X-15 was the last of the real exploratory aircraft. It was the last aircraft to probe unknown regions of flight where wind tunnel and other prediction techniques had not been verified. The X-15 probed the last major speed regime, the hypersonic speed regime, and verified the ability of the wind tunnel to accurately predict the flight characteristics of hypersonic aircraft. The space shuttle ultimately demonstrated the validity of these predictions at the high end of the hypersonic speed range.

  The one single individual responsible for the success of the X-15 flight program was Paul Bikle. Bikle was the director of the Dryden Right Research Center from September 1959 through May 1971. Richard Hoerner, the associate administrator of NASA in 1959 handpicked Bikle to succeed Walt Williams when Williams left to join the NASA Space Task Group. At the time he was selected, Bikle was the technical director of the Edwards Flight Test Center. In that position, Bikle had been involved in air force tests of many of the early jet and rocket aircraft. He came to Dryden well equipped for the job. Bikle had previously worked at Wright Field from 1940 to 1951. He flew as a flight test engineer during World War II and participated in the explosive growth of military aviation from biplanes to jets. Bikle had tried to become a pilot with the air corps during the war but he was rejected due to color blindness. He settled for the next best job, in his opinion, flight test engineer. Bikle accumulated over 3,000 flying hours as a flight test engineer and he agreed with my description of that job as the most demanding job that he ever had. He also felt it was the most satisfying job that he ever had. As a result of this experience and his experience at the Edwards Flight Test Center, Bikle was a recognized flight test expert before he came to Dryden.

  He was also a record holding sailplane pilot with over 3,000 hours in sailplanes. He established a world altitude record in sailplanes of 46,620 feet in February 1961. This record remained unbroken for almost 25 years. Bikle established that record without the use of a pressure suit or pressurized cockpit. His only high-altitude equipment was a pressure breathing oxygen system. He had no cockpit heat even though the outside air temperature was -65° F. He had donned an extra pair of cotton khaki pants for the flight, but had no exotic clothing to cope with the brutal temperatures. In some respects, that flight appeared to be a foolhardy attempt by a naive pilot. Bikle’s description of the flight indicated otherwise. It was well thought out and conducted in a relatively careful manner. That is not to say that all of his flights were conducted in that manner. Some of his flying stories raised the hair on the back of the neck. They were really hairy! Bikle also flew powered aircraft, accumulating over 1,100 hours in various propellor aircraft.

  It was very fortuitous that Bikle was available to replace Walt Williams. He was the right man for the job. He knew the flight test business as well as any of the pilots or engineers. He was a strong leader and an excellent judge of people. He exhibited a lot of confidence with no sign of indecisiveness and he could not be intimidated by egotistic pilots, Washington bureaucrats, or pompous generals. There was no
question of who was running the flight program.

  Bikle was not only the center director, he was also the unofficial project manager. He did not, however, try to dominate the flight program. He encouraged individual initiative and promoted friendly competition among the aircraft crews. He stimulated morale through personal contact with every member of the team and participated in many flight parties. He was an ideal leader of the flight test effort and the results verified this.

  There were many other key individuals involved in the flight program such as the project manager, the operations engineers, the flight planners, the research engineers, the instrumentation engineers, the crew chiefs, the many marvelous mechanics and technicians who kept the various systems working, the pressure suit technicians, the B-52 crew, the radar and telemetry engineers and technicians, the machinists, the sheet metal smiths, the welders, and on and on and on. I would love to list the names of these people, but that would require a second volume, and I would surely miss someone. As is the case in any major aircraft or spacecraft flight program, it is these people that make the pilot or the astronaut look good. The pilot gets the glory and the support team is lucky to hear an appreciative word. But that only seems to stimulate the team even more. It is almost unbelievable how hard these people worked to get a flight off. It was common to work overtime, paid or unpaid, to get an aircraft ready for flight the next day.