Category X-15 THE WORLD’S FASTEST ROCKET PLANE AND. THE PILOTS WHO USHERED IN THE SPACE AGE

North American Aviation’s dedicated group of engineers, set up by Vice President Ray Rice and supported later by Vice President Harrison Storms under the direction and leadership of Charlie Feltz, was tasked with designing the X-15.

North American Aviation was founded in 1928 by Clement Keys, a financier noted for aviation companies. In 1934, James H. “Dutch” Kindelberger became president, and he guided the organization through some of its most iconic high­speed airplane designs, such as the P-51 Mustang of World War II and America’s first swept-wing jet fighter, the F-86 Sabre.

In addition to the X-15, North American designed the Apollo Command and Service Module and the Space Shuttle. Through a series of sales and mergers, NAA became part of the Boeing Airplane Company in 1996.

single civilian test pilot, Scott Crossfield of the NAA, achieved all the objectives of the program.

Indeed, the X-15 program was as much about the people involved as it was about the data the airplane was designed to collect, or even the airplane itself. The twelve distinguished test pilots who flew this extraordinary aircraft worked hard to learn its characteristics and idiosyncrasies as well as the unknown character of the new flight regime they were investigating. Truly accomplished aviators and apt students of each mission, they bravely addressed each flight with knowledge gained from long hours at flight simulators and with a detailed flight plan. As with any new airplane, difficulties arose. Equipment problems, design unknowns, and other circumstances caused

problems on many flights, although the X-15 flight-testing program claimed only one life in its nine-year history.

The NASA (NACA) flight research crew at Edwards AFB, now known as Dryden Flight Research Center, was a unique and motivated group that built upon their experience with the X-1 airplanes. The first director of the flight research crew for the X-15 was Walt Williams, who was director of the NACA High Speed Research Section, later to become the NASA Flight Research Center. He was also in charge of the early X-1 research flight tests at Pinecastle, Florida. He and his successor, Paul Bickle, ran a rigorous professional organization that continued research begun in the 1920s, when engineers at Langley Memorial Lab wanted to determine the most desirable characteristics for an airplane, as well as innovations in aircraft design that could make flying better, more effective, and safer.

These questions included what data to measure, how to fly to obtain it, how to measure and record it, and, finally, the commitment to publishing this data for the betterment of the industry.

As an example of this research trajectory, the X-15, with Pete Knight at the controls, reached a Mach number of 6.76 on October 3, 1967. On August 22, 1963, the X-15 had gained an altitude of 354,200 feet, more than 67 miles high, with Joe Walker piloting. These incredible achievements were made possible by the use of a supplementary automatic stabilization system, which the successful X-15 test flights proved was necessary in much of the new flight region. Moreover, the X-15 tests also showed that the thermal protection provided by special materials yielded desired favorable results.

The X-15 featured unique design features, including a rolling tail. Each side of the horizontal tail operated separately in opposite directions to roll the aircraft, eliminating the need for ailerons

on the wings; ailerons would have induced shock waves at supersonic speeds that would have changed the airflow at the tail surfaces. These shock waves, produced at the deflection hinge lines, would have caused local regions of high aerodynamic heating at that location.

To provide longitudinal control, the two sides of the horizontal tail would operate together in the same direction. The airfoil of the vertical tail surfaces was slab-sided, with a blunt trailing edge; this configuration prevented separated flow on the surface and maintained control at supersonic speeds. The new materials included Inconel X, which maintains its strength at high Mach numbers. Also, the structure was designed

to minimize the effects of thermal gradients when the outside aircraft skin got hot and the inside stayed cool. The X-15 proved that each of these innovations was successful.

The X-15 was the third and last of a series of air-dropped rocket-powered aircraft designed to investigate high-speed flight regimes from transonic through supersonic to hypersonic velocities. At the time each airplane was conceived and built, there were inadequate wind tunnel or other test data available to assist in the design for flight at these speeds; or in the case of the X-15, the wind tunnel tests had yet to be validated by flight. The X-1, D-558-2, X-2, and X-15 were the first aircraft to fly at Mach 1, 2, 3.2, and 6.7, respectively.

The X-15 had to fly through all the flight regimes that had been pioneered by the earlier research aircraft before extending its speed and altitude range to include the hypersonic regime. These older research aircraft were essentially conventional configurations, with special design
variations required for their specific mission.

Like its predecessors, the X-15 had to be dropped successfully from a mother ship, which for the X-15 was the B-52. After drop, the X-15 had to accelerate from subsonic speed through Mach 1 with its attendant shock waves, flow changes, and trim changes. It then climbed and accelerated past the maximum speed of the X-2 to explore the hypersonic regime for which it was designed.

The X-15’s rocket engine was a new, much larger version of the RMI rocket engine that powered the X-1 and the X-1A. The new engine needed to increase its thrust from the 6,000 pounds used by the X-1 to the 57,000 pounds required by the X-15’s greater Mach number research goals. The new, larger engine was not ready for the early flights, which instead used two of the 6,000-pound engines, combined for 12,000 pounds of thrust. These placeholder engines allowed early flights to proceed, providing data and experience useful for the continuation of the

X-15; XLR-11 dual rockets. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

program. The full rocket thrust duration was limited by the quantity of fuel carried and lasted approximately 90 seconds. Since the total time of flight on most missions was about 10 minutes, measured from drop from the B-52 to touchdown on the lakebed at EAFB, this meant that the X-15 flew for about 8У2 minutes without any engine power. As in all the rocket research aircraft, the

fuel was exhausted in the accelerating portion of the flights so that deceleration, descent, approach, and landing were all performed without power. While the larger X-15 was modified to carry more fuel, this expanded capacity merely extended the plane’s speed further into the hypersonic range; it did not provide power for landing.

The X-15 program left an important legacy in the development of manned hypersonic flight. It was, and still is, the fastest, highest-flying piloted airplane in history, and there is no new airplane design being planned in the foreseeable future that could do better. The X-15 met all of its design goals, and the results from its research flights allowed the following, among many others:

1) A verification of existing hypersonic aerodynamic theory and hypersonic wind tunnel techniques

2) A study of aircraft structures under the influence of severe, sustained aerodynamic heating

3) An investigation of stability and control problems associated with acceleration to high altitude, and atmospheric entry at hypersonic speeds

4) A study of the biomedical effects of both weightless and high-acceleration flight

The X-15 was an important steppingstone in the development of the Space Shuttle, which was more space vehicle than airplane but which had to experience hypersonic flight through the atmosphere every time it came back to earth.

The spectacular success of the X-15 program is a testimonial to the vision and courage of the engineers and managers who initiated the idea in the first place, the designers who created the vehicle, and the pilots who flew the airplane in the face of many unknowns. It is one of the most important stories in the annals of aviation history in general and aeronautical engineering in particular.

X-15A-2, showing the extra fuel tanks. USAF, Air Force Flight Test Center Hlistory Office, Edwards Air Force Base

1924-1993

Bob Rushworth was the workhorse test pilot for the X-15, with thirty-four flights, more than the next most frequent flyer, Jack McKay, who flew the X-15 for twenty-nine flights. Rushworth set the high-speed record for the X-15-1 (the first X-15) on December 5, 1963, achieving Mach 6.06.

Bob Rushworth was born on October 9, 1924, in Madison, Maine. During World War II, he joined the Army Air Forces and flew C-46 and C-47 transports. He was called back into the Air Force to fly combat missions during the Korean War, after which he made the Air Force his career. He had graduated from Hebron Academy in 1943, and he continued his education at the University of Maine, where he received his bachelor of science degree in mechanical engineering in 1951. He followed this with a degree in aeronautical engineering from the Air Force Institute of Technology (AFIT) in Dayton in 1951. Much later, after completing his service as an X-15 test pilot, he graduated from the National War College at Fort McNair in Washington in 1967.

After receiving his AFIT degree in aeronautical engineering, Rushworth stayed at Wright Field in Dayton to start a flight-test career. In 1956, he was transferred to Edwards Air Force Base, where he graduated from the Experimental Test Pilot School just in time to join the X-15 program in 1958. His first flight in the X-15 was on November 4, 1960, an uneventful pilot-familiarization flight to obtain stability and control, and performance data, at Mach 1.95 at 48,900 feet. Rushworth was

Three X-15s were built and were unofficially labeled by people in the program as Ship 1,

Ship 2, and Ship 3. (This harks back to the early twentieth century when sometimes airplanes were referred to by the name of “ship.”) The official labels of the three X-15s were X-15-1, X-15-2 (later renamed the X-15A-2 after extensive modifications following an accident midway through the flight program), and X-15-3.

to fly thirty-three more times in the X-15, during which he achieved a maximum Mach number in the X-15-1 of 6.06, as noted earlier. Other accomplishments included the first ventral-off flight on October 3, 1961, and the highest dynamic pressure of 2,000 pounds per square foot (an aerodynamic high point that tested the structural integrity of the X-15) on May 8, 1962. When he attained an altitude of 285,000 feet on June 27, 1963, he qualified for Astronauts Wings.

Rushworth encountered numerous problems during his test flights. The right inner windshield
cracked during his Mach 6.06 flight, and it happened again six months later on May 12,

1964, after achieving Mach 5.72 and an altitude of 101,600 feet. On September 29, 1964, after achieving Mach 5.2, the nose gear scoop door came open at Mach 4.5 and 88,000 feet. Later, Rushworth calmly noted that the X-15 handled worse in that configuration than with the nose gear fully extended. On February 17, 1965, his right gear extended at Mach 4.3 at 85,000 feet, his inertial altitude indicator failed, and he momentarily lost engine power 23 seconds into the

flight. Despite all this, he continued with the flight, attaining Mach 5.27 at 95,000 feet and carrying out his test mission of stability and control evaluation, star tracker checkout, and advanced landing dynamics.

Rushworth’s last flight in the X-15 was on July 1, 1966, the 159th flight of the program, and again not without excitement. An indication of no propellant flow from one of the external tanks carried during that flight caused him to eject the external tanks and land prematurely, as he stripped off the top of a camper upon landing at Mud Lake.

Perhaps one of Rushworth’s most important contributions to the X-15 program was on the ground. Milton Thompson notes that because it was not a combat aircraft, the X-15 had low priority within the Department of Defense, and it was mainly due to Rushworth’s efforts that the X-15 schedule was reasonably maintained.

After leaving the X-15 program, Bob Rushworth moved to F-4 Phantom combat crew training at George AFB, and then assignment to Cam Ranh Bay Air Base in Vietnam as the assistant deputy commander for operations with the 12th Tactical Fighter Wing, where he flew 189 combat missions. He returned to the United States in 1969 as program director for the AGM-65 Maverick missile, and he became commander of the 4950th Test Wing at Wright-Patterson AFB in 1971. Two years later, he was inspector general for the Air Force Systems Command, and in 1974 he returned to Edwards as commander of the Air Force Flight Test Center. In 1975, he became commander of the Air Force Test and Evaluation Center at Kirtland Air Force Base in New Mexico. He was promoted to Major General on August 1, 1975. He retired from the Air Force in 1981 as a general and as vice commander of the Aeronautical Systems Division at Wright-Patterson Air Force Base.

On March 18, 1993, Bob Rushworth died of a heart attack in Camarillo, California. He left behind a stellar career as a test pilot and Air Force officer, and his expert handprints are all over the X-15 program.

NEIL A. ARMSTRONG

1930-2012

Neil Armstrong, by virtue of being the first man to step foot on the moon, is known and respected worldwide.

Armstrong was in many ways an anomaly among the X-15 test pilots. Following in the steps of Bob Rushworth, who flew the X-15 a total of thirty-four times, seventh X-15 pilot Armstrong made only seven flights in the airplane. Like Scott Crossfield, Neil Armstrong was first and foremost an aeronautical engineer. Even when he was working with NASA as a test pilot, he was known as one of their best engineering minds. Much later,

he said about himself, “I am, and ever will be, a white-socks, pocket-protector, nerdy engineer— born under the second law of thermodynamics, steeped in the steam tables, in love with free-body diagrams, transformed by Laplace, and propelled by compressible flow.”

Neil Armstrong was born on August 5, 1930, in Wapakoneta, Ohio. His interest in airplanes can
be traced back to the time when his father took him to the Cleveland Air Races when he was only two years old. When he was five, his father took him for his first airplane flight in Warren, Ohio, on a Ford Trimotor. Neil took flying lessons while attending high school, and he earned his flight certificate at the age of fifteen, before he had a driver’s license. He became an Eagle Scout, and for the remainder of his life he was a dedicated supporter of the Boy Scouts. (Among the few personal items that he carried with him to the moon was a World Scout Badge.)

In 1947, Armstrong began studies in aeronautical engineering at Purdue University, but he was interrupted by the Korean War. Armstrong became a Navy pilot, flying F9F Panthers for seventy-eight missions over Korea and achieving the Air Medal, the Gold Star, and the Korean Service Medal, all before the age of twenty-two. After leaving the Navy, he returned to Purdue and received his bachelor of aeronautical engineering degree in 1955. He joined NACA as an experimental research test pilot at the Lewis Flight Propulsion Laboratory in Cleveland, and he then moved to the NACA High Speed Flight Station (now the NASA Dryden Flight Research Center) as an aeronautical research scientist and test pilot. It was there that he attended the University of Southern California, earning a master’s degree in aeronautical engineering. And it was there that he became involved with the X-15 program as a test pilot.

Armstrong’s first flight in the X-15, the usual pilot-familiarization flight, took place on November 30, 1960, when he reached Mach 1.75 and an altitude of 48,840 feet. The upper No. 3 chamber of the rocket engine did not start, and the readout of inertial altitudes was incorrect.

His second flight came nine days later, when he evaluated a new ball nose for the airplane and measured stability and control data. His third flight was not until almost a year later, on

The X-15 was equipped with an air data inertial reference unit (ASIRU), which provided measurements based on air pressure, airspeed, angle of attack and altitude, and measurements based on inertial reference (accelerometer plus computer) of position and altitude. Hence, the altitude of the X-15 was measured using two separate techniques.

Radar data from the ground provided a third measurement of altitude.

(See NASA TM X-51000, The X-15 Flight Test Instrumentation, by Kenneth C. Sanderson, presented at the Third International Flight Test Instrumentation Symposium, Buckinghamshire, England, April 13-16, 1964.)

December 20, 1961, when he carried out the checkout flight of the No. 3 airplane.

On April 20, 1962, Armstrong carried out the longest flight of the X-15 program, a duration of 12 minutes and 28 seconds. On this same flight, he achieved his highest altitude, 207,500 feet. On his return, Armstrong inadvertently pulled too high an angle of attack during pullout. The flight path took a bounce in the atmosphere, and he overshot the Edwards Air Force Base, heading south at Mach 3 and at 100,000 feet. He was able to turn back while over the Rose Bowl in Pasadena. Almost out of kinetic and potential energy, he was just barely able to reach the south end of Rogers Dry Lake at Edwards.

Armstrong’s fastest flight in the X-15 was on July 26, 1962, when he achieved Mach 5.74. This was also his last flight in the airplane, because on September 13 he was selected for the Astronaut Corp by NASA, making him at that time the only civilian pilot in the astronaut program. With that, Armstrong’s career took a dramatic turn, culminating in his steps on the moon. The date was July 21, 1969, less than a year after the X-15 program came to an end.

After his Apollo 11 flight, Armstrong chose not to fly in space again. In 1971, he resigned from NASA and took a position with the University of Cincinnati as the distinguished university professor of aerospace engineering. He taught for eight years and then resigned without explaining his reason for leaving. He withdrew from public life and refused most speaking invitations. On August 7, 2012, in Cincinnati, he underwent bypass surgery for blocked coronary arteries. He died on August 25 from complications. Based on his request, his ashes were scattered in the Atlantic Ocean during a burial-at-sea ceremony aboard the USS Philippine Sea.

n 1933, a young aeronautical engineer at the NACA Langley Laboratory conceived the idea of a research airplane that would be designed, built, and flown strictly for the purpose of probing an unknown flight regime. John Stack, a research engineer working in Langley’s first high-speed wind tunnel, designed a hypothetical research airplane for the single purpose of collecting data in the subsonic flight regime near the speed of sound. In the early 1930s, little was known and understood about flight near the speed of sound. Because the governing flow equations were mathematically nonlinear in this region, no analytical solutions were available to predict the lift, drag, and stability characteristics for airplanes in this transonic regime. (Even today, the only reliable transonic flow solutions are numerical results obtained from computational fluid dynamics [CFD] using massive supercomputers.) In addition, no accurate transonic wind tunnel data could be obtained from existing high-speed tunnels due to adverse aerodynamic interactions between shock waves from the model, reflecting off the wind tunnel walls and impinging back on the model surfaces.

THE BELL X-1, X-1A, AND X-2 AND THEIR RELEVANCE TO THE X-15

In light of the complete lack of accurate theoretical results and wind tunnel data in the transonic regime, Stack turned to the airplane as the best way to study flight near Mach 1. One of his hand – drawn graphs from 1933, found in the John Stack archives at NASA Langley by one of the authors and replete with the original smudges and rusty paperclip marks, shows Stack’s calculations of the power required versus flight velocity for his propeller-driven design.

At the top of the graph, Stack drew a sketch of his research airplane. His ideas got as far as

Volume 1, Number 1 of the brand-new Journal of the Aeronautical Sciences, published by the newly formed Institute of the Aeronautical Sciences (IAS) in 1934 (now the American Institute of Aeronautics and Astronautics [AIAA]). The “Effects of Compressibility on High Speed Flight” both advances the concept and gives the results of his calculations for such an airplane. His idea, however, got no further than the journal at that time. He sent his results to the biannual meeting of the NACA in October 1933, but the committee chose not to help Stack find a developer for the airplane. His work, however, was the genesis of the idea that eventually resulted in the X-15 via three other research airplanes: the X-1, X-1A, and X-2.

John Stack’s hand-drawn graph showing the effects of compressibility on the power required for a high-speed airplane, 1933. NASA Langley Research Center Library, Stack archive file

1932-

Joe Engle is the only test pilot who has flown the two-winged vehicles to go into space, the X-15 and the Space Shuttle. He completed sixteen flights in the X-15 program before being chosen for the NASA astronaut program. His X-15 familiarization flight was on October 7,

1963. In a display of exuberance, at the end of this flight he slow-rolled the X-15 through 360 degrees, shocking the engineers in the control room who thought Engle had a control problem. He was thoroughly chastised by chief pilot Bob Rushworth. In the words of Milt Thompson, who was to be the next X-15 pilot, “Joe went on to

become a straight arrow after the flight.” Indeed, in the eyes of Milt Thompson, Engle was one of the better X-15 pilots.

Joe Engle was born in Abilene, Kansas, on August 26, 1932. He graduated from the University of Kansas at Lawrence in 1955 with a bachelor’s degree in aeronautical engineering. After a brief stint as a flight-test engineer for Cessna Aircraft, he was commissioned through the Air Force ROTC program, earning his pilot’s wings in 1958 and going on to fly F-100s. At that time, Engle had numerous opportunities to fly with then Lt. Col. Chuck Yeager, who in turn recommended Engle for admission to the Air Force Test Pilot School at Edwards. Graduating from the Test Pilot School in 1962, and getting a further recommendation from Yeager as “one of the sharpest pilots we had in the program,” Engle went on to the new Aerospace Research

Engle in the X-15 cockpit with a view of the instrument panel. It’s his first flight in the X-15 (October 7, 1963). USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

Engle standing beside the X-15. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

Engle in the X-15 cockpit for his first flight. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

Pilot School, also at Edwards. The purpose of that school was to train military pilots to be astronauts.

Graduating in 1963, Engle was selected as a project pilot for the X-15 program. After his first X-15 flight in October, he went on to achieve Mach 5.71 on February 2, 1965. Typical of the maturing X-15 research program, on this flight Engle tested a Martin 255 ablative material on the ventral and nose panels, made skin friction measurements, checked out a nose gear modification, and took boundary layer noise data. On June 29, 1965, he reached 280,600 feet, qualifying him for an astronaut rating. His last flight in the X-15 was on October 14, 1965, which was also his third flight above an altitude of 50 miles.

In 1966, Engle was selected for the NASA astronaut program. He was thirty-two years of age, the youngest man to become an astronaut.

He was also the only person in the program to have flown in space, by virtue of his X-15 experience. First assigned to the Apollo program, he was on the support crew for the Apollo 10 before becoming the backup lunar module pilot for Apollo 14.

Since the Apollo program was coming to an end, he moved to the Space Shuttle program. In 1977, he was commander of one of the two crews that conducted atmospheric approach and landing tests with the Space Shuttle Enterprise. In November 1981, he commanded the second flight of the Space Shuttle Columbia (STS-2), during which he intentionally flew manually large portions of the reentry flight path, performing twenty-nine flight – test maneuvers from Mach 25 through landing.

This was the first and only time a winged spacecraft has been manually flown from orbit to landing.

His last flight into space was as commander of the Space Shuttle Discovery (STS-27) in August 1985.

Engle retired from the Air Force as a major general on November 30, 1986. He went on to participate in the Challenger disaster investigation in 1986 and consulted for the shuttle program into the 1990s. He is enjoying his retirement as an aerospace and sporting goods consultant.

One of the more important aspects of the X-15 program was the providing of technical data for the design of the Space Shuttle. Joe Engel was the human link between the two programs, and he represents the rather smooth transition from the X-15 to the success of the Space Shuttle.

In 1939, Ezra Kotcher, an instructor at the U. S. Army Air Corps Engineering School (much later the Air Force Institute of Technology) at Wright Field near Dayton, Ohio, took up the banner for a high-speed research airplane. Like John Stack, Kotcher had come to the conclusion that viable technical data for the supersonic flight regime could be obtained only with a real airplane. In August 1939, after two years of analysis and study, Kotcher wrote a report describing his views on the problems that future aeronautical research and development would face. He concluded that a high-speed research airplane could be powered only by a gas turbine or a

rocket engine; a propeller-driven airplane would encounter insurmountable compressibility problems—shock waves on the blades—that simply could not be overcome. (To this day, no propeller-driven airplane has ever attained sustained supersonic speeds.) His report was reviewed by other engineers at Wright Field, and it eventually landed on the desk of Gen. Hap Arnold, who forwarded it to the NACA Langley Aeronautical Research Laboratory. There it met the same fate as John Stack’s memorandum. War clouds in Europe threatened, and the U. S. Army and the NACA had other pressing business.

By early 1944, the situation had changed completely. Germany was flying the twin-jet Me 262 jet fighter, against which Allied fighters and bombers were virtually helpless. The United States entered the jet age with the Bell P-59, a large, rather cumbersome jet with disappointing performance. Front-line propeller-driven fighters such as the North American P-51 and Republic P-47 flew faster. The Air Force had to face the
reality of flying into the transonic region, where there was no theoretical, wind tunnel, or flight data. Kotcher’s earlier proposal for a high-speed research airplane suddenly received priority attention. In January 1944, the Air Force issued “Confidential Technical Instruction 1568,” initiating a study for the development of an experimental airplane to probe the transonic flight regime. Starting with Kotcher’s original calculations, a small team of aeronautical engineers at Wright Field prepared a concept design of a rocket-powered airplane, soon to be labeled Mach 0.999. This design was vetted at a meeting of Air Force, Navy, and NACA engineers held at the Langley Aeronautical Laboratory in Hampton in mid-May 1944, where Kotcher reported the results of the Wright Field “Mach 0.999” study.

The final link in the development of a transonic research airplane took place in Ezra Kotcher’s office on November 30, 1944, when Robert Woods, Bell Aircraft’s chief of engineering, dropped by for a casual visit and expressed a

general interest in transonic developments. Kotcher seized the moment and shared the results of the “Mach 0.999” project, adding that the Air Force was having some difficulty finding an airplane company with enough time and interest to build such an airplane. Woods said that Bell Aircraft could do the job. The Bell X-1 was born.

The usual method for designing a new airplane is to first look at the previous one and then improve on it. The Bell designers had to start from scratch. Operating in a completely new design
space, Bell went to the Army’s Aberdeen Proving Ground in Maryland to study the aerodynamics of.50-caliber machine gun bullets, which were known to be slightly supersonic. The shape was stable, and the scatter of the bullets was minimal. The shape of the Bell X-1 fuselage is that of a.50-caliber machine gun bullet.

The concept of swept wings for high-speed airplanes originated with German engineer Adolf Busemann in 1935, and extensive wind tunnel research on the aerodynamics of swept wings advanced under German engineers under the shroud of secrecy of World War II. These swept – wing data were uncovered by the surprised Allied scientists who went into the German laboratories in May 1945. The data and its significance, however, were too late to be of direct use to the Bell designers. The Bell X-1 had straight wings.

From pioneering studies of the aerodynamic flow over airfoils at high subsonic speeds by the NACA in the 1930s, it was well known that thin airfoils delayed the formation of shock waves over

the airfoils to higher speeds, thus delaying the adverse compressibility effects of shock-induced flow separation, with the consequent large increase in drag, dramatic loss of lift, and almost instant change in stability characteristics. The wing of the Bell X-1, therefore, had a relatively thin airfoil. The precise airfoil thickness was, however, a compromise. Two wings for the X-1 were designed and utilized: an 8-percent thick wing using an NACA 65-108 laminar flow airfoil, and a 10-percent thick wing using an NACA 65-110 laminar flow airfoil.

The thinner wing was used for flights in which maximum speed was the object. The thicker wing, which would encounter compressibility effects at slower speeds, was used for detailed aerodynamic
research investigations of the physical nature of transonic flow over the wing. In this fashion, the Army could pursue the quest for supersonic speed using the thin wing, and the NACA could pursue its quest for obtaining detailed flight data using the thick wing. Because the Army was paying for the X-1, the early part of the X-1 flight program was focused on obtaining supersonic flight as a goal in itself.

The design of the X-1 set the mold for many of the research aircraft that followed. It was rocket – powered. The engine was especially designed for the X-1 by Reaction Motors and was labeled the XLR11, with a maximum of 6,000 pounds of thrust obtained from a total of four separate chambers. The thrust could be modulated by firing

any one or more of the chambers. The X-1 was air- launched from a B-29 bomber; the alternative of taking off from the ground would have consumed too much fuel and not allowed the airplane to reach transonic speeds. Some researchers in the NACA, John Stack included, argued that the research airplane should be powered by a turbojet, thus allowing ground takeoff. Ezra Kotcher and the Army strongly argued against this scenario, and as mentioned earlier, the Army was putting up the money.

Three X-1 aircraft were manufactured by Bell. The first rolled out the Bell factory door on December 27, 1945, without its rocket engine.

The unpowered X-1 was transported to the Air Force’s Pinecastle Field near Orlando, Florida, for a series of glide tests to examine stability and control characteristics, and to examine low – speed behavior. Carried aloft by a B-29 bomber, the X-1 successfully completed ten glide flights.

In each, the airplane behaved beautifully at low speeds. This airplane was then transported back to Bell’s factory in Niagara Falls, New York, for installation of its rocket engine. The center of activity now shifted to the Muroc Army Air Field in California, where the powered flights were to take place. There, Bell test pilot Chalmers H. “Slick” Goodlin continued flying the X-1, as called for in the contract. The second X-1 was delivered to Muroc on October 7, 1946, followed shortly thereafter by the first X-1. By May 27, 1947, Bell had completed all the contractually required test flights (all subsonic), and the airplanes were turned over to the Army Air Force.

The Army selected Capt. Charles (Chuck) Yeager to be the next test pilot for the X-1. The Army’s first flight, with Yeager at the controls, took place on August 6, when the X-1 was carried aloft by the B-29 carrier aircraft above Muroc for a pilot-familiarization flight. It was the thirty – eighth time that any of the X-1s had taken to the
air. Over the next two months the flight-testing program called for a slow increase in speed, gradually approaching the speed of sound. On October 8, Yeager squeezed the airplane to a Mach number of 0.925; two days later, he flew at Mach 0.997. The fiftieth flight took place on October 14,

1947. Although the flight plan did not officially call for it, Yeager brazenly pushed the X-1 through Mach 1, to Mach 1.06. On that day, aviation history was made. It was the first supersonic flight of a piloted airplane, perhaps the most important event in aviation history since the Wright brothers’ first successful flight at Kitty Hawk on December 17, 1903. Moreover, the flight was smooth with no technical problems. The existing myth of a “sound barrier” had been broken.

The Bell X-1 lived up to its role as the first airplane designed purely for the acquisition of research data. In total, there were 151 flights,

35 of which were supersonic. The highest Mach number reached by the X-1 was 1.45 on March 26,

1948, with Yeager at the controls. The X-1 was the progenitor of the X-15 in several respects. Both airplanes were rocket-powered. The X-1 proved the viability of a rocket engine for achieving high­speed flight at a time when no other powerplant was available to accomplish the mission. Both were air-launched for the same reason, namely

to conserve fuel to enable enough power for a long enough duration to achieve the design Mach number. Ezra Kotcher had argued forcefully for an air launch as opposed to taking off from the ground; he was proven right. This approach carried through to the X-15. The last flight of the X-1 took place on July 31, 1951, piloted by Scott Crossfield, who was also the first pilot to fly the X-15.

Differences in the interests of the three parties involved in the X-1 program were contentious at times. The NACA wanted slow, continuous testing below Mach 1 to fully and safely analyze transonic flow; the Army Air Force wanted to

reach supersonic capability quickly, to develop and build a fighter that would be faster than any enemy; Bell Aircraft wanted to meet its contract requirements and get paid, but also to reach the supersonic flight regime in a timely fashion and thus gain advantage in future procurements. The objectives of all parties were achieved. The NACA did its significant transonic testing and analysis, the Army Air Force had its supersonic airplane, and Bell Aircraft was rewarded for the design, building, and flight testing of the airplane.

1926-1993

Milt Thompson holds the distinction of being the only X-15 pilot to have written a book on the X-15 program. Entitled At the Edge of Space: The X-15 Flight Program, it was published by Smithsonian Institution Press in 1992, a year before Thompson’s death. It is a highly recommended read for anybody interested in the inside story of the X-15 flight program. As the ninth test pilot to join the X-15 program, Thompson flew the airplane fourteen times, beginning on October 29, 1963. On November 27, 1963, the inertials failed at launch. On January 16, 1964, he reached Mach 4.92, but the speed brakes were extremely hard to open during the high aerodynamic heating phase. On February 19, at Mach 5.29, he had a premature burnout due to a clogged liquid oxygen line. His highest Mach number was 5.48, reached on January 13, 1965, during which he lost the pitch-and-roll damping mechanism during the pull-up/roll maneuver after burnout and temporarily lost control. His last flight in the X-15 was on August 25, 1965, when he achieved his highest altitude of 214,100 feet. The technical difficulties encountered by Thompson were typical of those encountered by all of the X-15 test pilots; there were very few totally “good flights” during the 199 flights of the airplane.

Milt Thompson was born on May 4, 1926, in Crookston, Minnesota. He became a naval aviator

at age nineteen and served in China and Japan during World War II. After six years of active duty, he left the Navy and entered the University of Washington, where he graduated with a bachelor’s degree in aeronautical engineering in 1953. Following graduation, like many Washington graduates, he joined the Boeing Aircraft Company as a structural-test and flight-test engineer. He is one of only two X-15 pilots (along with Scott Crossfield) to have worked in the aircraft industry. One of the projects to which Boeing assigned him was testing the new B-52. In March 1956, he seized the opportunity to go to work for the

A lifting body is a wingless aerodynamic configuration that generates its lift from the body at high angle of attack, somewhat like the Space Shuttle. In the period between the X-15 and the Space Shuttle, several “lifting bodies” were designed and flown to explore principally the subsonic characteristics of this hypersonic aerodynamic shape in order to provide data for the subsonic portion of the Space Shuttle flight.

NACA’s High Speed Flight Station at Edwards Air Force Base as a research pilot.

At the time, the NACA had only five pilots, including future X-15 pilots Joe Walker, Jack McKay, and Neil Armstrong. Thompson worked on the early X-airplanes. Of this experience, he admitted that he “watched apprehensively as these programs wound down and were terminated.” He felt that the glory days of the X-airplanes were over and that he had missed it all. “In the next few years,” he later wrote, “I realized that I was wrong. The golden years were still to come.”

For Thompson, those glory years began when he was selected by the Air Force to be the only civilian pilot on the X-20 Dyna-Soar winged hypersonic vehicle project. Although he again witnessed yet another cancelation when the Dyna-Soar project was prematurely stopped, his participation on lifting entry bodies continued.

He was the first person to fly such a lifting body, the lightweight M2-F1. He continued to fly this

aircraft a total of forty-seven times, after which he made the first five flights in the all-metal M2-F2. He took all this experience to the X-15 program.

Thompson finished his active flying career in 1967. Two years later, he became chief of Research Projects, and in 1975 he was appointed chief engineer, a position he held until his death on August 6, 1993.

Exactly one month after Chuck Yeager had made history by breaking the sound barrier in the X-1, the Army Air Force began a new study with Bell

for an airplane to fly at Mach 2. Labeled the X-1A, the new airplane had the same wing and horizontal stabilizer and the same rocket engine as the X-1, but it had a completely new fuselage with a more slender shape (higher fineness ratio and increased propellant storage).

On December 12, 1953, Yeager flew the X-1A to a Mach number of 2.44 at 70,000 feet. This set an unofficial world speed record. During the flight, while at this Mach number and altitude, the airplane suddenly encountered inertial roll-coupling and went out of control. Yeager was knocked semi-conscious in the cockpit as the airplane wildly descended. Fortunately, at 25,000 feet, Yeager was able to regain control. Although not intended to be part of the research flight plan, this was the

X-1A on the lakebed. NASA Dryden Flight Research Center

first time that such roll-coupling at supersonic speeds had been encountered, although it had been predicted earlier by some aerodynamicists. The Air Force subsequently limited the top speed of the X-1A to Mach 2. Yeager, however, was not the first pilot to fly at Mach 2 or higher. This honor went to Scott Crossfield, who flew the swept – wing Douglas D-558-2 Skyrocket to Mach 2 on November 20, 1953.

The last flight of the X-1A was a captive flight in the carrier B-29. On August 8, 1955, as the B-29 was ascending to launch altitude, the X-1A suffered an internal explosion. The pilot, Joe Walker, in an act of heroism, saved himself and the crew of the B-29, but the X-1A was jettisoned and fell to the desert floor. Joe Walker was the second man to fly the X-15.

1929-2004

During the course of his sixteen flights in the X-15, William “Pete” Knight experienced perhaps the most notable event of all the pilots who flew the airplane. On October 3, 1967, he achieved Mach 6.7, the fastest speed attained in the X-15.

By virtue of this flight, Pete Knight still holds today the world’s speed record in a winged, powered aircraft.

On this same flight, the X-15 was coated with a white ablative heat shield. Attached underneath the X-15 was a dummy model of NASA’s high-speed research engine (HRE), part of a research program to develop a supersonic combustion ramjet engine (scramjet). During the course of the test, the shock wave from the engine cowling impinged on the bottom surface of the X-15. The intense aerodynamic heating in the impingement region burned through the attachment pylon, separating the dummy scramjet from the airplane. Had the dummy engine remained attached any longer to the airplane, the shock wave would have burned a hole into the primary structure of the fuselage
and most likely would have resulted in destruction of the X-15 in flight. Moreover, this was the last flight of the X-15A-2. The airplane is now on permanent display in the Air Force Museum at Wright-Patterson Air Force Base in Ohio.

Pete Knight was born on November 18, 1929, in Noblesville, Indiana. At the age of twenty – one, he enlisted in the Air Force, and he obtained his pilot’s wings in 1953. He was assigned to the 438th Fighter-Interceptor Squadron, flying Northrop F-89 Scorpions. While flying the F-89, he entered the National Air Show at Dayton,

Ohio, in 1954 and won the prestigious Allison Jet Trophy, becoming one of the youngest pilots to win the award. He then began his engineering study program, and he graduated from the Air Force Institute of Technology in 1958 with a

bachelor’s degree in aeronautical engineering.

With his career on a fast track, he graduated from the Air Force Test Pilot School that same year. Assigned to Edwards Air Force Base, he was a project test pilot for the F-100, F-101 Voodoo, F-104 Starfighter, T-38, and F-5.

The Air Force recognized Knight’s expert piloting ability by selecting him in 1960 to be one of the six test pilots for the X-20 Dyna-Soar, a winged orbital space vehicle that was an early precursor to the Space Shuttle. The X-20 program was canceled in 1963, but Knight went ahead to complete the Air Force astronaut training program at Edwards Air Force Base. With this background, Pete Knight became the tenth X-15 test pilot, and he had his first flight in the airplane on September 30, 1965. He flew the X-15 sixteen times. On October 17, 1967, he achieved an altitude of 280,500 feet, qualifying him for official astronaut status.

On June 29, 1967, Knight experienced total power failure while going through 107,000 feet at Mach 4.17. All onboard systems shut down.

He coasted to a maximum altitude of 173,000 feet and calmly set up a visual landing approach. He resorted to the old “seat-of-the-pants” flying and glided safely to an emergency landing at Mud Lake, Nevada. For this expert example of flying, he earned a Distinguished Flying Cross.

On July 16, 1968, Knight had a hydraulic gauge malfunction during boost, which required him to push over to an alternate flight profile, which is the planned variation of speed, altitude, and location for the flight of the aircraft. On his glide back to Edwards, he experienced unexpected shaking and vibrations. His last flight in the X-15 was on September 13, 1968; this was the 198th flight of X-15, the next to last flight of the program.

Pete Knight went on to a stellar Air Force career. He went to Southeast Asia in 1969 and completed a total of 253 combat flights in the F-100. His testing career was then extended to the F-15 program at Wright-Patterson Air Force Base as test director; he became the tenth pilot to fly the F-15 Eagle. He then returned to Edwards in 1979 as vice commander of the Air Force Flight Test Center. After thirty-two years of service and more than 6,000 hours in the cockpits of more than a hundred different aircraft, he retired from the Air Force as a colonel in 1982.

Knight became the only X-15 pilot to go into politics. In 1984, he was elected to the city council of Palmdale, California, and he became the city’s first elected mayor four years later. After becoming the fastest airplane pilot in the world, he thus became mayor of the fastest growing city in the United States. He was elected to the California State Assembly in 1992 and to the California State Senate in 1996. Knight achieved widespread public notice as the author of Proposition 22, the purpose of which was to ban same-sex marriage. He continued to serve in the California State Senate, representing the 17th District, until his death on May 7, 2004.

As far back as 1935, German aerodynamicist Dr. Adolf Busemann gave a paper at the fifth Volta Conference in Rome in which he introduced the swept wing as an idea for reducing the drag of supersonic airplanes. The conference topic, “High Velocities in Aviation,” was forward­looking. At that time, the speeds of typical aircraft were in the range of 250 miles per hour, and supersonic aircraft were not yet on the radar. Busemann’s swept-wing idea was virtually ignored by the audience, an “invitation only” crowd that consisted of some of the most important aerodynamicists of the day. Ignored, that is, by everybody except the German Luftwaffe, which classified the concept in 1936—one year after the Volta Conference. During World War II,

the Germans carried out secret, extensive aerodynamic research on swept wings, producing volumes of wind tunnel data, which was subsequently discovered by the Allied intelligence teams that went into German laboratories at the end of the war. Moreover, in 1944, R. T. Jones, one of the NACA’s best aerodynamicists, had independently proved the viability of the swept wing for reducing wave drag on high-speed wings.

This new information on swept wings, nevertheless, was too late for practical use by the designers of the X-1, who had already gone too far in the straight-wing design for the X-1.

But to compensate, the Air Force and Bell signed a new contract on December 14, 1945, for the design, development, and construction of an entirely new swept-wing X-aircraft, the X-2. This research aircraft was designed for speeds above Mach 3, which put it in the flight regime where aerodynamic heating becomes an important consideration. In fact, the investigation of aerodynamic heating at high Mach numbers was one of the principal drivers for the X-2. The skin temperature varies approximately as the square of the Mach number. Everything else being equal, the skin temperature of the X-2 (Mach 3) is nine times higher than that for the X-1 (Mach 1). This required that the X-2 be fabricated from K-monel and stainless steel alloys, rather than aluminum. Also, supersonic aerodynamics of the day dictated that an optimum supersonic airfoil shape be a thin bi-convex (circular arc) with an extremely sharp leading edge. The 40-degree swept wing of the X-2 had a thin, circular arc airfoil and an aspect ratio of 4.

Like its predecessors, the X-2 was rocket – powered. The X-2 contract included the engine, and Bell had designed for 15,000 pounds of thrust provided by two rockets, one with 5,000 pounds of thrust mounted above the other, which had 10,000 pounds of thrust. Each engine was oriented so that the thrust vector of each went through the center of gravity of the airplane. Each engine was throttleable to half, so there would be continuous thrust levels from 2,500 pounds to 15,000 pounds. The Air Force selected Curtiss-Wright, which— with Bell’s approval—took over the development of the rocket motors.

Bell Aircraft was responsible for the installation and testing of the rocket engine and for its operation during flight. Bill Smith, Bell’s chief of rocket engines, personally led these test efforts at Edwards Air Force Base, and he monitored the live static rocket testing on the ground from about 25 yards away from the airplane. (This was a far cry from modern rocket engine testing, which is conducted from concrete block houses to protect the test operators.)

X-2 with its B-50 mother ship, support vehicles, support personnel, support helicopter, and chase planes. NASA Dryden Flight Research Center

The Air Force wanted to involve another company in the aircraft rocket engine business in addition to Reaction Motors, and with the agreement of

Bell, they chose Curtiss-Wright. The Curtiss – Wright Corporation was formed in 1929 from the consolidation of the Wright Aeronautical and Glenn Curtiss companies, combining the two most important aeronautical pioneers in the history of early flight in the United States. Curtiss – Wright produced the famous P-40 during World War II, the airplane that was flown by the Flying Tigers in China. After the war, Curtiss-Wright fell behind in the design of jet airplanes and phased out of the aircraft design field, taking up the production of aircraft components and simulators.

Their excursion into aircraft rocket engines, prompted by the X-2, was transitory.

The decision to go with Curtiss-Wright for the engines ultimately resulted in delays. Although Bell produced two airframes by early 1953, only one engine was available, and that not until early 1953. Ironically, one X-2 was lost due to an engine explosion that took place in the bomb bay of the B-50 carrier aircraft. On May 12, 1953, company test pilot Jean Ziegler participated in a captive test flight that was intended to qualify the liquid oxygen top-off and jettisoning system. (A captive test flight is a flight in which the airplane is carried aloft by the mother plane but never released. The X-2 remained attached to the B-50 for the entire
flight—thus the name “captive” test flight.) When the B-50 was at 30,000 feet over the center of Lake Ontario, an explosion took place. The B-50 was tossed 650 feet upward, and the X-2 disintegrated. Both Ziegler and B-50 observer Frank Wolko were lost. The B-50 immediately returned to its home base. The weather was bad, with a heavy overcast ceiling of about 10 feet. It remained bad for a week, and searches to find the two men by Bell and government aircraft found nothing.

Test flying of the X-2 continued with the second aircraft. From August 8, 1954, to September 27, 1956, a total of seventeen flights were made in

the second X-2. On July 23, 1956, with Air Force Capt. Frank Everest at the controls, the X-2 set a new unofficial world speed record of Mach 2.87. Two months later, on September 27, 1956, the X-2 made aviation history, becoming the first airplane to fly faster than Mach 3. With Air Force Capt. Milburn Apt at the controls, the X-2 set an unofficial world speed record of Mach 3.196. This was Apt’s first flight in the X-2; he flew a near-perfect flight plan to a maximum altitude of 72,000 feet and nosed over, attaining the maximum Mach number at 66,000 feet. However, Apt then put the X-2 in a slight roll, not in the flight plan. The X-2, due to inertia coupling, was unstable in roll at that Mach number and went out of control. The airplane and pilot were both lost, underscoring the dangers of test flying in unknown flight regimes.

1930-

Bill Dana was the eleventh X-15 test pilot. He flew the X-15 sixteen times and was the pilot for the 199th flight, the last of the X-15 program.

Bill Dana was born in Pasadena, California, on November 3, 1930. He attended the United States Military Academy at West Point, graduating with a bachelor of science degree in 1952. He satisfied his military commitment by serving as a pilot in the U. S. Air Force for four years, after which he attended the University of Southern California. At USC, he graduated with a master of science degree in aeronautical engineering in 1958. He began his distinguished civilian career at the Dryden Flight Research Center on October 1, 1958.

This was the first day that NASA went into operation, and Dana proudly became NASA’s first employee. He was involved with the X-15 from that first day, initially as an engineer, then as a chase pilot, and finally as a project pilot. His first X-15 flight was on November 4, 1965, a checkout flight during which he reached Mach 4.22 and an altitude of 80,200 feet. At this point in the X-15 program, even the pilot checkout flights were relatively high – performance. This flight required two relights of the rocket engine. On October 4, 1967, Dana reached his highest speed, Mach 5.53, and on November 1, 1966, he achieved his highest altitude of 306,900 feet, one of two flights he made above 50 miles.

By the end of the X-15 program, Dana was just at the beginning of his distinguished career as a test pilot and aeronautical engineer. Building on his experience flying the X-15, he became a project pilot for NASA’s manned lifting body program, a precursor to the Space Shuttle. He completed one NASA M2-F1, nine Northrop HL-10, nineteen Northrop M2-F3, and two Martin Marietta X-24B flights, for a total of thirty-one lifting body missions. For this work, he received the NASA Exceptional Service Medal.

In 1976, Dana received the Haley Space Flight Award from the American Institute of Aeronautics and Astronautics. In 1986, he became the chief pilot at the Flight Research Center, and he then became the assistant chief of the Flight Operations Directorate. He continued to fly on several important research programs: the F-15 Highly Integrated Digital Electronic Control and the F-18 High Angle of Attack program. In August 1993, Dana became chief engineer of the NASA Dryden Flight Research Center, and he held that position until his retirement in 1998.

After retirement, Dana began a distinguished second career by working as a contractor with the NASA Dryden History Office. He was honored by the Smithsonian’s National Air and Space Museum in 1998 when he was selected to give the Charles A. Lindbergh Memorial Lecture, the most prestigious lecture at the museum. His lecture title was “A History of the X-15.” He still continues to lecture and write papers based on his experience in high-speed flight.