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

WILLIAM HARVEY “BILL” DANA

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.

THE GENESIS OF THE X-15

The first hypersonic vehicles in flight were missiles, not airplanes. On February 24, 1949, a WAC Corporal rocket mounted on top of a captured German V-2 boost vehicle was fired from the White Sands Proving Ground in New Mexico, reaching an altitude of 244 miles and a velocity of 5,150 miles per hour. After nosing over, the WAC Corporal careened back into the atmosphere at over 5,000 miles per hour, becoming the first object of human origin to achieve hypersonic flight. In this same period, a hypersonic wind tunnel capable of Mach 7, with an 11- by 11-inch cross-section test section, went into operation on November 26, 1947, the brainchild of NACA Langley researcher John Becker. For three years following its first run, this wind tunnel was the only hypersonic wind tunnel in the United States. It later provided key data for the design of the X-15.

The real genesis of the X-15, however, was human thinking, not test facilities. On January 8, 1952, Robert Woods of Bell Aircraft sent a letter to the NACA Committee on Aerodynamics in which he proposed that the committee undertake the study of basic problems in hypersonic and space flight. At that time, several X-airplanes were already probing the mysteries of supersonic flight: the X-1, X-1A, and X-2. Accompanying Woods’s letter was a document from his colleague at Bell, Dr. Walter Dornberger, outlining the development of a hypersonic research airplane capable of Mach 6 and reaching an altitude of 75 miles. By June 1952, the NACA Committee on Aerodynamics recommended that the NACA expand its efforts to study the problems of hypersonic manned and unmanned flight, covering the Mach number range from 4 to 10.

After two more years of deliberation, the committee passed a resolution during its October 1954 meeting recommending the construction of a hypersonic research airplane. Among the members of this committee were Walter Williams and Scott Crossfield, who would later play strong roles in the X-15 program. Kelly Johnson, who not only was the Lockheed representative to the committee but was considered to be the country’s most famous airplane designer, opposed any extension of the manned research program, arguing that to date

Подпись: Wright Flyer on its first flight at Kitty Hawk (Kill Devil Hills), North Carolina, December 17, 1903. NASM

image19the research airplane program was “generally unsatisfactory” and had not contributed to the practical design of tactical aircraft. Johnson was the only dissenter; he later appended a minority opinion to the majority report. The spectacular success of the X-15 program and the volumes of hypersonic data it contributed to the design of the Space Shuttle later proved Johnson wrong. The X-15 program was launched.

The X-15 was designed to be, purely and simply, a research vehicle to provide aerodynamic, flight dynamic, and structural response data

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Подпись: INCONEL X

for use in the development of future manned hypersonic vehicles, such as the Space Shuttle.

No hypersonic wind tunnels, past or present, can provide accurate data for the design of a full – scale hypersonic airplane. The frontiers of flight today are the same as they were in the 1950s: the exploration of hypersonic flight. The X-15 will ultimately be viewed as the Wright Flyer of hypersonic airplanes.

The X-15 was the third of a series of research aircraft that were designed specifically to obtain aerodynamic data, beginning with the Bell X-1, the first piloted airplane to fly faster than the speed of sound. The X-1 investigated aircraft behavior primarily in the transonic flight regime. The transonic regime is generally considered to be flight between Mach 0.8 and about 1.3. It begins when air is accelerated to Mach 1 at any local location on the airplane, usually when the airplane is flying at the subsonic airspeed of about Mach 0.8 The second research airplane, the Bell X-1A, investigated supersonic flight to a Mach number of 2.44. This was followed by the Bell X-2, a swept-wing aircraft of stainless steel construction designed to investigate the effects of sweepback and aerodynamic heating to a Mach number of 3.2.

Each of these aircraft, like the later X-15, was rocket-powered and carried aloft to be dropped at an altitude of about 30,000 feet. At these high altitudes, where the air is less dense and the drag is therefore low, the rocket provides maximum acceleration to the airplane following launch. This acceleration is sufficient to allow the airplane to reach the desired speeds and altitudes that allow scientists to study the flight regions between where aerodynamic forces are still useful, and outer space, where they are not, and to study speeds of almost Mach 7, which are solidly in the hypersonic regime.

The X-15 was designed with a very high thrust, 57,000 pounds, provided by an RMI rocket engine with enough fuel for about a minute and a half

Inconel X is a high-temperature alloy of 72.5 percent nickel, 15 percent chromium, and 1 percent columbium, the rest being iron. It has excellent strength at high temperatures, and it was a natural choice for the X-15 because it could withstand the high surface temperatures expected for the hypersonic flight regime up to Mach 7. Inconel X is a registered trademark of the Huntington Alloy Products Division, International Nickel Company, Huntington, West Virginia.

at full thrust. Researchers wanted to know if the analytical calculations and the wind tunnel data accurately predicted the performance, stability, and control of an airplane flying at Mach 7 at very high altitudes (over 250,000 feet); whether the aerodynamic heating at the high Mach numbers is as high as predicted theoretically; and if the Inconel X structure could maintain its strength at high temperatures.

They also wanted to learn whether the directional stability of the aircraft, which decreases at faster supersonic speeds, could be made sufficient by the X-15’s design and by the addition of a stabilization augmentation system (SAS) installed in the airplane. The 199 X-15 test flights evaluated all of these questions.

The risks of flying an airplane designed for testing in an unexplored flight regime are many, both for the known uncertainties and for the unknown. Any research airplane will have a new

Подпись: X-15 in captive flight; picture taken from the B-52. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base
design, new subsystems, and new materials, none of which have yet been tested in flight. Conversely, a new airliner that is intended to fly in a familiar speed range and with a conventional design usually has two years of test flights to prove that it is safe for passengers.

The X-15, designed to investigate hypersonic flight, needed to also fly in the supersonic, transonic, and subsonic regimes, and to land safely on the desert lakebed at about 220 miles per hour. Moreover, on its first flight it had to land after being dropped at altitude without any practice, so that the pilot and research team could learn the response of the airplane to the controls.

A risk example is the X-15’s first flight. Launched at 33,550 feet and without an engine, which otherwise would allow the pilot to go around again
if his approach was not right, the pilot had less than five minutes to learn how to handle the airplane in pitch, roll, and yaw, and to practice a simulated landing at altitude before doing the real thing. On this first flight, a longitudinal instability that caused the airplane to cycle up and down uncontrollably made it dangerously difficult to land. By good piloting, Scott Crossfield was able to touch down on the bottom part of this cycle, avoiding a serious, life-threatening crash. The problem was corrected later by merely resetting a valve.

Each flight was an adventure, with the pilot enduring up to 5 g of acceleration at full thrust for about 90 seconds until the fuel was used up. After burnout, the pilot had to fly, while coasting, to reach the speed and altitude required to conduct the necessary tests. Then the pilot would

X-15 landing. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

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return without fuel or power to the landing site at Edwards Air Force Base, which could be as far away as 300 miles from where the plane was originally dropped.

This history tells a single story, in two parts. The first details the goals and requirements of the X-15 program; the competition for the contract, eventually signed by North American Aviation (NAA) on December 5, 1955; NAA’s design and

Scott Crossfield, suited up prior to a flight. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

Подпись: RMI ROCKETS

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The XLR-11 and XLR-99 are liquid-fueled rocket engines designed by Reaction Motors, Inc., specifically for use with high-speed airplanes. The fuel is anhydrous ammonia, and the oxidizer is liquid oxygen.

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construction of the X-15; and NAA’s flight testing by Scott Crossfield, to show that the aircraft had met its contractual obligations. To demonstrate this achievement, Crossfield had to first fly the X-15 without an engine; then with two of the RMI-provided XLR11 rocket engines of the same type used in the X-1 flights (with 12,000 pounds thrust); and last, when it was ready, with the proposed RMI-provided XLR99 engine of 57,000 pounds thrust, all as specified in the contract.

The second part tells the thrilling story of the talented military pilots and NASA pilots who, under the direction of NASA’s Flight Research Center, were responsible for obtaining the data the X-15 was designed to provide. Eleven NASA and military pilots trained assiduously for each flight, flew each flight, managed the difficulties that arose with the aircraft or the engine in many of the flights, and dealt with crises that often placed them in mortal peril. They, along with the

PILOTS

T

Подпись: Bob White standing beside the X-15. USAF, Air Force Flight Test Center History Office, Edwards Air Force Basehe X-15 program was a success, thanks in no small part to the men who flew the airplane. Each of the X-15 test flights was an example of intense man-machine interaction, and each of the twelve pilots who flew the X-15 were as finely tuned and technologically sophisticated as the machine itself. They set speed and altitude records for a manned airplane that still stand today, and they pioneered new piloting techniques for hypersonic aircraft that were not only adapted for the Space Shuttle but will continue to be used for future manned hypersonic aircraft. The X-15 pilots were brave and professional, venturing into a totally unknown regime of flight, and they helped to write the book on manned hypersonic flight for the next generation.

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All of the X-15 pilots at one time or another were members of the elite NASA Flight Research Center at Edwards Air Force Base. The flight research team was under the direction of Walter C. Williams, who managed a group that planned all the flights, determined what data to acquire, gave the pilots what they needed to obtain the data in an effective and safe manner, and determined how to react in emergencies. Williams and his team were in charge of the flight testing of all the X-airplanes through transonic and supersonic regimes leading up to the X-15, namely the X-1, X1A, D558-2, and the X-2. This center had started out as a small group of about 27 people in 1946 dealing with the X-1 and grew to about 500 at the time of the X-15. These people collectively:

1) Maintained the aircraft, housed, repaired, modified, and prepared the airplane for each flight.

2) Provided for each flight. This included ground crew efforts to ready the airplane, provide the instrumentation, assure the safety for the airplane, provide the chase aircraft and their pilots, and provide emergency gear like the fire trucks and helicopters, as well as the communication links.

3) Provided plans and procedures for each flight, including a detailed pilot checklist for the X-15 and the B-52 mother ship.

4) Provided a flight plan for the X-15 to obtain the requisite data. This sequence included the drop from the B-52, rocket firing and powered flight, climb and transition to level flight, unpowered flight to the speed and altitude required for the data, and finally return to base and landing.

5) Provided a simulation plan to train the pilot for obtaining the data in flight, alternate flight paths to the desired data points if the airplane was over or under the speeds and altitudes planned, and emergency response to various potential problems during the flight. The Flight

Research Center had a special flight simulator designed for the hypersonic regime.

6) Conducted the flights with all the equipment, chase pilots and planes, and communication lines to assist the X-15 pilot to assure safety and performance.

7) Reduced and evaluated the flight data, and utilized the results in future activities.

In September 1959, Walter Williams left the Flight Research Center for the first of many executive positions in the space program, beginning with director of operations for Project Mercury. He was replaced at the Flight Research Center by Paul F. Bikle, who continued Williams’s rigorous professional standards. All the important accomplishments of the X-15 program were performed under Bikle.

The first flight of the X-15 took place on June 8, 1959. Carried aloft under the wing of a B-52, the experimental vehicle was released with its pilot at an altitude of 37,550 feet. Unlike all subsequent X-15 flights, however, there was no roar of the rocket engine. Indeed, there were no propellants aboard; this was intended to be a gliding flight, pure and simple. Its purpose was as a familiarization flight, the first checkout of the flight characteristics of the airplane in its glide down to landing, the response to the control system, the stability of the airplane, the handling of the control forces by the pilot, the response rate of the airplane to the controls, and its motion at touchdown and landing.

Nevertheless, the X-15 reached a speed of Mach 0.79 on its maiden descent to the desert floor. Moreover, as with all the other 198 X-15 test flights, a problem occurred. The airplane began to pitch up and down, a longitudinal oscillation that rapidly increased in amplitude.

The pitch damper designed to avoid this oscillation was discovered to be inoperable. Fortunately, the X-15 touched down safely at the bottom of an

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oscillation, suffering damage only to the landing gear. A. Scott Crossfield, the pilot who had the most influence of all the X-15 pilots on the design and flight performance of the airplane, performed the difficult maneuver. In all other aspects, the plane performed as anticipated by the designers.

MICHAEL J. ADAMS

1930-1967

Mike Adams was the twelfth (and last) pilot in the program, and he was the only pilot to lose his life flying the X-15.

On November 15, 1967, Michael Adams, veteran pilot with six previous X-15 flights, entered the aircraft for a flight to evaluate a guidance display and to conduct several experiments. He had spent more than 21 hours practicing the specifics of this flight in the simulator. The drop at about 10 a. m. and 45,000 feet was normal, and he climbed to 266,000 feet. While the aircraft climbed to higher altitude after launch, an electrical disturbance caused the MH – 96 dampers to trip out. Adams reset the dampers. He then switched the sideslip indicator to a vernier

Подпись: Mike Adams in the X-15 cockpit before his first flight, October 6, 1966. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base
attitude control mode to more accurately control the experiments. He planned to reset this back to indicate yaw angle when returning to base in order to see his sideslip during approach to landing. But this instrument change prevented him from seeing that the airplane was yawing at a critical time in the flight.

After burnout, as he soared upward, he conducted a wing-rocking experiment, in which the rocking became excessive as he approached his peak altitude, 266,000 feet. His yaw had drifted to 15 degrees, and he was unaware of this because his instrument was inadvertently set to show pitch attitude, not yaw. About 15 seconds later, the airplane was yawing wildly and Adams
communicated to Pete Knight that “the airplane seems squirrelly.” He soon after stated that he was in a spin, subjected to high accelerations. Since little was known about the hypersonic spin characteristics of the airplane, the ground crew was not able to offer advice. According to the ground data that was later correlated with the flight data, when Adams recovered, he was yawed 90 degrees, flying upside down, and descending at supersonic speed.

Adams pulled out of the spin, and he probably would have had a successful landing except that the MH-96, the Minneapolis-Honeywell adaptive flight control system, was on and locked in, causing the airplane to oscillate between its limits,

Подпись: Adams suited up and walking to the X-15 for his first flight. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base
up and down, preventing Adams from correcting his attitude and flying his way home. The loads on the airplane built up beyond the structural limits, and the X-15-3 aircraft broke up at approximately 62,000 feet and about 3,800 feet-per-second speed. It crashed to the desert floor near Johannesburg, California. There was talk about Adams having slight vertigo, which may have contributed to his not noticing the yaw buildup or resetting the yaw indicator to the yaw setting.

Adams’s death shows the dangers of flight testing a new aircraft in previously untested regions of flight, and of flying experiments in which certain research-data measuring instruments may have caused an electrical

disturbance that affected the MH-96 from operating at its top quality and in conditions it was not designed for. Any and all these things may have influenced the accident.

Because his flight was above 50 miles high, Adams was posthumously awarded an astronaut rating. For the X-15 program, the tragedy was a blight, but it was the only casualty in 199 flights. Since the objectives for the airplane had been accomplished, the accident was a major reason for the termination of the X-15 program. There were only seven subsequent flights.

Michael Adams was born on May 5, 1930, in Sacramento, California. After graduating from Sacramento Junior College, he enlisted in the Air

Подпись: ROTATIONAL MOTION

The rotational motion of an airplane in flight takes place centered around the airplane’s center of gravity. It is a combination of three rotational directions: the nose up or down rotation, called pitch; the wing rotation about the fuselage, called roll; and the nose swinging right or left, called yaw.

Force in November 1950. The Korean War was in full force at that time, and Adams flew forty – nine combat missions as a fighter-bomber pilot in Korea. In 1958, he earned an aeronautical engineering degree from the University of Oklahoma, and he went on to eighteen months of study at MIT in astronautics. In 1962, he was selected to attend the Experimental Test Pilot School at Edwards Air Force Base. He excelled at the school, winning the Honts Trophy as the best scholar and pilot in his class. In December 1963, he graduated with honors from the Aerospace Research Pilot School. His first flight in the X-15 was on October 6, 1966. On June 8, 2004, a memorial monument to Adams was erected near the crash site, northwest of Randsburg, California.

Test pilots are a special breed. They face risks above and beyond those faced by conventional pilots. The X-15 pilots, however, are in a special class. They were research test pilots, putting their lives on the line to prove the viability of a pioneering hypersonic airplane and to obtain research data on an unknown regime of flight.

This data was invaluable to the subsequent design of the Space Shuttle.

image148On almost every flight of the X-15, some type of technical problem or failure occurred, sometimes multiple problems on the same flight.

Signed photo of six of the X-15 pilots standing beside the X-15. From left to right: Rushworth, McKay, Peterson, Walker, Armstrong, and White. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

 

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X-15 mounted under the wing of the B-52 prior to a flight. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

 

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image152X-15 on the lakebed of Rogers Dry Lake. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

It is remarkable that only one pilot, Mike Adams lost his life during the whole X-15 program of 199 flights. Ten of the twelve had formal college degrees in aeronautical engineering and took pride in their status as dedicated, professional aeronautical engineers. All served at one time or another in the military, and six (Crossfield, Walker, McKay, Armstrong, Thompson, and Dana) were in civilian status when they flew the X-15. Of the career military officers who flew the X-15, three retired as major generals in the Air Force and one as a vice admiral in the Navy.

B-52 flying over the X-15 on the ground. USAF, Air Force Flight Test Center Flistory Office, Edwards Air Force Base

 

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AVIATION

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.

Подпись: F-86. USAF 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.

Подпись: X-15, rear; XRL-99 rocket. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base Подпись: X-15 and HL-10 lifting body. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

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

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Walter C. Williams. NASA

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

Подпись: X-15 in flight an instant after drop. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base
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

Подпись: КЕШ'Подпись: Space Shuttle Columbia, launching. NASAimage32

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

 

AVIATION

 

 

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1921-2006

Scott Crossfield was more than just the first man to fly the X-15; he was the only one of the twelve test pilots who contributed directly to the
airplane’s design and to the design of its flight-test program. Crossfield successfully combined his master’s degree in aeronautical engineering with his exceptional piloting ability and experience to enhance the design and operation of an experimental vehicle that would go far beyond the known atmospheric flight spectrum, to speeds of almost Mach 7 and to altitudes higher than 350,000 feet.

Scott Crossfield was born on October 2, 1921, in Berkeley, California, and attended college at the University of Washington in Seattle, beginning in 1940. The outbreak of World War II interrupted

Подпись: Scott Crossfield in his pressure suit for a preflight Crossfield in the X-15 cockpit. USAF, Air Force briefing. USAF, Air Force Flight Test Center History Flight Test Center History Office, Edwards Office, Edwards Air Force Base Air Force Base
his studies in 1942, when he joined the Navy. After he received his pilot’s wings and ensign’s commission in 1943, the Navy assigned him to be a flight instructor and maintenance officer.

He served in the South Pacific for six months but did not see combat duty. His piloting skills put him at the helm of a Navy aerobatic team, and he flew Corsair fighters for a short period following the war. Crossfield was, however, an aeronautical engineer at heart, and he returned to the University of Washington in 1946 to finish his bachelor’s degree in aeronautical engineering, as well as his M. S., in 1949. During that time, he obtained valuable experience working in the Kirsten Wind Tunnel at Washington.

It was not a good time to graduate with an aeronautical engineering degree; the industry

was suffering from large government cutbacks in defense after World War II. However, the advent of the Korean War in 1950 reversed this situation, and suddenly the aircraft industry was back on its feet. Crossfield found a position as an aeronautical research pilot with the NACA High Speed Flight Station (now the NASA Dryden Flight Research Center) at Edwards Air Force Base in June 1950. The time and opportunity were ripe for Crossfield; over the next five years, he was to fly virtually all the experimental airplanes at Edwards, including the Bell X-1, the delta-wing XF-92, the X-4, the X-5, and the Douglas D-558- 1 Skystreak. On November 20, 1953, he became the first person to fly at Mach 2 while piloting the rocket-powered Douglas D-558-2 Skyrocket to a speed of 1,291 miles per hour in a shallow dive.

Подпись: DOUGLAS D-558-2 Powered by a rocket engine, and developed by Douglas for the U. S. Navy, the Douglas D-558-2 explored transonic and supersonic flight and the flight characteristics of swept-wing supersonic aircraft. Flight tested at the Muroc Flight Test Facility alongside other research aircraft such as the X-1, X-1A, and X-2, the D-558-2 was the Navy’s venture into the mysteries of supersonic flight. Controversy persists as to who deserves credit for the first Mach 2 flight. Crossfield reached Mach 2 in the D-558-2, but in a shallow dive. Just twenty-two days later, Chuck Yeager flew the Bell X-1A to Mach 2.44 in level flight.

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This beautiful, swept-wing airplane now hangs in the Milestones of Flight Gallery at the National Air and Space Museum.

On June 24, 1952, the NACA Committee on Aerodynamics called for an airplane that could probe the unknown problems of flight at Mach numbers between 4 and 10 and at altitudes between 12 and 50 miles. On October 5, 1954, this same committee, in executive session, made the final decision to proceed with this manned hypersonic research airplane, which would eventually become the X-15; Crossfield was a
member of the committee. On May 9, 1955, four aircraft companies submitted proposals to the Air Force (which was paying for the airplane):

Bell, Douglas, North American, and Republic. After North American won the contract, Scott Crossfield left the NACA and joined North American as chief engineering test pilot and design consultant on the X-15.

After piloting the first test flight of the X-15 on June 8, 1959, Crossfield flew the airplane thirteen more times, his last X-15 flight taking place on December 6, 1960—the thirtieth test flight of the X-15 program. At this point, North American finished its contractor check flights and turned the aircraft over to the Air Force. Although Crossfield had expected to fly the X-15 during its entire program, because he was a NAA employee, not a NACA employee, his flight participation in the X-15 came to an end.

Crossfield continued with North American, first as the director responsible for systems tests, reliability engineering, and quality assurance for several aircraft and space vehicles, and then as its technical director, Research Engineering and Test. In 1967, he left the company to serve as a division vice president for Research and Development for Eastern Airlines until 1973, and he then served as senior vice president for Hawker Siddeley Aviation in 1974 and 1975. In 1977, nine years after the X-15 program ended, he became a technical consultant to the House Committee on Science and Technology. He served in this capacity for sixteen years, during which he was a steadfast proponent of manned hypersonic flight. He especially supported the massive U. S. X-30 supersonic combustion ramjet engine-(scramjet) powered single-stage to orbit aerospace plane project during the 1980s and early ’90s. He retired in 1993.

Scott Crossfield earned a number of prestigious awards during his life, including being a joint recipient of the 1961 Collier Trophy, the

Подпись: X-15 at rollout. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

International Clifford B. Harmon Trophy for 1960, the Lawrence Sperry Award for 1954, the Octave Chanute Award for 1954, and the Iven C. Kincheloe Award for 1960. He was inducted into the National Aviation Hall of Fame in 1983 and the International Space Hall of Fame in 1988. As a reflection on his aeronautical engineering accomplishments, the American Institute of Aeronautics and Astronautics elected him to the rank of Honorary Fellow in 1999, the highest recognition in that society.

In 2000, the National Air and Space Museum awarded him its most prestigious award, the Lifetime Achievement Award. An elementary school in Herndon, Virginia, and the terminal of the Chehalis-Centralia Airport in Washington State both bear his name.

On April 19, 2006, Crossfield got into his Cessna 210A to return home from Maxwell Air Force Base in Montgomery, Alabama, where he had just finished giving a speech to a class of young Air Force officers. Amid severe thunderstorms, his airplane broke up in midair; recovery teams found wreckage in three different locations within a quarter-mile region. Later, the National Transportation Board ruled the probable cause of his crash to be a combination of two failures: Crossfield had not obtained updated weather information en route, and the air traffic controller failed to provide adverse-weather avoidance assistance. Crossfield was survived by his wife of sixty-three years, Alice Crossfield, as well as six children and nine grandchildren. He is buried in Arlington National Cemetery.

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Joe Walker in his flight suit going to the X-15. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

 

Crossfield was unique among the X-15 pilots. He always considered himself an aeronautical engineer, although he was also an exceptional test pilot. Being an honorary fellow of the AIAA is indicative of his status within the aeronautical engineering profession. Although he flew the X-15 only fourteen times, never exceeded Mach 2.97 (Flight 26, November 15, 1960), and never flew any higher than 88,116 feet (Flight 6, February 11, 1960), he was arguably the most influential of all the pilots in the X-15 program.

FLIGHTS

RISKS IN RESEARCH AIRCRAFT

When we talk about risk, we mostly mean the life of the pilot, the dangers to the man who governs the airplane through its flight path to the new conditions in flight that the new airplane will investigate. This is the life of a person who is talented, productive, and well experienced in test flying—and a human being unique in his flying abilities in high-speed and high-altitude flight.

Подпись: Bob White after a flight in the X-15. USAF, Air Force Flight Test Center History Office, Edwards Air Force BaseThese characteristics are in addition to all the other attributes that pertain to each person’s life. We also mean the risk to the airplane, which is important enough to have had many years of development, thousands of man-hours of workmanship, and millions of dollars in cost. If the airplane is lost, the research program for which it was designed is jeopardized.

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X-15 at the end of Jack McKay’s flight on May 6, 1966, during which the rocket engine failed after 35.4 seconds. The X-15 landed at Delamar and skidded off the smooth lakebed. McKay was not injured, and the X-15 sustained only slight damage. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

 

This research aircraft was designed to explore regions for flight at altitudes and speeds not yet then achieved. It incorporated a new, advanced design using new materials that allowed it to operate at higher temperatures than previously experienced. It had new and multiple control systems. It used a new rocket engine with a new fuel-oxidizer combination. It had twin skids rather than wheels for a landing gear.

Wind tunnel data existed for the aerodynamics in this new speed region, but it had not been evaluated and confirmed in flight. Moreover, not all the conditions of hypersonic flight that it experienced in the wind tunnel had been previously analyzed or fully understood. Some problems were not known until they were

discovered in flight. Therefore, they could not have been addressed in advance.

Before this aircraft could achieve hypersonic speeds and high altitudes, it still had to traverse all the flight regions previously explored. The new design had to prove that it could safely fly in those known flight regions. For example, it had to be able to take off on its own or be air-dropped in the subsonic regime. It then need to accelerate to high subsonic speeds, go through transonic flight, experience shock waves beginning at Mach 1, and accelerate to supersonic speeds, experiencing stability changes longitudinally, and thereafter in regions of reduced lateral-directional stability with increasing Mach number. It also had to decelerate and return to the landing site with normal

Подпись: Discussion before a flight. USAF, Air Force Flight Test Center Flistory Office, Edwards Air Force Base approach, descent, and landing, all without using thrusting power. It should be noted that although low-speed subsonic flight and landing had been analyzed for the X-15 for these conditions, they were not the primary focus of the design.

The pilots controlled many aspects of the flight, such as the handling and control actions about the three axes of the airplane and the application of thrust. But the pilots could not control other factors, such as the strength of materials at high temperatures and the effect of temperature gradients on the design and strength caused by high aerodynamic heating on the outside and cool internal temperatures.

The characteristics of the X-15 would not be definitively known and understood until verified or determined in actual flight. The handling characteristics in these regions were unique, controlled by the pilot with three different control systems: a traditional stick on the floor between the legs and a rudder; a small control stick on the right console, with power assist or electronic force amplification when experiencing dynamic pressures too high for normal pilot forces; and a rocket power control on the left console for use in space where the air is too thin and the dynamic pressure too low for aerodynamic control surfaces to be effective. In the X-15, the pilot experienced for the first time these new controls, designed for this airplane, following his drop from the B-52 at altitudes of about 40,000 feet and speeds of about Mach 0.8. There were no ground trials with the controls during taxiing or on short hops prior to a real test flight, as is possible while familiarizing oneself with the controls of a conventional aircraft that has wheels rather than skids and that has a jet or reciprocating engine instead of a rocket.

Pilots had to address new interfaces in each new test aircraft. For example, the X-15 was taken aloft by the B-52 and attached under the

B-52’s right wing, unlike the other rocket research aircraft. These previous X-airplanes were attached under the fuselage, allowing the test pilot to ride in the mother craft’s cabin and enter the test aircraft only after everything had been checked out. In the X-15, located out on the wing, the pilot had to enter his aircraft before B-52 takeoff, and he was at risk as the two airplanes climbed to altitude. He had to also check out the X-15 systems while riding in the X-15 after takeoff and prior to drop. He thus had to deal with the interface with the mother airplane mechanically and electronically, including communications, and also operationally by topping off the liquid oxygen and checking other conditions before separation and drop occurred in midair. In

Подпись: Crossfield in discussion after a flight. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base image160
both the X-15 and B-52, the interfaces included the mechanical ties between subsystems and components, each of which had their own requirements, as well as continuity of electrical and electronic signals that signaled to and from the pilot’s cabin and the operating systems, as well as the thermal interfaces that controlled the heating and cooling needs of particular subsystems and components. Repairs could have adversely affected these initial design conditions. An example of this occurred when extra cooling was needed for a component after problems in flight and an extra cooling line was installed to fix the problem. This new cooling line ran alongside an APU hydraulic line that caused its hydraulic fluid to freeze, preventing the APU from functioning.

A conventional aircraft has a shakedown period in which the newly installed subsystems first operate together as a complete aircraft system. Then interfaces with other elements of the airplane are tested mechanically, electrically, and thermally in actual flight conditions when they can be fine- tuned. Experience has shown that many changes

Подпись: DAVID CLARK PRESSURE SUITor improvements are necessary in a new airplane. Routinely, there are bugs to work out, safety issues to resolve, and procedures to establish. For the flight-test program, there is a new support team from management through inspection. For an airliner, it may take two years of testing before it is put into use. There is no such luxury for these high-performance research aircraft. They start out in their very first flight at 40,000 feet in the air. Experience also has shown that unexpected difficulties are uncovered in air-launched research aircraft such as the X-1, X-1A, and X-2, in the increasing velocity regions of transonic and supersonic flight.

Such a flight program is necessarily risky.

This was a new airplane. The old flight regimes in which this plane had to traverse were not the prime focus in design. New equipment, previously untested in flight, was necessary, and the exploration was conducted in a new flight regime to ascertain the validity and shortcomings of the applicable theories, which were approximated with many simplifying assumptions and the use of wind tunnel test data.

Since the X-15 followed the course of the previous X-aircraft, it also had numerous difficulties with equipment—such as the auxiliary power units, landing gear, windshield and cockpit seals, stability in landing, and so forth—that required pilot experience, fortitude, and ingenuity to overcome. In the 199 flights, the problems were frequent, unanticipated, and in many instances life-threatening. It was the piloting excellence, the prior experience of the pilots and engineers, and the extensive preparation for each flight—including hours of simulation—that permitted these many flights to be completed with only one fatality.

Another difference from conventional aircraft testing relates to the lack of any power when the rocket fuel is expended. The fuel is used up in just about 90 seconds of flight. Conventionally

The David Clark full-pressure suit was developed by Dr. David Clark and produced in his small factory in Worcester, Massachusetts. Unlike previous partial-pressure suits that pressurized only parts of the human body, Clark’s full – pressure suit provided pressurization for the whole body. It was made from his patented Link-Net nylon fabric, which consisted of two layers of nylon arranged with opposite bias that provided maximum strength in high-stress areas while also allowing the suit to deform easily to the pilot’s movement. It was lightweight, but it held its shape under pressure. The suits were custom-made for each pilot, who had to make several trips to Worchester for fitting. Clark made improvements to the suit throughout the X-15 program. It became the standard full-pressure suit for the Air Force and NASA, being used by pilots of the U-2 and SR-71 high-altitude spy planes as well as the Space Shuttle astronauts. Several photographs in Chapter 5 show some X-15 pilots in their David Clark full-pressure suits.

powered aircraft can reposition themselves if in trouble or when in descent, approach to landing, and during the landing itself. All X-15 flight positions and corrections have to be done with aerodynamic controls alone, not with power. If the landing approach is too high or too low, the pilot must bring it down safely without power. He cannot go around the field a second time to try again. His first attempt must be successful.

ADVENT OF THE RESEARCH AIRPLANE

I

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.

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X-15 in flight. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

 

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X-15A-2 in captive flight under the B-52. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

 

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

JOSEPH A. WALKER

1921-1966

Подпись:Joe Walker flew the X-15 for his first time on March 25, 1960, during which he achieved Mach

Walker in the cockpit preparing for a flight. USAF, Air Force Flight Test Center History Office,

Edwards Air Force Base

 

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2.0 and an altitude of 48,630 feet. He was the first NASA pilot in the test program.

Joe Walker was born on February 20, 1921, in Washington, Pennsylvania. He graduated with a bachelor of arts degree in physics from Washington & Jefferson College in 1942. He was caught up in the storm of World War II, joined the Army Air Force, and flew P-38 fighters in North Africa, for which he earned the Distinguished Flying Cross and the Air Medal with seven Oak Leaf clusters. In March 1945, he joined the NACA and became involved in the icing research program at the Aircraft Engine Research Laboratory
(now the NASA Glenn Research Laboratory) in Cleveland, Ohio. There, in the words of Milton Thompson, himself an X-15 pilot, Joe Walker “spent many hours droning around in the crappiest winter weather that they could find in the Great Lakes region.” [citation: Milton O. Thompson,

At the Edge of Space: The X-15 flight Program (Smithsonian Institution Press, 1992, p. 4)]

Walker transferred to the NACA High Speed Flight Station (later the Dryden Flight Research Center) in 1951, and his flying skills earned him the position of chief pilot in 1955. He flew as project pilot on some of the early, important
high-speed experimental airplanes, including the Douglas D-558-1 and 2 and the Bell X-1A and X-1E, X-3, X-4, and X-5. During his first year at the NACA High Speed Flight Station, Walker received an NACA medal for heroism. He was in the cockpit of the X-1A mounted in the bomb bay of a B-29 in flight. In preparation for his research flight, he pressurized the X-1A’s propellant tank. An explosion immediately occurred, and Walker passed out. Regaining consciousness as the B-29 crew opened the X-1A canopy and pulled him out, Walker realized that the X-1A had to be deactivated before a bigger explosion occurred. Risking his life, Walker crawled back into the cockpit and depressurized the remaining tanks. The smell of hot peroxide started to fill the B-29. The X-1A now resembled a bomb about ready to go off. Scrambling back into the B-29, Walker decided to jettison the X-1A. The experimental airplane spun down to the desert floor and was destroyed, but the B-29 and its crew returned safely.

In 1959, the NACA became part of the newly formed National Aeronautics and Space Administration. Hence, on March 25, 1960, Walker became the first NASA pilot to fly the X-15. Remarkably, on his first flight, Walker took the X-15 to Mach 2 and an altitude of 48,630 feet. During the course of his remaining twenty – four flights in the X-15, Walker achieved the highest altitude of all the X-15 flights, 354,200 feet on Flight 91, August 22, 1963. This is still the unofficial world record for winged vehicles.

During his twenty-five flights in the X-15, Walker collected data on stability and control, aerodynamic heating, flight performance, aerodynamics, thermostructural response, maximum speed, and maximum altitude characteristics. On Flight 91, in addition to setting the unofficial world altitude record, he obtained data on reentry flight with the ventral fin off, checked out an altitude predictor, and took physical atmospheric measurements with a Barnes spectrometer and a photometer. Collecting this scientific and engineering data was the core of the X-15’s research mission.

After his last X-15 flight on August 22, 1963, Walker continued in his position as chief pilot at the NASA High Speed Flight Station. Prior to his involvement with the X-15, he had logged a number of flights in the Lockheed F-104, the first airplane designed for sustained supersonic flight at Mach 2. It was in this airplane that he first carried out pioneering tests using reaction controls, taking the F-104 to altitudes of 90,000 feet. So it was natural that on June 8, 1966, he chose to pilot an F-104 on a routine photo shoot with the North American XB-70. General Electric had requested some promotional photographs of a family of airplanes powered by GE engines. Flying too close to the XB-70, his F-104 was caught in the trailing vortex of the large airplane and flipped onto the top of the bomber. Walker perished in the ensuing fireball. The XB-70 pilot, Al White, ejected, sustaining serious injury but surviving. Carl Cross, the copilot, was killed.

HIGHLIGHTS OF THE FLIGHTS

What is it like for a research test pilot to fly an X-15 airplane into unknown areas of speed and altitude? He arrives early in the morning, a good time for flight since the winds and temperature are lower at that time in this desert area. He goes to the physiological van at Edwards Air Force Base and there puts on his David Clark full-pressure

suit. He walks across the ramp to the airplanes, the B-52 and X-15. He climbs a large ladder to a platform next to the X-15, and then he enters the small X-15 cockpit. He prepares the airplane and himself for takeoff while the X-15 is attached to the B-52 mother plane.

The B-52 crew goes through a preflight list that includes the location, altitude, and velocity at which the X-15 was to be launched. They then start the engines and check that everything is okay with the pilot, who is captive in the X-15 under the wing. (All this was a much less severe routine than that required by the X-15 pilot in preparation for the flight, but their job to make sure the X-15 was safely launched was just as important.)

The B-52 takes off and climbs to altitude, about 45,000 feet. There the flight crew inside the B-52 prepares for the drop launch of the X-15, going through their checklist and topping off the liquid oxygen in the X-15, some of which has boiled off during the climb to launch altitude.

When all is ready, the B-52 drops the X-15, located underneath its right wing. The X-15 smoothly separates from the mother ship, usually with a roll to the right to compensate for the local airflow located under the right wing of the B-52. The X-15 pilot levels his airplane and lights up his engine. He accelerates away from the B-52 and, once clear, the pilot rotates his airplane to increase the angle of attack for climb to altitude.

Although the primary purpose of the X-15 was the acquisition of research data on the aerodynamics, thermodynamics, and flight dynamics of hypersonic flight, the quest for speed and altitude has been the driving force in the historical advancement of the airplane over the past 120 years. Therefore, obtaining maximum speed and maximum altitude was also important. However, the flight conditions required to obtain maximum speed are different than those to obtain maximum altitude.

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MAXIMUM SPEED

Here, the pilot continues his climb to altitude, then pushes over at zero lift until the airplane is in level flight at the desired altitude. He continues to fly at that altitude at full thrust until the maximum speed is obtained, which occurs when the fuel is used up. Zero lift means that the pilot adjusts the orientation of the airplane relative to the airflow ahead of the airplane (the angle of attack) so that the aerodynamic lift becomes zero, and he holds this until the X-15 is now moving in horizontal flight (level flight).

The airplane then starts to fall back to earth under the force of gravity, and it decelerates as the aerodynamic drag builds up at lower altitudes. During this return to earth, the airplane is in a steep glide, with a plan to reach an altitude of about 35,000 feet with a velocity of 290 to 350 miles per hour (called high key, which was the highest approach to the runway at Edwards Air Force Base). From there, he descends to an altitude of 18,000 feet, flying in the opposite direction of the landing runway (called low key on the flight trajectory). At this point, the airplane is about 4 miles from touchdown. The pilot continues in a 180-degree turn and then lands, probably at a speed of 200 miles per hour.

MAXIMUM ALTITUDE

After launch from the B-52, the X-15 continues to climb until the fuel is used up and then continues in an upward ballistic trajectory, reaching a maximum altitude determined by its kinetic energy at the point of engine burnout and the force of gravity. The airplane then begins to descend. The pilot then heads for home, reaches high key above Edward, descends, and lands as above. Because of the high altitude, the glide return is over a larger distance than the lower-altitude flights. For these flights, the airplane would be dropped at a greater distance from Edwards Air Force Base, sometimes as far as away as 300 miles, so that his glide ends at Edwards.

For most of the X-15 flights, the data gathering was done in the regions bounded by the maximum speed and the maximum altitude flights. The variation of Mach number and altitude during these flights is shown in the two Mach number/ altitude versus time-of-flight figures shown, one for a maximum speed flight and one for a maximum altitude flight.

The data obtained in the hypersonic region of these flights provided vital flight data points that were calibrated against analytical predictions and against wind tunnel data. The designing of aircraft

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ROLLOUT FROM THE NORTH AMERICAN FACTORY

Arrival of the first X-15 to Edwards Air Force Base. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

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Unloading the X-15 upon arrival at Edwards Air Force Base. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

The welcoming crowd upon arrival of the X-15 to Edwards Air Force Base. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

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PREPARATION FOR FLIGHT-UNDER THE WING OF THE B-52

X-15 being mated to the B-52. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

TAKEOFF

 

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Detail of the mating of the X-15 with the B-52 for its first flight with external fuel tanks (empty), November 3, 1965. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

Takeoff of the B-52 with the X-15 with external tanks, November 3, 1965. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

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MATED WITH B-52 IN FLIGHT

 

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X-15 mated with the B-52 for one of its early contractor flights. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

Takeoff of the B-52 with the X-15 mounted under the wing. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

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LAUNCH

X-15 mounted under the wing of the B-52 mother ship at altitude. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

IN FLIGHT

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X-15 in flight after launch. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

LANDING ON LAKEBED AND RESTING ON

X-15 in flight. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

LAKEBED

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X-15 landing with the F-104 chase plane alongside. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

X-15 after landing. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

Подпись: X-15A-2 with external fuel tanks on the ramp of the NASA Flight Research Center at Edwards. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

to fly in these regions, as well as vehicles to return from space, could proceed with confidence by knowing what corrections to make to the analyses and wind tunnel data. This data gathering and its correlation to analysis and wind tunnel results was the purpose of the X-15 research airplane program.

On October 3, 1967, Pete Knight achieved the maximum Mach number for the X-15, and he did it flying the modified version of the X-15, the X-15-A2, with additional fuel in the extended fuel tanks and with extra external fuel tanks. The extra fuel allowed more full thrust time, totaling 141 seconds—50 seconds more than the basic X-15 Nos. 1 and 2. After being in the X-15 for more than an hour under the wing of the B-52 while on the ground, Knight performed the preflight checklist and was lifted when the B-52 took off at 1:20 p. m. They headed for Mud Lake, over which the B-52 dropped him an hour later.

It took two launch attempts before the drop actually worked. Knight stated later that he “reached up and hit the launch switch and immediately took my hand off to [go] back to the throttle and found that I had not gone anywhere. It did not launch.” [citation: Jenkins, X-15: Extending the Frontiers of Flight, NASA SP-2007-562, 1967, p. 459] A second attempt 2 minutes later resulted in a smooth release. Pete then accelerated and climbed at an angle of attack of 12 degrees (angle between the wing chord and the free-stream airflow direction) at high lift until he reached a climb angle (angle between the horizontal and the flight path) of 32 degrees. He leveled off at 102,100 feet and reached a speed of 6,600 feet per second (Mach 6.7). This speed remains the fastest for a manned-powered airplane forty-seven years later, with no competitor airplane in sight.

Then, some unpleasant excitement occurred after burnout. Pete performed some rudder pulses to get data with the yaw damper off. As he decelerated through M=5.5, the “Hot Peroxide” warning light came on. On this particular flight, the X-15 was carrying a dummy supersonic combustion ramjet engine (scramjet) below its fuselage as part of a NASA hypersonic propulsion project. This was not an operating engine; it was a dummy engine being carried under the X-15 to examine the aerodynamic characteristics of the engine shape in full-scale hypersonic flight. The warning was caused by the aerodynamic heating generated by the shock wave from the dummy scramjet impinging on the bottom surface of the X-15. It severely damaged the airplane. Pete jettisoned the remaining peroxide to prevent it from exploding. The dummy scramjet was externally mounted in anticipation of future experiments. Shock waves also impinged on the vertical tail, with some melting and skin rollback.

The hot-peroxide event distracted Knight from energy management of the X-15, and he arrived at high key at supersonic speed rather than the desired, slower, subsonic speed. With this airspeed, the X-15 had too much kinetic energy. Pete then tried to jettison the ramjet, but nothing seemed to happen. He dissipated the excess kinetic energy by flying past the landing site, allowing aerodynamic drag to slow the airplane, and then landed at the proper speed. The dummy ramjet didn’t release at once when jettisoned, and it was later located on the lakebed after some clever reasoning and analysis by Johnny Armstrong of the Flight Planning Group.

Joe Walker flew the maximum altitude flight on August 22, 1963. In his prior flight on July 19, 1963, the maximum altitude planned by NASA for that flight had been 315,000 feet, but he unintentionally overshot that mark and achieved an altitude of 347,800 feet, close to the maximum altitude of 360,000 feet that NASA was ultimately seeking for the X-15. The airplane could go over

400.0 feet, but there was concern about the reentry from that altitude. It was deemed difficult but possible for the pilot to make a successful reentry from there, but NASA set a limit at

400.0 feet. Because of the risks of reentry from higher altitudes, they set the flight at 360,000 feet to allow for the inaccuracies of the engine and the ability of the pilot to hold to the tight limits of controlling the angle of attack.

The flight path was selected, with climb angles and fuel cut-off that were calculated to achieve their goal. The engine thrust could vary from 57,000 pounds to 60,000 pounds, and a difference of 1,500 pounds would result in a 7,500-feet altitude change. One second in fuel cut-off time would result in a 4,000-foot altitude change, and if the climb angle were off by one degree, a 7,500-foot change in altitude would result. The planned maximum altitude of the flight was set at 360,000 feet because it allowed a factor of safety. If some of the slight variations in engine thrust, fuel cut-off time, and climb angle took place, the inadvertent increase in altitude would not take the X-15 to over 400,000, where reentry was more dangerous.

This flight was delayed for about two weeks because of weather and airplane APU problems. The actual launch went well, and Walker stayed close to the flight plan. The propellants were depleted at 176,000 feet at a speed of 5,600 feet per second. The airplane continued to soar upward on a ballistic trajectory to 354,200 feet—two minutes after fuel burnout. At that point, Walker and the X-15 were 67 miles high.

After reaching peak altitude, the airplane headed home, some 306 miles away, and was moving at 5,500 feet per second when it passed through 176,000 feet. This was a mirror image of

Подпись:

Подпись: ASTRONAUT WINGS

its ballistic climb after fuel burnout. The pullout force at 5 g occurred at 95,000 feet, and the pilot maintained the high g pullout in order to level flight at 70,000 feet. The rest of the flight back to landing at Edwards Air Force Base was uneventful. The total time of flight was 11 minutes and 8 seconds. While 67 miles is well above the 50 miles required for the pilot to achieve official astronaut rating, it was not awarded to Joe Walker until forty-two years later, after he had died.

There was only one fatal accident during the whole X-15 flight-test program. On November 15, 1967, Michael Adams lost his life when a possible electrical disturbance affected his flight control

The Air Force pilots who flew the X-15 to altitudes above 50 miles all received Astronaut Wings, but NASA had decided not to give the same award to the civilian pilots who had made the same achievement. This caused controversy within the aerospace community. Finally, NASA reversed this policy, and in a ceremony on August 23,

2005, the three NASA pilots who flew the X-15 above 50 miles—William Dana, Jack McKay, and Joe Walker—were awarded Astronaut Wings.

image180Only Bill Dana was alive at that time to receive the certificate. However, the families of McKay and Walker were present to receive the honor.

Подпись: ABLATIVE COATING

Подпись: Mike Adams in the cockpit of the X-15 (mated to the B-52), in preparation for his first X-15 flight, October 6, 1966. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base Подпись: The X-15A-2 with its ablation coating. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base
image181

system. This, combined with his possible vertigo, caused his X-15 to go out of control and break up at an altitude of approximately 62,000 feet during descent and crash to the desert floor. This flight underscored the risk involved in such flight testing. The details of this flight are given in Chapter 5.