NASA 

National Aeronautics and

Facts

Space Administration

Dryden Flight Research Center 

P.O. Box 273

Edwards, California 93523

In the joint X-15 hypersonic research program that 

NASA conducted with the Air Force, the Navy, and 

North American Aviation, Inc., the aircraft flew over 

a period of nearly 10 years and set the world’s unofficial speed and altitude records of 4,520 mph (Mach 

6.7) and 354,200 feet in a program to investigate all 

aspects of piloted hypersonic flight. Information 

gained from the highly successful X-15 program 

contributed to the development of the Mercury, 

Gemini, and Apollo piloted spaceflight programs as 

well as the Space Shuttle program.

 Manufactured by North American Aviation, Inc., 

three rocket-powered X-15s flew a total of 199 times, 

with North American (and former National Advisory 

Committee for Aeronautics or NACA) pilot Scott 

Crossfield making the first, unpowered glide flight on 

June 8, 1959. NASA’s William H. Dana was the pilot 

for the final flight in the program on Oct. 24, 1968. 

All of these flights took place within what was called 

the “High Range” surrounding but mostly to the east 

of Edwards Air Force Base, Calif., and NASA’s Flight 

Research Center (later called the NASA Dryden Flight 

Research Center).

 There were 10 other pilots in the program for a total 

of 12: five from NASA, five from the Air Force, one 

from the Navy, and one, Crossfield, from North 

American. Generally, pilots used one of two types of 

flight profiles — a speed profile that called for the 

pilot to maintain a level altitude until time for descent 

to a landing, and a high-altitude flight plan that reVoice 661-276-3449 

FAX 661-276-3566 

pao@dfrc.nasa.gov FS-2002-09-052 DFRC

Hypersonic Research Program

X-15 #2 just after launch NASA photo EC88 0180-1. 

quired maintaining a steep rate of climb until reaching 

altitude and then decending.

 Because of the large fuel consumption of its rocket 

engine, the X-15 was air launched from a B-52 aircraft 

at about 45,000 feet and speeds upward of 500 mph. 

Depending on the mission, the rocket engine provided 

thrust for the first 80 to 120 seconds of flight. The 

remainder of the normal 8- to 12-minute flight was 

without power and ended in a 200-mph glide landing. 

Because the nose landing wheel lacked steering and 

the main landing gear employed skids, the X-15 had to 

land on a dry lakebed. The Rogers Dry Lake adjacent 

to Edwards and Dryden was the intended landing 

location for all flights, but there were numerous 

emergency lakebeds selected in advance for emergency landings. 

(see drawing on page 4) 
Design

 The X-15 was a follow-on research aircraft to the 

early X-planes, which had explored the flight regime 

from just below the speed of sound (Mach 1) to Mach 

3.2. In 1952 the NACA had begun preliminary research into space flight and associated problems. Two 

years later, NACA’s Research Airplane Projects Panel 

discussed the need for a new research airplane to study 

hypersonic and space flight. The NACA established 

the characteristics of what became the X-15 and 

presented them to the Air Force and Navy in July 

1954. The two services and NACA signed a memorandum of understanding for the joint project in 

December 1954, and the Air Force selected North 

American to develop three X-15 research aircraft in 

September 1955.

 A North American team headed by Chief Project 

Engineer Charles Feltz designed the aircraft, with 

technical guidance from the NACA’s Langley Aeronautical Laboratory (later NASA’s Langley Research 

Center), Hampton, Va., and High-Speed Flight Station 

(as Dryden was then called).

 Although the number two aircraft was later modified, 

the basic X-15 was a single-seat, mid-wing monoplane 

designed to explore the areas of high aerodynamic 

heating rates, stability and control, physiological phenomena, and other problems relating to hypersonic flight 

(above Mach 5). Because the Reaction Motors Division 

of Thiokol Chemical Corp. did not have the throttleable 

XLR-99 engine ready for the early flights of the aircraft, 

the X-15 initially flew with two XLR-11 engines, producing a thrust of 16,380 lb. Once the XLR-99 was 

installed, the thrust became 57,000 lb.

 The X-15 used conventional aerodynamic controls for 

flight in the dense air of the usable atmosphere. The 

controls consisted of rudder surfaces on the vertical 

stabilizers to control yaw (movement of the nose left or 

right) and canted horizontal surfaces on the tail to control 

pitch (nose up and down) when moving in synchronization or roll when moved differentially.

 For flight in the thin air outside the Earth’s atmosphere, the X-15 used a reaction control system. Hydrogen peroxide thrust rockets on the nose of the aircraft 

provided pitch and yaw control. Those on the wings 

furnished roll control. 

The outer skin of the X-15 consisted of a nickelchrome alloy called Inconel X, employed in a heat 

sink structure to withstand the results of aerodynamic 

heating when the aircraft was flying within the atmosphere. The cabin was made of aluminum and was 

isolated from the outer structure to keep it cool. 

Program History

 The first X-15 arrived at the NASA High-Speed 

Flight Station in the early months of 1959, and Scott 

Crossfield, who had helped with the design of the 

aircraft, soon began the contractor demonstration 

flights. During its research program, the aircraft set 

unofficial world speed and altitude records of 4,520 

mph (Mach 6.7—on Oct. 3, 1967, with Air Force pilot 

Pete Knight at the controls) and 354,200 feet (on Aug. 

22, 1963, with NASA pilot Joseph Walker in the 

cockpit).

X-15A-2 (with ablative coating and external tanks) in 

which Pete Knight set the unofficial world speed record. 

NASA photo EC68 1899 

More important than records, however, were the

 X-15’s probing of hypersonic aerodynamic performance and heating rates, research into structural 

behavior during high heating and high flight loads, 

study of hypersonic stability and control during exit 

from and reentry of the atmosphere, and examination 

of pilot performance and physiology.

 In the course of its flight research, the X-15’s 

pilots and instrumentation yielded data for more than 

765 research reports. As Dryden Chief Scientist Ken 

Iliff and his wife, aerospace research engineer Mary 

Shafer, have written, “The aircraft returned benchmark 

hypersonic data for aircraft performance, stability and 

2
control, materials, shock interaction, hypersonic 

turbulent boundary layer, skin friction, reaction control jets, 

aerodynamic heating, and heat transfer.” (The boundary 

layer is the thin layer of air next to the body of the aircraft that 

has distinctive flow characteristics because of friction 

between the air and the surface of the aircraft; control of the 

flow in the boundary layer is critical to improving aircraft 

performance.)

 environment.

 •Discovery that hypersonic boundary layer flow is

 turbulent and not laminar.

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

 •First direct measurement of hypersonic aircraft skin friction and discovery that skin friction is lower than had

 been predicted.

 The distinguished Langley aeronautical researcher John 

Becker, who had been an early advocate of the X-15 program, identified 25 specific accomplishments of the effort. 

These included:

 • Discovery of hot spots generated by surface

 irregularities. (These last four discoveries led to improved

 design tools for future hypersonic vehicles, including the Space Shuttle.)

 • Discovery of methods to correlate base drag

 measurements with tunnel test results so as to

 •First application of hypersonic theory and wind correct wind tunnel data (and thereby improve

 tunnel work to an actual flight vehicle. design criteria for future air- and spacecraft).

 •First use of reaction controls for attitude control in • Demonstration of a pilot’s ability to control a

 space. rocket boosted aerospace vehicle through atmo-

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

 hypersonic reentry.

 spheric exit.

 • Successful transition from aerodynamic controls

 to reaction controls and back again.

 • Development of (a servo-actuated ball) nose flow

 direction sensor for operation over an extreme range

 of dynamic pressure and a stagnation air temperature of 1,900 ° F (for accurate measurement of air speed

 and flow angle at supersonic and

 hypersonic speeds).

 •First application of energy-management techniques

 (for the positioning of the vehicle for landing; these

 were essential for the landing of the Space Shuttle and

 all future reusable launch vehicles following their

 reentry from space.)

 •Use of the three X-15 aircraft as testbeds to carry a

 • Development of the first practical full pressure wide variety of experimental packages.

suit for pilot protection in space.

 •Development of inertial flight data systems capable

 of functioning in a high dynamic pressure and space

XLR-99

 ENGINE 

ANHYDROUS-AMMONIA 

TANK (FUEL) 

LIQUID - OXYGEN 

TANK (OXIDIZER) 

LIQUID NITROGEN 

AUXILIARY 

POWER UNITS 

HYDROGEN 

ATTITUDE ROCKETS 

PEROXIDE 

ATTITUDE ROCKETS HYDROGEN 

PEROXIDE HELIUM 

TANKS 

EJECTION SEAT 

Cutaway drawing of X-15, showing location of components.

3 
 Typical X-15 Research Flight Paths

astronauts.

 These experiments —28 of them— ranged from astronomy to micrometeorite collection. They included tests of 

horizon definition and proposed insulation that bore fruit in 

the navigation equipment and thermal protection used on the 

Saturn launch vehicles in the Apollo program, which dispatched 12 astronauts to the moon and back. Among the 12 

was Neil Armstrong, the first human to step on the moon’s 

surface and a former X-15 pilot who also flew many other 

research aircraft at the Flight Research Center.

 In the area of physiology, researchers learned that the heart 

rates of X-15 pilots ranged from 145 to 185 beats per minute 

during flight. This greatly exceeded the normal 70 to 80 

beats per minute experienced on test missions for other 

aircraft. The cause of the difference proved to be the stress 

X-15 pilots encountered during prelaunch in anticipation of 

each mission. As it turned out, the higher rates proved 

typical for the future physiological behavior of pilotMore intangibly but no less importantly, in the words of 

John Becker, the X-15 project led to “the acquisition of new 

piloted aerospace flight ‘know how’ by many teams in 

government and industry. They had to learn to work together, face up to unprecedented problems, develop solutions, and make this first manned [today, we would say 

piloted] aerospace project work. These teams were an 

important national asset in the ensuing space programs.”

 As the partial list of accomplishments suggests, the 

X-15 brilliantly achieved its basic purpose of supporting 

piloted hypersonic flight within and outside the Earth’s 

atmosphere. In addition, it carried out the “explorations to 

separate the real from the imagined problems and to make 

known the overlooked and the unexpected problems” that 

Hugh Dryden had called for in 1956 when the X-15 was 

still in the design and development phase. 

4
The Aircraft

 Except for the number two X-15 when modified 

as the X-15A-2, the X-15s were roughly 50 ft long, 

with a 22-ft wing span. The wedge-shaped vertical 

tail was 13 ft high. Because the lower vertical tail 

extended below the landing skids when they were 

deployed, a part of the lower vertical tail was jettisoned just before landing and recovered by a parachute. The aircraft was powered by a Thiokol (Reaction Motors Division) XLR-99 throttleable rocket 

engine powered by anhydrous ammonia and liquid 

oxygen. It provided a maximum thrust of 57,000 lb 

and a minimum thrust of 28,000 lb. Launch weight 

of the aircraft was 31,275 lb, decreasing to 12,295 lb 

at burnout.

 The X-15A-2, modified from the number two 

aircraft and delivered to NASA in February 1964, 

included among other new features, a 28-in. fuselage extension to carry liquid hydrogen for a supersonic combustion ramjet that was flown (as a 

dummy) but never tested. It also had external tanks 

for liquid ammonia and liquid oxygen. These tanks 

provided roughly 60 seconds of additional engine 

burn and were used on the aircraft’s Mach 6.7 

flight. While adding to the speed the X-15 did 

achieve, the tanks also increased the aircraft’s 

weight to almost 57,000 lb and added significantly 

to the drag experienced by the aircraft in flight. 

Sky 

hatch Windshield 

Fuselage 

extension Helium 

tank 

Removable wing tip 

External tank Ramjet 

Extended 

main gear 

H2O2 tanks 

Three-view drawing of X-15 aircraft. Two-view drawing of X-15A-2 with modifications.

5
Pilots Sources: 

X-15 pilots in order by dates of first flight # of 

Flights 

A. Scott Crossfield, North American Aviation 14 

Joseph A. Walker, NASA 25 

Robert M. White, United States Air Force (USAF) 16 

Forrest S. Petersen, United States Navy 05 

John B. McKay, NASA 29 

Robert A. Rushworth, USAF 34 

Neil A. Armstrong, NASA 07 

Joe H. Engle, USAF 16 

Milton O. Thompson, NASA 14 

William J. Knight, USAF 16 

William H. Dana, NASA 16 

Michael J. Adams, USAF 07 

total 199

 The X-15 had its share of emergency landings and 

accidents, but only two produced serious injuries or 

death. On Nov. 9, 1962, Jack McKay experienced an 

engine failure and landed at Mud Lake, Nev. The 

landing gear collapsed, flipping him and the aircraft 

on its back. Although he recovered from his injuries 

sufficiently to fly again, he eventually had to retire 

because of them. On Nov. 15, 1967, on Michael 

Adams seventh flight, he entered a spin from which 

he was able to recover but could not bring it out of an 

inverted dive because of a technical problem with the 

adaptive flight control system. He died in the resultant 

crash of the X-15 number three. 

Milton O. Thompson, At the Edge of Space: The X-15 

Flight Program (Washington, D.C., and London: 

Smithsonian Institution Press, 1992). 

Richard P. Hallion, On the Frontier: Flight Research 

at Dryden, 1946-1981 (Washington, D.C.: NASA SP4303, 1984). 

Wendell H. Stillwell, X-15 Research Results (Washington, D.C.: NASA SP-60, 1965). 

John V. Becker, “The X-15 Program in Retrospect,” 

3rd Eugen Sänger Memorial Lecture, Bonn, Germany, 

Dec. 4-5, 1968, copy in the NASA Dryden Historical 

Reference Collection. 

Kenneth W. Iliff and Mary F. Shafer, Space Shuttle 

Hypersonic Aerodynamic and Aerothermodynamic 

Flight Research and the Comparison to Ground Test 

Results (Washington, D.C.: NASA Technical Memorandum 4499, 1993), p. 2 for quotation and see also their “A 

Comparison of Hypersonic Flight and Prediction Results,” AIAA-93-0311, paper delivered at the 31st 

Aerospace Sciences Meeting & Exhibit, Jan. 11-14, 

1993, in Reno, Nev. 

R. L. Schleicher, “Structural Design of the X-15,”

Journal of the Royal Aeronautical Society (Oct. 1963): 

618-636. 

Proceedings of the X-15 First Flight 30th Anniversary 

Celebration (Washington, D.C.: NASA Conference 

Publication 3105, 1991). 

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