The X-15 was a high-altitude research aircraft designed for flight conditions in the upper ionosphere, at the very fringe of outer space. Data obtained from X-15 flights were later incorporated into aerospace research applications as well as high-altitude surveillance and fighter aircraft designs. The X-15 project contributed significant knowledge to the piloted space programs that followed, especially to the Apollo and space shuttle programs.

In the early years of the space program, aerospace scientists and engineers considered the possibility of constructing a so-called space plane capable of traveling outside Earth’s atmosphere and returning safely from orbit to land on a conventional aircraft runway. Such an aerospace vehicle, boosted into the upper ionosphere on rocket motors with detachable fuel tanks that could be jettisoned after takeoff, held certain obvious advantages over the single-use ballistic missiles then currently in use, not the least of which was the clear economic advantage of such a vehicle being reusable for dozens, possibly hundreds, of such flights. X-15 aircraft[X 15 aircraft]
Aircraft;high-altitude flight[high altitude flight]
High-altitude flight[High altitude flight]
Experimental aircraft
[kw]X-15 Rocket Aircraft Program (June 8, 1959-Dec. 31, 1968)[X 15 Rocket]
[kw]Rocket Aircraft Program, X-15 (June 8, 1959-Dec. 31, 1968)
[kw]Aircraft Program, X-15 Rocket (June 8, 1959-Dec. 31, 1968)
X-15 aircraft[X 15 aircraft]
Aircraft;high-altitude flight[high altitude flight]
High-altitude flight[High altitude flight]
Experimental aircraft
[g]North America;June 8, 1959-Dec. 31, 1968: X-15 Rocket Aircraft Program[06120]
[g]United States;June 8, 1959-Dec. 31, 1968: X-15 Rocket Aircraft Program[06120]
[c]Space and aviation;June 8, 1959-Dec. 31, 1968: X-15 Rocket Aircraft Program[06120]
[c]Engineering;June 8, 1959-Dec. 31, 1968: X-15 Rocket Aircraft Program[06120]
Adams, Michael
Armstrong, Neil
Bridgeman, William
Crossfield, A. Scott
McKay, John B.[Mackay, John B.]
Rushworth, Robert
Walker, Joe
White, Alvin
White, Robert

It was in this context that the X-15 rocket plane was developed specifically for the purpose of in-flight testing of materials and structural designs that had previously only been capable of being modeled mathematically or static-tested inside a wind tunnel. Since these limited models sometimes yielded contradictory or inconclusive results, it became imperative to design an actual piloted research craft capable of testing firsthand the effects of dynamic fight maneuvers at extreme altitudes and at hypersonic speeds. In addition, X-15 flights would provide valuable human-factor data on the physiological effects associated with piloting aircraft under such extreme conditions.

An X-15 aircraft sits on the dry lake bed near Dryden Flight Research Center, in Southern California, in 1961.

(NASA)

Even while the X-15 research program was under way, the Soviet Union successfully launched Yuri Gagarin, the first person in space, on April 12, 1961, utilizing a ballistic missile to place his Vostok spacecraft into orbit. At that point, policy makers at the National Aeronautics and Space Administration National Aeronautics and Space Administration;reusable spacecraft (NASA) realized that if the United States intended to keep pace with the Soviets in space, it could not afford to pursue the space-plane concept as its primary goal. Certain key materials and systems essential to the success of the space-plane project had not yet been invented and, moreover, it had a minimum estimated development time of about five years. Clearly, if the United States hoped to beat the Soviet Union to the Moon, the space program would need to shift its focus quickly, toward the development of rockets Rockets capable of lifting heavy payloads outside Earth’s atmosphere. In fact, such heavy-lift rocket systems had been under development by German scientists in the closing years of World War II, most notably the V-2 rocket originally envisioned as an intercontinental ballistic missile weapon. Fortunately, many of the top German scientists involved in the V-2 program had been brought to the United States after the war. Although the space-plane concept was never abandoned entirely, it assumed a relatively lower priority below single-stage ballistic missile research.

The X-15 research project continued to receive research funding as a useful adjunct to missile research. The U.S. military had an ongoing interest in developing reconnaissance Espionage aircraft that would be capable of flying higher and faster than Soviet surface-to-air interceptor missiles of the type that had shot down Francis Gary Powers’s U-2 reconnaissance plane over Sverdlovsk in 1960. Such high-performance aircraft would operate at the very fringe of outer space at altitudes so high that air density was no longer sufficient to support standard aircraft control surfaces, such as ailerons and flaps. It was at the fringe of space that the X-15 proved its worth as a research craft.

To overcome the control problem, the nose and wingtips of the X-15 were outfitted with small thrusters, reaction motors that operated on hydrogen peroxide fuel, to control the vehicle’s pitch, roll, and yaw axes. Similar X-15 research data contributed significantly to the design of the Mercury spacecraft as well as many later high-performance aircraft, such as the Lockheed F-104 Starfighter, the SR-71 Blackbird, and its counterpart, the YF-12 Interceptor, each of which bears a certain resemblance to the X-15.

The X-15 rocket plane, the development cost of which was split between the U.S. Navy and the U.S. Air Force, was designed by Harrison Storm’s Storm, Harrison design team at North American Aviation, with rocket engines built by Reaction Motors of New Jersey. Generally speaking, the X-15 approximated a jet fighter in overall shape and size. The rocket plane weighed 15,000 pounds with empty fuel tanks and 50,914 pounds at maximum gross load, with a length of just over 52 feet, a wingspan of 22.3 feet, and a wing area of 200 square feet. The design of the all-black X-15 had some distinctive features, most obvious of which was a wedge-shaped tail rudder (thicker than any plane built before it) that extended both above and below the fuselage. This gave the X-15 a pronounced resemblance to a V-2 rocket when seen in horizontal flight (the lower/ventral rudder, which housed the rear landing gear skids, was jettisoned before landing). The X-15’s elongated fuselage and short, stubby wings were made of a special nickel-steel alloy, Inconal-X, which possessed unusual structural stability and heat-resistance sufficient to withstand the extreme friction heating and thermal distortion that affected airframes at supersonic speeds. Since the X-15 typically operated at maximum speeds from four to six times the speed of sound, thermal effects from atmospheric friction heating were significant stress factors; at top speed the external hull temperature of the X-15 rose to almost 1,200 degrees Fahrenheit, hot enough to melt stainless steel.

Three X-15A’s were built; the first flew a dead-stick, or powerless, test flight on June 8, 1959, piloted by A. Scott Crossfield. The first powered flight, using twin XLR-11 rocket motors brought over from the X-1 program, occurred in November, 1960. Later, a single-nozzle XLR-99/RM2 rocket motor was outfitted to the second X-15, along with two underwing “saddle” fuel tanks to extend the craft’s powered flight time. The power plant of the rocket plane generated 57,000 pounds of thrust, equivalent to more than 600,000 horsepower at 4,000 miles per hour. With the upgraded XLR-99/RM2 engine, the X-15 set speed records (mach 6.72 at 102,100 feet), and altitude records (354,200 feet/70 miles) that remained unbroken until the first space shuttle, Columbia, was launched in 1981.

In a typical flight, the X-15 was carried aloft by a so-called mother ship, a B-29, B-50, or B-52 bomber, specially modified with an attachment fairing mounted to the undersurface of the inboard right wing. At around 45,000 feet altitude, the X-15 was air-dropped from the wing fairing and fell away in a freefall glide. Once clear of the bomber, the X-15’s pilot ignited the main rocket motor for a burn that lasted approximately eighty seconds. This accelerated the rocket plane to altitudes of more than 60 miles. After the rocket plane’s fuel had been expended, the X-15 made a supersonic reentry into the upper atmosphere and, after dissipating excess speed, glided to a 200 mile-per-hour dead-stick landing on a dry lake bed runway at Edwards Air Force Base in the Mojave Desert of Southern California.

The X-15 enjoyed a reputation for dependability among its pilots; nevertheless, there were mishaps. On November 5, 1959, a small onboard explosion and subsequent engine fire forced a landing in which pilot Crossfield’s fuel-heavy X-15 literally “broke its back” when the nose wheel was dropped too quickly. In November, 1962, John B. McKay suffered four crushed vertebrae when his X-15 rolled onto its back upon landing at Mud Lake, Nevada (McKay later recovered). One fatality occurred in the X-15 program. On November 15, 1967, Michael Adams, distracted by malfunctioning equipment, misread his instruments, causing his X-15 to reenter the atmosphere, scudding sideways. The aircraft spun out of control and disintegrated, killing Adams instantly. The program’s last flight took off on October 24, 1968, and the X-15 program was shut down officially at the end of that year, on December 31.

Over a span of ten years, three X-15’s made a total 199 flights in what was one of the most successful flight research programs in history. Many of the X-15 pilots qualified as astronauts during their fights, having reached altitudes of more than 50 miles. These astronauts included Neil Armstrong (who became the first person to set foot on the moon), William Bridgeman, Crossfield, McKay, Robert Rushworth, Joe Walker, Alvin White, and Robert White. In the years following the Apollo program (which placed astronauts on the Moon), NASA again was able to concentrate on the idea of a space plane, which was reborn as the space shuttle. X-15 aircraft[X 15 aircraft]
Aircraft;high-altitude flight[high altitude flight]
High-altitude flight[High altitude flight]
Experimental aircraft

• Chant, Christopher. The World’s Greatest Aircraft. Edison, N.J.: Chartwell Books, 1991. Well illustrated, with plan drawings in color; basic flight data; and aircraft specifications.
• Crouch, Tom. Wings: A History of Aviation from Kites to the Space Age. New York: Smithsonian National Air and Space Museum, 2003. Detailed narrative history of the X-planes and their contribution to aerospace research.
• Hallion, Richard P., and Michael H. Gorn. On the Frontier: Experimental Flight at NASA Dryden. Washington, D.C.: Smithsonian Institution Press, 2003. A 554-page history of the research aircraft operated by NASA. Contains detailed accounts of record-setting aircraft and flights, from those of the X-1 through the X-15.
• Jenkins, Dennis. Hypersonic: The Story of the North American X-15. North Branch, Minn.: Speciality Press, 2003. A detailed history of the X-15 program from its genesis in the 1950’s to its end in 1968. Includes photographs and illustrations.
• Wallace, Lane E. Flights of Discovery. NASA History Series, NASA SP-4309. Washington, D.C.: National Aeronautics and Space Administration, 1996. Exceptionally detailed summary of major aspects of the X-15 program. Illustrated with color photographs.

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