Prior to 1952 the National Advisory Committee for Aeronautics (NACA) had sponsored a progressive series of experimental aircraft—the X-planes—that had achieved speeds approaching the speed of sound (Mach 1) to Mach 3.2. The next step would be an aircraft capable of flight beyond the Earth’s atmosphere and speeds in excess of Mach 5—hypersonic flight. In 1954, the NACA selected North American to develop three X-15 hypersonic aircraft.
The X-15 research aircraft were developed to determine the effects of hypersonic on aero-thermodynamics, aerodynamics, structures, flight controls and the physiological aspects of high-speed, high-altitude flight.
Note: In 1958 before the first flight of the X-15, the NACA was absorbed into the National Aeronautics and Space Administration (NASA).
The X-15 was a small rocket-powered aircraft, 50 feet long with a wingspan of 22 feet. It had a conventional fuselage, but an unusual wedge-shaped vertical tail, thin stubby wings and unique side fairings that extended along the fuselage. The X-15 weighed about 14,000 pounds empty and approximately 34,000 pounds at launch. Because the lower vertical tail extended below the landing skids, a part of the lower vertical tail was jettisoned before landing and recovered by a parachute.
During its first powered flights, the X-15 used two un-throttled XLR-11 engines, producing a combined thrust of 16,380 lb. With no throttles, engine power was set on the ground before each flight. The intended Thiokol XLR-99 rocket engine, which was not available at the beginning of the X-15’s flight program, produced up to 60,000 pounds of thrust and could be controlled by the pilot.
Depending on the mission and engine throttle setting, the rocket engine provided thrust for the first 80 to 120 seconds of flight. The remainder of a normal 10-11minute flight was unpowered and ended with a glide to a 200 mile-per-hour landing. One X-15, rebuilt and lengthened after an accident, also had external tanks for additional fuel. These tanks provided about 60 seconds of additional engine burn. and were used on the aircraft’s Mach 6.7 flight.
Because of the rapid fuel consumption, the X-15 was air launched from under the wing of a B-52 aircraft at 45,000 feet at a speed of about 500 miles per hour. In 1958, B-52 Stratofortress aircraft NB-52B, S/N52-008 “Balls 8,” and its sister ship, NB-52A, S/N52-0003, “The High and the Mighty,” were modified for the mission of launching the X-15.
Three aircraft were built and delivered to the NASA. During a ground test later in the program, the number two X-15, with Scott Crossfield in the cockpit, experienced a serious explosion just behind the cockpit. Crossfield was not seriously hurt, and the aircraft was rebuilt and lengthened by 28 inches to provide added space for fuel and designated X-15A-2.
The X-15A-2, was also modified to carry and test a supersonic ramjet engine. Although the engine was flown as a dummy, it was never powered or tested.
The airframe manufacturer was North American Rockwell, Inc. Thiokol Chemical Corp. manufactured the rocket engine. The program was a joint NASA-USAF-USN effort.
For flight in the Earth’s atmosphere, the X-15 used conventional aerodynamic controls. The controls consisted of the rudder on the vertical stabilizers to control yaw. The canted horizontal stabilizers controlled both pitch (when operated in unison) and roll (when moved differentially by lateral movements of the control stick). For flight outside of the Earth’s atmosphere, i.e., in “space,” the X-15 used eight thrust rockets; four on the nose of the aircraft to control pitch and yaw, and four on the wings for roll control.
In 1955, Scott Crossfield left the NACA to become the chief engineering test pilot for North American, where he played a major role in the design and development of the X-15. The X-15 was an entirely new and unproven design, and flight operations were considered extremely hazardous. It was Crossfield’s job to demonstrate its airworthiness at speeds up to Mach 3 (2290 mph).
A total of 12 pilots participated in the X-15 program: five from NASA, five from the Air Force, one from the Navy, and one, Crossfield, from North American. Among these pilots were two future astronauts, Neil Armstrong and Joe Engle.
Pilots generally flew one of two flight profiles: (1) a speed profile that called for the pilot to maintain a level altitude until time for descent to a landing, or (2) a high-altitude flight plan that required maintaining a steep rate of climb until reaching altitude and then descending—these flights were eventually recognized as flights into space and that the pilots as astronauts.
NASA’s X-15 hypersonic research program lasted nearly 10 years and set the unofficial world speed and altitude records of 4,520 mph (Mach 6.7) and 354,200 feet. Information gathered during the program contributed directly to science and technologies used on the Mercury, Gemini, and Apollo piloted spaceflight programs as well as the Space Shuttle program.
Scott Crossfield, made the first unpowered flight (June 8, 1959) and the first powered flight. The X-15’s last flight was on Oct. 24, 1968. The three X-15s flew a total of 199 powered flights. A 200th flight was scheduled, but due to weather and technical difficulties, it was delayed and eventually cancel, ending the program. All X-15 flight operations originated from what is now the NASA Dryden Flight Research Center on Edwards Air Force Base, Calif.
In the course of its flight research, the X-15’s pilots and instrumentation yielded data for more than 765 research reports. Dryden’s Chief Scientist Ken Iliff and his wife, aerospace research engineer Mary Shafer, wrote, “The [X-15] aircraft returned benchmark hypersonic data for aircraft performance, stability and control, materials, shock interaction, hypersonic turbulent boundary layer, skin friction, reaction control jets, aerodynamic heating, and heat transfer.”
Total cost of the X-15 program, including development of the three aircraft, was about $300 million.
Although the X-15 had its share of emergency landings and accidents, only two resulted in serious injuries or death. On Nov. 9, 1962, after the engine failed, pilot Jack McKay landed at Mud Lake, NV. The landing gear collapsed and the aircraft flipped onto its back. McKay recovered sufficiently to fly again.
On Nov. 15, 1967, on Michael Adams, flying the number three aircraft on his seventh flight, entered a spin from which he recovered, but could not bring the aircraft out of an inverted dive due control problems. He died in the resultant crash and the aircraft was destroyed.
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.
Crossfield cited the X-15 as one of few aircraft that caused grown men to cry upon its retirement.
Of the three X-15 aircraft built, yhe number 1 aircraft is on display in the Smithsonian’s National Air and Space Museum, Washington, D.C., and aircraft number 2 is part of the research and development display that has been recently moved to the new fourth wing of the Air Force Museum at Wright-Patterson Air Force Base, Ohio.