The initiative for development of a high-speed high-altitude research aircraft was taken by NACA during 1952 when studies were initiated into problems associated with manned space flight. By 1954 these studies indicated that an airplane was required to explore both civil and military aspects of flight operation at very high speeds and altitudes. Some of the areas explored included construction techniques and materials applicable to high temperature structures, hypersonic aerodynamics, hypersonic stability and control, and fundamental piloting and pilot survival problems.
In July 1954 the USAF and USN became involved and it was agreed that the USAF and Navy would finance the prototypes, the USAF would administer design and construction, and the NACA would have overall technical direction of the project. On December 30, 1954, twelve aircraft manufacturers were invited to submit proposals and on February 4th, 1955, four prospective engine manufacturers were asked to submit proposals.
Eventually the Research Airplane Committee of the USAF selected the North American NA-240 design on November 18, 1955, and three aircraft were ordered under contract AF33 (600)-31693 on June 11, 1956, allotted s/n 56-6670 to 56-6672 and the designation X-15, the project was also known as System 605A. On February 14, 1956, the Reaction Motors XLR-99 engine was selected, shortly thereafter Reaction Motors became a division of Thiokol Chemical, hence the engines were ordered from Thiokol under contract AF33 (600)-32248 on September 7, 1956.
As the aircraft would be air-launched, two B-52s were adopted as carriers, the X-15A being slung beneath the starboard wing inboard of the engines. Originally a B-52A-1-BO (52-003 c/n 16493) and a RB-52B-10-BO (52-008 c/n 16498), these two aircraft became the NB-52A and NB-52B respectively.
The North American design team, headed by Harrison Storms with Charles Feltz as project engineer, designed the X-15A as a mid-wing monoplane with a long tubular body fitted with side fairings containing propellant. The tail unit was cruciform, provision being made for the ventral fin to be jettisoned shortly before touchdown, and the retractable landing gear consisted of a twin-wheel nose gear and twin steel skids under the rear fuselage.
The 57,000 lb (25,855 kg) s.t XLR-99 rocket engine was not ready for service in time, so the first X-15A was fitted with two 8,000 lb (3,629 kg) s.t XLR-11 rocket engines and was rolled out at the Los Angeles, California plant on October 15, 1958. Transferred to Edwards AFB, the first flight beneath a B-52 was made on March 10, 1959. It was not released on this flight and several more captive flights were made before the first release, on June 8, 1959, for the sole unpowered X-15A flight, with North American pilot Scott Crossfield at the controls.
Meanwhile the second X-15A had been completed, and in this aircraft Scott Crossfield made the first powered flight of an X-15A, on September 17, 1959, reaching 1,393 mph (2,242 kmh). The first X-15A made its powered flight on January 23, 1960. The definitive XLR-99 engine was fitted in the third X-15A, but during preliminary ground runs this aircraft was severely damaged on June 3, 1960, and the first flight of the new engine was made in the second X-15A on November 15, 1960.
The third X-15A, rebuilt and fitted with an adaptive control system, made its first flight on December 20, 1960, powered by the XLR-99. On August 11, 1961, the first X-15A flew again after being re-engined with the XLR-99. By mid-1962, the three X-15As progressively reached out to the limits of their design envelope, achieving a maximum speed of 4,104 mph (6,605 kmh) on June 27, 1962, and an altitude of 354,200 ft (67.08 mls, 107.96 km) on August 22, 1962.
On November 9, 1962, the second X-15A was damaged in a landing accident, and it was then rebuilt with modifications designed to permit even higher speeds to be achieved. Modifications included a 29 in (0.737m) lengthened fuselage, external fuel tanks, and an external ablative coating to help protect the aircraft from high temperatures. The aircraft was redesignated X-15A-2, and first flew on June 28, 1964. On October 3, 1967, this aircraft reached a new high speed of 4,534 mph (7,297 kmh). The last and 199th flight was made on October 24, 1968.
As the program developed and the flight envelope expanded, the X-15 became a test bed vehicle for a large number of other high speed, high altitude test programs. Among these were some of the first airborne tests of a scramjet engine (supersonic combustion ramjet), thermodynamic metallurgical sample testing on various leading edges, hypersonic weapons deployment tests, cockpit environment improvements tests, attitude indicator systems tests, and reentry configuration tests.
Two of the aircraft are preserved, 56-6670 is presently on display in the NASM Museum at the National Air & Space Museum, Udvar Hazy Center, Chantilly, Virginia, while 56-6671 is presently on display at the National Museum of the USAF, Wright-Patterson AFB, Dayton, Ohio. The third aircraft was destroyed in a fatal accident on November 15, 1967, USAF test-pilot Major Michael J. Adams was killed. The NB-52A is on display at the Pima Air and Space Museum, Tucson, Arizona, while the NB-52B is presently on display at west gate of Edwards AFB.
X-15A Johan Visschedijk Collection X-15A Johan Visschedijk Collection
22 ft 0 in (6.71 m)
50 ft 0 in (15.24 m)
13 ft 0 in (3.96 m)
200 sq.ft (18.58 sq.m)
15,000 lb (6,804 kg)
33,300 lb (15,105 kg)
4,159 mph (6,693 kmh)
354,200 ft (107,960 m)
275 mls (443 km)
22 ft 0 in (6.71 m)
52 ft 5 in (15.98 m)
14 ft 0 in (4.27 m)
200 sq.ft (18.58 sq.m)
18,340 lb (8,319 kg)
56,130 lb (25,460 kg)
4,534 mph (7,297 kmh)
275 mls (443 km)