Altitudes attained by X-15 aircraft do not match those of Alan Shepard's and Gus Grissom's Project Mercury space capsules in 1961, nor of any other manned spacecraft. However, the X-15 ranks supreme among manned rocket-powered aircraft, becoming the world's first operational spaceplane in the early 1960s.
Before 1958, USAF and NACA officials discussed an orbital X-15 spaceplane, the X-15B that would launch into outer space from atop an SM-64 Navaho missile. This was canceled when the NACA later became NASA and NASA adopted Project Mercury instead.
By 1959, the Boeing X-20 Dyna-Soar space-glider program became the USAF's preferred means for launching military manned spacecraft into orbit; however, this program was canceled in the early 1960s before an operational vehicle could be built. Various configurations of the Navajo were considered, and another proposal involved a Titan I stage.
Three X-15s were built, flying 199 test flights, the last on 24 October 1968.
The first X-15 flight was an unpowered test flight by Albert Scott Crossfield, on 8 June 1959. Crossfield also piloted the first powered flight, on 17 September 1959, and his first flight with the XLR-99 rocket engine on 15 November 1960. Twelve test pilots flew the X-15. Among these were Neil Armstrong, later a NASA astronaut, and Joe Engle, later a commander of NASA Space Shuttle test flights.
In a 1962 proposal, NASA considered using the B-52/X-15 as a launch platform for a Blue Scout rocket to place satellites up to 150 pounds (68 kg) into orbit.
In July and August 1963, pilot Joseph A. Walker exceeded 100 km in altitude, joining NASA astronauts and Soviet cosmonauts as the first human beings to cross that line on their way to outer space. The USAF awarded astronaut wings to anyone achieving an altitude of 50 miles (80 km), while the FAI set the limit of space at 100 kilometers (62.1 mi).
On 15 November 1967, U.S. Air Force test pilot Major Michael J. Adams was killed during X-15 Flight 191 when the (X-15-3) entered a hypersonic spin while descending, then oscillated violently as aerodynamic forces increased after re-entry. As his aircraft's flight control system operated the control surfaces to their limits, acceleration built to 15 g vertical and 8.0 g lateral. The airframe broke apart at 60,000 feet (18 km) altitude, scattering the X-15's wreckage for 50 square miles (130 km2). On 8 June 2004, a monument was erected at the cockpit's locale, near Randsburg, California.[14] Major Adams was posthumously awarded Air Force astronaut wings for his final flight in X-15-3, which had reached an altitude of 50.4 miles (81.1 km). In 1991, his name was added to the Astronaut Memorial.
The second X-15A was rebuilt after a landing accident. It was lengthened 2.4 feet (0.73 m), a pair of auxiliary fuel tanks attached beneath its fuselage and wings, and a complete heat-resistant ablative coating was added. Renamed the X-15A-2, this plane first flew on 28 June 1964, reaching a maximum speed of 4,520 miles per hour (7,274 km/h). in October 1967, flown by William "Pete" Knight of the U.S. Air Force.
Five aircraft were used during the X-15 program: three X-15s planes and two B-52 bombers:
X-15A-1 – 56-6670, 82 powered flights
X-15A-2 – 56-6671, 53 powered flights
X-15A-3 – 56-6672, 64 powered flights
NB-52A – 52-003 (retired in October 1969)
NB-52B – 52-008 (retired in November 2004)
A 200th flight over Nevada was first scheduled for 21 November 1968, to be flown by William "Pete" Knight. Numerous technical problems and outbreaks of bad weather delayed this proposed flight six times, and it was permanently canceled on 20 December 1968. This X-15 was detached from the B-52 and then put into indefinite storage. The aircraft was later donated to the museum at Wright-Patterson Air Force Base for display.
General characteristics
Crew: one
Length: 50 ft 9 in (15.45 m)
Wingspan: 22 ft 4 in (6.8 m)
Height: 13 ft 6 in (4.12 m)
Wing area: 200 ft2 (18.6 m2)
Empty weight: 14,600 lb (6,620 kg)
Loaded weight: 34,000 lb (15,420 kg)
Max. takeoff weight: 34,000 lb (15,420 kg)
Powerplant: 1 × Thiokol XLR99-RM-2 liquid-fuel rocket engine, 70,400 lbf at 30 km (313 kN)
Performance
Maximum speed: 4,520 mph (7,274 km/h)
Range: 280 mi (450 km)
Service ceiling: 67 mi (108 km, 354,330 ft)
Rate of climb: 60,000 ft/min (18,288 m/min)
Wing loading: 170 lb/ft2 (829 kg/m2)
Thrust/weight: 2.07
