The X-15 set speed and altitude records in the 1960s, crossing the edge of outer space and returning with valuable data used in aircraft and spacecraft design.

[5][6] The X-15 was based on a concept study from Walter Dornberger for the National Advisory Committee for Aeronautics (NACA) of a hypersonic research aircraft.

[8] The X-15 fuselage was long and cylindrical, with rear fairings that flattened its appearance, and thick, dorsal and ventral wedge-fin stabilizers.

Without the main rocket engine thrust, the X-15's instruments and control surfaces remained functional, but the aircraft could not maintain altitude.

[11] The automatic mode used a feature called "Reaction Augmentation System" (RAS) that helped stabilize the vehicle at high altitude.

[12] Other features of the cockpit included heated windows to prevent icing and a forward headrest for periods of high deceleration.

[12] Above 35,000 feet (11 km) altitude, the cockpit was pressurized to 3.5 psi (24 kPa; 0.24 atm) with nitrogen gas, while oxygen for breathing was fed separately to the pilot.

[14] The X-15 reaction control system (RCS), for maneuvering in the low-pressure/density environment, used high-test peroxide (HTP), which decomposes into water and oxygen in the presence of a catalyst and could provide a specific impulse of 140 s (1.4 km/s).

[16] Before 1958, United States Air Force (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.

By 1959, the Boeing X-20 Dyna-Soar space-glider program was to become the USAF's preferred means for launching military crewed spacecraft into orbit.

In a 1962 proposal, NASA considered using the B-52/X-15 as a launch platform for a Blue Scout rocket to place satellites weighing up to 150 pounds (68 kg) into orbit.

[17][18] In July and August 1963, pilot Joe 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).

[19] 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).

[1] It was lengthened by 2.4 feet (73 cm), had a pair of auxiliary fuel tanks attached beneath its fuselage and wings, and a complete heat-resistant ablative coating was added.

It reached its maximum speed of 4,520 miles per hour (7,274 km/h) in October 1967 with pilot William "Pete" Knight of the U.S. Air Force in control.

Numerous technical problems and outbreaks of bad weather delayed this proposed flight six times, and it was permanently canceled on 20 December 1968.

During 13 of the 199 total X-15 flights, eight pilots flew above 264,000 feet (50.0 mi; 80 km), thereby qualifying as astronauts according to the US Armed Forces definition of the space border.

All five Air Force pilots flew above 50 miles and were awarded military astronaut wings contemporaneously with their achievements, including Adams, who received the distinction posthumously following the flight 191 disaster.