On October 3, 1967, U.S. Air Force pilot William "Pete" Knight made aerospace history when he piloted the X-15A-2 to an astonishing speed of Mach 6.7, approximately 4,520 miles per hour (7,274 km/h), at an altitude of 102,100 feet (31,120 m). This incredible feat, which still stands as the highest speed ever recorded by a crewed, powered aircraft, was just one of the many groundbreaking achievements of the North American X-15 rocket-powered aircraft program.
Operated by the U.S. Air Force and NASA from 1959 to 1968, the X-15 program aimed to achieve hypersonic flight and reach the edge of space. The aircraft's innovative design, featuring a unique wedge-shaped tail and an uninsulated, exposed Inconel X hot structure, allowed it to withstand the extreme temperatures associated with high-speed flight and reentry.
Throughout its operational life, the three X-15s built were flown by 12 pilots for a total of 199 missions. Remarkably, 13 of these flights, piloted by eight individuals, met the U.S. Air Force's criteria for spaceflight, earning those pilots their astronaut wings. NASA pilot Joseph Walker set an unofficial altitude record of 354,200 feet (107.96 km) on August 22, 1963, in the X-15-3, demonstrating the aircraft's ability to reach the edge of space.
The X-15 program's contributions to aerospace technology and human spaceflight were significant. As former NASA historian Dr. Roger Bilstein noted in his book "Testing Aircraft, Exploring Space," "The X-15's survival encouraged extensive use of comparatively exotic alloys, such as titanium and Inconel-X, which led to machining and production techniques that became standard in the aerospace industry." The program also pioneered the development of the first practical full-pressure suit for pilot protection in space, reaction controls for attitude control, and reentry techniques that would later be applied to the Mercury, Gemini, Apollo, and Space Shuttle programs.
Although the X-15 program ended in 1968, its legacy continues to inspire modern aerospace research and development. Private companies like Virgin Galactic and XCOR have cited the X-15 as an inspiration for their suborbital spaceplanes, which aim to make space tourism a reality. The lessons learned from the X-15 program are also being applied to ongoing research into hypersonic flight and the development of new experimental aircraft.
