17 September 2026 marks 67 years since the first powered flight of the X-15 rocket plane, one of the most important experimental programmes in American aeronautical history. In 1959 the programme was being run by NASA with the US Air Force and US Navy, and with North American Aviation building the aircraft.

After initial unpowered flights, test pilot A. Scott Crossfield flew the first powered X-15 mission on 17 September 1959. NASA records that, at the controls of X-15-2, Crossfield reached about Mach 2.11 — 1,393 miles per hour — and an altitude of 52,341 feet. The flight lasted just over nine minutes and covered 88 miles.

The X-15 did not take off from a runway in the ordinary way. It was carried to altitude beneath the wing of a modified B-52 bomber and then released for the experimental portion of the flight. The engine at that stage was not yet the 57,000-pound-thrust XLR-99 designed for the aircraft: the early missions used eight weaker XLR-11 engines because of development delays.

The programme was designed to study aircraft behaviour at extreme speeds and altitudes, including aerodynamic heating, control at hypersonic speeds and the pilot’s reactions. In later years the X-15 far exceeded the performance of its first powered mission. It reached speeds above Mach 6, and the maximum record, set by William J. “Pete” Knight in 1967, was Mach 6.70.

Some flights went high enough for their pilots to receive astronaut recognition under the US criteria then in use. Joseph A. Walker reached 354,200 feet in 1963. Among the X-15 pilots was Neil A. Armstrong, before he joined NASA’s astronaut corps.

X-15 research generated data that influenced later aerospace vehicles, including the space shuttle, and improved understanding of flight at the boundary between atmosphere and space. Between 1959 and 1968, 12 pilots completed 199 missions.

RoAdevăr has also written about NASA programmes that link experimental flight and space exploration, including preparations for Artemis III.

Photo: a Boeing NB-52A just after releasing an X-15. US government photograph, public domain via Wikimedia Commons.

Source consulted: NASA — 65 Years Ago: First Powered Flight of the X-15 Hypersonic Rocket Plane.