North American X-15

The North American X-15 holds world records that remain unbroken more than 55 years after the programme ended: Mach 6.72 (7,274 km/h) for the highest speed of a winged, manned aircraft, and 107,960 metres for the highest altitude reached by a non-orbital, winged, manned aircraft. These marks were set in 1967, the final year of the programme, by William ‘Pete’ Knight (speed) and Joseph Walker (altitude, in 1963). No aircraft since has come close to either figure.

The X-15 was not a jet aircraft. It was powered by a single Reaction Motors XLR99-RM-2 rocket engine burning anhydrous ammonia and liquid oxygen, producing 253 kN of thrust for roughly 85 seconds before fuel exhaustion. The aircraft was carried aloft under the wing of a Boeing B-52 and released at approximately 14,000 metres, at which point the pilot ignited the rocket engine. After burnout, the aircraft coasted to its peak altitude or speed before an unpowered glide back to Edwards Air Force Base.

The X-15 programme made fundamental contributions to the understanding of hypersonic flight, aerodynamic heating, and reaction control systems (small rocket thrusters used for attitude control in near-space where aerodynamic surfaces are ineffective). Thirteen of the 199 flights qualified as spaceflight under the USAF/NASA definition of 50 miles (80 km) altitude — meaning eight X-15 pilots earned military astronaut wings. Several X-15 pilots later flew in the Apollo and Space Shuttle programmes.