The NASA X-43 was an experimental unmanned hypersonic aircraft with multiple planned scale variations meant to test various aspects of hypersonic flight. It was part of the X-plane series and specifically of NASA's Hyper-X program developed in the late 1990s. It set several airspeed records for jet aircraft. The X-43 is the fastest jet-powered aircraft on record at approximately Mach 9.6.
NASA X-43
Artist's concept of X-43A with scramjet attached to the underside
NASA's B-52B launch aircraft takes off carrying the X-43A hypersonic research vehicle (March 27, 2004)
Full-scale model of the X-43 plane in Langley's 8-foot (2 m), high-temperature wind tunnel
In aerodynamics, a hypersonic speed is one that exceeds five times the speed of sound, often stated as starting at speeds of Mach 5 and above.
CFD image of the NASA X-43A at Mach 7