The X-43 was an unmanned experimental hyper-sonic aircraft with multiple planned scale variations meant to test various aspects of hyper-sonic flight. It was part of the X-plane series and specifically of NASA’s Hyper-X program. It set several airspeed records for jet-propelled aircraft. The X-43 is the fastest aircraft on record at approximately 7,000 miles per hour (11,000 km/h). A winged booster rocket with the X-43 placed on top, called a ‘stack’, was drop launched from a larger carrier plane. After the booster rocket (a modified first stage of the Pegasus rocket) brought the stack to the target speed and altitude, it was discarded, and the X-43 flew free using its own engine, a scramjet.