NASA's B-52B launch aircraft takes off from Edwards Air Force Base, California, carrying a X-43A hypersonic research vehicle attached to a modified Pegasus rocket. The X-43 was an unmanned experimental 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. It set several airspeed records for jet-propelled aircraft. The X-43 is the fastest aircraft on record at approximately Mach 9.2 (7,000 mph). 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. The first plane in the series, the X-43A, was a single-use vehicle. Three of them were built. NASA flew the third version of the X-43A on November 16, 2004. The modified Pegasus rocket which was launched from a B-52 mother ship at an altitude of 13,157 meters (43,166 ft). The X-43A set a new speed record of 10,617 km/h (6,598 mph) or Mach 9.65.