The X-51A WaveRider is an unmanned U.S. Air Force experimental aircraft capable of reaching hypersonic speeds (specifically, six times the speed of sound, or 4000 miles per hour) using new scramjet engine technology. Test flights began in December 2009 and continue today. – Design – The end of the Cold War and the transition from manned spyplanes like the U-2 and the SR-71 Blackbird to satellite surveillance and unmanned, remote-operated spyplanes somewhat reduced the military's long-lived desire to develop a hypersonic high-altitude aircraft. Nevertheless, the air force did begin to fund the studies that led to the X-51 in the 1990s, taking advantage of NASA's own hypersonic research on previous prototypes like the X-43. To reach its extremely high velocities, the X-51 takes advantage of a new jet propulsion technology known as the scramjet. Previous high-speed aircraft, like the SR-71 Blackbird, used ramjets, which were once the speed kings of the jet world. However, within a ramjet engine, the air actually slows down to subsonic speeds, is then combusted with jet fuel and accelerated back to supersonic speeds, and finally used to propel the aircraft forward. A scramjet design allows the engine to burn fuel without slowing down the air as it enters the engine. Ramjet-powered aircraft never exceeded a few times the speed of sound, but this innovation means that future scramjet aircraft will travel several times as fast, perhaps even Mach 20 or more. added by: Armageddon_Now
