The U.S. military on Tuesday conducted an unmanned test flight of its hypersonic Waverider aircraft, designed to move at six times the speed of sound using technology that bridges the gap between planes and rocketships. Reuters reports that Pratt & Whitney Rocketdyne designed the X-51A’s “scramjet” engine, which uses the forward motion of the craft to compress air for fuel combustion, according to a description of the project from the military.