A scramjet is a supersonic combustion ramjet engine that operates – in fact, can only operate – at hypersonic (greater than Mach 5) speeds. Like its comparatively slower ramjet counterpart, the scramjet has engines with a simple mechanical design and no moving parts. However, scramjet combustion occurs at supersonic air speeds in the engine. Rather than using a rotating compressor like a turbojet engine, the forward velocity and vehicle aerodynamic design compress air into the engine. There, fuel, usually hydrogen, is injected and the expanding hot gases from combustion accelerate the exhaust air and create thrust. Experimental scramjets include NASA's X-43A and a DARPA (Defense Advanced Research Projects Agency) scale model that can be fired from a special gun. The later achieved the first-ever free flight of a scramjet in September 2001 when it was gun-launched at Mach 7, then used its scramjet engine to travel 80 meters in just over 30 milliseconds.