Scramjets, of course, can only operate at hypersonic speeds, as they rely on extreme intake air velocity to compress and heat the air before combustion can take place. Where a turbojet uses a compressor and a piston engine uses a compression stroke to achieve this effect, scramjets in some cases need no moving parts at all. Air comes in at hypersonic speeds, and is then forced into a narrowing channel which compresses it, and then fuel is added at the narrowest point, igniting and producing thrust as it leaves the chamber.

In this case, the test unit put out more than 13,000 pounds of thrust – a US Air Force record test figure for an air-breathing hypersonic engine. That might not sound very impressive, what with Boeing gearing up to put GE9X jets on its next model airliners that make more than a 100,000 pounds of thrust each, but hypersonic flight starts at five times the speed of sound and theoretically goes up to as fast as Mach 24, and this makes everything considerably more challenging.