a scramjet (supersonic combustion ramjet) is a special type of ramjet jet engine where fuel combustion takes place while the air in the engine is moving faster than the speed of sound. in a ramjet, air is slowed to subsonic (slower than the speed of sound) speeds before fuel is burned. unlike ramjets, scramjets do not use shock cones to slow down incoming air; instead, the scramjet uses shock waves from combustion to slow down the air before fuel is burned. this means that scramjets can operate very efficiently at extremely high speeds. however, a scramjet relies on the aircraft moving much faster than the speed of sound in order to work; scramjets do not work at subsonic, or even low supersonic, speeds.