It isn't universally true that military and commercial aircraft use different engines. The military and commercial versions of the Boeing 707/C135 used similar engines as do the military and commercial versions of the DC10. Combat aircraft however are usually powered by low bypass turbofan engines rather than the high bypass turbofans used in commercial jetliners. The low bypass engines are less fuel efficient but are more compact and have a higher thrust-to-weight ratio than the high bypass engines as well as including an afterburner which can be used to increase thrust at critical periods although at the expense of even greater fuel consumption. The design of the Lockheed SR71 "Blackbird" reconnaissance aircraft featured turbo-ramjet engines which at high speeds generated most of their thrust in the bypass ducts surrounding the central turbojet engine, using the ramjet principle in which the incoming air is compressed by ram pressure alone without the use of a compressor. The requirement for supersonic maximum speeds has traditionally been a major factor determining the choice of engines for combat, although such speeds have in practice only been routinely used by reconnaissance aircraft, most notably the SR71. Most manned fighters use so much fuel in afterburner that this capability is for practical purposes almost useless. The F22 Raptor however has a "supercruise" capability of achieving supersonic speed without the use of afterburner.