The ramjet is the simplest possible type of jet engine involving, as it does, no moving parts. It was invented in 1913 by René Lorin in France.

Air entering the ramjet is compressed solely by the forward movement of the vehicle. Also known as a "flying drainpipe," it consists essentially of a long duct into which fuel is fed at a controlled rate. The fuel is ignited by the incoming heated, compressed air. Although straightforward in principle, the ramjet demands careful design if it is to work efficiently and significant design variations are called for depending on the operating speed of the vehicle. In any case, a ramjet will only start to work above a certain speed – typically about 485 km/h (300 mph) – so that below this speed some form of auxiliary propulsion system is needed. The ramjet is more fuel efficient than turbojets or turbofans above about Mach 3 (three times the speed of sound) making them very attractive for use on missiles.