Ramjet
A ramjet is a type of jet engine which uses the forward motion of the engine to produce thrust. Ramjets are most often attached to aircraft, and require that the aircraft is moving above the speed of sound (Mach 1). They are different from turbojets ("normal" aircraft engines) because they do not have any moving parts which compress the air. Instead, they use a shock cone to compress the air. The ramjet gets its name from this fact; it "rams" the air hard enough to compress it to a combustible pressure.