From SKYbrary Wiki
A ramjet is a variant of an air breathing jet engine that does not include a rotary compressor; rather, it uses the engine's forward motion to compress the incoming air. A ramjet cannot function at zero airspeed and therefore cannot be used to power an aircraft in all phases of flight. A ramjet equipped aircraft requires another type of propulsion to accelerate it to a speed at which the ramjet is capable of producing thrust. A ramjet can theoretically be started at speeds as low as 100 knots but it does not start to produce any significant thrust until the airspeed reaches approximately mach 0.5. Even at this speed, efficiency is very low and peak efficiency will not be attained until reaching supersonic speeds in the realm of mach 3. Ramjet engines are limited to a maximum speed of about mach 6 due to the shockwave induced pressure loss which occurs when slowing the intake air to subsonic speed.
In its most basic form, a ramjet has very few moving parts and, because of this simplicity, is often referred to as a "flying stovepipe". The engine consists of an air intake, a combustor and an exhaust nozzle. A high speed object moving through the air generates a high pressure region in front of it. The ramjet engine intake takes in high dynamic pressure air from this region and, using shockwaves created by the intake configuration, slows the air to subsonic speed before it enters the combustion chamber. In the combustor, fuel is mixed with the compressed air and ignited to reaccelerate the exhaust to supersonic speed. The exhaust gas is further accelerated as it exits the engine due to the geometric configuration of the exhaust nozzle.