Ramjet engines used in aerospace applications ingest air into an engine inlet at supersonic speeds caused by the forward motion of an airplane or missile. The air is rammed into a smaller opening between a center-body and the engine side wall generating a series of shock waves. These shock waves compress and decelerate the air to subsonic speeds while, at the same time, dramatically raising working flow pressure and temperature. The ramjet effect may also be achieved in a stationary platform by passing an accelerated flow of air over raised sections machined on the rim of a rotor disc. Combined with the high rotation rate of the rotor, this produces a supersonic flow relative to the rotor rim. Interaction between the raised sections of the rim which are rotating at supersonic speeds and the stationary engine case creates a series of shock waves that compress the air stream in a manner similar to ramjet inlets on a supersonic missile or aircraft.
The advent of carbon composite and like materials has enabled the introduction of a reinforcement wall, called rim-rotor, for compensating centrifugal forces generated by rotating components of the ramjet engine. In a rim-rotor rotary ramjet engine (R4E), inlet blades compress the air and fuel mixture with shockwaves, combustion takes place to increase the flow enthalpy and finally the products are accelerated by outlet blades at a high tangential speed to generate shaft power.
Improvements to the rim-rotor rotary ramjet engine are still required in order to reach better fuel efficiency and power output.