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A jet engine that uses solely ram pressure to compress the air it takes in, rather than using a turbocompressor.

A ramjet is a kind of with no moving parts; rather than compressing air using a turbocompressor, it compresses the air simply by converting its speed into pressure (ram effect). As a result, ramjets are best suited for use at high speeds; they produce negligible thrust below about Mach 0.5, and are very inefficient below approximately Mach 2 (because they can't compress the air as well as a or can, thus limiting efficiency). Above Mach 3, ramjets become more efficient than turbojets, as the high speed allows enough ram compression and their lack of moving parts minimises disruption to the airflow through the engine. Ramjets remain the optimal choice up to about Mach 5, where compression heating from deceleration to below Mach 1 does not allow enough temperature margin for combustion; for flight, a (Supersonic Combustion RAMJET), which uses very fast-burning fuel (such as hydrogen) to avoid having to decelerate the airflow to subsonic speeds, is more efficient than a ramjet (although most hypersonic aircraft instead use engines, as scramjets are still in their infancy).

For more ramjet information, see Wikipedia.