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Flight Applications of Shock Compression

Since the sound barrier was broken in the late 1940's, ramjet engines have been widely used as a means to propel aerospace vehicles at supersonic speeds. The technology is very well understood and fully characterized.

All conventional, subsonic jet engines feature discrete compression, combustion and turbine/expansion sections to create the thrust used to propel an aircraft. In operation, hot pressurized exhaust gas expands through the turbine to drive the compressor, and then further expands through a nozzle, creating forward thrust.

Ramjet engines feature these same discrete compression, combustion and expansion sections. The significant difference in ramjet engines is that the compressor section does not rotate and the turbine section is therefore eliminated. There are no rotating components in the engine. At supersonic velocities, air is ingested into the engine and flows around a fixed obstructing body in the center of the engine duct, "ramming" the air flow into channels between the center-body and the engine's sidewall. Inside these channels, the airflow is almost instantaneously slowed to subsonic speeds, creating "shock waves." These shock waves are associated with a dramatic increase in pressure, or, in other words, "shock compression." As with conventional subsonic turbine engines, fuel is then added and the hot, pressurized exhaust gas expands through a nozzle to create forward thrust.


Typical Ramjet Engine

Ramjets are simple, with no moving parts, but the aircraft has to be moving at supersonic speeds to initiate the shock necessary for effective operation. As a result, all ramjet experience has been in the context of supersonic planes and missiles.

One well-known application of shock compression is its use in the F-15 fighter jet. The pictures and illustrations below show how a supersonic shock compression inlet acts to boost the inlet pressure, while at the same time reducing the air flow to the subsonic velocity required by the engine.


Picture and location of F-15 Supersonic Inlet



F-15 Supersonic Inlet Cross Section Detail

 

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