Classification of jet propulsion engines

All jet engines produce thrust by increasing the momentum of a stream of fluid and ejecting it at the rear. They are grouped by the way the working fluid is compressed and by whether oxygen is taken from the atmosphere.

Air-breathing engines (take oxygen from the air)

1. Turbine-powered (continuous combustion with a compressor):
   • Turbojet: all the air passes through the compressor, combustor, turbine and nozzle. Thrust comes from the hot jet.
   • Turbofan: a fan driven by the turbine pushes extra (bypass) air around the core. Good efficiency at subsonic speed.
   • Turboprop/turboshaft: most of the turbine power drives a propeller or shaft; the jet thrust is small.
2. Ram-powered:
   • Ramjet: compression only by the ram effect of the inlet diffuser; no compressor or turbine; works at $Ma > 1$ but cannot start from rest.
   • Scramjet: combustion in supersonic flow, for $Ma > 5$.
3. Non-continuous (intermittent) combustion: the pulsejet, with a valved inlet; fuel is burnt in repeated cycles of intake, ignition and exhaust (close to the Humphrey cycle).

Non-air-breathing engine

4. Rocket: carries both fuel and oxidiser; works in space; very high thrust and very high fuel consumption. Liquid, solid and hybrid propellant types exist.

Hybrid (combined) engines

5. These combine two principles in one unit, e.g. the turbo-ramjet (turbojet at low speed, ramjet at high speed), the air-turbo-rocket, or the hybrid rocket (solid fuel with a liquid oxidiser).

 Turbojet:   air->[C]->[Comb]->[T]->nozzle->jet
 Ramjet:     air->diffuser->[Comb]->nozzle->jet
 Rocket:     fuel+oxidiser->[Comb]->nozzle->jet