What causes propulsion engines to generate thrust
Any kind of reaction engine generates thrust because of the forces acting on the engine components. These pressure forces within the engine act on the compressor, turbine and casing. Thrust is not directly caused by the flow of gas, but by the direct 'static pressure' of the gas on engine surfaces.
The lower the net force acting on the surfaces in the engine, the less thrust is developed.
When the reaction fluid is moving supersonically, pressure waves in the flow cannot push directly backwards, so the fluid must be expanded laterally against a nozzle surface of some kind to gain more thrust.
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