Piston engines, jet engines, and rocket engines all depend on the same basic principles to produce thrust.
1. The engine mixes fuel with oxygen or another oxidizer in a combustion chamber.
2. The mixture is ignited.
3. The burning mixture creates hot, expanding gases.
4. The expanding gases either produce thrust directly (in jet and rocket engines), or are used to push a piston or drive a turbine.
Newton’s Third Law of Motion is often given as an explanation for the generation of thrust. But it explains the effect of thrust, not the cause of thrust. Pressure and shear stress are the only two ways nature exerts an aerodynamic force on an object. Pressure is the basic source of thrust produced by a propeller, jet engine, or rocket engine.
A piston engine cannot produce thrust on its own. It provides power to a spinning propeller, which produces thrust by creating a pressure difference between the front and back of the propeller, resulting in a forward force. Jets or rocket engines produce thrust by increasing the pressure inside the engine. This increased pressure in the jet or rocket engine exerts more force in the forward direction than the rear direction.
The exhaust gases produced by a propeller, jet or rocket, due to Newton’s Third Law, are feeling a force opposite and equal to the thrust, and therefore are moved in the direction opposite to the thrust of the engine. Hence, the exhaust is the effect of thrust.