Jet engines operate on the application of Newton’s third law of motion: for every action there is an equal and opposite reaction.
The most common type of jet engine is the turbojet engine. Air from the atmosphere enters the fan section at the front of the engine where it is compressed in the compressor section. Then it is forced into combustion chambers where fuel is sprayed into it and ignited. Gases that form expand rapidly and are exhausted out the rear of the combustion chambers. The energy from these gases spins the fan-like set of blades called a turbine, which rotates the turbine shaft. This shaft, in turn, rotates the compressor, thereby bringing in a fresh supply of air through the intake at the front of the engine. The rest of the energy is expelled out the tail pipe, providing forward thrust.
Adding an afterburner section, where extra fuel is sprayed into the gases as they are exhausted and the fuel burns, adds thrust.
In a turboprop engine, the exhaust gases rotate a propeller that is attached to the turbine shaft. The propeller provides increased fuel economy at lower altitudes.
Another type of turbine engine, called a turbofan or bypass engine, uses a fan to produce additional thrust. This is most efficient at high altitudes.
The fourth type of jet engine is the ramjet. The ramjet is a simple engine that lacks a turbine and compression chambers. Ramjets do not function at speeds below the speed of sound, although there are new variations of these called SCRAMjets that are designed to generate supersonic speeds. The engine must be on an aircraft already traveling above the speed of sound (supersonic speed) before it can be used. Supersonic air enters the front of the ramjet and is automatically compressed due to the shape of the engine’s opening. This compressed air is mixed with fuel in the combustion chamber and ignited, causing the resulting gases and energy to be expelled out of the back and providing forward thrust.
A rocket engine operates on the same principle as a jet engine but carries its oxygen with it rather than using oxygen from the atmosphere.