The first type of jet engine is known as ‘Turbojet’ engine. Air is drawn into the turbojet by a compressor, which is basically a large spinning fan. The compressor slows down the incoming air, raising its pressure, and delivers it to the combustion chamber. Fuel is injected into the high-pressure air in the combustion chamber and ignited. The resulting hot gases expand and rush first through a turbine and then through a jet pipe and nozzle at the rear. Forward thrust is generated as a reaction to the rearward momentum of the exhaust gases.
A rocket engine doesn’t need air to function, since the rocket carries both its own fuel and oxygen supplies, whereas a jet or car only carries fuel, obtaining its’ oxygen from the air – it’s air breathing. In a rocket, the combination of the fuel and oxygen source is called the propellant. Since a rocket carries it’s own oxygen, it can be used where jet engines can’t, such as at altitudes above 30 miles. Above 30 miles, there is not enough air for jet engines to work. Thus rockets are used to carry payloads (cargo such as satellites, or people) to very high altitudes, into orbit around earth, to the moon, or even through interplanetary space.
There are many types of rockets, characterized by both the form of the propellant and the kind of fuels and oxygen sources (oxidizer) they use. There are rockets that use liquid propellants, others that use solid propellants, and still others that are variants of the above types. Two of the important variants are called gel propellants and hybrid propellants.