Jet Engine Sub-Types
	Even before the first Jet Engine ran for the first time, it became clear that there were several ways in which gas turbine engines could be used to produce their thrust.
	The Turbojet or Pure Jet
	In the turbojet engine, all the air entering the engine passes through the combustion chamber and turbines. All the thrust is produced from a single stream of air which has gone through the entire engine. This type of design offers a high level of performance and its low frontal area produces less drag. However it suffers from a high level of fuel consumption and high noise levels. The noise is caused by the fast exhaust air tearing through the still outside air. This type of engine is not common on modern aircraft.
	Turboprop
	Before the advent of the jet engine, a conventional piston engine was used to drive the aircraft's propeller. The jet engine can be used to do the same job and this type of engine is known as a turboprop engine. Unlike a turbojet engine, very little of the thrust comes from the jet stream leaving the engines exhaust. Instead, an extra turbine stage is fitted which absorbs most of the energy from the gas stream to drive a gearbox which in turn drives the propeller. The turboprop is very fuel efficient at low to medium flying speeds but it cannot operate efficiently at higher speeds because of the increased drag of the propellers. The turbo-prop engine is quite noisy but this is due to the propeller and not the engine.
	Turbofan
	The turbofan is the trade off between the high power of the turbojet and the good economy of the turboprop. Like the turboprop, the turbofan does not use the jet stream to produce its thrust but uses an additional turbine to power a fan at the front of the engine. The fan operates like a propeller in many respects. It pushes large amounts of air rearwards just like the propeller. The fan blades are smaller than propeller blades but there are more of them and they are completely enclosed in their own fan casing. Some of the air that goes through the fan is used for the operation of the "core" jet engine but most of the air is ducted rearwards to provide the thrust. Turbofan engines are quiet in operation as the slower "bypass" air is mixed with the faster exhaust air before it leaves the engine.
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