The basis of most jet engines is: air is drawn in at the front. The air is then is compressed by rotating fans. Fuel is then added and combusted. The combustion greatly increases the volume of the gases which are then exhausted out of the rear of the engine.
The gas turbine was the earliest form of jet propulsion. The idea was developed independently in England (Whittle) and Germany (von Ohain). They did not invent the gas turbine but both realised the promise the design held for flight. Whittle filed his first patent in 1930 and ran his first engine in April 1937. Almost a month earlier von Ohain ran an experimental hydrogen powered engine. The first jet aircraft - a Heinkel He-178, flew on 27th August 1939. An English Gloster-Whittle E28/39 flew 21 months later.
There are a number of different types of jet engines:
- Turbojet - the simplest jet engine. It consists of an air intake at the front, a compressor section, a combustion chamber, a turbine (to extract some of the energy of the exhaust to drive the compressor), and an exhaust nozzle.
- Turbofan or By-Pass Engine - Improves the Froude efficiency of the turbojet by increasing the size of the compressor stage and by-passing some of the compressed air around the combustion chamber and the turbine. This acts as a jet-powered ducted fan propeller. Efficiency can be further improved by by-passing even more air around the core -- high by-pass turbo-fans and even ultra high by-pass (UHB) engines -- where with an unducted fan it is a grey area whether the engine is a jet or an advanced type of propellor. UHB engines are noisy and only capable of high subsonic speeds.
- Ramjet - When air enters a jet engine its speed decreases and its pressure increases, the ram compression effect. So as an aircraft's speed rises the compressor becomes less and less necessary. At Mach three and above no compressor (or turbine) is needed for efficient combustion. All that is needed is a suitably shaped tube, with a variable internal geometry to create a combustion chamber where fuel is injected. A ramjet thus contains no moving parts and can run much more reliably at supersonic speeds than a turbofan. However ramjets work inefficiently below Mach 3, and not at all from stand still so some kind of booster is required to get the aircraft up to speed. One possibility is a dual-mode turbo-ramjet (the HOTOL space vechicle) but rocket boosters, a drop from a mother ship or even a catapult are more common today. At hypersonic speeds the compression produced by the shock waves also allows the possibility of an external ramjet - fuel could be injected directly into the airstream and burned, integrating lift and propulsion onto the surface of an aircraft.
- Scramjet (supersonic combusting ramjet) - When the air inside a ramjet exceeds the speed of sound (an external aircraft speed of around Mach 5+) combustion fails to occur properly. This is overcome in a scramjet: the inlet is much wider so the compression is less and the air remains at supersonic speeds. But conventional fuels are unusable so reactive chemicals or gases are used and the design of the jet is much more complex. Like a ramjet the scramjet must already be moving extremely fast before it will start working but, theoretically, speeds in excess of Mach 20 are possible.
Air-breathing Rocket Hybrid