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What are Jet Engines? Although lighter than reciprocating engines, these jet engines provide much more thrust. The only downside to these engines is that they are not as efficient at lower speeds. There are four types of jet engines, turbojet, turboprop, turbofan, and ramjet. However, the ramjet is primarily used for unmanned craft, such as guided missiles. 
Jet engines operate on the basis of Newton’s Third Law of Motion. The gases are expelled from the nozzle at incredible speeds, so as a reactive force, the engine is pushed forward with the same force - thrust. However, because jet engines accelerate a relatively small amount of air to very high speeds, there is much energy in the form of heat that is unused for thrust. Different Types The turbojet was the first jet to be developed. Like most of the other jet engines, this engine intakes relatively low velocity air at the front and compresses it. Compression is accomplished by a series of wheels with many blades attached to them. These turbines are configured in such a way to squeeze the air closer together, making them more combustible. This higher energy air flows into the combustion chamber where it is mixed with fuel and burned. Since only a fraction of the air is burned, the rest is used to cool off the engine since the burned mixture reached scorching temperatures and immense pressure. This hot gas rushes through and turns the turbine at the end, which powers the combustion chamber. Finally the gases reach the nozzle where they are accelerated out at tremendous speeds. Turboprop engines use the force of jet engines to spin a propeller. The internal structure of a turboprop is much like that of the turbojet. The difference is that in a turboprop, there is a second turbine behind the one that turns the combustion chamber. This second turbine, known as the power turbine spins the propeller at the front of the engine by means of shafts. The remaining high energy gas still left in the combustion chamber is used for jet propulsion. A turboprop engine is quieter and smoother than conventional jet engines, but these are limited only to subsonic flight. The most common jet engine, the turbofan combines the efficiency of the propeller without losing much of the raw thrust power of a turbojet. Like the turboprop, this engine has two turbines at the end of the engine. The last turbine powers a large propeller-like fan enclosed within the engine. This huge fan accelerates some of the air into the normal jet propulsion sequence, but most of the air is forced back out of the engine at high speeds to create further thrust. Since its exhaust velocity is slower than that of a turbojet, the turbofan is a relatively quiet yet powerful engine. Four turboprops provide the thrust for enormous planes like the Boeing 747. The final most simple type of jet is the ramjet. This is the simplest type because it lacks a compression chamber as well as a turbine. One downside to this type of engine is that it does not function at subsonic speeds. A plane must first accelerate it to supersonic speeds in order for this jet to have enough compression of the air entering the ducts. First the air entering is slowed down by a diffuser. This air is automatically compressed by the action of more air trying to squeeze in the ducts. Immediately the air is mixed and burned and flies out the nozzle. 
Afterburners Afterburners are special devices fitted between the turbine and the nozzle of some turbojets and turbofans let the thrust become especially strong for short bursts of time. When the combusted mixture is about the enter the nozzle, a lot of the concentration is still oxygen. Inside the afterburner, more fuel is mixed, increasing the fuel concentration. When this new mixture is burned, much higher temperatures are reached and the thrust becomes much stronger. Afterburners consume a lot of fuel, so an "afterburn" only lasts for a short period of time. Normal jet engine nozzles narrow as it get further away from the engine, but some nozzles designed for planes that fly at supersonic speeds become narrow then widen out. This shape of the nozzle helps accelerate the gases at speeds faster than the speed of sound. 
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