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What are Rocket Engines?
If you've ever seen a movie with fighter planes in it, or you've ever launched a model rocket, you've probably seen a rocket engine in action. However, you've probably never actually thought about what's going on inside. 
Differences Rocket engines operate in a very different manner than that of reciprocal engines and jet engines, in that its main component of thrust is rocket fuel, and it operates with different characteristics. In short, rocket engines operate better at higher speeds, burn more fuel, and are more dangerous. Rocket engines, unlike other types, do not require an external supply of oxygen like jet engines, since they burn fuel internally. Rocket engines burn some combination of fuels, either fluid or solid. With engines which burn solid fuel, there is one type of fuel, but with fluid-propellant engines, two fuels are often used, such as liquid hydrogen (the fuel) and liquid oxygen (the oxidizer). Rocket engines are much more expensive than jet engines to operate; this should be somewhat intuitive. We know that for either a jet or a rocket engine to propel a plane, they both must provide the same amount of thrust. However, the jet engine is propelling backwards both fuel and oxygen, while the rocket engine is propelling only fuel. Since the jet and rocket engines must both provide the same amount of thrust, but one is using oxygen (free!) for part of it, the jet engine, which uses less fuel to accomplish the same amount of thrust, must be cheaper. Rocket engines, unlike jet engines, are slightly dangerous, since they involve combustion. In commercial airlines, the possibility of a fire is something not to be considered, so rocket engines are not used as the main source of thrust in commercial planes. However, a few jet and turboprop planes use small rockets, to help with extra thrust at takeoff, when there is a short runway, for example. In this case, the small rockets are attached to the plane's belly or under its wings, in a system called JATO, short for Jet-Assisted Take-Off. In some places, however, rocket engines are used frequently. The military, for example, uses rocket engines in all of the high-speed fighter planes. In addition, many supersonic test planes have used rocket engines. 
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