The Difference Between Jet & Rocket Engines
Jet engines are propulsion devices that produce power by directing a 'jet' of a fluid (usually a gas) in order to produce motion at a desired velocity. Rocket engines are propulsion devices in which the energy of propulsion is provided exclusively by the chemical reaction in the fuel, and is not augmented by any moving parts.
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Clarifying Definitions
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Formally, rocket engines are a subset of jet engines. But in standard terminology a "jet engine" refers to an aircraft engine wherein the combustion of fuel drives a turbofan or turbojet that provides vehicle, thrust as well as the energy needed by the engine to maintain the reaction. These jet engines require air for fuel combustion. Rocket engines also produce a 'jet' of exhaust that drives them forward, but require no moving parts in the combustion area and usually do not need air.
How Jet Engines Work
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A jet engine first draws air into a compressor where a staggered series of moving and stationary fanblades compress and heat the air. It is then pushed into the combustion chamber, where fuel injectors deliver liquid fuel (such as kerosene or Jet A) that mixes with the air and is ignited. The energy of combustion creates very hot exhaust gases that push out of the chamber at very high speed. The gases now have one more task: exert force on the turbine blades that guard their exit to the atmosphere. The rear turbine is connected to the front turbofan that draws air into the engine; so the power of the exhaust not only propels the aircraft forward, but provides the power that keeps the engine running.
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How Rocket Engines Work
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In a rocket engine, there are no moving parts such as turbofans, compressors or turbines involved in producing thrust. Instead the energy comes exclusively from the chemical reaction in the combustion chamber. The exhaust gases produced by that reaction are directed out of a nozzle. In accordance with Newton's Third Law of Motion (that for every action there is an equal but opposite reaction), the exhaust gases pushing upon the inside of the nozzle push the rocket upwards while they exit downwards.
Key Differences
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rocket Jet engines require air (i.e. oxygen) to enable the burning of jet fuel that produces the energy. Rocket engines do not require air. Instead they carry their oxygen (in the form of an oxidizer) with them.
Jet engines require liquid fuel to function. Rocket engines, however, may be designed to use either liquid or solid fuel.
In liquid-fueled rockets, the oxidizing agent may be supercold liquid oxygen or another chemical like nitrogen tetroxide.
Solid-fueled rockets tend to be easier to handle because the fuel can be loaded during construction, making the rocket compact and quick to launch. Liquid-fueled rockets require more complicated handling and storage systems because the fuel can only be loaded close to launch.
Applications
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Jet engines use simple fuels that are relatively safe to handle. Because they are built to have rapidly adjustable power, they are well suited to sectors like aircraft where safety, responsiveness and passenger comfort are critical considerations.
Jet engines can only be used where there is sufficient oxygen to support combustion, which means they are limited by altitude.
Rocket engines function where jet engines can't: in the high upper atmosphere and outer space. Because their fuels and requirements are more complex, they cost much more.
Rocket engines also produce huge amounts of thrust in a very short time, meaning they exhaust their fuel supply quickly, and are suited to applications where comfort and adjustability are secondary considerations, like astronaut, satellite or missile launch.
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References
Resources
- Photo Credit Image by Flickr.com, courtesy of Steve Jurvetson Image by Flickr.com, courtesy of chuckyeager Image by Flickr.com, courtesy of Cliff
