Rocket engine

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Image:Rocket-test-chamber-NASA-med.jpg A rocket engine is a heat engine that can be used for spacecraft propulsion as well as terrestial uses, such as missiles. Rocket engines take their reaction mass from one or more tanks and form it into a hypersonic jet, obtaining thrust in accordance with Newton's third law. Most rocket engines are internal combustion engines, although non combusting forms exist.

1 Principle of operation
2 Thermal issues
3 Non chemical rockets
4 Types of rocket engines

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Principle of operation

Classic rocket engines produce a high temperature gaseous exhaust. This is achieved by the combustion of solid, liquid or gaseous propellant, containing oxidiser and a fuel, within a combustion chamber at high pressure. The hot gas produced is then allowed to escape through a narrow hole (the 'throat'), into a high-expansion ratio nozzle. The large bell or cone shaped expansion nozzle gives a rocket engine its characteristic shape. The effect of the nozzle is to dramatically accelerate the mass, converting most of the thermal energy into kinetic energy. Exhaust speeds as high as 10 times the speed of sound at sea level are not uncommon.

Part of the rocket engine's thrust comes from the gas pressure inside the combustion chamber but the majority comes from the pressure against the inside of the expansion nozzle. Inside the combustion chamber the gas produces a similar force against all the sides of the combustion chamber but the throat gives no force producing an unopposed resultant force from the diametrically opposite end of the chamber. As the gases (adiabatically) expand inside the nozzle they press against the bell's walls forcing the rocket engine in one direction, and accelerating the gases in the opposite direction.

For optimum performance hot gas is used because it maximises the speed of sound at the throat-for aerodynamic reasons the flow goes sonic ("chokes") at the throat, so the highest speed there is desirable. By comparison, at room temperature the speed of sound in air is about 340m/s, the speed of sound in the hot gas of a rocket engine can be over 1700m/s.

The expansion part of the rocket nozzle then multiplies the speed of the flow by a further factor, typically between 1.5 and 4 times, giving a highly collimated exhaust jet. The speed ratio of a rocket nozzle is mostly determined by its area expansion ratio—the ratio of the area of the throat to the area at the exit, but details of the gas properties are also important. Larger ratio nozzles are more massive and bulkier, but they are able to extract more heat from the combustion gases, which become lower in pressure and colder, but also faster.

A significant complication arises when launching a vehicle from the Earth's surface as the ambient atmospheric pressure changes with altitude. For maximum performance it turns out that the pressure of the gas leaving a rocket nozzle should be the same as ambient pressure; if lower the vehicle will be slowed by the difference in pressure between the top of the engine and the exit, if higher then this represents pressure that the bell has not turned into thrust. To achieve this ideal, the diameter of the nozzle would need to increase with altitude, which is difficult to arrange. A compromise nozzle is generally used and some percentage reduction in performance occurs. To improve on this, various exotic nozzle designs such as the plug nozzle, stepped nozzles, the expanding nozzle and the aerospike have been proposed, each having some way to adapt to changing ambient air pressure and each allowing the gas to expand further against the nozzle giving extra thrust at higher altitude.

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Thermal issues

The reaction mass's combustion temperature is not atypically far higher than the melting point of the nozzle and combustion chamber materials. Indeed many construction materials can make perfectly acceptable propellants in their own right. It is important that these materials be prevented from combusting, melting or vapourising to the point of failure. Materials technology could potentially place an upper limit on the exhaust temperature of chemical rockets.

To avoid this problem rockets can use ablative materials that erode in a controlled fashion, or very high temperature materials, such as graphite, ceramics or certain exotic metals.

Alternatively, rockets may use more common construction materials such as aluminum, steel or copper alloys and employ cooling systems to prevent the construction material itself becoming too hot. Regenerative cooling, where the propellant is passed through tubes around the combustion chamber or nozzle, and other techniques such as curtain cooling or film cooling, may be employed to give essentially unlimited nozzle and chamber life.

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Non chemical rockets

Rockets emitting plasma can potentially carry out reactions inside a magnetic bottle and release the plasma via a magnetic nozzle, so that no solid matter need come in contact with the plasma. Of course, the machinery to do this is complex, but research into nuclear fusion has developed methods, some of which have been proposed to be used in speculative propulsion systems.

Image:H1Rocket.JPG Image:Twin Linear Aerospike XRS-2200 Engine.jpg

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