Rocket engine
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A rocket engine is a heat engine that can be used for spacecraft propulsion as well as terrestrial 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.
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.
Ignition
With liquid propellants immediate ignition of the propellants as they first enter the combustion chamber is essential.Failure to ignite within milliseconds causes too much liquid propellant to be within the chamber, and if/when ignition occurs the amount of hot gas created will often exceed the maximum design pressure of the chamber. The pressure vessel will often fail catastrophically. This is sometimes called a hard start.
Ignition can be achieved by a number of different methods; a pyrotechnic charge can be used, the propellants can ignite spontaneously on contact (hypergolic), a plasma torch can be used, or electric spark plugs may be employed.
Gaseous propellants generally will not cause hardstarts, with rockets the total injector area is less than the throat thus the chamber pressure tends to ambient prior to ignition and high pressures cannot form even if the entire chamber is full of flammable gas at ignition.
Solid propellants are usually ignited with one-shot pyrotechnic devices.
Chemistry
Contrary to popular belief, whilst rocket propellants require reasonably high energy per kilogram, many common materials are more energetic; for example petrol/gasoline for cars has more energy. Largely this discrepancy is due to the necessity of the propellant containing large amounts of oxidiser, normal fuels used on earth react with the atmosphere and hence can have higher energy density.
Good rocket propellants require large quantities of hydrogen in the propellant, as this gives the highest exhaust speeds primarily due to the low molecular mass; but this is not the whole story.
Programs exist to predict the performance of propellants in rocket engines.(e.g.[Cpropep-Web]).
See also Liquid rocket propellants.
Thermal issues
The reaction mass's combustion temperatures can fairly typically reach 3500K (4500F) which is often far higher than the melting point of the nozzle and combustion chamber materials (~1200K for copper). 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, nickel or copper alloys and employ cooling systems that 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. These techniques ensure that the gas boundary layer touching the material is kept below the point where the material would fail.
Mechanical issues
The combustion chamber is often under substantial pressure, typically 10-200 bar, higher pressures giving better performance. This causes the outermost part of the chamber to be under very large hoop stresses.
Worse, due to the high temperatures created in rocket engines the materials used tend to have a significantly lowered working tensile strength.
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.
Safety
Rockets have a reputation for unreliability and danger; particularly catastrophic failures.
In fact, carefully designed rockets can probably be made arbitrarily reliable. In military use, rockets are not unreliable. However one of the main uses of rockets is for orbital launch. There the premium is on minimum weight, and it is difficult to achieve high reliability and low weight simultaneously. In addition the number of flights launched is low, thus there is a very high chance of a design, operations or manufacturing error causing destruction of the vehicle. Essentially, as of 2006 all launch vehicles are test vehicles by normal aerospace standards.
The X-15 rocket plane achieved a 0.5% failure rate, with a single catastrophic failure during ground test, and the SSME has managed to avoid catastrophic failures in over 300 engine-flights.
Noise
Rockets are incredibly noisy! The Saturn V launch was detectable on seismometers a considerable distance from the launch site.
As the hypersonic exhaust mixes with the ambient air, shock waves are formed. The sound intensity from these shock waves depends on the size of the rocket, and on large rockets can actually kill. The Space Shuttle generates over 200 dBA of noise around its base.
Generally speaking the noise is most intense at the ground, since the noise from the engines radiate up away from the plume, as well as reflecting off the ground.
This noise can be reduced somewhat by flame trenches with roofs, by water injection around the plume and by deflecting the plume at an angle.
Types of rocket engines
Low temperature
- cold gas thruster
- water rocket
Chemical heating
- Solid rocket
- Hybrid rocket
- Monopropellant rocket
- Bipropellant rocket
- Tripropellant rocket
- Dual mode propulsion rocket
Electric heating
- Resistojet rocket (electric heating)
- Arcjet rocket (chemical burning aided by electrical discharge)
- Pulsed plasma thruster (electric arc heating; emits plasma)
Solar heating
Nuclear heating
- Nuclear thermal rocket (nuclear fission energy)
- Radioisotope rocket/"Poodle thruster" (radioactive decay energy)
- Antimatter catalyzed nuclear pulse propulsion (fission and/or fusion energy)
- Gas core reactor rocket (nuclear fission energy)
- Fission-fragment rocket (nuclear fission energy)
- Fission sail (nuclear fission energy)
- Nuclear salt-water rocket (nuclear fission energy)
- Nuclear pulse propulsion (exploding fission/fusion bombs)
- Fusion rocket (nuclear fusion energy)
- Antimatter rocket (annihilation energy)
References
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