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A rocket engine is a Reaction Engine that can be used for Spacecraft Propulsion as well as terrestrial uses, such as Missile s. Rocket engines take all their reaction mass from within tankage and form it into a high speed Jet , obtaining thrust in accordance with Newton's Third Law . Most rocket engines are Internal Combustion Engine s, although non combusting forms exist. PRINCIPLE OF OPERATION Most rocket engines produce thrust by the expulsion of a high-temperature, high-speed gaseous exhaust. This is typically created by high pressure (10-200 bar) combustion of solid or liquid Propellants , consisting of Fuel and Oxidizer components, within a Combustion Chamber . Liquid-fueled Rockets typically pump separate fuel and oxidizer components into the combustion chamber, where they mix and burn. Solid Rocket propellants are prepared as a mixture of fuel and oxidizing components and the propellant storage chamber becomes the combustion chamber. Hybrid Rocket engines use a combination of solid and liquid or gaseous propellants. Alternatively, a chemically inert Reaction Mass can be heated using a high-energy power source. The hot gas produced escapes through a narrow opening (the "throat"), into a High Expansion-ratio 'de Laval Nozzle' . The nozzle dramatically accelerates the gas, converting most of the thermal energy into kinetic energy. The large bell or cone shaped expansion nozzle gives a rocket engine its characteristic shape. Exhaust speeds as high as ten times the Speed Of Sound at sea level are not uncommon. A portion of the rocket engine's thrust comes from the unbalanced pressures inside the combustion chamber but the majority comes from the pressures against the inside of the nozzle. As the gas expands ( Adiabatically ) the pressure against the nozzle's walls forces the rocket engine in one direction while accelerating the gas in the other. The highest exhaust speed possible is highly desirable for rocket engines to minimise propellant usage. For aerodynamic reasons the flow goes sonic (" Chokes ") at the narrowest part of the nozzle, the 'throat'. Since the Speed Of Sound in gases increases with the square root of temperature, the use of hot exhaust gas greatly improves performance. By comparison, at room temperature the speed of sound in air is about 340m/s while the speed of sound in the hot gas of a rocket engine can be over 1700m/s; much of this performance is due to the higher temperature, but additionally rocket propellants are chosen to be of low molecular mass, and this also gives a higher velocity compared to air. Expansion in the rocket nozzle then further multiplies the speed, typically between 1.5 and 4 times, giving a highly Collimated hypersonic exhaust jet. The speed increase 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 detailed properties of the gas are also important. Larger ratio nozzles are more massive but are able to extract more heat from the combustion gases, increasing the exhaust velocity. Nozzle efficiency is affected by operation in the atmosphere because atmospheric pressure changes with altitude. For optimal performance the pressure of the gas at the end of the nozzle should just equal the 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 maintain 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 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. PERFORMANCE Rocket technology can combine very high thrust (Mega Newtons), very high exhaust speeds (around 10 times the speed of sound at sea level) and very high thrust/weight ratios (>100) ''simultaneously'' as well as being able to operate outside the atmosphere. Rockets can be further optimised to even more extreme performance along one or more of these axes at the expense of the others. Rocket engine nozzles are surprisingly efficient Heat Engines for generating a high speed jet, as a consequence of the high combustion temperature and high Compression Ratio in accordance with the Carnot Cycle . For a vehicle employing a rocket engine the energetic efficiency is very good if the vehicle speed approaches or somewhat exceeds the exhaust velocity (relative to launch); but at low speeds the efficiency asymptotically approaches 0% at zero speed (as with all Jet Propulsion .) THERMAL ISSUES The reaction mass's combustion temperatures can fairly typically reach ~3500 K (~5800 F) 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. Carbon based materials such as graphite, diamond, carbon nanotubes or certain metals such as Tantalum , Tungsten are able to take even these temperatures, but require protection from oxidation. 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, are employed to give longer nozzle and chamber life. These techniques ensure that a gaseous thermal Boundary Layer touching the material is kept below the temperature which would cause the material to catastrophically 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. SAFETY Rocket engines are tested at a Test Facility before being put into production. Rocket s have a reputation for unreliability and danger; especially catastrophic failures. Contrary to this reputation, carefully designed rockets can be made arbitrarily reliable. In military use, rockets are not unreliable. However, one of the main non-military uses of rockets is for orbital launch. In this application, the premium is on minimum weight, and it is difficult to achieve high reliability and low weight simultaneously. In addition, if the number of flights launched is low, there is a very high chance of a design, operations or manufacturing error causing destruction of the vehicle. Essentially all launch vehicles are test vehicles by normal aerospace standards ( As Of 2006 ). 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 350 engine-flights. NOISE The Saturn V launch was detectable on Seismometer s a considerable distance from the launch site. As the Hypersonic exhaust mixes with the ambient air, Shock Wave s 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 DB(A) of noise around its base. Generally speaking noise is most intense when a rocket is close to the ground, since the noise from the engines radiates 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. CHEMISTRY Although Rocket Propellant s require relatively high energy density (energy per unit mass) many common materials are more energetic. For example, petrol/gasoline or paraffin has as much energy as a typical rocket fuel and far more than the fuel/oxidiser mix used in a rocket engine. This is because the rocket propellant carries its own oxidiser. Fuels for automobile or Turbojet Engine s, utilise atmospheric oxygen and can have much higher energy density. Many rocket propellants use 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) Newsgroup correspondence , 1998-99. Computer programs that predict the performance of propellants in rocket engines are available. Complex chemical equilibrium and rocket performance calculations , Cpropep-Web. 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 hard starts, 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. Once ignited, rocket chambers are self sustaining and igniters are not needed. Indeed chambers often spontaneously reignite if they are restarted after being shut down for a few seconds. However, when cooled, many rockets cannot be restarted without at least minor maintenance, such as replacement of the pyrotechnic igniter. TYPES OF ROCKET ENGINES See Also: Liquid rocket propellants Electric heating See Also: Ion thruster Solar heating The Solar Thermal Rocket would make use of solar power to directly heat Reaction Mass , and therefore does not require an electrical generator as most other forms of solar-powered propulsion do. A solar thermal rocket only has to carry the means of capturing solar energy, such as Concentrator s and Mirror s. The heated propellant is fed through a conventional rocket nozzle to produce thrust. The engine thrust is directly related to the surface area of the solar collector and to the local intensity of the solar radiation. Beamed power Nuclear heating Nuclear Propulsion includes a wide variety of Propulsion methods that use some form of Nuclear Reaction as their primary power source. Various types of nuclear propulsion have been proposed, and some of them tested, for spacecraft applications: HISTORY OF ROCKET ENGINES According to the writings of the Roman Aulus Gellius , in c. 400 BC , a Greek Pythagorean named Archytas , propelled a wooden bird along wires using steam.Leofranc Holford-Strevens, ''Aulus Gellius: An Antonine Author and his Achievement'' (Oxford University Press; revised paperback edn. 2005)
The '' Aeolipile '' ( 50 / 62 / 70 ) (known as '' Hero's Engine '') was a Rocket-like Reaction Engine and the first recorded Steam Engine . It essentially consists of a hot water rocket on a bearing. It was created almost two millennia before the industrial revolution. Apparently Hero's steam engine was taken to be little more than a toy, the principles behind it were not well understood, and its full potential not realized for a millenium. The availability of Black Powder to propel projectiles was a precursor to the development of the first solid rocket. Ninth Century Chinese Taoist Alchemists discovered Black Powder in a search for the Elixir Of Life ; this accidental discovery led to Fire Arrow s which were the first rocket engines to leave the ground. Slow development of this technology continued up to the later 20th Century, when the writings of Konstantin Tsiolkovsky first talked about Liquid Fuelled Rocket Engines . These independently became a reality thanks to Robert Goddard . REFERENCES SEE ALSO
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