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| {{For|a general overview of aircraft engines|Aircraft engine}}
| | == Louis Vuitton Sac A Main 2014 rouge ou bleu == |
| {{pp-semi|small=yes}}
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| [[File:F100 F-15 engine.JPG|thumb|right|A [[Pratt & Whitney F100]] [[turbofan]] engine for the [[F-15 Eagle]] being tested in the [[hush house]] at [[Florida Air National Guard]] base. The tunnel behind the engine muffles noise and allows exhaust to escape]]
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| [[Image:Jet engine simulation.jpg|right|thumb|Simulation of a low-bypass turbofan's airflow.]]
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| <!--When editing intro please try to keep it general -->
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| A '''jet engine''' is a [[reaction engine]] discharging a fast moving [[jet (fluid)|jet]] that generates [[thrust]] by ''[[jet propulsion]]'' in accordance with [[Isaac Newton|Newton's]] [[Newton's laws of motion|laws of motion]]. This broad definition of jet engines includes [[turbojet]]s, [[turbofan]]s, [[rocket engine|rocket]]s, [[ramjet]]s, and [[pulse jet engine|pulse jets]]. In general, jet engines are combustion engines but non-combusting forms also exist.
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| In common parlance, the term ''[[wiktionary:jet engine|jet engine]]'' loosely refers to an [[internal combustion engine|internal combustion]] [[airbreathing jet engine]] (a ''[[wiktionary:duct engine|duct engine]]''). These typically consist of an engine with a rotary (rotating) air compressor powered by a [[turbine]] ("[[Brayton cycle]]"), with the leftover power providing thrust via a [[propelling nozzle]]. [[Jet aircraft]] use these types of engines for long-distance travel. Early jet aircraft used [[turbojet]] engines which were relatively inefficient for subsonic [[flight]]. Modern subsonic jet aircraft usually use [[High-bypass turbofan|high-bypass turbofan engines]]. These engines offer high speed and greater fuel efficiency than piston and propeller aeroengines over long distances.<ref name=hist8/>
| | Soyez à l'affût près des arbres, des arbustes et le long des clôtures. Pourvu que je [http://www.bistrochapoo.be/includes/menukaart.asp?g=78-Louis-Vuitton-Sac-A-Main-2014 Louis Vuitton Sac A Main 2014] ne frappe pas un oiseau de passage et de perturber totalement la prédiction (un important facteur de confusion totalement inattendue) Pour la plupart des fins quoique cette première "balle dans le vide" estimation, avec [http://www.stobbeldeen.be/download/spelletjes.asp?d=28-Acheter-New-Balance-Wr996 Acheter New Balance Wr996] des barres d'erreur suffisantes, est parfaitement servicable ..<br><br>Les enfants voient leurs parents étant membres de la culture de la misère et se développer en elle une dette courante incroyable, tout comme leurs parents. Et pourrais-je mentionné que les soins infirmiers a été un jeu d'enfant par la suite; toute la douleur a disparu, et nous avons eu [http://www.bistrolarmagnac.be/history/foto/contact.asp?g=46-Basket-Isabel-Marant-Vestiaire-Collective Basket Isabel Marant Vestiaire Collective] une relation agréable infirmiers de longue jusqu'à ce que Lee était âgé de 19 mois et j'étais enceinte de 5 mois, avec Ike ..<br><br>Ce qu'il dit, c'est que les «lois des États-Unis fait en application" de la Constitution sont la loi suprême de la land.In application de celui-ci, pas de violation de celui-ci. La question qui se posera donc est de savoir comment, le cas échéant, si l'engagement financier de la seconde épouse d'affecter l'évaluation des tribunaux de la nécessité de soutenir la seconde épouse.<br><br>Que vous vous en premier ou de prendre ce qui est disponible à la fin du mois, vous aurez besoin de réduire les dépenses ou augmenter les revenus afin de sauver. StylistIt est un travail qui est à la fois magnifique et difficile. Je suppose que je pourrais commander mes propres pépites de poulet, comme je l'ai suggéré à mon propre maman depuis des décennies, mais qui veut faire cela? <br><br>Questions essentielles pour First Grade Math. J'ai parlé de cela plusieurs fois chez mon petit cercle d'amis, et j'ai la nostalgie de tout dirigeant, rouge ou bleu, à avoir le courage de dire que je crois que cette nation devrait s'engager à atteindre les objectifs, avant que cette décennie soit, <br><br>Westbury totales de santé est une pratique de la réadaptation spécialisé multiples spécialisée dans la thérapie complexe lymphoedème, médecine physique de réadaptation et services chiropratiques dans le comté de Nassau, New York. Si c'était votre enfant u ne voudraient le même support aider à arrêter ce canular en partageant cet article [http://www.slagerij-vasseur.be/backoffice/class/ligging.asp?id=31-Nike-Blazer-Homme-Pas-Cher Nike Blazer Homme Pas Cher] 08/12/11 mise à jour:!! Nouvelle version de la fausse alerte ne précise plus Edmonton, KY comme l'emplacement, mais revendique toujours un 3 <br><br>Je viens donc de penser que vous êtes vraiment une source d'inspiration pour un peuple de lot et les gens qui travaillent à mon lieu de travail, l'Institut Téléthon, qui font beaucoup de travail autour de l'alcool et de la grossesse et de l'ETCAF, ils vous aiment tous. Ses parents sont arrivés à la porte de classe pour leurs quelques heures plus tard, couvert de la tête aux pieds en frêne blanc ..<ul> |
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| ==History==
| | <li>[http://iehdi.org/wiki/index.php?title=User:Ieoqktvp#Beats_By_Dr_Dre_Fnac_il_inclut_la_croyance_en_Dieu http://iehdi.org/wiki/index.php?title=User:Ieoqktvp#Beats_By_Dr_Dre_Fnac_il_inclut_la_croyance_en_Dieu]</li> |
| {{Main|History of the jet engine}}
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| {{See also|Timeline of jet power}}
| | <li>[http://en.camelliachine.com/news/html/?773765.html http://en.camelliachine.com/news/html/?773765.html]</li> |
| Jet engines date back to the invention of the [[aeolipile]] before the first century AD. This device directed steam power through two nozzles to cause a sphere to spin rapidly on its axis. So far as is known, it did not supply mechanical power and the potential practical applications of this invention did not receive recognition. Instead, it was seen as a curiosity.
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| | | <li>[http://www.mdj.cze.as/forum.php?mod=viewthread&tid=584759 http://www.mdj.cze.as/forum.php?mod=viewthread&tid=584759]</li> |
| Jet propulsion only took off, literally and figuratively, with the invention of the [[gunpowder]]-powered [[rocket]] by the Chinese in the 13th century as a type of [[fireworks]], and gradually progressed to propel formidable weaponry. However, although very powerful, at reasonable flight speeds rockets are very inefficient and so jet propulsion technology stalled for hundreds of years.
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| | | <li>[http://tcxs.cn/forum.php?mod=viewthread&tid=1419056&fromuid=56748 http://tcxs.cn/forum.php?mod=viewthread&tid=1419056&fromuid=56748]</li> |
| The earliest attempts at airbreathing jet engines were hybrid designs in which an external power source first compressed air, which was then mixed with fuel and burned for jet thrust. In one such system, called a ''thermojet'' by [[Secondo Campini]] but more commonly, [[motorjet]], the air was compressed by a fan driven by a conventional piston engine. Examples of this type of design were the [[Caproni Campini N.1]], and the Japanese [[Tsu-11]] engine intended to power [[Ohka]] [[kamikaze]] planes towards the end of [[World War II]]. None were entirely successful and the N.1 ended up being slower than the same design with a traditional engine and [[Propeller (aircraft)|propeller]] combination.
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| [[Image:Albert Fono's ramjet-cannonball in 1915.png|thumb|left|[[Albert Fonó]]'s [[ramjet]]-cannonball from 1915]]
| | <li>[http://www.xh688.com/bbs/forum.php?mod=viewthread&tid=880178 http://www.xh688.com/bbs/forum.php?mod=viewthread&tid=880178]</li> |
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| Even before the start of World War II, engineers were beginning to realize that engines driving propellers were self-limiting in terms of the maximum performance which could be attained; the limit was due to issues related to propeller efficiency,<ref>[http://selair.selkirk.bc.ca/aerodynamics1/Performance/Page8.html propeller efficiency]{{Dead link|date=March 2010}}</ref> which declined as blade tips approached the [[speed of sound]]. If aircraft performance were ever to increase beyond such a barrier, a way would have to be found to use a different propulsion mechanism. This was the motivation behind the development of the gas turbine engine, commonly called a "jet" engine, which would become almost as revolutionary to aviation as the [[Wright brothers]]' first flight.
| | </ul> |
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| The key to a practical jet engine was the gas turbine, used to extract energy from the engine itself to drive the [[gas compressor|compressor]]. The [[gas turbine]] was not an idea developed in the 1930s: the patent for a stationary turbine was granted to John Barber in England in 1791. The first gas turbine to successfully run self-sustaining was built in 1903 by Norwegian engineer [[Ægidius Elling]]. Limitations in design and practical engineering and metallurgy prevented such engines reaching manufacture. The main problems were safety, reliability, weight and, especially, sustained operation.
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| The first patent for using a gas turbine to power an aircraft was filed in 1921 by Frenchman [[Maxime Guillaume]].<ref>Maxime Guillaume, "Propulseur par réaction sur l'air," French patent no. 534,801 (filed: 3 May 1921; issued: 13 January 1922). Available on-line (in French) at: http://v3.espacenet.com/origdoc?DB=EPODOC&IDX=FR534801&F=0&QPN=FR534801 .</ref> His engine was an axial-flow turbojet. [[Alan Arnold Griffith]] published ''An Aerodynamic Theory of Turbine Design'' in 1926 leading to experimental work at the RAE.
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| [[Image:Whittle Jet Engine W2-700.JPG|thumb|right|The [[Whittle W.2]]/700 engine flew in the [[Gloster E.28/39]], the first British aircraft to fly with a turbojet engine, and the [[Gloster Meteor]]]] | |
| In 1928, [[RAF College Cranwell]] cadet <ref>{{cite web|url=http://www.pbs.org/kcet/chasingthesun/innovators/fwhittle.html |title=Chasing the Sun - Frank Whittle |publisher=PBS |date= |accessdate=2010-03-26}}</ref> [[Frank Whittle]] formally submitted his ideas for a turbo-jet to his superiors. In October 1929 he developed his ideas further.<ref>{{cite web|url=http://www.bbc.co.uk/history/historic_figures/whittle_frank.shtml |title=History - Frank Whittle (1907 - 1996) |publisher=BBC |date= |accessdate=2010-03-26}}</ref> On 16 January 1930 in England, Whittle submitted his first patent (granted in 1932).<ref>Frank Whittle, "Improvements relating to the propulsion of aircraft and other vehicles," British patent no. 347,206 (filed: 16 January 1930). Available on-line at: http://v3.espacenet.com/origdoc?DB=EPODOC&IDX=GB347206&F=0&QPN=GB347206 .</ref> The patent showed a two-stage [[axial compressor]] feeding a single-sided centrifugal compressor. Practical axial compressors were made possible by ideas from [[Alan Arnold Griffith|A.A.Griffith]] in a seminal paper in 1926 ("An Aerodynamic Theory of Turbine Design"). Whittle would later concentrate on the simpler centrifugal compressor only, for a variety of practical reasons. Whittle had his first engine running in April 1937. It was liquid-fuelled, and included a self-contained fuel pump. Whittle's team experienced near-panic when the engine would not stop, accelerating even after the fuel was switched off. It turned out that fuel had leaked into the engine and accumulated in pools, so the engine would not stop until all the leaked fuel had burned off. Whittle was unable to interest the government in his invention, and development continued at a slow pace.
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| [[Image:Ohain USAF He 178 page61.jpg|thumb|left|[[Heinkel He 178]], the world's first aircraft to fly purely on turbojet power]]
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| In 1935 [[Hans von Ohain]] started work on a similar design in [[Germany]], initially unaware of Whittle's work.<ref>[http://inventors.about.com/library/inventors/bljetengine.htm The History of the Jet Engine - Sir Frank Whittle - Hans Von Ohain<!-- Bot generated title -->] Ohain said that he had not read Whittle's patent and Whittle believed him ([http://www.solarnavigator.net/inventors/frank_whittle.htm Frank Whittle 1907-1996]).</ref>
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| Von Ohain's first device was strictly experimental and could only run under external power, but he was able to demonstrate the basic concept. Ohain was then introduced to [[Ernst Heinkel]], one of the larger aircraft industrialists of the day, who immediately saw the promise of the design. Heinkel had recently purchased the Hirth engine company, and Ohain and his master machinist [[Max Hahn]] were set up there as a new division of the Hirth company. They had their first [[Heinkel HeS 1|HeS 1]] centrifugal engine running by September 1937. Unlike Whittle's design, Ohain used [[hydrogen]] as fuel, supplied under external pressure. Their subsequent designs culminated in the gasoline-fuelled [[Heinkel HeS 3|HeS 3]] of 1,100 lbf (5 kN), which was fitted to Heinkel's simple and compact [[Heinkel He 178|He 178]] airframe and flown by [[Erich Warsitz]] in the early morning of August 27, 1939, from [[Rostock]]-Marienehe [[aerodrome]], an impressively short time for development. The He 178 was the world's first jet plane.<ref>Warsitz, Lutz: [http://www.pen-and-sword.co.uk/?product_id=1762 ''THE FIRST JET PILOT - The Story of German Test Pilot Erich Warsitz'' (p. 125), Pen and Sword Books Ltd., England, 2009]</ref>
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| [[Image:Junkers Jumo 004.jpg|thumb|A cutaway of the Junkers Jumo 004 engine]]
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| [[Austria]]n [[Anselm Franz]] of [[Junkers (Aircraft)|Junkers]]' engine division (''Junkers Motoren'' or "Jumo") introduced the [[axial-flow compressor]] in their jet engine. Jumo was assigned the next engine number in the [[Reich Air Ministry|RLM]] 109-0xx numbering sequence for gas turbine aircraft powerplants, "004", and the result was the [[Junkers Jumo 004|Jumo 004]] engine. After many lesser technical difficulties were solved, mass production of this engine started in 1944 as a powerplant for the world's first jet-fighter aircraft, the [[Messerschmitt Me 262]] (and later the world's first jet-bomber aircraft, the [[Arado Ar 234]]). A variety of reasons conspired to delay the engine's availability, causing the fighter to arrive too late to improve Germany's position in World War II. Nonetheless, it will be remembered as the first use of jet engines in service.
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| Meanwhile, in Britain the [[Gloster E28/39]] had its maiden flight on 15 May 1941 and the [[Gloster Meteor]] finally entered service with the [[RAF]] in July 1944.
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| Following the end of the war the German jet aircraft and jet engines were extensively studied by the victorious allies and contributed to work on early Soviet and US jet fighters. The legacy of the axial-flow engine is seen in the fact that practically all jet engines on [[fixed-wing aircraft]] have had some inspiration from this design.
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| By the 1950s the jet engine was almost universal in combat aircraft, with the exception of cargo, liaison and other specialty types. By this point some of the British designs were already cleared for civilian use, and had appeared on early models like the [[de Havilland Comet]] and [[Avro Canada Jetliner]]. By the 1960s all large civilian aircraft were also jet powered, leaving the piston engine in low-cost niche roles such as [[cargo]] flights.
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| The efficiency of turbojet engines was still rather worse than piston engines but by the 1970s, with the advent of [[high bypass]] turbofan jet engines, an innovation not foreseen by the early commentators such as [[Edgar Buckingham]], at high speeds and high altitudes that seemed absurd to them, fuel efficiency was about the same as the best piston and propeller engines.<ref name=hist8>{{cite web|url=http://www.hq.nasa.gov/pao/History/SP-468/ch10-3.htm |title=ch10-3 |publisher=Hq.nasa.gov |date= |accessdate=2010-03-26}}</ref>
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| ==Uses==
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| [[File:JT9D on 747.JPG|thumb|right|A [[Pratt & Whitney JT9D|JT9D]] turbofan jet engine installed on a [[Boeing 747]] aircraft.]]
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| Jet engines power aircraft, [[cruise missile]]s and [[unmanned aerial vehicle]]s. In the form of rocket engines they power [[fireworks]], [[model rocketry]], [[spaceflight]], and military [[missile]]s.
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| Jet engines have propelled high speed cars, particularly [[drag racer]]s, with the all-time record held by a [[rocket car]]. A turbofan powered car [[ThrustSSC]] currently holds the [[land speed record]].
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| Jet engine designs are frequently modified for non-aircraft applications, as industrial gas turbines. These are used in electrical power generation, for powering water, natural gas, or oil pumps, and providing propulsion for ships and locomotives. Industrial gas turbines can create up to 50,000 shaft horsepower. Many of these engines are derived from older military turbojets such as the Pratt & Whitney J57 and J75 models. There is also a derivative of the P&W JT8D low-bypass turbofan that creates up to 35,000 HP.
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| ==Types==
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| There are a large number of different types of jet engines, all of which achieve forward thrust from the principle of ''jet propulsion''.
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| ===Airbreathing===
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| {{Main|Airbreathing jet engine}}
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| Commonly aircraft are propelled by airbreathing jet engines. Most airbreathing jet engines that are in use are [[turbofan]] jet engines which give good efficiency at speeds just below the speed of sound.
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| ====Turbine powered====
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| {{Main|Gas turbine}}
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| [[Gas turbines]] are rotary engines that extract energy from a flow of combustion gas. They have an upstream compressor coupled to a downstream turbine with a combustion chamber in-between. In aircraft engines, those three core components are often called the "gas generator."<ref>{{Cite book|last1=Mattingly|first1=Jack D.|title=Elements of Propulsion: Gas Turbines and Rockets|series=AIAA Education Series|year=2006|publisher=American Institute of Aeronautics and Astronautics|location=Reston, VA |page=6|isbn=1-56347-779-3}}</ref> There are many different variations of gas turbines, but they all use a gas generator system of some type.
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| =====Turbojet=====
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| {{Main|Turbojet}}
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| [[File:Jet engine.svg|thumb|right|Turbojet engine]]
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| A [[turbojet]] engine is a [[gas turbine]] engine that works by compressing air with an inlet and a compressor ([[Axial compressor|axial]], [[Centrifugal compressor|centrifugal]], or both), mixing fuel with the compressed air, burning the mixture in the [[combustor]], and then passing the hot, high pressure air through a [[turbine]] and a [[propelling nozzle|nozzle]]. The compressor is powered by the turbine, which extracts energy from the expanding gas passing through it. The engine converts internal energy in the fuel to kinetic energy in the exhaust, producing thrust. All the air ingested by the inlet is passed through the compressor, combustor, and turbine, unlike the [[turbofan]] engine described below.<ref>Mattingly, pp. 6-8</ref>
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| =====Turbofan=====
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| [[File:Turbofan operation lbp.svg|thumb|right|Schematic diagram illustrating the operation of a low-bypass turbofan engine.]]
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| {{Main|Turbofan}}
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| A [[turbofan]] engine is a gas turbine engine that is very similar to a turbojet. Like a turbojet, it uses the gas generator core (compressor, combustor, turbine) to convert internal energy in fuel to kinetic energy in the exhaust. Turbofans differ from turbojets in that they have an additional component, a fan. Like the compressor, the fan is powered by the turbine section of the engine. Unlike the turbojet, some of the flow accelerated by the fan [[Bypass ratio|bypasses]] the gas generator core of the engine and is exhausted through a nozzle. The bypassed flow is at lower velocities, but a higher mass, making thrust produced by the fan more efficient than thrust produced by the core. Turbofans are generally more efficient than turbojets at subsonic speeds, but they have a larger frontal area which generates more drag.<ref>Mattingly, pp. 9-11</ref>
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| There are two general types of turbofan engines, [[Low-bypass turbofan|low bypass]] and [[High-bypass turbofan|high bypass]]. Low bypass turbofans have a [[bypass ratio]] of around 2:1 or less, meaning that for each kilogram of air that passes through the core of the engine, two kilograms or less of air bypass the core.{{Citation needed|date=July 2010}} Low bypass turbofans often used a [[mixed exhaust nozzle]] meaning that the bypassed flow and the core flow exit from the same nozzle.<ref name="M12">Mattingly, p. 12</ref> High bypass turbofans have larger bypass ratios, sometimes on the order of 5:1 or 6:1. These turbofans can produce much more thrust than low bypass turbofans or turbojets because of the large mass of air that the fan can accelerate, and are often more fuel efficient than low bypass turbofans or turbojets.{{Citation needed|date=July 2010}}
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| =====Turboprop and turboshaft=====
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| {{Main|Turboprop|Turboshaft}}
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| [[File:Turboprop operation-en.svg|thumb|right|Turboprop engine]]
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| [[Turboprop]] engines are jet engine derivatives, still gas turbines, that extract work from the hot-exhaust jet to turn a rotating shaft, which is then used to produce thrust by some other means. While not strictly jet engines in that they rely on an auxiliary mechanism to produce thrust, turboprops are very similar to other turbine-based jet engines, and are often described as such.
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| In turboprop engines, a portion of the engines' thrust is produced by spinning a [[Propeller (aircraft)|propeller]], rather than relying solely on high-speed jet exhaust. As their jet thrust is augmented by a propeller, turboprops are occasionally referred to as a type of hybrid jet engine. While many turboprops generate the majority of their thrust with the propeller, the hot-jet exhaust is an important design point, and maximum thrust is obtained by matching thrust contributions of the propeller to the hot jet.{{sfn |Hill|Peterson|1992| pp=190}} Turboprops generally have better performance than turbojets or turbofans at low speeds where propeller efficiency is high, but become increasingly noisy and inefficient at high speeds.{{sfn |Mattingly|2006| pp=12-14}}
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| Turboshaft engines are very similar to turboprops, differing in that nearly all energy in the exhaust is extracted to spin the rotating shaft, which is used to power machinery rather than a propeller, they therefore generate little to no jet thrust and are often used to power [[helicopters]].<ref name="M12"/>
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| =====Propfan=====
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| {{Main|Propfan}}
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| [[File:Nasa ge udf.jpg|thumb|right|A propfan engine]]
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| A [[propfan]] engine (also called "unducted fan", "open rotor", or "ultra-high bypass") is a jet engine that uses its gas generator to power an exposed fan, similar to turboprop engines. Like turboprop engines, propfans generate most of their thrust from the propeller and not the exhaust jet. The primary difference between turboprop and propfan design is that the propeller blades on a propfan are highly swept to allow them to operate at speeds around [[Mach number|Mach]] 0.8, which is competitive with modern commercial turbofans. These engines have the fuel efficiency advantages of turboprops with the performance capability of commercial turbofans.<ref>Sweetman, Bill (2005). [http://www.airspacemag.com/history-of-flight/prop-fan.html The Short, Happy Life of the Prop-fan]. ''Air & Space Magazine''. 1 September 2005.</ref> While significant research and testing (including flight testing) has been conducted on propfans, no propfan engines have entered production.
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| ====Ram powered====
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| Ram powered jet engines are airbreathing engines similar to gas turbine engines and they both follow the [[Brayton cycle]]. Gas turbine and ram powered engines differ, however, in how they compress the incoming airflow. Whereas gas turbine engines use axial or centrifugal compressors to compress incoming air, ram engines rely only on air compressed through the inlet or diffuser.<ref name="m14">Mattingly, p. 14</ref> Ram powered engines are considered the most simple type of air breathing jet engine because they can contain no moving parts.<ref>*{{Cite book
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| |last1=Flack |first1=Ronald D.|title=Fundamentals of Jet Propulsion with Applications|series=Cambridge Aerospace Series|year=2005|publisher=Cambridge University Press|location=New York, NY|page=16|isbn=978-0-521-81983-1}}</ref>
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| =====Ramjet=====
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| {{Main|Ramjet}}
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| [[File:Ramjet operation.svg|thumb|right|A schematic of a ramjet engine, where "M" is the [[Mach number]] of the airflow.]]
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| Ramjets are the most basic type of ram powered jet engines. They consist of three sections; an inlet to compress incoming air, a combustor to inject and combust fuel, and a nozzle to expel the hot gases and produce thrust. Ramjets require a relatively high speed to efficiently compress the oncoming air, so ramjets cannot operate at a standstill and they are most efficient at [[supersonic]] speeds. A key trait of ramjet engines is that combustion is done at subsonic speeds. The supersonic incoming air is dramatically slowed through the inlet, where it is then combusted at the much slower, subsonic, speeds.<ref name="m14"/> The faster the incoming air is, however, the less efficient it becomes to slow it to subsonic speeds. Therefore ramjet engines are limited to approximately Mach 5.<ref>Benson, Tom. [http://www.grc.nasa.gov/WWW/K-12/airplane/ramjet.html Ramjet Propulsion]. NASA Glenn Research Center. Updated: 11 July 2008. Retrieved: 23 July 2010.</ref>
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| =====Scramjet=====
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| {{Main|Scramjet}}
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| [[File:Scramjet operation.png|thumb|right|Scramjet engine operation]]
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| Scramjets are mechanically very similar to ramjets. Like a ramjet, they consist of an inlet, a combustor, and a nozzle. The primary difference between ramjets and scramjets is that scramjets do not slow the oncoming airflow to subsonic speeds for combustion, they use supersonic combustion instead. The name "scramjet" comes from "supersonic combusting ramjet." Since scramjets use supersonic combustion they can operate at speeds above Mach 6 where traditional ramjets are too inefficient. Another difference between ramjets and scramjets comes from how each type of engine compresses the oncoming air flow: while the inlet provides most of the compression for ramjets, the high speeds at which scramjets operate allow them to take advantage of the compression generated by [[shock waves]], primarily [[oblique shocks]].<ref>{{Cite book|last1=Heiser|first1=William H.|last2=Pratt|first2=David T.| title=Hypersonic Airbreathing Propulsion|series=AIAA Education Series|year=1994|publisher=American Institute of Aeronautics and Astronautics|location=Washington, D.C.|pages=23–4|isbn=1-56347-035-7}}</ref>
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| Very few scramjet engines have ever been built and flown. In May 2010 the [[Boeing X-51]] set the endurance record for the longest scramjet burn at over 200 seconds.<ref>[http://www.af.mil/news/story.asp?id=123206525 X-51 Waverider makes historic hypersonic flight]. United States Air Force. 26 May 2010. Retrieved: 23 July 2010.</ref>
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| ====Non-continuous combustion====
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| {| class="wikitable"
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| |-
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| !'''Type'''
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| !'''Description'''
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| !'''Advantages'''
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| !'''Disadvantages'''
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| |-
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| ![[Motorjet]]
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| |Obsolete type that worked like a turbojet but instead of a turbine driving the compressor a piston engine drives it.
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| |Higher exhaust velocity than a propeller, offering better thrust at high speed
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| |Heavy, inefficient and underpowered. Example: [[Caproni Campini N.1]].
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| |-
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| ![[Pulsejet]]
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| |Air is compressed and combusted intermittently instead of continuously. Some designs use valves.
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| |Very simple design, commonly used on model aircraft
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| |Noisy, inefficient (low compression ratio), works poorly on a large scale, valves on valved designs wear out quickly
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| |-
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| ![[Pulse detonation engine]]
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| |Similar to a pulsejet, but combustion occurs as a [[detonation]] instead of a [[deflagration]], may or may not need valves
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| |Maximum theoretical engine efficiency
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| |Extremely noisy, parts subject to extreme mechanical fatigue, hard to start detonation, not practical for current use
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| |}
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| ===Rocket===
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| {{Main|Rocket engine}}
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| [[File:Rocket thrust.svg|thumb|right|Rocket engine propulsion]]
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| The rocket engine uses the same basic physical principles as the jet engine for propulsion via thrust, but is distinct in that it does not require atmospheric air to provide oxygen; the rocket carries all components of the reaction mass. This allows them to operate at arbitrary altitudes and in space.
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| This type of engine is used for launching satellites, [[space exploration]] and manned access, and permitted [[landing on the moon]] in 1969.
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| Rocket engines are used for high altitude flights, or anywhere where very high accelerations are needed since rocket engines themselves have a very high [[thrust-to-weight ratio]].
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| However, the high exhaust speed and the heavier, oxidizer-rich propellant results in far more propellant use than turbofans although, even so, at extremely high speeds they become energy-efficient.
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| An approximate equation for the net thrust of a rocket engine is:
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| :<math>F_N = \dot m\, g_0\, I_{sp-vac} - A_e\, p \;</math>
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| Where <math>F_N</math> is the net thrust, <math>I_{sp(vac)}</math> is the [[specific impulse]], <math>g_0</math> is a [[standard gravity]], <math>\dot m</math> is the propellant flow in kg/s, <math>A_e</math> is the cross-sectional area at the exit of the exhaust nozzle, and <math>p</math> is the atmospheric pressure.
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| | |
| {| class="wikitable"
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| |-
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| ! Type
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| ! Description
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| ! Advantages
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| ! Disadvantages
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| |-
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| ![[Rocket]]
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| |Carries all propellants and oxidants on board, emits jet for propulsion<ref>{{cite web|url=http://www.grc.nasa.gov/WWW/K-12/airplane/rockth.html |title=Rocket Thrust Equation |publisher=Grc.nasa.gov |date=2008-07-11 |accessdate=2010-03-26}}</ref>
| |
| |Very few moving parts, Mach 0 to Mach 25+, efficient at very high speed (> Mach 5.0 or so), thrust/weight ratio over 100, no complex air inlet, high compression ratio, very high speed ([[hypersonic]]) exhaust, good cost/thrust ratio, fairly easy to test, works in a vacuum-indeed works best exoatmospheric which is kinder on vehicle structure at high speed, fairly small surface area to keep cool, and no turbine in hot exhaust stream. Very high temperature combustion and high expansion ratio nozzle gives very high efficiency- at very high speeds.
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| |Needs lots of propellant- very low [[specific impulse]]—typically 100–450 seconds. Extreme thermal stresses of combustion chamber can make reuse harder. Typically requires carrying oxidizer on-board which increases risks. Extraordinarily noisy.
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| |}
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| ===Hybrid===
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| Combined cycle engines simultaneously use 2 or more different jet engine operating principles.
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| {| class="wikitable"
| |
| |-
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| !'''Type'''
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| !'''Description'''
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| !'''Advantages'''
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| !'''Disadvantages'''
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| |-
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| ![[Air turborocket|Turborocket]]
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| |A turbojet where an additional [[oxidizer]] such as [[oxygen]] is added to the airstream to increase maximum altitude
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| |Very close to existing designs, operates in very high altitude, wide range of altitude and airspeed
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| |Airspeed limited to same range as turbojet engine, carrying oxidizer like [[LOX]] can be dangerous. Much heavier than simple rockets.
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| |-
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| ![[Air-augmented rocket]]
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| |Essentially a ramjet where intake air is compressed and burnt with the exhaust from a rocket
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| |Mach 0 to Mach 4.5+ (can also run exoatmospheric), good efficiency at Mach 2 to 4
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| |Similar efficiency to rockets at low speed or exoatmospheric, inlet difficulties, a relatively undeveloped and unexplored type, cooling difficulties, very noisy, thrust/weight ratio is similar to ramjets.
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| |-
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| ![[Precooled jet engine|Precooled jets]] / [[Liquid air cycle engine|LACE]]
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| |Intake air is chilled to very low temperatures at inlet in a heat exchanger before passing through a ramjet and/or turbojet and/or rocket engine.
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| |Easily tested on ground. Very high thrust/weight ratios are possible (~14) together with good fuel efficiency over a wide range of airspeeds, Mach 0-5.5+; this combination of efficiencies may permit launching to orbit, single stage, or very rapid, very long distance intercontinental travel.
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| |Exists only at the lab prototyping stage. Examples include [[RB545]], [[Reaction Engines SABRE]], [[ATREX]]. Requires liquid hydrogen fuel which has very low density and requires heavily insulated tankage.
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| |}
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| ===Water jet===
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| {{Main|Pump-jet}}
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| A water jet, or pump jet, is a marine propulsion system that utilizes a jet of water. The mechanical arrangement may be a ducted [[Propeller (aircraft)|propeller]] with nozzle, or a [[centrifugal compressor]] and nozzle.
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| [[File:Pump jet.PNG|thumb|right|A pump jet schematic.]]
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| {| class="wikitable"
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| |-
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| !'''Type'''
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| !'''Description'''
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| !'''Advantages'''
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| !'''Disadvantages'''
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| |-
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| ![[Pump-jet|Water jet]]
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| |For propelling [[water rocket]]s and [[jetboat]]s; squirts water out the back through a nozzle
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| |In boats, can run in shallow water, high acceleration, no risk of engine overload (unlike propellers), less noise and vibration, highly maneuverable at all boat speeds, high speed efficiency, less vulnerable to damage from debris, very reliable, more load flexibility, less harmful to wildlife
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| |Can be less efficient than a propeller at low speed, more expensive, higher weight in boat due to entrained water, will not perform well if boat is heavier than the jet is sized for
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| |}
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| ==General physical principles==
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| All jet engines are reaction engines that generate thrust by emitting a [[jet (fluid)|jet]] of fluid rearwards at relatively high speed. The forces on the inside of the engine needed to create this jet give a strong thrust on the engine which pushes the craft forwards.
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| Jet engines make their jet from propellant from tankage that is attached to the engine (as in a 'rocket') as well as in '''duct engines''' (those commonly used on aircraft) by ingesting an external fluid (very typically air) and expelling it at higher speed.
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| ===Propelling nozzle===
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| The [[propelling nozzle]] is the key component of all jet engines as it creates the exhaust [[jet (fluid)|jet]]. Propelling nozzles turn pressurized, slow moving, usually hot gas, into lower pressure, fast moving, colder gas by [[adiabatic expansion]].<ref>GFC Rogers, and Cohen, H. ''Gas Turbine Theory'', p.108 (5th Edition), HIH Saravanamuttoo</ref> Propelling nozzles can be subsonic, sonic, or supersonic,<ref>Rocket propulsion elements, Sutton, Biblarz- table 3-1</ref> but in normal operation nozzles are usually sonic or supersonic. Nozzles operate to constrict the flow, and hence help raise the pressure in the engine, and physically the nozzles are very typically convergent, or convergent-divergent. Convergent-divergent nozzles can give supersonic jet velocity within the divergent section, whereas in a convergent nozzle the exhaust fluid cannot exceed the speed of sound of the gas within the nozzle.
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| ===Thrust===
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| The net thrust ('''''F<sub>N</sub>''''') of a turbojet is given by:<ref name=Cumpsty>{{cite book|author=Nicholas Cumpsty|title=Jet Propulsion|edition=2nd|publisher=Cambridge University Press|year=2003|isbn=0-521-54144-1}}</ref>
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| :<math>F_N =( \dot{m}_{air} + \dot{m}_{fuel}) v_e - \dot{m}_{air} v</math>
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| {| border="0" cellpadding="2"
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| |-
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| |align=right|where:
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| |-
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| |align=right|'''''ṁ<sub> air</sub>'''''
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| |align=left|= the mass rate of air flow through the engine
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| |-
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| |align=right|'''''ṁ<sub> fuel</sub>'''''
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| |align=left|= the mass rate of fuel flow entering the engine
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| |-
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| |align=right|'''''v<sub>e</sub>'''''
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| |align=left|= the velocity of the jet (the exhaust plume) and is assumed to be less than sonic velocity
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| |-
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| |align=right|'''''v'''''
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| |align=left|= the velocity of the air intake = the true airspeed of the aircraft
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| |-
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| |align=right|('''''ṁ<sub> air</sub>''''' + '''''ṁ<sub> fuel</sub>''''')'''''v<sub>e</sub>'''''
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| |align=left|= the nozzle gross thrust ('''''F<sub>G</sub>''')
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| |-
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| |align=right|'''''ṁ<sub> air</sub> v'''''
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| |align=left|= the ram drag of the intake air
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| |}
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| The above equation applies only for air-breathing jet engines. It does not apply to rocket engines. Most types of jet engine have an air intake, which provides the bulk of the fluid exiting the exhaust. Conventional rocket engines, however, do not have an intake, the oxidizer and fuel both being carried within the vehicle. Therefore, rocket engines do not have ram drag and the gross thrust of the rocket engine nozzle is the net thrust of the engine. Consequently, the thrust characteristics of a rocket motor are different from that of an air breathing jet engine, and thrust is independent of velocity.
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| If the velocity of the jet from a jet engine is equal to sonic velocity, the jet engine's nozzle is said to be choked. If the nozzle is choked, the pressure at the nozzle exit plane is greater than atmospheric pressure, and extra terms must be added to the above equation to account for the pressure thrust.<ref name=Cumpsty/>
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| The rate of flow of fuel entering the engine is very small compared with the rate of flow of air.<ref name=Cumpsty/> If the contribution of fuel to the nozzle gross thrust is ignored, the net thrust is:
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| :<math>F_N = \dot{m}_{air} (v_e - v)</math>
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| The velocity of the jet ('''''v<sub>e</sub>''''') must exceed the true airspeed of the aircraft ('''''v''''') if there is to be a net forward thrust on the aircraft. The velocity ('''''v<sub>e</sub>''''') can be calculated thermodynamically based on adiabatic expansion.<ref>[http://web.mit.edu/16.unified/www/SPRING/propulsion/notes/node139.html 16.Unified: Thermodynamics and Propulsion, Prof. Z. S. Spakovszky]. Scroll down to "Performance of Turbojet Engines, Section 11.6.4. (Obtained from the website of the Massachusetts Institute of Technology)</ref>
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| ====Thrust augmentation====
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| Jet thrust can be increased by injecting additional fluids and it is then called ''wet thrust''.{{clarify|date=January 2012}} Early engines and some current non-afterburning engines use water injection to temporarily increase thrust. Water is injected at the air compressor inlet or the diffuser to cool the compressing air which permits an increase in pressure for higher burning. A 10 to 30% additional thrust can thus be gained. Methyl or ethyl alcohol (or a mixture of one or both of these with water) has been used in the past for injection. However, water has a higher heat of evaporation, and is therefore the only liquid generally used for thrust augmentation today.
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| Today's military combat engines use an [[afterburner]] for increased thrust.
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| ===Energy efficiency===
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| [[Image:Propulsive efficiency.png|upright=1.2|thumb|Dependence of propulsion efficiency (η) upon the vehicle speed/exhaust velocity ratio (v/v<sub>e</sub>) for air-breathing jet and rocket engines.]]
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| The energy efficiency (<math>\eta</math>) of jet engines installed in vehicles has two main components:
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| *''propulsive efficiency'' (<math>\eta_p</math>): how much of the energy of the jet ends up in the vehicle body rather than being carried away as [[kinetic energy]] of the jet.
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| *''cycle efficiency'' (<math>\eta_{v_e}</math>): how efficiently the engine can accelerate the jet
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| Even though overall energy efficiency <math>\eta</math> is simply:
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| :<math>\eta= \eta_p \eta_{v_e}</math>
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| For all jet engines the ''propulsive efficiency'' is highest when the engine emits an exhaust jet at a velocity that is the same as, or nearly the same as, the vehicle speed as this gives the smallest residual kinetic energy.<ref>'''Note:''' In Newtonian mechanics kinetic energy is frame dependent. The kinetic energy is easiest to calculate when the speed is measured in the ''center of mass frame'' of the vehicle and (less obviously) its ''reaction mass'' / air (i.e., the stationary frame '''before''' takeoff begins.</ref> The exact formula for air-breathing engines moving at speed <math>v</math> with an exhaust velocity <math>v_e</math> is given in the literature as:<ref>{{cite book|author=K.Honicke, R.Lindner, P.Anders, M.Krahl, H.Hadrich and K.Rohricht| title=Beschreibung der Konstruktion der Triebwerksanlagen (English: Description of the design of engine plants)|edition=|publisher=Interflug GmbH (East Germany state airline), Berlin| year=1968|isbn=}}</ref>
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| :<math>\eta_p = \frac{2}{1 + \frac{v_e}{v}}</math>
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| And for a rocket:<ref name="RPE">{{cite book|author=George P. Sutton and Oscar Biblarz|title=Rocket Propulsion Elements|edition=7th Edition|publisher=John Wiley & Sons|year=2001|pages=37–38|isbn=0-471-32642-9 }}</ref>
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| :<math>\eta_p= \frac {2\, (\frac {v} {v_e})} {1 + ( \frac {v} {v_e} )^2 }</math>
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| In addition to propulsive efficiency, another factor is ''cycle efficiency''; essentially a jet engine is typically a form of heat engine. Heat engine efficiency is determined by the ratio of temperatures reached in the engine to that exhausted at the nozzle, which in turn is limited by the overall pressure ratio that can be achieved. Cycle efficiency is highest in rocket engines (~60+%), as they can achieve extremely high combustion temperatures and can have very large, energy efficient nozzles. Cycle efficiency in turbojet and similar is nearer to 30%, the practical combustion temperatures and nozzle efficiencies are much lower.
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| [[Image:Combustion efficiency of aircraft gas turbines.svg|thumb|left|Typical combustion efficiency of an aircraft gas turbine over the operational range.]]
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| [[Image:Combustion stability limits of aircraft gas turbine.svg|thumb|right|Typical combustion stability limits of an aircraft gas turbine.]]
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| The combustion efficiency of most aircraft gas turbine engines at sea-level takeoff conditions
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| is almost 100%. It decreases nonlinear to 98% at altitude cruise conditions. Air-fuel ratio ranges from 50:1 to 130:1. For any type of combustion chamber there is a ''rich'' and ''weak limit'' to the air-fuel ratio, beyond which the flame is extinguished. The range of air-fuel ratio between the rich and weak limits is reduced with an increase of air velocity. If the
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| increasing air mass flow reduces the fuel ratio below certain value, flame extinction occurs.<ref>Claire Soares, "Gas Turbines: A Handbook of Air, Land and Sea Applications", pp. 140.</ref>
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| | |
| In aircraft turbines, the regular fuel ratio is less than the most efficient fuel ratio of 15%. Therefore, only a part of the air is being used in the combustion process. Part of the fuel isn't completely burned, leaving a mix of carbon monoxide, soot, and hydrocarbon behind. At idle these amount to 50-2000 [[Parts-per notation|ppm]], and decreases during cruising to 1-50 ppm. That is why the air around airports is bad.<ref>Klaus Huenecke, "Die Technik des modernen Verkehrsflugzeuges", pp. 111-117.</ref>
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| | |
| [[Image:Specific-impulse-kk-20090105.png|thumb|upright=1.3|[[Specific impulse]] as a function of speed for different jet types with kerosene fuel (hydrogen I<sub>sp</sub> would be about twice as high). Although efficiency plummets with speed, greater distances are covered, it turns out that efficiency per unit distance (per km or mile) is roughly independent of speed for jet engines as a group; however airframes become inefficient at supersonic speeds]]
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| ===Consumption of fuel or propellant===
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| A closely related (but different) concept to energy efficiency is the rate of consumption of propellant mass. Propellant consumption in jet engines is measured by '''[[Thrust specific fuel consumption|Specific Fuel Consumption]]''', '''[[Specific impulse]]''' or '''[[Effective exhaust velocity]]'''. They all measure the same thing. Specific impulse and effective exhaust velocity are strictly proportional, whereas specific fuel consumption is inversely proportional to the others.
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| For airbreathing engines such as turbojets energy efficiency and propellant (fuel) efficiency are much the same thing, since the propellant is a fuel and the source of energy. In rocketry, the propellant is also the exhaust, and this means that a high energy propellant gives better propellant efficiency but can in some cases actually can give ''lower'' energy efficiency.
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| It can be seen in the table (just below) that the subsonic turbofans such as General Electric's CF6 turbofan uses a lot less fuel to generate thrust for a second than does the Concorde's Rolls-Royce/Snecma Olympus 593 turbojet. However, since energy is force times distance and the distance per second is greater for Concorde, the actual power generated by the engine for the same amount of fuel is higher for Concorde at Mach 2 than the CF6. Thus, the Concorde's engines are more efficient in terms of thrust per mile.
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| {{Thrust engine efficiency}}
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| ===Thrust-to-weight ratio===
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| {{Main|Thrust-to-weight ratio}}
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| The thrust to weight ratio of jet engines of similar principles varies somewhat with scale, but mostly is a function of engine construction technology. Clearly for a given engine, the lighter the engine, the better the thrust to weight is, the less fuel is used to compensate for drag due to the lift needed to carry the engine weight, or to accelerate the mass of the engine.
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| As can be seen in the following table, rocket engines generally achieve very much higher thrust to weight ratios than [[wiktionary:duct engine|duct engines]] such as turbojet and turbofan engines. This is primarily because rockets almost universally use dense liquid or solid reaction mass which gives a much smaller volume and hence the pressurisation system that supplies the nozzle is much smaller and lighter for the same performance. Duct engines have to deal with air which is 2-3 orders of magnitude less dense and this gives pressures over much larger areas, and which in turn results in more engineering materials being needed to hold the engine together and for the air compressor.
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| {{Engine thrust to weight table}}
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| ===Comparison of types===
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| [[Image:JetSuitabilityEn.png|thumb|Comparative suitability for (left to right) [[turboshaft]], [[Low-bypass turbofan|low bypass]] and [[turbojet]] to fly at 10 km altitude in various speeds. Horizontal axis - speed, m/s. Vertical axis displays engine efficiency.]]
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| Propeller engines are useful for comparison. They accelerate a large mass of air but by a relatively small maximum change in speed. This low speed limits the maximum thrust of any propeller driven airplane. However, because they accelerate a large mass of air, propeller engines, such as turboprops, can be very efficient.
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| On the other hand, turbojets accelerate a much smaller mass of intake air and burned fuel, but they emit it at the much higher speeds which are made possible by using a [[de Laval nozzle]] to accelerate the engine exhaust. This is why they are suitable for aircraft traveling at supersonic and higher speeds.
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| Turbofans have a mixed exhaust consisting of the bypass air and the hot combustion product gas from the core engine. The amount of air that bypasses the core engine compared to the amount flowing into the engine
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| determines what is called a turbofan’s bypass ratio (BPR).
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| While a turbojet engine uses all of the engine's output to produce thrust in the form of a hot high-velocity exhaust gas jet, a turbofan's cool low-velocity bypass air yields between 30 percent and 70 percent of the total thrust produced by a turbofan system.<ref>{{cite book|author=Federal Aviation Administration (FAA)|url=http://www.faa.gov/library/manuals/aircraft/airplane_handbook/media/FAA-H-8083-3B.pdf|title=FAA-H-8083-3B Airplane Flying Handbook Handbook|publisher=Federal Aviation Administration|edition=|year=2004|isbn= }}</ref>
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| The net thrust ('''''F<sub>N</sub>''''') generated by a turbofan is:<ref>[http://www.grc.nasa.gov/WWW/K-12/airplane/turbfan.html Turbofan Thrust, Glenn Research Center, National Aeronautics and Space Administration (NASA)]</ref>
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| :<math>F_N = \dot{m}_e v_e - \dot{m}_o v_o + BPR\, (\dot{m}_c v_f)</math>
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| where:
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| {| border="0" cellpadding="2"
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| |-
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| |align="right"|'''''ṁ<sub> e</sub>'''''
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| |align="left"|= the mass rate of hot combustion exhaust flow from the core engine
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| |-
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| |align=right|'''''ṁ<sub>o</sub>'''''
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| |align=left|= the mass rate of total air flow entering the turbofan = '''''ṁ<sub>c</sub>''''' + '''''ṁ<sub>f</sub>'''''
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| |-
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| |align=right|'''''ṁ<sub>c</sub>'''''
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| |align=left|= the mass rate of intake air that flows to the core engine
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| |-
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| |align=right|'''''ṁ<sub>f</sub>'''''
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| |align=left|= the mass rate of intake air that bypasses the core engine
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| |-
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| |align=right|'''''v<sub>f</sub>'''''
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| |align=left|= the velocity of the air flow bypassed around the core engine
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| |-
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| |align=right|'''''v<sub>e</sub>'''''
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| |align=left|= the velocity of the hot exhaust gas from the core engine
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| |-
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| |align=right|'''''v<sub>o</sub>'''''
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| |align=left|= the velocity of the total air intake = the true airspeed of the aircraft
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| |-
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| |align=left|'''''BPR'''''
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| |align-right|= Bypass Ratio
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| |}
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| [[Rocket engine]]s have extremely high exhaust velocity and thus are best suited for high speeds ([[hypersonic]]) and great altitudes. At any given throttle, the thrust and efficiency of a rocket motor improves slightly with increasing altitude (because the back-pressure falls thus increasing net thrust at the nozzle exit plane), whereas with a turbojet (or turbofan) the falling density of the air entering the intake (and the hot gases leaving the nozzle) causes the net thrust to decrease with increasing altitude. Rocket engines are more efficient than even scramjets above roughly Mach 15.<ref>{{cite web|url=http://www.energy.kth.se/courses/4A1346/2ndLecture/KTH%20High%20Speed.pdf |title=Microsoft PowerPoint - KTHhigspeed08.ppt |format=PDF |date= |accessdate=2010-03-26}}</ref>
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| | |
| ===Altitude and speed===
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| With the exception of [[scramjet]]s, jet engines, deprived of their inlet systems can only accept air at around half the speed of sound. The inlet system's job for transonic and supersonic aircraft is to slow the air and perform some of the compression.
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| | |
| The limit on maximum altitude for engines is set by flammability- at very high altitudes the air becomes too thin to burn, or after compression, too hot. For turbojet engines altitudes of about 40 km appear to be possible, whereas for ramjet engines 55 km may be achievable. Scramjets may theoretically manage 75 km.<ref>{{cite web|url=http://www.orbitalvector.com/Orbital%20Travel/Scramjets/Scramjets.htm |title=Scramjet |publisher=Orbitalvector.com |date=2002-07-30 |accessdate=2010-03-26}}</ref> Rocket engines of course have no upper limit.
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| | |
| At more modest altitudes, flying faster [[dynamic pressure|compresses the air in at the front of the engine]], and this greatly heats the air. The upper limit is usually thought to be about Mach 5-8, as above about Mach 5.5, the atmospheric nitrogen tends to react due to the high temperatures at the inlet and this consumes significant energy. The exception to this is scramjets which may be able to achieve about Mach 15 or more{{Citation needed|date=April 2010}}, as they avoid slowing the air, and rockets again have no particular speed limit.
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| ===Noise===
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| Noise is due to shockwaves that form when the exhaust jet interacts with the external air. The intensity of the noise is proportional to the thrust as well as proportional to the fourth power of the jet velocity.{{Citation needed|date=April 2011}} Generally then, the lower speed exhaust jets emitted from engines such as high bypass turbofans are the quietest, whereas the fastest jets, such as rockets, turbojets, and ramjets, are the loudest.
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| | |
| Because of noise restrictions, civilian turbofans have specially shaped nozzles that limit the exhaust to subsonic speeds. This leads to a thermic clogging termed [[Nozzle#High velocity|''choked nozzle'']], where the mass flow cannot be increased beyond a certain amount. The mass flow can only be increased through the bypass airstream. In a military combat engine, the variable nozzle allows the exhaust to reach supersonic speeds. This is the primary difference between normal ''dry thrust'' and ''military power''.
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| | |
| Although some variation in jet speed can often be arranged from a jet engine (such as by throttling back and adjusting the nozzle) it is difficult to vary the jet speed from an engine over a very wide range. Engines for supersonic vehicles, such as Concorde, military jets, and rockets, need to have supersonic exhaust to support their top speeds, making them especially noisy, even at low speeds.
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| | |
| ==See also==
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| * [[Air turboramjet]]
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| * [[Balancing machine]]
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| * [[Jet engine performance]]
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| * [[Reverse thrust]]
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| * [[Jetboat]]
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| * [[Variable Cycle Engine]]
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| * [[Pulse jet]]
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| * [[Turborocket]]
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| * [[Rocket turbine engine]]
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| * [[Rocket engine nozzles]]
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| * [[Spacecraft propulsion]]
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| * [[Water injection (engines)]]
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| * [[Turbojet development at the RAE]]
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| * [[Components of jet engines]]
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| | |
| ==References==
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| | |
| ===Notes===
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| {{Reflist|2}}
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| | |
| ===Bibliography===
| |
| *{{Cite book
| |
| |last1=Brooks
| |
| |first1=David S.
| |
| |title=Vikings at Waterloo: Wartime Work on the Whittle Jet Engine by the Rover Company
| |
| |year=1997
| |
| |publisher=Rolls-Royce Heritage Trust
| |
| |isbn=1-872922-08-2
| |
| }}
| |
| *{{Cite book
| |
| |last1=Golley
| |
| |first1=John
| |
| |title=Genesis of the Jet: Frank Whittle and the Invention of the Jet Engine
| |
| |year=1997
| |
| |publisher=Crowood Press
| |
| |isbn=1-85310-860-X
| |
| }}
| |
| *{{Citation
| |
| | first = Philip | last = Hill
| |
| | first2 = Carl | last2 = Peterson
| |
| | title = Mechanics and Thermodynamics of Propulsion
| |
| | edition = 2nd
| |
| | publisher = Addison-Wesley
| |
| | year = 1992
| |
| | location = New York
| |
| | isbn = 0-201-14659-2}}
| |
| *{{Cite book
| |
| |last1=Kerrebrock
| |
| |first1=Jack L.
| |
| |title=Aircraft Engines and Gas Turbines
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| |edition=2nd
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| |year=1992
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| |publisher=The MIT Press
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| |location=Cambridge, MA
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| |isbn=978-0-262-11162-1
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| }}
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| | |
| ==External links==
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| {{Wiktionary|rocket}}
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| {{Wiktionary|duct engine}}
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| {{Wiktionary|jet engine}}
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| {{Commons category|Jet engines}}
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| * [http://www.rolls-royce.com/about/education/ Media about jet engines from Rolls-Royce]
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| * [http://travel.howstuffworks.com/turbine.htm How Stuff Works article on how a Gas Turbine Engine works]
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| * [http://www.generalatomic.com/jetmakers/chapter15.html Influence of the Jet Engine on the Aerospace Industry]
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| * [http://www.rand.org/publications/MR/MR1596/MR1596.appb.pdf ''An Overview of Military Jet Engine History''], Appendix B, pp. 97–120, in ''Military Jet Engine Acquisition'' (Rand Corp., 24 pgs, PDF)
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| * [http://www.geae.com/education/engines101/ Basic jet engine tutorial (QuickTime Video]
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| {{Heat engines|state=uncollapsed}}
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| {{DEFAULTSORT:Jet Engine}}
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| [[Category:Energy conversion]]
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| [[Category:Gas turbines]]
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| [[Category:Jet engines| ]]
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| [[Category:Compressors]]
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| [[Category:Turbomachinery]]
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| [[Category:Engineering thermodynamics]]
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| [[Category:Fluid dynamics]]
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| [[Category:Aerodynamics]]
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| [[Category:English inventions]]
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| {{Link FA|de}}
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| {{link FA|hr}}
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| {{Link FA|ar}}
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