Editing Afterburner (section)

== Principle ==

The first jet engine with after-burner was the E variant of [[Jumo 004]].<ref>{{cite book | url=https://books.google.com/books?id=eJ7oCAAAQBAJ&dq=Jumo+004+afterburner&pg=PA242 | isbn=978-3-642-18484-0 | title=Aeronautical Research in Germany: From Lilienthal until Today | date=December 6, 2012 | publisher=Springer }}</ref>
[[File:Rear view of afterburner in sectioned Rolls-Royce Turboméca Adour turbofan.jpg|thumb|Rear part of a sectioned [[Rolls-Royce Turbomeca Adour]].

The afterburner with its four combustion rings is clearly seen at the center.]]Jet-engine thrust is an application of Newton's reaction principle, in which the engine generates thrust because it increases the [[momentum]] of the air passing through it.<ref>{{cite web|url=https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/thrsteq.html|title=General Thrust Equation|website=www.grc.nasa.gov|access-date=19 March 2018}}</ref> Thrust depends on two things: the velocity of the [[exhaust gas]] and the mass of the gas exiting the nozzle. A jet engine can produce more thrust by either accelerating the gas to a higher velocity or ejecting a greater mass of gas from the engine.<ref name="DingleTooley2013">{{cite book|author1=Lloyd Dingle|author2=Michael H Tooley|title=Aircraft Engineering Principles|url=https://books.google.com/books?id=-Vb7AAAAQBAJ&pg=PA189|date=23 September 2013|publisher=Routledge|isbn=978-1-136-07278-9|pages=189–}}</ref> Designing a basic [[turbojet]] engine around the second principle produces the [[turbofan]] engine, which creates slower gas, but more of it. Turbofans are highly fuel efficient and can deliver high thrust for long periods of time, but the design tradeoff is a large size relative to the power output. Generating increased power with a more compact engine for short periods can be achieved using an afterburner. The afterburner increases thrust primarily by accelerating the exhaust gas to a higher velocity.<ref name="Lancaster2015">{{cite book|author=Otis E. Lancaster|title=Jet Propulsion Engines|url=https://books.google.com/books?id=0TbWCgAAQBAJ&pg=PA176|date=8 December 2015|publisher=Princeton University Press|isbn=978-1-4008-7791-1|pages=176–}}</ref>

The following values and parameters are for an early jet engine, the [[Pratt & Whitney J57]], stationary on the runway,<ref>The Aircraft Gas Turbine Engine and its operation, Part No. P&W 182408, P&W Operating Instruction 200, revised December 1982, United Technologies Pratt & Whitney, Figure 6-4</ref> and illustrate the high values of afterburner fuel flow, gas temperature and thrust compared to those for the engine operating within the temperature limitations for its turbine.

The highest temperature in the engine (about {{convert|3,700|F|C}}<ref>AGARD-LS-183, Steady and Transient Performance Prediction, May 1982, {{ISBN|92 835 0674 X}}, section 2-3</ref>) occurs in the combustion chamber, where fuel is burned (at an approximate rate of {{convert|8,520|lb/h|kg/h|abbr=on}}) in a relatively small proportion of the air entering the engine. The combustion products have to be diluted with air from the compressor to bring the gas temperature down to a specific value, known as the Turbine Entry Temperature (TET) ({{convert|1,570|F|C}}), which gives the turbine an acceptable life.<ref name="Warhaft1997">{{cite book|author=Zellman Warhaft|title=An Introduction to Thermal-Fluid Engineering: The Engine and the Atmosphere|url=https://books.google.com/books?id=yFAYV9MUg0kC&pg=PA97|year=1997|publisher=Cambridge University Press|isbn=978-0-521-58927-7|pages=97–}}</ref> Having to reduce the temperature of the combustion products by a large amount is one of the primary limitations on how much thrust can be generated ({{convert|10,200|lb-f|N|abbr=on}}). Burning all the oxygen delivered by the compressor stages would create temperatures ({{convert|3,700|F|C}}) high enough to significantly weaken the internal structure of the engine, but by mixing the combustion products with unburned air from the compressor at ({{convert|600|F|C}}) a substantial amount of oxygen ([[fuel–air ratio|fuel/air ratio]] 0.014 compared to a no-oxygen-remaining value 0.0687) is still available for burning large quantities of fuel ({{convert|25,000|lb/h|kg/h|abbr=on}}) in an afterburner. The gas temperature decreases as it passes through the turbine (to {{convert|1,013|F|C}}). The afterburner combustor reheats the gas, but to a much higher temperature ({{convert|2,540|F|C}}) than the TET ({{convert|1,570|F|C}}). As a result of the temperature rise in the afterburner combustor, the gas is accelerated, firstly by the heat addition, known as [[Rayleigh flow]], then by the nozzle to a higher exit velocity than that which occurs without the afterburner. The mass flow is also slightly increased by the addition of the afterburner fuel. The thrust with afterburning is {{convert|16,000|lb-f|N|abbr=on}}.

The visible exhaust may show ''[[shock diamond]]s'', which are caused by [[shock waves]] formed due to slight differences between ambient pressure and the exhaust pressure. This interaction causes oscillations in the exhaust jet diameter over a short distance and causes visible banding where pressure and temperature are highest.