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Encyclopedia > Afterburner (engine)
SR-71 in flight with J58 on full afterburner
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SR-71 in flight with J58 on full afterburner

An afterburner is an additional component added to some jet engines, primarily those on military aircraft. The Jumo 004 engine variation, 004C, included an afterburner for increased thrust in the design, but was never built. It was also considered for the Miles M.52 project during the last years of World War II (never completed nor tested) where it was called a reheat jetpipe. Image File history File links Ec93-03092-5. ... Image File history File links Ec93-03092-5. ... A Pratt and Whitney turbofan engine for the F-15 Eagle is tested at Robins Air Force Base, Georgia, USA. The tunnel behind the engine muffles noise and allows exhaust to escape. ... Military aircraft are airplanes used in warfare. ... The Jumo 004 was the worlds first turbojet engine in production and operational use. ... Miles M.52 The Miles M.52 was a supersonic research aircraft project which was undertaken in top secret conditions between 1942 and 1945. ... Combatants Major Allied powers: United Kingdom Soviet Union United States Republic of China and others Major Axis powers: Nazi Germany Italy Japan and others Commanders Winston Churchill Joseph Stalin Franklin Roosevelt Harry Truman Chiang Kai-Shek Adolf Hitler Benito Mussolini Hideki Tojo Casualties Military dead: 17,000,000 Civilian dead...


Its purpose is to provide a temporary increase in thrust for situations such as take-off, or in military aircraft, combat or supersonic flight. This is achieved by injecting additional fuel into the jet pipe downstream of (i.e. after) the turbine. This fuel is ignited by the hot exhaust gasses and adds greatly to the thrust of the engine. The advantage of afterburning is significantly increased thrust; the disadvantage of afterburning is its very high fuel consumption and inefficiency but this is acceptable for the short periods in which reheat is usually used. Thrust is a reaction force described quantitatively by Newtons Second and Third Law. ... Combat has been fought in the air since 1911. ... Jet fuel is a type of aviation fuel designed for use in jet-engined aircraft. ... WWII era steam turbine used for ship propulsion. ... The word exhaust can mean:- A verb meaning tire out, as in After the long gallop, his horse was exhausted. ...


Jet engines are referred to as operating wet when reheat is being used, and dry when the engine is used without afterburner.

Contents

Design

A statically mounted Pratt & Whitney J58 engine with full afterburner on disposing of the last of the SR-71 fuel prior to program termination
A statically mounted Pratt & Whitney J58 engine with full afterburner on disposing of the last of the SR-71 fuel prior to program termination

A jet engine afterburner is an extended exhaust section containing extra fuel injectors, and since the jet engine upstream (i.e., before the turbine) will use little of the oxygen it ingests, the afterburner is, at its simplest, a type of ramjet. When the afterburner is turned on, fuel is injected, which ignites readily, owing to the relatively high temperature of the incoming gases. The resulting combustion process increases the afterburner exit (nozzle entry) temperature significantly, resulting in a steep increase in engine net thrust. Image File history File links J58_AfterburnerT.jpeg http://www. ... Image File history File links J58_AfterburnerT.jpeg http://www. ... Fuel is any material that is capable of releasing energy when its chemical or physical structure is changed or converted. ... A ramjet, sometimes referred to as a stovepipe jet, is a type of jet engine. ... Combustion or burning is a complex sequence of chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames. ... Rocket Nozzle A nozzle is a mechanical device designed to control the characteristics of a fluid flow as it exits from an enclosed chamber into some medium. ...


In order to accommodate the resulting increase in afterburner exit volume flow, the nozzle throat area must be increased. Otherwise, the upstream turbomachinery will rematch (probably causing fan surge in a turbofan application). CFM56-3 turbofan, lower half, side view. ...


Limitations

Due to their high fuel consumption, afterburners are not used for extended periods (a notable exception is the Pratt & Whitney J58 engine used in the SR-71 Blackbird). Thus, they are only used when it is important to have as much thrust as possible. This includes takeoffs from short runways (as on an aircraft carrier) and air combat situations. The Pratt & Whitney J58 (also known as the JT11D) was the jet engine used on the Lockheed SR-71 Blackbird. The J58 produced 32,000 lbf (142 kN) of thrust. ... The Lockheed SR-71, unofficially known as the Blackbird and by its crews as the Habu, was an advanced, long-range, Mach 3 strategic reconnaissance aircraft developed from the Lockheed YF-12A and A-12 aircraft by the Lockheed Skunk Works (also responsible for the U-2). ... Runway 13R/31L of El Dorado International Airport, Bogotá, D.C. Aerial picture of a runway of Chennai International Airport, Tamil Nadu A runway is a strip of land on an airport, on which aircraft can take off and land. ... Four aircraft carriers, Principe-de-Asturias, USS Wasp, USS Forrestal and HMS Invincible (front-to-back), showing the difference in size between a supercarrier, light V/STOL carriers, and an amphibious carrier. ... Combat has been fought in the air since 1911. ...


Efficiency

Since the exhaust gas already has reduced oxygen due to previous combustion, and since the fuel is not burning in a highly compressed air column, the afterburner is generally inefficient compared with the main combustor. Afterburner efficiency also declines significantly if, as is usually the case, the tailpipe pressure decreases with increasing altitude. General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Atomic mass 15. ...


However, as a counter-example the SR-71 had reasonable efficiency at high altitude in afterburning mode ("wet") due to its high speed (mach 3.2) and hence high pressure due to ram effect. An F/A-18 Hornet approaching the sound barrier. ...


Afterburners do produce markedly enhanced thrust as well as (typically) a very large, impressive flame at the back of the engine. This exhaust flame may show shock-diamonds, which are caused by shock waves being formed due to slight differences between ambient pressure and the exhaust pressure. These imbalances cause oscillations in the exhaust jet diameter over distance and cause the visible banding where the pressure and temperature is highest Shock diamonds in a Pratt & Whitney J58 engine on testbed with full afterburner Shock diamonds or mach diamonds are a formation of shock waves in the exhaust plume of an aerospace propulsion system, such as a supersonic jet engine, rocket, ramjet, or scramjet. ... Shock Waves is a horror movie from 1977 directed by Ken Wiederhorn. ...


Influence on cycle choice

Afterburning has a significant influence upon engine cycle choice. The Brayton cycle is a cyclic process generally associated with the gas turbine. ...


Lowering fan pressure ratio decreases specific thrust (both dry and when afterburning), but results in a lower temperature entering the afterburner. Since the afterburning exit temperature is effectively fixed, the temperature rise across the unit increases, raising the afterburner fuel flow. The total fuel flow tends to increase faster than the net thrust, resulting in a higher afterburning thrust-specific fuel consumption (TSFC). However, the corresponding dry power TSFC improves (i.e. lower specific thrust). The high temperature ratio across the afterburner results in a good thrust boost.


If the aircraft burns a large percentage of its fuel with the afterburner alight, it pays to select an engine cycle with a high specific thrust (i.e. high fan pressure ratio/low bypass ratio). The resulting engine is relatively fuel efficient with afterburning (i.e. Combat/Take-off), but thirsty in dry power. If, however, the afterburner is to be hardly used, a low specific thrust (low fan pressure ratio/high bypass ratio) cycle will be favored. Such an engine has a good dry TSFC, but a poor afterburning TSFC at Combat/Take-off. In aeronautical engineering, and jet engine design in particular, bypass ratio is a common measurement that compares the amount of air deliberately blown past the engine to that moving through the core. ...


Often the engine designer is faced with a compromise between these two extremes.


Usage

McDonnell F3H Demon and the Douglas F4D Skyray — were designed around the Westinghouse J-40 turbojet engine, rated at 8,000 lb. thrust without afterburner. The new Pratt & Whitney J-48 turbojet, at 8,000 lb. thrust with afterburner, would power the Grumman sweptwing fighter F9F-6, which was about to go into production. Other new Navy fighters included the highspeed Chance Vought F7V-3 Cutlass, powered by two 6,000-lb.-thrust Westinghouse J-46 engines, and the Douglas F3D Skynight, an all-weather fighter, powered by two 3,600-lb.-thrust Westinghouse J-34 turbojets


The only civilian passenger transport aircraft to use afterburners were Concorde and the Tupolev Tu-144 supersonic transport, which used them at takeoff and to minimise the time in the high drag transonic flight regime. The Aérospatiale-BAC Concorde supersonic transport (SST), along with the Tupolev Tu-144, was one of only two models of supersonic passenger airliners to have seen commercial service. ... The Tupolev Tu-144 (NATO reporting name: Charger) was a supersonic airliner constructed under management of the Soviet Tupolev design bureau headed by Alexei Tupolev (1925–2001). ... A supersonic transport (SST) is a civil aircraft designed to transport passengers at speeds greater than the speed of sound. ... Transonic is an aeronautics term referring to a range of velocities just below and above the speed of sound. ...


Except for some NASA research aircraft and the White Knight of Scaled Composites, afterburners are in the regime of military fighter jets. Modern design supercruise engines have inherently high thrust and this has lessened the need for afterburner. A turbojet engine equipped with an afterburner is called an "afterburning turbojet," whereas a turbofan engine similarly equipped is called an "augmented turbofan." SpaceShipOne and White Knight during a captive carry test flight The Scaled Composites Model 318 White Knight is a jet-powered carrier aircraft used to launch the SpaceShipOne experimental spacecraft. ... Scaled Composites (often abbreviated as Scaled) was founded in 1982 in Mojave, California by famous aircraft designer Burt Rutan out of what used to be the Rutan Aircraft Factory. ... A supercruising aircraft is able to cruise at supersonic speeds efficiently without the use of afterburners. ... Turbojets are the simplest and oldest kind of general purpose jet engine. ... CFM56-3 turbofan, lower half, side view. ...


See also

A ramjet, sometimes referred to as a stovepipe jet, is a type of jet engine. ... A supercruising aircraft is able to cruise at supersonic speeds efficiently without the use of afterburners. ...

External links

  • Photo of the reheat fuel spray nozzles of a Bristol Siddeley Olympus (picture at bottom left of page)

  Results from FactBites:
 
Howstuffworks "How does an afterburner work?" (452 words)
The idea behind an afterburner is to inject fuel directly into the exhaust stream and burn it using this remaining oxygen.
An afterburner is nothing but a set of fuel injectors, a tube and flame holder that the fuel burns in, and an adjustable nozzle.
The disadvantage of an afterburner is that it uses a lot of fuel for the power it generates.
Reheat @ iCookClub.com (567 words)
Afterburning is extravagant on fuel and inefficient but this is acceptable for the short periods in which reheat is usually used.
A jet engine afterburner is an extended exhaust section containing extra fuel injectors, and since the jet engine upstream (i.e., before the turbine) will use little of the oxygen it ingests, the afterburner is, at its simplest, a type of ramjet.
Afterburner efficiency also declines significantly if, as is usually the case, the tailpipe pressure decreases with increasing altitude.
  More results at FactBites »

 
 

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