Afterburner
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- For other uses of afterburner, see Afterburner (disambiguation).
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| A U.S. Navy F/A-18 Hornet being launched from the catapult on full afterburner |
An afterburner (or reheat) is an additional component added to some jet engines, primarily those on military supersonic aircraft. Its purpose is to provide a temporary increase in thrust, both for supersonic flight and for takeoff (as the high wing loading typical of supersonic aircraft designs means that take-off speed is very high). On military aircraft the extra thrust is also useful for combat situations. 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 gases and adds greatly to the thrust of the engine. The advantage of afterburning is significantly increased thrust; the disadvantage is its very high fuel consumption and inefficiency, though this is acceptable for the short periods during which it is usually used.
Jet engines are referred to as operating wet when afterburning is being used and dry when the engine is used without afterburning. An engine producing maximum thrust wet is at maximum power (this is the maximum power the engine can produce); an engine producing maximum thrust dry is at military power.
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[edit] Design
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. As well as an increase in afterburner exit stagnation temperature, there is also an increase in nozzle mass flow (i.e. afterburner entry mass flow plus the effective afterburner fuel flow), but a decrease in afterburner exit stagnation pressure (owing to a fundamental loss due to heating plus friction and turbulence losses).
The resulting increase in afterburner exit volume flow is accommodated by increasing the throat area of the propulsion nozzle. Otherwise, the upstream turbomachinery rematches (probably causing a compressor stall or fan surge in a turbofan application).
To a first order, the gross thrust ratio (afterburning/dry) is directly proportional to the root of the stagnation temperature ratio across the afterburner (i.e. exit/entry).
[edit] Limitations
Due to their high fuel consumption, afterburners are not used for extended periods, a notable exception being 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.
[edit] 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.
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.
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.
[edit] Influence on cycle choice
Afterburning has a significant influence upon engine cycle choice.
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.
Often the engine designer is faced with a compromise between these two extremes.
[edit] Usage
As early as during the Second World War, the principle was in development for the British Power Jets W.2/700 with what was termed at the time a "a reheat jetpipe" for the Miles M.52 supersonic aircraft project. Post war, the McDonnell F3H Demon and the Douglas F4D Skyray were designed around the Westinghouse J-40 turbojet engine, rated at 8,000 lbf (36 kN) thrust without afterburner. The new Pratt & Whitney J-48 turbojet, at 8,000 lbf (36 kN) 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 lbf (27 kN) thrust Westinghouse J-46 engines, and the Douglas F3D Skynight, an all-weather fighter, powered by two 3,600 (16 kN) thrust Westinghouse J-34 turbojets.
In the United Kingdom, the Rolls-Royce Avon was available with reheat and in such configuration powered the famous English Electric Lightning, making it the first supersonic aircraft in RAF service and the first in the world capable of "supercruise". The Bristol-Siddeley Olympus was also given reheat for the TSR-2 and was fitted to Concorde in such a state (Bristol Siddeley had by then become part of Rolls-Royce and the nozzle and reheat system was developed by Snecma). This would be the only civilian application of an afterburner, aside from Concorde's counterpart the Tupolev Tu-144, both of which were capable of supersonic cruising and used them at takeoff and to minimise time spent in the high drag transonic flight regime.
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".
A dump-and-burn is a fuel dumping procedure where the fuel is intentionally ignited using the plane's afterburner. A spectacular flame combined with high speed makes this a popular display for airshows, or as a finale to fireworks.
[edit] See also
- Ramjet
- Supercruise
- Bristol Siddeley BS100, an engine which was intended to use Plenum Chamber Burning, similar but not identical to an afterburner.
[edit] External links
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