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An afterburner (or reheat in British English) is an additional combustion component used on some , mostly those on military supersonic aircraft. Its purpose is to increase thrust, usually for supersonic flight, takeoff, and aerial combat. The afterburning process injects additional Jet fuel into a combustor ("burner") in the jet pipe behind (i.e., "after") the turbine, "reheating" the exhaust gas. Afterburning significantly increases thrust as an alternative to using a bigger engine with its added weight penalty, but at the cost of increased fuel consumption (decreased fuel efficiency) which limits its use to short periods. This aircraft application of "reheat" contrasts with the meaning and implementation of "reheat" applicable to gas turbines driving electrical generators and which reduces fuel consumption.Gas Turbine Design, Components and System Design Integration, Meinhard T. Schobeiri, , p. 12/24
Jet engines are referred to as operating wet when afterburning and dry when not. An engine producing maximum thrust wet is at maximum power, while an engine producing maximum thrust dry is at military power.
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. 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. 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.
The following values and parameters are for an early jet engine, the Pratt & Whitney J57, stationary on the runway,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 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 AGARD-LS-183, Steady and Transient Performance Prediction, May 1982, , section 2-3) occurs in the combustion chamber, where fuel is burned (at an approximate rate of ) 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) (), which gives the turbine an acceptable life. 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 (). Burning all the oxygen delivered by the compressor stages would create temperatures () high enough to significantly weaken the internal structure of the engine, but by mixing the combustion products with unburned air from the compressor at () a substantial amount of oxygen (fuel/air ratio 0.014 compared to a no-oxygen-remaining value 0.0687) is still available for burning large quantities of fuel () in an afterburner. The gas temperature decreases as it passes through the turbine (to ). The afterburner combustor reheats the gas, but to a much higher temperature () than the TET (). 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 .
The visible exhaust may show , 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.
Figure 2 schematic of afterburner In comparison, the afterburning Rolls-Royce Spey used a twenty chute mixer before the fuel manifolds.
Plenum chamber burning (PCB) was partially developed for the Thrust vectoring Bristol Siddeley BS100 engine for the Hawker Siddeley P.1154 until the program was cancelled in 1965. The cold bypass and hot core flows were split between two pairs of nozzles, front and rear, in the same manner as the Rolls-Royce Pegasus, and fuel was burned in the fan air before it left the front nozzles. It would have given greater thrust for take-off and supersonic performance in an aircraft similar to, but bigger than, the Hawker Siddeley Harrier.
Duct heating was used by Pratt & Whitney for their JTF17 turbofan proposal for the U.S. Supersonic Transport Program in 1964 and a demonstrator engine was run.The Engines of Pratt & Whitney: A Technical History, Jack Connors2009, . p.380 The duct heater used an annular combustor and would be used for takeoff, climb and cruise at Mach 2.7 with different amounts of augmentation depending on aircraft weight.
The resulting increase in afterburner exit volume flow is accommodated by increasing the throat area of the exit nozzle. Otherwise, if pressure is not released, the gas can flow upstream and re-ignite, possibly causing a compressor stall (or fan surge in a turbofan application). The first designs, e.g. Solar afterburners used on the F7U Cutlass, F-94 Starfire and F-89 Scorpion, had 2-position eyelid nozzles.SAE 871354 "The First U.S. Afterburner Development" Modern designs incorporate not only variable-geometry (VG) nozzles but multiple stages of augmentation via separate spray bars.
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).
An afterburner has a limited life to match its intermittent use. The J58 was an exception with a continuous rating. This was achieved with
p.5 and by cooling the liner and nozzle with compressor bleed airhttp://roadrunnersinternationale.com/pw_tales.htm, p.3 instead of turbine exhaust gas.
Since the exhaust gas already has a reduced oxygen content, owing to previous combustion, and since the fuel is not burning in a highly compressed air column, the afterburner is generally inefficient in comparison to the main combustion process. Afterburner efficiency also declines significantly if, as is usually the case, the inlet and tailpipe pressure decreases with increasing altitude.
This limitation applies only to turbojets. In a military turbofan combat engine, the bypass air is added into the exhaust, thereby increasing the core and afterburner efficiency. In turbojets the gain is limited to 50%, whereas in a turbofan it depends on the bypass ratio and can be as much as 70%."Basic Study of the Afterburner" Yoshiyuki Ohya, NASA TT F-13,657
However, as a counterexample, the SR-71 had reasonable efficiency at high altitude in afterburning ("wet") mode owing to its high speed (Mach number 3.2) and correspondingly high pressure due to Ram-air intake.
Lowering the fan pressure ratio decreases specific thrust (both dry and wet 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 specific fuel consumption (SFC). However, the corresponding dry power SFC 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 SFC, but a poor afterburning SFC at Combat/Take-off.
Often the engine designer is faced with a compromise between these two extremes.
Early British afterburner ("reheat") work included flight tests on a Rolls-Royce W2/B23 in a Gloster Meteor I in late 1944 and ground tests on a Power Jets W2/700 engine in mid-1945. This engine was destined for the Miles M.52 supersonic aircraft project."Fast Jets-the history of reheat development at Derby". Cyril Elliott p14,16
Early American research on the concept was done by NACA, in Cleveland, Ohio, leading to the publication of the paper "Theoretical Investigation of Thrust Augmentation of Turbojet Engines by Tail-pipe Burning" in January 1947.
American work on afterburners in 1948 resulted in installations on early straight-wing jets such as the Pirate, Starfire and Scorpion."Afterburning: A Review of Current American Practice" Flight magazine 21 November 1952 p648
The new Pratt & Whitney J48 turbojet, at 8,000 lbf (36 kN) thrust with afterburners, would power the Grumman swept-wing fighter F9F-6, which was about to go into production. Other new Navy fighters with afterburners included the Chance Vought F7U-3 Cutlass, powered by two 6,000 lbf (27 kN) thrust Westinghouse J46 engines.
In the 1950s, several large afterburning engines were developed, such as the Orenda Iroquois and the British de Havilland Gyron and Rolls-Royce Avon RB.146 variants. The Avon and its variants powered the English Electric Lightning, the first supersonic aircraft in RAF service. The Bristol-Siddeley/Rolls-Royce Olympus was fitted with afterburners for use with the BAC TSR-2. This system was designed and developed jointly by Bristol-Siddeley and Solar of San Diego."Bristol/Solar reheat" Flight magazine 20 September 1957 p472 The afterburner system for the Concorde was developed by Snecma.
Afterburners are generally used only in military aircraft, and are considered standard equipment on fighter aircraft. The handful of civilian planes that have used them include some NASA research aircraft, the Tupolev Tu-144, Concorde and the White Knight of Scaled Composites. Concorde flew long distances at supersonic speeds. Sustained high speeds would be impossible with the high fuel consumption of afterburner, and the plane used afterburners at takeoff and to minimize time spent in the high-drag transonic flight regime. Supersonic flight without afterburners is referred to as supercruise.
A turbojet engine equipped with an afterburner is called an "afterburning turbojet", whereas a turbofan engine similarly equipped is sometimes called an "augmented turbofan".
A "dump-and-burn" is an airshow display feature where fuel is jettisoned, then intentionally ignited using the afterburner. A spectacular flame combined with high speed makes this a popular display for , or as a finale to fireworks. Fuel dumping is used primarily to reduce the weight of an aircraft to avoid a heavy, high-speed landing. Other than for safety or emergency reasons, fuel dumping does not have a practical use.
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