Afterburner (engine)
For other uses of afterburner
, see Afterburner (disambiguation). |
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. It was originally developed for the
Miles M.52 project (during the last years of World War II) where it was called a
reheat jetpipe.
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. Afterburning is extravagant on fuel and inefficient but this is acceptable for the short periods in which reheat is usually used.
Jet engines are referred to as operating
wet when reheat is being used, and
dry when the engine is used without afterburner.
 |
Pratt & Whitney J58 engine on testbed with full afterburner |
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.
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).
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.
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.
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.
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.
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."
*
Ramjet*
Supercruise*
Photo of the reheat fuel spray nozzles of a Bristol Siddeley Olympus (picture at bottom left of page)
