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ReheatAn 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. 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. Check out the following recipes that are tagged "Reheat":
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