Afterburning

An afterburner (or a reheat) is a component present on some jet engines, mostly those used on military supersonic aircraft. Its purpose is to provide an increase in thrust, usually for supersonic flight, takeoff and for combat situations. Afterburning is achieved by injecting additional fuel into the jet pipe downstream of (i.e. after) the turbine. The advantage of afterburning is significantly increased thrust; the disadvantage is its very high fuel consumption and inefficiency, though this is often regarded as acceptable for the short periods during which it is usually used.

Pilots can activate and deactivate afterburners in-flight, and jet engines are referred to as operating wet when afterburning is being used 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.

Jet-engine thrust is governed by the general principle of mass flow rate. Thrust depends on two things: the velocity of the exhaust gas and the mass of that gas. A jet engine can produce more thrust by either accelerating the gas to a higher velocity or by having a greater mass of gas exit 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, but the design trade-off is a large size relative to the power output. To generate increased power with a more compact engine for short periods, an engine requires an afterburner. The afterburner increases thrust primarily by accelerating the exhaust gas to a higher velocity. While the mass of the fuel added to the exhaust does contribute to an increase in exhaust mass, this effect is negligible compared to the increase in exhaust velocity.

The temperature of the gas in the engine is highest just before the turbine, and the ability for the turbine to withstand these temperatures is one of the primary restrictions on total dry engine thrust. This temperature is known as the Turbine Entry Temperature (TET), one of the critical engine operating parameters. Because a combustion rate high enough to consume all the intake oxygen would create temperatures high enough to overheat the turbine, the flow of fuel must be restricted to an extent that fuel rather than oxygen becomes the limiting factor in the reaction, leaving some oxygen to flow past the turbine. After passing the turbine, the gas expands at a near constant entropy, thus losing temperature. The afterburner then injects fuel downstream of the turbine and reheats the gas. In conjunction with the added heat, the pressure rises in the tailpipe and the gas is ejected through the nozzle at a higher velocity. The mass flow is also slightly increased by the addition of the fuel.

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