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How Things Work: Afterburners

Jets get no kick from champagne, but a little fuel in the tailpipe...

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(Continued from page 1)

The stable flow ensures that the flame ignites quickly and burns at a consistent location. If the flame moves around, it could set up oscillations that eventually could burn through the jetpipe or damage the end of the exhaust nozzle.

Designers often also add pilot lights downstream from the ignition spot to make sure that the flame burns evenly and consumes all of the fuel that flows into the afterburner.
Another challenge is keeping the metal jetpipe cool in the afterburner’s high temperatures, which can reach 3,000 degrees Fahrenheit.

“People keep pushing the limit between the gas temperature and the melting point” of the engine components, says Povinelli. “The materials aren’t any different than other parts of the engine, and the walls aren’t especially thick.”
Cold fuel flowing through tubes at the top of the afterburner absorbs some of the heat, Povinelli explains.

More recent turbofan engines add a flow of cold air through a ring around the barrel-shaped engine, bypassing its combustion chamber. At high altitudes the temperature is well below zero, and the influx of cold air into the afterburner pipe helps protect it against the flaming
exhaust.

As the exhaust races out the back, the engine’s nozzle is designed to open wider to accommodate the extra volume of hot gas, preventing any increase in pressure inside the engine.

One problem with this arrangement, engineers note, is that things that tend to be good for combustion are bad for stealth, and vice versa.

With afterburners, the open tailpipe welcomes enemy radar waves, which enter the hole and bounce back a strong signal even when the afterburner is not lit.
It is also nearly impossible to hide the infrared emissions from a lit afterburner and its nozzle—structures that stealth airplanes like the B-2 and F-117 don’t have.
Using a turbofan, which mixes cool air with the turbine exhaust gases, helps decrease the signature a little.

Future designs, featuring afterburner nozzles built into the fuselage and cooled with bypass air, may mask the jetpipe’s infrared emissions. Engineers also are evaluating construction materials that absorb heat, similar to space shuttle thermal tiles, and other engine designs that would create stealthier afterburners.

But there is no way to fully hide a plume of hot air roaring from the back of a warplane. The only way to preserve stealth is to reduce the reliance on afterburners. The F-22A, for example, can cruise at about 1.5 times the speed of sound without lighting the afterburner.

Still, aviation engineers say afterburners will remain in use well into the 21st century. Although carrier takeoffs are the most common of current uses of the afterburner, extra speed is always useful in combat. As long as military pilots might need extra bursts of power, afterburners are likely to remain the solution.

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