How Things Work - Afterburners
Jets get no kick from champagne, but a little fuel in the tailpipe...
- By Damond Benningfield
- Air & Space magazine, July 2007
Airman Apprentice Nathan Laird, U.S. Navy
(Page 2 of 3)
“You need to insure that when you spray fuel into the high-velocity air stream, it doesn’t just blow out the tailpipe,” says Louis Povinelli, chief scientist for turbomachinery and propulsion systems at NASA’s Glenn Research Center in Cleveland, Ohio. The ignition process is “still somewhat of a black art,” he says.
The afterburner is designed so that the flame flows along its axis, away from its walls. Careful placement of the fuel tubes and the ignition source at the front end of the jetpipe (the four- to seven-foot-long tube at the back of the engine), where hot but not burning exhaust gas is flowing out of the engine, creates a stable zone in the airflow where air and fuel can mix.
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
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.