Have you ever gone to an airshow and seen a fighter aircraft take off with a spectacular flame shooting out the exhaust with a noise louder than you can imagine what that plane should sound like? If so, you were witness to an afterburner take off. Afterburners are typically installed in military fighter aircraft as well as a few civilian aircraft, such as the Concorde, that are no longer in operation. The ability to reburn the air in the exhaust allows the aircraft to reach supersonic speeds much faster then a conventional turbine engine. It also makes for a great airshow display on takeoff.
Afterburner Development and Operation
Afterburners have been around in the development phase from as early as 1943 and forward. They were first designed for military applications in order to increase speed. This would give underpowered aircraft turbine engines an extra boost and give the fighter pilots an added advantage during times of war. Combat maneuvering requires skill but now with the added thrust it gave another leg up to our pilots.
An afterburner in the simplest of terms is the addition/extension of a ramjet engine on the backside of a turbojet or turbofan engine. This allows the afterburner to be used and stopped when needed allowing the aircraft to have the ability to have a fast mode and an efficient mode of flight. It consists of an afterburner duct, fuel spray nozzles, flame holders and an adjustable exhaust nozzle. (Figure 1) The adjustable exhaust nozzle allows the pressure to be regulated when the air/fuel mixture is ignited. The nozzle opens when with afterburner is in use and closes when it is not. All parts are made to exact tolerances due the sensitive nature of working to relight passing air.
Keeping the flame and igniting the air is complicated. According to Grogh (2017) “It’s like lighting a butane lighter when you’re sticking it out the window of your car and holding it behind the side mirror” (p. 1) The air is moving at several hundred feet per second so maintaining that flame is paramount.
After the flame has been established an afterburner now adds fuel through a series of small tubes that form a ring around the engine. The fuel sprays from these tiny holes in the tubes into the air stream, where it’s ignited, by a sparking device. Afterburning is incredibly fuel-inefficient, yet it is the best answer for allowing huge amounts of additional thrust to be created. How fuel efficient you ask? According to Crane (2011) It can be “up to a threefold increase in the normal fuel consumption” (p. 376) The only other option would be to increase the size of the engine, yet this comes with its own problems, mainly in the form of addition drag when reaching supersonic speeds.
Now with the extra thrust that is being produced by the addition of reburning the exhaust air, we now have to think about the heat that is going to be produced. The afterburner materials are the same materials as the engine, so a way to cool them need to be found. Cooling via the fuel that is passing to the area is one option, but it is not enough so other ways have been developed to allow cooling to happen
On recent afterburning engines Benningfield (2007) notes:
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. (p. 2)
The afterburner is an amazing piece of engineering that allows our military fighters an advantage over those aircraft that do not possess them. It also allows us the opportunity to further explore the idea of supersonic flight in new and developing aircraft. The downside of using an afterburner, such as tripling the use of fuel and the noise that is created by the use of afterburners, the idea that relighting bypass air is genius and the technology is only going to get better as the years go on.