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Airplane Propulsion 147

Thrust Augmentation

Military fighters and interceptors sometimes need extra power, called
military power. One solution is to install a larger engine. But a larger
engine weighs more and so is not a practical choice. Instead, some mil-
itary engines “augment” their thrust. The principle is to add more
power to the air after the turbine has removed most of the power to
drive the compressor and fan. Fuel injectors are added between the tur-
bine and the nozzle, which injects fuel to mix and burn with the excess
oxygen. This device is called an afterburner. The energy added by the
afterburner is transferred to kinetic energy through the nozzle. The net
effect is an increase in thrust and a loss of fuel efficiency. A fighter on
afterburners may only be able to maintain this power level for 15 min-
utes or less.
Visually, an afterburner is impressive. Because the combustion is
taking place just before the nozzle, the flame actually extends through
the nozzle and out the rear of the engine. You can see the
flame when the afterburners are working. As an example,
The YF-22 (Figure 3.36) is the
Figure 5.26 shows an SR-71 Blackbird on afterburners. The
first airplane that can cruise
flame is easily visible. The bright spots are called diamond
supersonically without the use of
shocks. Because there is so much energy left in the exhaust,
afterburners.
afterburners are incredibly noisy.

Wrapping It Up

Aircraft propulsion systems involve the same physics as wings. In
the case of the propeller, it is nothing more than a rotating wing. The

Fig. 5.26. The SR-71 on afterburners. (Photo courtesy of NASA.)