Afterburning (AB) is another way of thrust augmentation intended exclusively for the supersonic combat aircraft category (the Concorde is the only civil aircraft that used AB). Figure 10.6 is a cutaway diagram of a modern AB engine intended for combat aircraft.
Figure 10.7. Schematic diagram of a turboprop engine
The simple straight-through turbojets have a relatively small frontal area resulting in low drag and excess air in the exhaust flow. If additional fuel can be burned in the exhaust nozzle beyond the turbine exit plane, additional thrust can be generated to propel an aircraft at a considerably higher speed and acceleration, thereby also possibly improving propulsive efficiency. However, the reason for using AB arises from the mission demand, such as at takeoff with a high payload and acceleration to engage or disengage during combat and evasion maneuvers. Mission demand overrides the fact that there is a high level of energy rejection in the high exhaust velocity. Fuel economy degrades with AB – it takes 80 to 120% more fuel burn to gain a 30 to 50% increase in thrust. Currently, most supersonic aircraft engines have some BPR when AB is done in the cooler mixed flow past the turbine section of the primary flow.