What is the purpose of an afterburner on jets?

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What is the purpose of an afterburner on jets?

In: Engineering

It provides a huge increase in thrust (at the cost of a similarly huge consumption of fuel).

It injects fuel directly into the exhaust of the aircraft, resulting in a large increase in thrust, at the cost of a lot more fuel being used.

Afterburners add a ton of additional thrust at the cost of very poor efficiency.

Air coming into the engine is compressed by fans at the front. Fuel needs air to burn, so if you pack your air tightly it’s more efficient, because any individual molecule of fuel will find a molecule of oxygen quicker. However, you’re limited by the mechanics of the engine to just how compressed you can make the air, and how much pressure the engine can take from the burning, rapidly expanding fuel. That limits how much thrust you can make with that engine.

The afterburner dumps fuel into the already burning exhaust just before it exits the engine. This bypasses those limitations – the air *isn’t* compressed and all that extra force doesn’t need to be contained much, it’s already leaving the engine. But because that air isn’t compressed and is already partially “used up” by the fuel already burning from the combustion chamber, a lot of that afterburner fuel doesn’t burn until after it’s left the engine, which means it produces no thrust for the engine.

The afterburner gives the engine a big extra kick of thrust when needed, like when jets that aren’t the F-22 or SR-71 need to go supersonic briefly, but the fuel consumption while using the afterburner just isn’t sustainable. It’s horribly inefficient and burns through fuel very quickly.

Additionally, the added heat and corrosive fuel puts more energy in the nozzle, which also isn’t sustainable. If the afterburner is used for too long, it could damage the engine. Which is fine because, again, it uses too much fuel to do it for too long anyway.