Afterburners

Everything in this course so far has been a hymn to efficiency. This lesson is the opposite: the afterburner is the jet engine's deliberate, glorious act of waste — a way to buy a huge slug of extra thrust right now, at a fuel cost that would bankrupt any airline in an afternoon. Fighters carry one because in combat, "right now" outbids "affordable" every time.

Burning the leftovers

The trick exploits a leftover: the gas leaving the turbine still contains most of its original oxygen — the combustor could only burn ~20% of the air (its own lesson explained why), and the turbine merely cooled the flow without consuming any. So: extend the jet pipe, add rings of fuel injectors behind the turbine, and light the mixture again — this time with no fragile blades downstream to protect. Freed of the turbine's temperature ceiling, the reheat flame can run past 1,700 °C, and the far hotter, faster exhaust — through a nozzle that visibly widens its petals to accommodate it — delivers up to 50% more thrust at the press of the throttle past its detent.

Holding a flame in a jetpipe hurricane needs the combustor's cave trick again — here a ring of V-shaped flame holders whose sheltered wakes anchor the burn. The glowing rings you see inside a fighter's nozzle at night are exactly these, and the diamond pattern in the plume outside is the supersonic exhaust's shock cells made visible.

The bill

Reheat roughly doubles or triples the fuel flow for that ~50% more thrust — catastrophic thermodynamics, because the extra heat is added at low pressure (after the turbine, not before it), where the cycle can convert little of it to useful work. That is precisely why the main combustor sits upstream of the turbine, and why afterburner time is rationed: take-off from short runways, transonic acceleration, combat. Minutes, not hours. An F-16 in full reheat can drain its internal fuel in around ten minutes — burning a family car's annual fuel budget before the kettle boils.

Cousins and one famous exception

Because reheat hates high bypass (all that cold air would quench it), fighter engines are low-bypass turbofans — the two rivers remix before the afterburner so the burn has one hot stream to feast on. And one airliner used reheat in revenue service: Concorde, whose four Olympus engines lit their burners for take-off and the push through Mach 1, then cruised — uniquely — supersonic on dry thrust alone. Feel the trade yourself on the very engine: