Afterburner Exergy Analysis: What Reheat Really Costs
The afterburner is the most dramatic device on a fighter jet: a tube of flame that adds half again the engine's thrust at the flick of a switch. It is also the most thermodynamically brutal. A first-law analysis tells you reheat is expensive; a second-law (exergy) analysis tells you why, where, and exactly how much — which is what you need when deciding whether a mission segment justifies it.
What the afterburner actually does
Between the turbine exit and the nozzle sits the jet pipe. The gas there still holds usable oxygen — all of it in a turbojet's leftover margin, plus the entire bypass stream's oxygen in a mixed turbofan. Spray fuel into that stream, anchor the flame on V-gutter flameholders, and you get a second combustion stage with no turbine behind it:
- Exhaust temperature jumps from ~900–1,000 K to 1,700–2,000 K,
- the nozzle (opened wider) converts that enthalpy to velocity,
- and thrust rises ~50–70% on a turbojet, sometimes ~100% on a mixed low-bypass turbofan.
No spool accelerates, no compressor works harder — that is the appeal. Reheat is thrust on demand, decoupled from the core engine's limits.
The second-law indictment
Fuel exergy is nearly equal to its heating value (ech/LHV ≈ 1.04 for JP-8) — every kilogram of fuel arrives as almost pure work potential. What happens to it in the afterburner is a case study in irreversibility:
Ėd,AB = T₀·Ṡgen,AB- Low-pressure combustion: the main combustor burns at 15–30 atm; the afterburner burns at 2–4 atm. Chemical reaction irreversibility scales badly with pressure — burning the same fuel at low pressure destroys a much larger share of its exergy.
- No work extraction: heat added in the main combustor passes through a turbine that converts a large share to shaft work. Heat added in the jet pipe has one exit: the nozzle. Whatever the nozzle cannot convert to directed kinetic energy leaves as a hot plume.
- The plume: reheat raises exhaust velocity far above flight speed. That velocity mismatch is kinetic exergy thrown overboard — the same propulsive-efficiency penalty that punishes turbojets, amplified.
Typical numbers (static sea level, military turbojet)
| Dry | Full reheat | Change | |
|---|---|---|---|
| Net thrust | ~14.7 kN | ~43.9 kN | ≈ 3× (reheat-heavy config) |
| TSFC | ~22 g/(kN·s) | ~57 g/(kN·s) | ≈ 2.6× |
| Overall efficiency (static) | 0 (by definition) | 0 | — |
| Second-law efficiency ηII | baseline | drops sharply | AB is the largest single destroyer when lit |
The pattern is universal: thrust grows slower than fuel flow, so TSFC roughly doubles to triples. In the exergy budget the afterburner instantly becomes the engine's largest irreversibility — often exceeding the main combustor that runs continuously.
Design rule: an afterburner is a thrust-per-minute device. If the mission need is measured in seconds (takeoff, dash, combat), reheat is unbeatable. If it is measured in hours, no amount of reheat engineering makes it efficient — that is what bypass ratio is for.
Why the nozzle has to open
Reheat raises temperature at nearly constant mass flow; to keep the same corrected flow through the turbine and fan, nozzle throat area must increase. Fixed-geometry nozzles and reheat do not mix — back-pressure rises, the fan operating point drifts toward surge, and much of the gain evaporates. This is why afterburning engines carry variable convergent or convergent-divergent (C-D) nozzles, and why any honest reheat model must model the nozzle schedule too.
When reheat is the right answer
- Takeoff: 30–60 seconds of reheat replaces a much larger, heavier dry engine that would be dead weight for the rest of the flight.
- Transonic acceleration: wave drag peaks near Mach 1; punching through quickly on reheat can use less total fuel than crawling through dry.
- Supersonic dash / combat: thrust margin is survival; efficiency is secondary.
- Never for cruise: Concorde — the canonical supersonic cruiser — reheated for takeoff and transonic acceleration, then flew dry at Mach 2.
Frequently asked questions
How does an afterburner work?
An afterburner (reheat) injects and burns extra fuel in the jet pipe, downstream of the turbine. Because the turbine has already taken its work, the gas still contains unused oxygen — especially in turbofans with bypass air mixed in. Burning more fuel there raises exhaust temperature and velocity, increasing thrust by roughly 50–70% on a dry turbojet.
Why are afterburners so inefficient?
Combustion in the afterburner happens at much lower pressure than in the main combustor, and the added heat leaves almost immediately through the nozzle instead of driving a turbine. Low-pressure heat addition has inherently poor second-law quality: most of the fuel's exergy is destroyed by combustion irreversibility or carried away in the hot exhaust plume rather than converted to thrust.
How much fuel does an afterburner use?
A rough rule: lighting the afterburner doubles to triples total fuel flow while adding about 50–70% more thrust on a turbojet. In TSFC terms a dry military engine around 20–24 g/(kN·s) jumps to roughly 50–70 g/(kN·s) with full reheat. It is a thrust-per-minute purchase, not an efficiency device.
Why does an afterburner need a variable nozzle?
Reheat raises exhaust temperature enormously at nearly constant mass flow, so the nozzle throat area must open to keep the pressure balance and avoid disturbing the fan/turbine operating point. A convergent nozzle opens its exit; convergent-divergent (C-D) nozzles additionally expand the supersonic jet efficiently. Running a fixed-area nozzle with reheat would back-pressure the engine and lose most of the benefit.
When is using an afterburner worth it?
When thrust-per-minute matters more than fuel-per-thrust: takeoff from short or hot-and-high runways, transonic acceleration, supersonic dash, and combat maneuvers. Fighters accept the enormous fuel burn for minutes at a time. For cruise, reheat is never efficient — Concorde used it for takeoff and transonic acceleration, then cruised dry.
Flip the switch and watch the budget
ExergyJet models afterburner on/off for the same engine configuration: net thrust, TSFC, station states and the component-level exergy budget side by side. The afterburner row in the destruction table makes the cost impossible to miss — and the compare view makes the trade quantitative.
Run a turbojet dry, then lit — and see exactly where the extra fuel exergy goes.
Run a free exergy analysis →