Afterburner Thrust

The idea in one sentence

You have just spent enormous energy compressing air, heating it, and extracting work to drive the compressor. The exhaust leaving the turbine is hot, fast, and — crucially — still rich in oxygen. The combustor upstream ran lean on purpose so the turbine blades would not melt. Downstream of the turbine there are no rotating parts to protect, so you can light it all up again: spray fuel, ignite, and let the now far hotter gas expand through the nozzle. The kinetic-energy content of the jet jumps; thrust jumps with it. That is an afterburner.

The trade is brutal but for short bursts irresistible. A modern fighter engine producing 17,000 lbf dry will produce 26,000 lbf wet — about 53 percent more — at roughly 3.5× the fuel flow. For five minutes of supersonic acceleration through transonic drag, or 20 seconds of combat dash to a missile-launch position, the calculus pays. For an eight-hour ocean crossing, it is suicide.

Why oxygen survives the combustor

Stoichiometric combustion of Jet-A or JP-8 — the ratio at which every fuel molecule meets exactly the oxygen it needs — produces a flame temperature near 2300 °C. No turbine blade material can survive that. Even the latest single-crystal nickel superalloys with film cooling and thermal-barrier coatings are limited to about 1700 °C metal temperature, and the gas they see needs to be below about 1700 °C at the turbine inlet to keep blade life acceptable.

The standard fix is to run the main combustor extremely lean overall — a global fuel-air ratio of about 0.020 versus the stoichiometric 0.067 for kerosene. Only the inner core of each combustor "can" reaches stoichiometric; outer dilution holes inject huge quantities of cooler compressor air to dilute the products before they reach the turbine. The result is exhaust gas with an average temperature of 1300–1700 °C and an oxygen content of roughly 21 → 15 percent by volume — still over two-thirds of what fresh air carries. Plenty to burn again.

The hardware

An afterburner section sits between the last turbine stage and the propulsive nozzle. From upstream to downstream it contains:

• Diffuser. A short divergent duct that slows the post-turbine gas to a velocity where combustion can stabilise (Mach 0.2–0.3). Combustion cannot anchor in a fast-moving stream.
• Spray bars. Concentric fuel rings — typically three to seven of them — that distribute jet fuel across the duct cross-section. Each ring has dozens of orifices sized for atomisation rather than total flow.
• Flame holders. V-shaped or annular bluff bodies (often called "vee-gutters") downstream of the spray bars. They generate a recirculating low-velocity wake in which the flame anchors. Without them the flame would simply blow downstream and extinguish.
• Afterburner liner. A perforated inner shell that carries air around the flame zone for cooling — the same trick used in main combustors, scaled up for the larger envelope.
• Igniters. Spark plugs or hot-streak igniters that fire briefly to start combustion, then drop out once the flame is anchored.
• Variable convergent-divergent nozzle. The propulsive nozzle whose throat area and exit area can both change. On modern fighters it is built from interlocking petals driven by hydraulic actuators that synchronise with the throttle.

Why the nozzle has to open

Adding heat to a flow at constant area chokes it. Specifically, in Rayleigh flow — a frictionless duct with heat addition — adding heat to a subsonic flow drives it toward Mach 1. Past that, more heat cannot be added without unchoking the duct upstream. In an afterburner, "unchoking the duct upstream" would mean back-pressuring the turbine into a stall, with catastrophic consequences for engine integrity.

The fix is the variable nozzle. When reheat lights, the gas density drops (hot gas is less dense, T₃/T₂ rises by ~2) and the volumetric flow rate roughly doubles at the same mass flow. To pass that doubled volumetric flow at the same Mach number through the throat, the throat area must roughly double — or at least open by 40–60 percent on real designs. Hydraulic actuators tied to throttle position do this in tens of milliseconds.

The divergent section downstream of the throat performs the supersonic expansion. At subsonic flight speeds (takeoff, landing) the divergent section is barely needed — the petals nest into a near-convergent geometry. At Mach 2+ the divergent section opens to a much larger exit area, giving the gas a longer expansion run from throat conditions down toward ambient. The geometry that matches both regimes is the source of the petal-and-actuator complexity you see on every modern fighter nozzle.

Performance numbers

The canonical comparison is dry versus wet thrust at sea level static conditions:

Engine	Aircraft	Dry thrust	Wet thrust	SFC dry	SFC wet	Boost
Pratt & Whitney F100-220	F-15, F-16	14,670 lbf	23,830 lbf	0.74	2.05	+62 %
Pratt & Whitney F119-100	F-22	26,000 lbf	35,000+ lbf	~0.79	~2.5	+35 %
General Electric F110-129	F-16	17,150 lbf	29,400 lbf	0.77	2.07	+71 %
Pratt & Whitney J58	SR-71	25,000 lbf	34,000 lbf*	—	~1.9	+36 %*
Tumansky R-15B-300	MiG-25	16,500 lbf	24,700 lbf	—	~2.4	+50 %
Rolls-Royce / Snecma Olympus 593	Concorde	32,000 lbf	38,000 lbf	—	—	+19 %

* The J58's quoted boost at sea level understates the engine's real contribution. At Mach 3.2 cruise, with the bypass tubes open and most of the airflow being burned in the afterburner rather than in the main combustor, afterburner contribution to total thrust exceeds 70 percent.

Two patterns stand out. First, the percentage boost varies widely — Concorde's relatively low 19 percent reflects that the Olympus 593 was designed for steady supersonic cruise with continuous "minimum reheat", not for headline boost. Second, SFC roughly triples wet versus dry for every engine on the list. There is no afterburner that breaks that rule.

Famous applications

• Concorde — takeoff and transonic climb. The Olympus 593 ran reheat for the first 14 minutes of every flight: full reheat for takeoff and rotation, then "minimum reheat" through Mach 0.95 to 1.7 to power through transonic drag. From Mach 1.7 to cruise Mach 2.0 the engine ran dry. This is the only commercial use of reheat in scheduled passenger service — the Tu-144 used a similar profile.
• SR-71 Blackbird — sustained Mach 3 cruise. The Pratt J58 used a bypass scheme that sent air around the compressor at high speed, with that air being burned in the afterburner along with the core flow. The result is that at Mach 3+ the engine effectively ran as a ramjet with the turbojet section serving as a starter and a pre-compressor. Afterburner ran continuously during the supersonic cruise phase.
• F-22 Raptor — boost on top of supercruise. The F119 is one of the rare fighter engines whose dry thrust is high enough to give supersonic cruise (about Mach 1.8) without afterburner. Reheat is reserved for combat acceleration, vertical climb, and post-stall manoeuvres in thrust vectoring.
• MiG-25 Foxbat — reheat-only top speed. The Tumansky R-15 is the case study for an engine that lived on its afterburner. Maximum recorded speed (Mach 2.83 in service, Mach 3.2 in one famous reconnaissance overflight that destroyed the engines) was reached almost entirely on reheat. The compressor was modest; the afterburner was huge.
• F-15, F-16, F/A-18, Eurofighter, Rafale, Su-27 family. Every fourth- and fifth-generation fighter uses reheat for takeoff, climb to altitude, transonic acceleration, and combat manoeuvres. Without it, none of them is a "supersonic" aircraft in any meaningful sense — most cannot even reach Mach 1 dry.

Bypass air, mixed-flow, and modern afterburners

Modern fighter engines are low-bypass turbofans (bypass ratios 0.3–0.7) rather than pure turbojets. Some of the inlet air goes around the core through a bypass duct and rejoins the core exhaust just upstream of the afterburner. This bypass air carries more oxygen than the core exhaust — it has not been combusted at all — and contributes substantially to afterburner mass flow. The F100, F110, F119, and EJ200 all use this mixed-flow architecture; the F119 in particular relies heavily on bypass-stream reheat to get its peak augmentation ratio.

High-bypass civil turbofans (bypass ratios 5–12) cannot usefully be reheated. Their thrust comes overwhelmingly from the cool bypass stream, and igniting that stream in a downstream burner would not work efficiently — the bypass air has not been pre-heated by the core. There were research programmes in the 1970s for afterburning high-bypass engines (the "VCE", variable-cycle engine) for an American supersonic transport, but none flew commercially.

Screech, rumble, and combustion instability

Afterburners are prone to dramatic combustion instabilities. The two named modes are:

• Screech. A high-frequency (300–1500 Hz) acoustic resonance in the afterburner duct. Pressure oscillations can reach 10 percent of mean pressure, enough to destroy liners and rupture nozzle hardware in seconds. Mitigated with perforated screech liners that absorb the resonant modes.
• Rumble or buzz. Low-frequency (50–250 Hz) instability driven by coupling between fuel-injection delay and flame heat release. Felt as a deep, audible thumping through the airframe.

Engineers tune spray-bar location, flame-holder geometry, and liner perforation pattern to stay clear of these modes across the full flight envelope. Off-design points — for example, light-off at very high altitude where ambient pressure is low — are where afterburner stability problems most commonly bite. Decades of flight test have gone into the simple-looking ring of vee-gutters in any modern afterburner.

Downsides

• Fuel burn 3–5×. The fundamental trade. An F-15 with both engines in afterburner consumes about 200 lb/s of fuel — its internal fuel load lasts less than a minute in maximum burner. Mission planning treats afterburner as a budgeted resource.
• Infrared signature. Plume temperature rises from ~600 °C to 1500 °C; total IR radiance (T⁴) rises by an order of magnitude. The plume length grows from feet to tens of feet. IR-guided missiles like AIM-9X and R-73 lock dramatically more easily.
• Noise. Reheat is roughly 10–15 dB louder than dry power. Concorde takeoffs were banned at several airports under Stage 2 noise rules; the SR-71's afterburner takeoff at Mildenhall was famous as a regional event.
• Engine wear. Hot section temperatures and acoustic loads in reheat reduce afterburner liner life dramatically; many designs require liner inspection every few hundred flight hours of reheat use.
• Weight and complexity. A variable con-di nozzle, hydraulic actuator ring, and afterburner ducting add roughly 15–20 percent to engine weight. On a fighter this is acceptable; on a transport it would be ruinous.

Worked example: thrust boost in Rayleigh flow

To first order the thrust ratio scales as the square root of the temperature ratio for a choked nozzle. If T₆ is the gas temperature entering the afterburner and T₇ the temperature after reheat,

F_wet / F_dry ≈ √(T₇ / T₆)

For typical numbers — T₆ = 900 K (post-turbine, low-bypass fighter) and T₇ = 2000 K (post-reheat) — the ratio is √(2000/900) = √2.22 ≈ 1.49. A 49 percent thrust boost. The match to real engines on the table above (35–71 percent) is good given the simplifying assumptions: fixed-area inlet, choked nozzle, ideal Rayleigh heat addition, neglecting ram drag changes.

The corresponding SFC increase comes from the heat added per unit mass: about (T₇ − T₆) × c_p worth of fuel, which is roughly 2.4× the heat added in the main combustor. Divide by the modest thrust increase and you recover the factor-of-three rise in pounds of fuel per pound of thrust per hour.

Variants and extensions

• Water injection. An older pre-afterburner technique (used on early jet airliners like the 707-100 and KC-135) where water-methanol is injected into the compressor inlet to cool the air and let the engine ingest a denser mass flow on hot days. Less drastic than reheat, used only for takeoff, replaced by larger engines on later types.
• Hot-streak augmentation. A simplified afterburner that injects fuel through a single radial line rather than a full spray ring, used in some helicopter turboshaft IR-decoy systems.
• Minimum reheat / partial augmentation. Continuous afterburner at low fuel flow — the Olympus 593's transonic mode. Sits between dry and full reheat to bridge the transonic drag rise efficiently.
• Plug nozzle. An alternative variable-area nozzle that uses an axial centerbody (plug) rather than petals. Tested on the J58 and J85, never made it into series fighter production.
• Variable-cycle engine (VCE). Research engines (GE YF120, Pratt & Whitney VCE) that can switch between turbojet and turbofan modes by varying bypass ratio in flight, with reheat available in both modes. NGAD-class engines (GE XA100, Pratt & Whitney XA101) are the current generation of this idea.

Common pitfalls

• Conflating afterburner with thrust vectoring. They are independent. The F-22 has both; the F-16 has only afterburner; the Su-30 family has only thrust vectoring on some variants. A nozzle can have either, both, or neither.
• Assuming reheat works on any turbine engine. High-bypass civil turbofans cannot meaningfully be afterburned. The bypass air is not pre-heated, and the core stream is a small fraction of the total mass flow.
• Forgetting that ram drag rises with afterburner. At high subsonic and supersonic speeds, the inlet captures more mass flow when the engine is in reheat; ram drag (the momentum drag of decelerating that mass) goes up. The net thrust gain is smaller than the gross gain — sometimes substantially.
• Treating SFC as a single number. Afterburner SFC varies strongly with altitude and Mach number. The 2.5 lb/(lb·hr) figure is sea-level static; at altitude and high Mach, SFC drops because the inlet pressure recovery delivers air essentially for free.
• Designing a fixed nozzle for reheat. Without a variable throat, lighting reheat back-pressures the turbine into a stall — sometimes a hard stall with mechanical damage. Every operational afterburner has a variable nozzle.