page_title: Afterburner
section_title: Principle
hierarchical_section_title: Afterburner / Principle
caption_attribution_description: English: NASA’s SR-71 streaks into the twilight with full afterburner on the first night/science flight from the Dryden Flight Research Facility. Onboard were research pilot Steve Ishmael and flight engineer Marta Bohn-Meyer.Mounted in the nose of the SR-71 was an ultraviolet video camera aimed skyward to capture images of stars, asteroids and comets. This flight checked the operation of the camera to insure the air turbulence at Mach speeds and the vibration of the aircraft did not interfere with the operation of the camera. The science portion of the flight is a project of the Jet Propulsion Laboratory, Pasadena, California. Deutsch: Eine NASA SR-71 der Dryden Flight Research Facility steigt mit zugeschaltetem Nachbrenner im ersten Nacht-Wissenschaftsflug in der Dämmerung auf. An Bord befanden sich der Forschungspilotin Steve Ishmael und die Flugingenieurin Marta Bohn-Meyer.In der Nase der SR-71 befand sich eine ultraviolette Videokamera, die zum Himmel gerichtet war, um Bilder von Sternen, Asteroiden und Kometen aufzunehmen. Dieser Flug überprüfte den Betrieb der Kamera, um sicherzustellen, dass die Luftturbulenzen bei Mach-Geschwindigkeiten und die Vibration des Flugzeugs nicht den Betrieb der Kamera stören. Der wissenschaftliche Teil des Fluges ist ein Projekt des Jet Propulsion Laboratory, Pasadena, Kalifornien.
context_page_description: An afterburner is an additional combustion component used on some jet engines, mostly those on military supersonic aircraft. Its purpose is to increase thrust, usually for supersonic flight, takeoff, and combat. Afterburning injects additional fuel into a combustor in the jet pipe behind the turbine, "reheating" the exhaust gas. Afterburning significantly increases thrust as an alternative to using a bigger engine with its attendant weight penalty, but at the cost of very high fuel consumption which limits its use to short periods. This aircraft application of reheat contrasts with the meaning and implementation of reheat applicable to gas turbines driving electrical generators and which reduces fuel consumption.
  Jet engines are referred to as operating wet when afterburning is being used and dry when not. An engine producing maximum thrust wet is at maximum power, while an engine producing maximum thrust dry is at military power.
context_section_description: Jet-engine thrust is an application of Newton's reaction principle where the engine generates thrust because it increases the momentum of the air passing through it. Thrust depends on two things: the velocity of the exhaust gas and the mass of the gas. A jet engine can produce more thrust by either accelerating the gas to a higher velocity or ejecting a greater mass of gas from the engine. Designing a basic turbojet engine around the second principle produces the turbofan engine, which creates slower gas, but more of it. Turbofans are highly fuel efficient and can deliver high thrust for long periods, but the design tradeoff is a large size relative to the power output. Generating increased power with a more compact engine for short periods can be achieved using an afterburner. The afterburner increases thrust primarily by accelerating the exhaust gas to a higher velocity.
  The values in the following are for an early jet engine the Pratt & Whitney J57, stationary on the runway and illustrate the high values of afterburner fuel flow, gas temperature and thrust compared to those for the engine operating within the temperature limitations for its turbine.
  The highest temperature in the engine (about 3,700 °F (2,040 °C)) occurs in the combustor where fuel (8,520 lb/h (3,860 kg/h)) is burned completely in a relatively small proportion of the air entering the engine. The combustion products have to be diluted with air from the compressor to bring the gas temperature down to a value, called the Turbine Entry Temperature (TET) (1,570 °F (850 °C)), which gives the turbine an acceptable life. Having to reduce the temperature of the combustion products by a large amount is one of the primary limitations on how much thrust can be generated (10,200 lbf (45,000 N)). Burning all the oxygen delivered by the compressor would create temperatures (3,700 °F (2,040 °C)) high enough to destroy everything in its path, but by mixing the combustion products with unburned air from the compressor at 600 °F (316 °C) a substantial amount of oxygen (fuel/air ratio 0.014 compared to a no-oxygen-remaining value 0.0687) is still available for burning large quantities of fuel (25,000 lb/h (11,000 kg/h)) in an afterburner. The gas temperature drops as it passes through the turbine to 1,013 °F (545 °C). The afterburner combustor reheats the gas but to a much higher temperature (2,540 °F (1,390 °C)) than the TET (1,570 °F (850 °C)). As a result of the temperature rise in the afterburner combustor, the gas is accelerated, firstly by the heat addition, known as Rayleigh flow, then by the nozzle to a higher exit velocity than occurs without the afterburner. The mass flow is also slightly increased by the addition of the afterburner fuel. The thrust with afterburning is 16,000 lbf (71,000 N).
  The visible exhaust may show shock diamonds, which are caused by shock waves formed due to slight differences between ambient pressure and the exhaust pressure. This interaction causes oscillations in the exhaust jet diameter over a short distance and cause visible banding where the pressure and temperature is highest.
caption: SR-71 Blackbird in flight with J58 engines on full afterburner, with numerous shock diamonds visible in the exhaust.
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