Afterburner

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Illustration of an afterburner

An afterburner is an accessory to a turbine jet engine that can be used to increase the outlet velocity of the working medium and thus the thrust of the engine by burning fuel injected behind the turbine . The afterburner use can often be regulated by the pilot in several power levels.

Sectional view of a Rolls-Royce Turboméca Adour with the four clearly visible rings of the afterburner
Adjustable, convergent-divergent nozzle of a Eurojet EJ200 turbofan engine with afterburner system

Turbine jet engines are operated with excess air in order to keep the exhaust gas temperature at a level that is compatible with the materials of the turbine blades. After passing through the turbine, there is still enough oxygen available in the exhaust gas to burn additional injected fuel in the jet pipe. The beam temperature is no longer limited by downstream components in the stream.

Since the fuel consumption in afterburner operation is up to a factor of 10 higher than the normal consumption of the engine, it is only switched on in special situations when more thrust is required for a short time. At subsonic speed, the additional fuel expenditure is badly proportioned to the thrust gained. As the Mach number increases, the afterburner becomes increasingly efficient.

Most supersonic aircraft rely on afterburners to achieve speeds higher than Mach  1 in level flight . The ability to fly supersonic without using an afterburner is known as a super cruise .

Mode of action

Due to the isobaric increase in temperature, the density of the gas decreases and the exit velocity increases to the same extent. The mass throughput remains largely unaffected. When the afterburner is used, an increase in the jet speed is achieved without increasing the pressure of the compressor as is the case with the nozzle. The afterburner replaces the load of the subsonic convergent nozzle.

Constructive details

The afterburner usually has an inner tube that is separated from the outer skin of the thrust tube by a narrow gap. The gap is used to cool the inner afterburner tube through the flow of flue gas or a mixture of flue gas and air. The injection of the fuel after exiting the turbine (“flame holder”) creates local backflow areas and thus stabilizes the flame (temperature up to 2,000 K).

The thrust tube cross-section is usually significantly larger than the nozzle cross-section and is intended to reduce the pressure loss in the flow. The fuel is usually ignited by:

  • Catalyst ignition by platinum elements
  • Spark plug or ignition torch (endurance burn)
  • "Hot Shot", i.e. H. Additional fuel injection into the combustion chamber. As a result, the flame generated in this way extends into the afterburner tube; it ignites the additional fuel injected there.
  • Regulation of the nozzle position or the fuel supply into the afterburner, with the other parameter being adapted so that the ratio of the (total) pressures in the combustion chamber and turbine remains unaffected by the afterburner operating state.

Afterburners are used almost exclusively on military aircraft types. Two of the few civil aircraft with an afterburner were the Tupolev Tu-144 and the Concorde .

If, in turbofan engines , the hot exhaust gas jet is first mixed with the cold bypass flow and then directed into the afterburner, the afterburner type is known as an augmentor .

literature

  • Ernst Götsch: Aircraft technology. Motorbuchverlag, Stuttgart 2003, ISBN 3-613-02006-8 .
  • Kyrill von Gersdorff, Helmut Schubert, Stefan Erbert: The German aviation. Aircraft engines and jet engines. Bernard & Graefe, Bonn 2004, ISBN 978-3-7637-6128-9 .

Web links

Commons : Afterburner  - collection of images, videos and audio files
Wiktionary: Afterburner  - explanations of meanings, word origins, synonyms, translations