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War emergency power (WEP) is a throttle setting that was present on some American World War II military
aircraft engine An aircraft engine, often referred to as an aero engine, is the power component of an aircraft propulsion system. Most aircraft engines are either piston engines or gas turbines, although a few have been rocket powered and in recent years many ...
s. For use in emergency situations, it produced more than 100% of the engine's normal rated power for a limited amount of time, often about five minutes. Similar systems used by non-US forces are now often referred to as WEP as well, although they may not have been at the time, as with the German Luftwaffe's ''Notleistung'' and Soviet VVS' ''forsazh'' systems.


WEP in World War II aircraft

Maximum normal power would be limited by a mechanical stop, for instance a wire across the throttle lever slot, but a more forceful push would break the wire, allowing extra power. In normal service, the P-51H Mustang was rated at , but WEP would deliver up to , an increase of 61%. In the
P-51D Mustang Over twenty variants of the North American P-51 Mustang fighter were produced from 1940, when it first flew, to after World War II, some of which were employed also in the Korean War and in several other conflicts. Allison-engined Mustangs ...
, the model most produced and used during World War II, the WEP increased engine power from . The Vought F4U Corsair, not originally equipped for WEP, later boasted a power increase of up to (17%) when WEP was engaged. Several methods were used to boost engine power by manufacturers, including water injection and methanol-water injection. Some earlier engines simply allowed the throttle to open wider than normal, allowing more air to flow through the
intake An intake (also inlet) is an opening, structure or system through which a fluid is admitted to a space or machine as a consequence of a pressure differential between the outside and the inside. The pressure difference may be generated on the ins ...
. All WEP methods result in greater-than-usual stresses on the engine, and correspond to a reduced engine lifetime. For some airplanes, such as the P-51D, use of WEP required that the engine be inspected for damage before returning to the air.AAF Manual 51-127-3, ''Pilot Training Manual for the P-51 Mustang'', USAAF, August 1945, p.14. 5 hours' total use of WEP on the P-51D required a complete tear-down inspection of the engine. British and Commonwealth aircraft could increase power by increasing the supercharger boost pressure. This modification was common by the summer of 1940, with the widespread availability of 100 octane fuel. Raising supercharger boost pressure from increased the Merlin III engine rating to , an increase of over . Pilots had to log the use of emergency boost and were advised not to use it for more than 5 minutes continuously. The German
MW 50 MW 50 (Methanol-''Wasser'' 50) was a 50-50 mixture of methanol and water (German: ''Wasser'') that was often sprayed into the supercharger of World War II aircraft engines primarily for its anti-detonation effect, allowing the use of increased b ...
methanol-water injection system required additional piping, as well as a storage tank, increasing the aircraft's overall weight. Like other boost techniques, MW 50 was restricted by capacity and engine temperatures and could only be used for a limited time. The GM 1 nitrous oxide injection system, also used by the Luftwaffe, provided extreme power benefits of 25 to 30 percent at high altitude by adding oxidizer gases but required cooling on the ground and, like the MW 50 boost system, added significant weight. One of the few German aircraft that could be equipped with both ''Notleistung'' systems, the late war
Focke-Wulf Ta 152 The Focke-Wulf Ta 152 is a World War II German high-altitude fighter-interceptor designed by Kurt Tank and produced by Focke-Wulf. The Ta 152 was a development of the Focke-Wulf Fw 190 aircraft. It was intended to be made in at least three ve ...
H high-altitude fighter, could attain a velocity of some with both systems used together. Kurt Tank reportedly once did this, using both boost systems simultaneously when he was flying a Junkers Jumo 213E-powered Ta 152H prototype fitted with both MW 50 and GM-1, to escape a flight of P-51D Mustangs in April 1945.


Modern times


MiG-21

Perhaps the most dramatic WEP feature was found in the
MiG-21 The Mikoyan-Gurevich MiG-21 (russian: Микоян и Гуревич МиГ-21; NATO reporting name: Fishbed) is a supersonic jet fighter and interceptor aircraft, designed by the Mikoyan-Gurevich Design Bureau in the Soviet Union. Its nickna ...
bis fighter jet. This late variant of the standard Soviet light fighter plane was built as a stopgap measure to counter the newer and more powerful American F-16 and F/A-18 fighters until the next-generation
MiG-29 The Mikoyan MiG-29 (russian: Микоян МиГ-29; NATO reporting name: Fulcrum) is a twin-engine fighter aircraft designed in the Soviet Union. Developed by the Mikoyan design bureau as an air superiority fighter during the 1970s, the M ...
could be introduced to service. The MiG-21bis received the upgraded
Tumansky Sergei Konstantinovich Tumansky (russian: Серге́й Константинович Туманский; – 9 September 1973) was a designer of Soviet aircraft engines and the chief designer in the Tumansky Design Bureau, OKB-300. He worked i ...
R-25 engine, which retained the standard normal and forsazh power settings of earlier R-13 powerplants, but added a second fuel pump to supply the new super-
afterburning An afterburner (or reheat in British English) 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 ...
system with jet fuel. Use of this "diamond regime" provided a massive of thrust for no more than 3 minutes in actual wartime use. Use of this temporary power gave the MiG-21bis slightly better than 1:1
thrust-to-weight ratio Thrust-to-weight ratio is a dimensionless ratio of thrust to weight of a rocket, jet engine, propeller engine, or a vehicle propelled by such an engine that is an indicator of the performance of the engine or vehicle. The instantaneous thrust-to-w ...
and a climbing rate of , equalling the F-16's nominal capabilities in close-quarters dogfight. In air combat practice with the MiG-21bis, use of WEP thrust was limited to one minute, to reduce impact on the engine's 800 flight hours lifetime, since every second of super-afterburner use counted as several minutes of regular power run due to extreme thermal stress. When WEP was on, the R-25 engine produced a huge long blowtorch exhaust - the six or seven brightly glowing rhomboid "
shock diamond Shock diamonds (also known as Mach diamonds or thrust diamonds) are a formation of standing wave patterns that appear in the supersonic exhaust plume of an aerospace propulsion system, such as a supersonic jet engine, rocket, ramjet, or scramje ...
s" visible inside the flames gave the emergency-power setting its "diamond regime" name.


F-15C

The Vmax switch on the
F-15 The McDonnell Douglas F-15 Eagle is an American twin-engine, all-weather tactical fighter aircraft designed by McDonnell Douglas (now part of Boeing). Following reviews of proposals, the United States Air Force selected McDonnell Douglas's ...
fighter jet allows the engines to burn 22 degrees hotter and about 2 percent more revolutions per minute. It is safety-wired shut. During combat, pulling the Vmax switch would provide the pilot with a little more thrust. However, the engines would then need to be serviced and rebuilt.


WEP in surface vehicles

Some modern military surface vehicles also employ WEP features. The US Marine Corps
Expeditionary Fighting Vehicle The Expeditionary Fighting Vehicle (EFV) (formerly known as the Advanced Amphibious Assault Vehicle (AAAV)) was an amphibious assault vehicle developed by General Dynamics during the 1990s and 2000s for use by the U.S. Marine Corps. It would h ...
(cancelled in 2011) sported a 12-cylinder diesel engine developed by the German company MTU. When the EFV is swimming the powerplant can be boosted to via the use of open circuit seawater-cooling. Such extreme war power setting allows the MTU engine to drive four massive water-jet exhausts which propel the surface-effect riding EFV vehicle at sea speeds reaching . Although the EFV prototypes demonstrated revolutionary performance on water and land, the reliability of their extremely boosted powerplants never met stringent military standards and the vehicle failed to enter Marine Corps service.


Boost systems

* Water injection * MW50 (German, methanol/water mixture) * GM 1 (German, nitrous oxide injection) * Forsazh (Russian) * Propane injection


See also

*
Afterburner An afterburner (or reheat in British English) 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 comba ...
*
Flank speed Flank speed is an American nautical term referring to a ship's true maximum speed but it is not equivalent to the term ''full speed ahead''. Usually, flank speed is reserved for situations in which a ship finds itself in imminent danger, such as ...


References

{{reflist Aircraft propulsion components