Supercruise

A supercruising aircraft is able to maintain supersonic speeds in level flight without the use of afterburners and with a cargo/weapons load.

Advantages

Afterburners, which most military aircraft use to travel at supersonic speeds, are very inefficient compared to conventional jet engine operation due to the low pressures typically found in the exhaust section. Therefore, in general, an aircraft which can supercruise has greater endurance at supersonic speeds than one which cannot. Furthermore, without a requirement to carry such a large quantity of fuel, a supercruise-capable aircraft can have a more favourable fuel fraction, the proportion of the plane's overall mass which is devoted to fuel. Supercruise also increases the aircraft's stealthiness, as an afterburner plume reflects radar signals and creates a significant infra-red signature. [ [http://www.fighter-planes.com/stealth.htm Stealth design of airplanes / stealth aircraft ] ]

History

The first turbine-powered aircraft to exceed Mach 1 in level flight without afterburners was the P.1 prototype of the English Electric Lightning, on August 4, 1954. However, this early demonstration of supercruise was extremely limited; the Lightning could supercruise at approximately Mach 1.02, while later versions were able to achieve much higher speeds.

Only the civilian SSTs Concorde and Tupolev Tu-144 spent most of their time supercruising. Many of the fighters listed as capable of supercruise can only marginally exceed the speed of sound without afterburners and may only be able to do so without an external weapons load. In day-to-day operation Concorde used reheat (afterburners) to accelerate through the high-drag transonic flight regime; although it was capable of reaching its top speed without the use of reheat, the excessive fuel consumption involved in doing so made this impractical for commercial flights. Unlike Concorde's Soviet counterpart, the Tu-144, which was capable of supercruise at Mach 1.6, Concorde could supercruise at Mach 2.0. For the Tupolev to attain this speed, it required continuous reheat, and a cruising speed of Mach 1.6 was adopted for the service period of the 10 first-generation Tu-144s. Later Tu-144s had more powerful military engines fitted which were not as restrictive, giving only slightly less range but higher speeds than Concorde.

Military use

The term supercruise was originally used to describe a fighter performance requirement set forth by USAF Col. John Boyd, Pierre Sprey, and Col. Everest Riccioni, designers of the F-16 Falcon. Following the entry into production of the F-16, they began work on an improved fighter design with the ability to cruise supersonically over enemy territory for a minimum of twenty minutes. As air combat is often the result of surprise, and the speed of the combat is determined by the speed of the surprising aircraft, this would have given a supercruise-capable design a worthwhile performance advantage in many situations. The postulated fighter would have had a top speed of just over Mach 1, and a fuel fraction in excess of 40%, the minimum required to successfully meet the twenty-minute requirement. Meeting the fuel fraction reqirement necessitated a very austere design with few advanced electronics. The Air Force showed no interest in the proposal at that time, but years later revived the term and redefined it to apply to the new F-22 Raptor.

The F-22 Raptor's supercruise capabilities are touted, with or without justification, as a major performance advantage over other fighters. Even so, supercruising uses much more fuel to travel the same distance than at subsonic speeds: [http://www.afa.org/magazine/Jan2005/0105raptor.asp The Air Force Association] estimates that use of supercruise for a convert|100|nmi|km|-1|sing=on dash as part of a mission would cut the F-22's combat radius from about 600 to about convert|450|nmi|km|-1. However, this is still unconfirmed as the altitude and flight profile are classified (as are most of the F-22's capabilities). There is no way to compare this with other aircraft; most aircraft using afterburner to fly at supersonic speeds will have exhausted their fuel supplies very rapidly, typically tripling their fuel consumption during that portion of the flight.

The F-22 has demonstrated supercruise speeds of at least Mach 1.7, a difference of convert|320|kn|km/h|0 indicated airspeed (KIAS) at convert|40000|ft|m|-3|abbr=on. [ [http://www.f-22raptor.com/af_airframe.php F-22 Raptor.com] ] Supercruise in militarily significant parlance is meant to imply a significant increase in effective combat speed with a full weapons load over existing types. Virtually all current and past jet fighters, prior to the F-22, cruise at approximately Mach 0.8~0.9 with a militarily significant weapons load. The F-22 represents a significant advance in cruise speed over previous types (for performance of current USAF types, see Air Force Magazine, May 2006, "Gallery of USAF Weapons," pp. 147-155 - [ [http://www.afa.org/magazine/may2006/0506weapons.pdf AFA.org] ] ). Nonetheless, it can maintain this speed, even in supercruise, for only about five minutes, falling far short of the original performance requirement.

The key challenge in attaining supercruise is not simply a high thrust to weight ratio vis-à-vis the aircraft but a radical redesign of the engine because the air entering a jet engine must always travel at subsonic speeds, regardless of aircraft speed. Otherwise compressibility waves (or shock waves) will create uncontrollable vibrations among the compressor vanes. Engine inlet design therefore can effectively limit the speed of the aircraft, regardless of thrust. The SR-71 Blackbird's distinctive nosecone is designed to funnel air around the J-58 engine for that very reason.

There are few engines in production that are able to facilitate supercruise.
* The PW F119 in combination with the F-22 Raptor (the PW F135 and the RR/GE F136 used on the Joint Strike Fighter have higher thrust but the JSF does not supercruise).
* The EJ200 engine built by EuroJet Turbo GmbH adds the supercruise capability in the Eurofighter Typhoon, and is capable of supercruising at mach 1.5 [http:// [http://www.eurofighter.at/austria/td_lu.asp Austrian Eurofighter Site in German] ] . Typhoon pilots have stated that mach 1.3 is attainable in combat configuration with external stores.

Independently Russia is working on an all new Al41 engine with a complete redesign underway to add supercruise ability to the PAK FA. This is yet to give fruit, but the stop-gap 117S engine, produced by this program, seems to achieve the supercruise goal already. It was recently announced [ [http://www.lenta.ru/articles/2008/07/04/su35/ Su-35BM testing report on Lenta.ru (in Russian)] ] that during testing of a Su-35BM fighter equipped with these engines it was traveling at the ~1.1-1.2M airspeed at nominal power and was still accelerating, thus suggesting that the supercruise was possible at even higher speed. Further testing will show the extent of this possibility.

upersonic, but non supercruising aircraft

The Pratt & Whitney J58 engines of the Lockheed A-12 and SR-71 Blackbird were designed for sustained and efficient operation at supersonic speeds using afterburners with air that was diverted past the turbojet core of the engine. This gave a good compression ratio and higher efficiency simply due to the ram effect at the high operating speed of the aircraft. The afterburners acted essentially as ramjets and these types of engines achieve peak efficiency at around Mach 3.

In a somewhat similar vein, the XB-70A Valkyrie made use of specially designed turbojets (six General Electric YJ-93 engines) to sustain speeds in excess of Mach 3. Unlike the J-58 engines powering the SR-71, the YJ-93 engines of the XB-70A did not require the use of special fuel, and did not radically modify the intake/exhaust geometry in order to achieve Mach 3 flight. The YJ-93 engines did operate in afterburner at Mach 3; however, the engines were specifically designed to be very efficient in afterburner, and the XB-70A AV-2 prototype sustained speeds in excess of Mach 3 for 32 minutes on one flight. Furthermore, the type was designed to operate at such speeds for periods of hours over intercontinental ranges. [cite book|title=Valkyrie: North American's Mach 3 Superbomber|last=Jenkins|first=Dennis R.|coauthors=Tony R. Landis, Alvin S. White, and Fitzhugh L. Fulton|publisher=Speciality Press|date=2005|page=various pages|isbn=1580070728]

Aircraft with supercruise

Aircraft with supercruise include:
* English Electric Lightning (The first aircraft capable of supercruise)
* BAC TSR-2
* Eurofighter Typhoon
* JAS 39 Gripen [ [http://www.gripen.com/NR/rdonlyres/FE463B06-8C9B-4A49-A382-999C6AF1E53B/0/gripen_news_2001_01.pdf Gripen News, June 2001] "Gripen is supersonic at all altitudes and can cruise supersonically with an external load including fuel tank, four AMRAAM and two Sidewinder missles without the need to engage the afterburner." A Totally Superior Product (p.2)]
* F-14D Super TomcatFact|date=October 2008
* F-22 Raptor
* YF-23 Black Widow II
* XB-70 Valkyrie
* Concorde
* Tupolev Tu-144S

References

ee also

*Fuel fraction
*Supersonic transport