Supercruise

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The English Electric Lightning was the first aircraft to be capable of supercruise.

Supercruise is sustained supersonic flight of an aircraft with a useful cargo, passenger, or weapons load performed efficiently and without the use of afterburners ("reheat").

Due to its combination of decades long scheduled service and length of time spent at supersonic speeds, the main user of supercruise has been Concorde, with more time spent in supersonic, largely supercruise flight, than all of the other aircraft put together.^[1]

Advantages

Most military aircraft use afterburners (or reheats) to travel at supersonic speeds. Afterburners are inefficient compared to conventional jet engine operation due to the low pressures typically found in the exhaust section. Therefore an aircraft that can supercruise has generally 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 favorable fuel fraction, the proportion of the plane's overall mass which is devoted to fuel. Supercruise also increases the aircraft's stealth, as an afterburner plume reflects radar signals and creates a significant infra-red signature. ^[2]

History

Concorde routinely supercruised most of the way over the Atlantic.

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 11, 1954. However, this early demonstration of supercruise was extremely limited; the Lightning could supercruise at approximately Mach 1.22^[3] while later versions were able to achieve much higher speeds.

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 and the Tupolev Tu-144 both used afterburners to accelerate quickly through the high-drag transonic flight regime before deactivating them to supercruise. Doing this minimized fuel use, even though afterburners are relatively inefficient. Both aircraft were capable of achieving supersonic flight without the use of afterburners; however, doing so meant that they spent much longer in the high-drag transonic flight envelope, and this made the use of afterburners more fuel efficient.

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 requirement necessitated a very austere design with few advanced electronics. The United States Air Force showed no interest in the proposal at that time, but years later revived the term and redefined it to apply to the requirements for the Advanced Tactical Fighter, which resulted in the F-22 Raptor.

The F-22 Raptor is capable of supercruise (seen here running afterburner)

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: The Air Force Association estimates that use of supercruise for a 100-nautical-mile (190 km) dash as part of a mission would cut the F-22's combat radius from about 600 to about 450 nautical miles (830 km). However, this is still unconfirmed as the altitude and flight profile are classified (as are most of the F-22's capabilities). With other aircraft; aircraft using afterburner to fly at supersonic speeds will typically have exhausted their fuel supplies very rapidly, often 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 320 knots (593 km/h) indicated airspeed (KIAS) at 40,000 ft (12,000 m).^[4] 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 - ^[5]). 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 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 engine inlet spikes and a system of bleed doors are designed to funnel air around the J-58 engines 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^[6]. 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 and the Indo-Russian Sukhoi/HAL FGFA. This is yet to bear fruit, but the stop-gap 117S engine, produced by this program, seems to achieve the supercruise goal already.^{[citation needed]} It was recently announced^[7] 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.^{[citation needed]}

Aircraft Designed To Cruise On Afterburner

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.^[8]

Ramjets and Scramjets

Ramjet and scramjet powered aircraft have to date been purely experimental, but these engines operate most efficiently at supersonic speeds and therefore would be theoretically ideal for an aircraft intended to spend long periods in supersonic flight. Due to the exotic nature of the engines, whether this would be considered "supercruise" is largely semantic.

Aircraft with supercruise

Aircraft with supercruise include:

• English Electric Lightning (The first aircraft capable of supercruise)
• Concorde^[9]
• BAC TSR-2
• Eurofighter Typhoon
• JAS 39 Gripen^[10][11]
• F-14D Super Tomcat^{[citation needed]}
• F-22 Raptor
• YF-23 Black Widow II

References

See also

• Fuel fraction
• Supersonic transport