Supercruise

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

Supercruise is sustained supersonic flight of a supersonic aircraft with a useful cargo, passenger, or weapons load performed efficiently, which typically precludes the use of highly inefficient afterburners (reheat). Many well known supersonic military aircraft are not capable of supercruise as they are only able to maintain supersonic flight in short bursts (typically with afterburners) while they cruise at subsonic speeds. Aircraft such as the SR-71 Blackbird are designed to cruise at supersonic speed with afterburners enabled.

One of the most prominent and well-known examples of this type of aircraft was Concorde. Due to its long service in commercial airlines, Concorde has the record for the most time spent in supercruise; it has spent more time in supercruise than all other aircraft combined.^[1]

Advantages

Most military aircraft use afterburners (or reheat) to travel at supersonic speeds and cannot reach supersonic speeds using the dry engine thrust. Afterburners are highly inefficient compared to conventional jet engine operation due to the low pressures typically found in the exhaust section. This limits most aircraft to using afterburners for only very short periods. Therefore, an aircraft that can supercruise has generally greater endurance at supersonic speeds than one which cannot. Supercruise capability is also an advantage for stealth aircraft, as an afterburner plume both reflects radar signals and creates a significant infrared signature^[2] in addition to being visually conspicuous.

History

Concorde routinely supercruised most of the way over the Atlantic.

One of the first turbine-powered aircraft to exceed Mach 1 in level flight without afterburners was the un-reheated Armstrong Siddeley Sapphire powered P.1 prototype of the English Electric Lightning, on 11 August 1954. (On 3 August 1954 a Gerfaut aircraft powered by an ATAR 101D2A engine exceeded Mach 1 on the level without afterburner.^[3][4]) Previously the P.1, WG760, flown by Roland Beamont had unknowingly exceeded Mach 1 in the climb on its first flight on 4 August 1954, although due to position error, the Mach meter had only shown a maximum of Mach 0.95, and Beamont, who had not noticed any change in behaviour of the aircraft, was surprised when informed of the fact after the flight data had been analysed.^[5] However, this early demonstration of supercruise was extremely limited; the Lightning could supercruise at approximately Mach 1.02^[6] while later versions were able to achieve much higher speeds.

The British Aircraft Corporation Tactical Strike/Reconnaissance 2 (TSR-2), which first flew on 27 September 1964, was one of the first military aircraft specifically designed to cruise supersonically; one of the planned mission profiles was for a supersonic cruise at Mach 2.00 at 50–58,000 ft (15-18 km.^[7] Supersonic cruise at lower levels was at Mach 1.1 at 200 ft (60 m).^[8] The TSR-2 used Bristol Olympus engines, a later version of which would also power Concorde.

Qualitative variation in Cd factor with Mach number for aircraft, supercruising above Mach 2.15 is efficient.

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. The Concorde's Soviet counterpart, the Tu-144, could supercruise at Mach 1.6, while the 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 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, proponents 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 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 meet the twenty-minute requirement. 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 (but is seen here running afterburner).

The Eurofighter Typhoon is capable of supercruise at Mach 1.5.^[9]

The F-22 Raptor's supercruise capabilities are touted as a major performance advantage over other fighters, with supercruise being demonstrated up to at least Mach 1.58.^[10] 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.^[11] However, supercruising uses more fuel to travel the same distance than at subsonic speeds, with the Air Force Association estimating 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 nautical miles (1,110 km) to about 450 nautical miles (830 km). This reduction is unconfirmed because the altitude and flight profile are classified, as are most of the F-22's capabilities, but it is still far less of a reduction than would result from the use of afterburner.

There are a few engines in production that are designed to facilitate tactically significant supercruise:

• Pratt & Whitney F119 engines are used in the F-22 Raptor.
• The EJ200 engine built by EuroJet Turbo GmbH gives the Eurofighter Typhoon supercruise capability. It is capable of supercruising at Mach 1.5 in clean configuration.^[9] Typhoon pilots have stated that Mach 1.3 is attainable in combat configuration with external stores.^[12]
• The General Electric F414G in JAS 39 Gripen NG is designed for supercruise and has been shown to achieve Mach 1.2.^[13]

Although the Pratt & Whitney F135 F-35 engine was not designed to achieve a supercruise capability,^[14] the F-35 is able to maintain Mach 1.2 for a dash of 150 miles without using afterburners.^[15]

Independently Russia is working on an all new AL-41 engine with a complete redesign underway to add supercruise ability to the PAK FA. This has yet to bear fruit, but the stop-gap 117S engine, produced by this program, seems to achieve the supercruise goal already. It was recently announced that during testing of a Su-35BM fighter equipped with these engines it was travelling at just past supersonic speed it continued to accelerate without the use of the afterburner, thus suggesting that it had supercruise capability, though it has yet to be seen whether this will be possible with a combat load.^[16]

All known supercruise aircraft can only do so at considerable altitude (where the air is thinner and so offers less resistance), which restricts the use of terrain mask for eluding detection.

Ramjet missiles cruised supersonically in the 20th century but went out of use. Scramjets have cruised in experiments.

Aircraft designed to cruise on afterburner

The Pratt & Whitney J58 engine used in the Lockheed A-12 and SR-71 Blackbird was designed for sustained operation at supersonic speeds using an afterburner. SR-71 missions were flown, with in-flight refuelling, at different combinations of speed and altitude. Maximum range cruise, with afterburner settings in the lower portion of the range, was flown about 98% of the time.^[17]

The XB-70A Valkyrie used six General Electric YJ-93 engines for sustained flight at Mach 3.0, its design point.^[18] Unlike the J-58 engine in the SR-71 the YJ-93 did not need special fuel. It used JP-6.^[18] Partial afterburner was used for cruise.^[19] The XB-70A AV-2 prototype sustained speeds in excess of Mach 3 for 32 minutes on one flight. The type was designed to operate at its design point speed for periods of hours over intercontinental ranges.^[20]

Aircraft with supercruise ability

In service:

• Dassault Rafale
• Eurofighter Typhoon
• Gripen NG^[21][22]
• F-22 Raptor

Formerly in service:

• English Electric Lightning
• SR -71 Blackbird
• A-12

Prototype/experimental only:

• Sukhoi PAK FA
• YF-12
• BAC TSR-2
• General Dynamics F-16XL
• Northrop YF-23
• Lockheed YF-22
• North American XB-70 Valkyrie

Civilian:

• Concorde^[23]
• Tu-144

Future (proposed):

• TAI TFX
• Mikoyan LMFS
• Zero Emission Hyper Sonic Transport
• SonicStar
• HAL Advanced Medium Combat Aircraft
• Flygsystem 2020

References

See also

• Fuel fraction
• Supersonic transport