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 or "reheat". Many well known supersonic military aircraft not capable of supercruise must maintain supersonic flight in short bursts typically with afterburners. 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 occasionally allow travel at supersonic speeds and, therefore, cannot reach supersonic speeds using the dry engine thrust. The inefficiency of afterburners see low pressures typically in the exhaust section and higher fuel usage that limits most aircraft to using afterburners for short periods. Therefore, efficiency increases when afterburners are not operational. Moreover, an aircraft that can supercruise has greater endurance at supersonic speeds than one that cannot. Also, supercruise capability gives advantages for stealth aircraft because an afterburner plume reflects radar signals and creates a significant infrared signature.^[2]

History

The Concorde routinely supercruised most of the way over the Atlantic, enabling it to travel from London to New York in 3 hours, a record which has yet to be surpassed by any commercial aircraft.

On 3 August 1954 a Gerfaut research aircraft powered by an ATAR 101D2A engine exceeded Mach 1 on the level without afterburner.^[3][4] The first production intent 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. 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 shown a maximum of only 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 only cruise without reheat 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 50,000–58,000 ft (15–18 km).^[7] Supersonic cruise at lower levels was at Mach 1.1 at 200 ft (61 m).^[8] The TSR-2 used Bristol Olympus engines, a later version of which would also power the Concorde.

Qualitative variation in Cd factor (Drag coefficient) with Mach number (speed) for aircraft; supercruising above Mach 2.15 is efficient.

Only the SSTs, the Concorde and the last version of the Tu-144 (the Tu-144M), spent most of their time cruising at their design speeds without needing afterburning. Some combat aircraft that are listed as capable of supercruise may only be able to do so without an external weapons load, for example. Reheat was added to the Concorde for take-off to cope with weight increases that came after the initial design. As it was available, it was also used for transonic acceleration to reduce the time taken and fuel used to reach cruise. The Tu-144M had more economical, non-afterburning, engines than the Tu-144, which increased the full payload range from 3,080 to 5,330 km (the Concorde's operational range was 6,470 km).^[9]

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,^[10] which resulted in the F-22 Raptor.

The F-22 Raptor is capable of supercruise at Mach 1.8 (but is seen here running afterburner).

The Dassault Rafale is capable of supercruising with four missiles and a belly drop tank.^[11]

The Eurofighter Typhoon is capable of supercruise at Mach 1.5.^[12]

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 1.8^[13] Virtually all current and past jet fighters, prior to the F-22, cruise at approximately Mach 0.8–0.9 with militarily significant weapons loads.^[14] 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:

• Two Pratt & Whitney F119 power the F-22 Raptor, making it the most capable supercruiser fighter aircraft in service. The F-22 Raptor can supercruise up to Mach 1.8 without external stores.^[15][16] (Note: General Electric YF120 supercruise capable engines also powered an F-22 prototype during this fly off, but were not selected for production.)
• The EJ200 engine built by EuroJet Turbo GmbH gives the Eurofighter Typhoon supercruise capability. It is capable of supercruising at Mach 1.5 with an air superiority missile load.^[12] Typhoon pilots have stated that Mach 1.3 is attainable in combat configuration with external stores.^[17]
• The General Electric F414G in JAS 39 Gripen NG is designed for supercruise and has been shown to achieve Mach 1.2.^[18]
• Two Snecma M88 engines power the Dassault Rafale. The twin Snecma M88s enable the Rafale to supercruise with four missiles and a belly drop tank.^[11]
• Two AL-41F-1S engines power the Sukhoi Su-35, these can achieve supersonic speed without engaging afterburner .^[19]

Independently Russia is working on Izdelje 30 (after AL31F and AL41F derivatives modifications, like 117S turbofan) and RD33MKRU Morskaja Osa; an all new AL-41 engine with a complete redesign underway to add supercruise ability to the Sukhoi Su-57. 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.^[20]

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.^[21]

The XB-70A Valkyrie used six General Electric YJ-93 engines for sustained flight at Mach 3.0, its design point.^[22] Unlike the J-58 engine in the SR-71, the YJ-93 did not need special fuel, as it used JP-6.^[22] Partial afterburner was used for cruise.^[23] 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.^[24]

Aircraft with supercruise ability

See also

• Fuel fraction
• Supersonic transport

References