Asupersonic aircraft is anaircraft capable ofsupersonic flight, that is, flying faster than thespeed of sound (Mach 1).Supersonic aircraft were developed in the second half of the twentieth century. Supersonic aircraft have been used for research and military purposes, but only two supersonic aircraft, theTupolev Tu-144 (first flown on December 31, 1968) and theConcorde (first flown on March 2, 1969), ever entered servicefor civil use as airliners.Fighter jets are the most common example of supersonic aircraft.
Theaerodynamics of supersonic flight is calledcompressible flow because of thecompression associated with theshock waves or "sonic boom" created by any object traveling faster than sound.
Aircraft flying at speeds above Mach 5 are calledhypersonic aircraft. Supersonic speed is reckoned with respect toair speed; higher speeds can be achieved in terms ofground speed when flying in the same direction as fast-moving winds such as thejet stream.[1]
The first aircraft to fly supersonic in level flight was the AmericanBell X-1 experimental plane which was powered by a 6,000-pound (2,700 kg) thrust rocket powered by liquid oxygen and ethyl alcohol. Most supersonic aircraft have been military or experimental aircraft.
Aviation research during World War II led to the creation of the first rocket- and jet-powered aircraft. Several claims of breaking the sound barrier during the war subsequently emerged. However, the first recognized flight exceeding the speed of sound by a manned aircraft in controlled level flight was performed on October 14, 1947 by the experimentalBell X-1 research rocket plane piloted byChuck Yeager. The first aircraft to break the sound barrier with a female pilot was an F-86Canadair Sabre withJacqueline Cochran at the controls.[2] According to David Masters,[3] theDFS 346 prototype captured in Germany by the Soviets, after being released from a B-29 at 32800 ft (10000 m), reached 683 mph (1100 km/h) late in 1951, which would have exceeded Mach 1 at that height. The pilot in these flights was the German Wolfgang Ziese.
On August 21, 1961, aDouglas DC-8-43 (registration N9604Z) exceededMach 1 in a controlled dive during a test flight at Edwards Air Force Base. The crew were William Magruder (pilot), Paul Patten (copilot), Joseph Tomich (flight engineer), and Richard H. Edwards (flight test engineer).[4] This was the first intentional supersonic flight by a civilian airliner, and the only one ever performed by a civilian airliner other than theConcorde orTu-144.[4]
In the 1960s and 1970s, many design studies for supersonic airliners were done and eventually two types entered service, the SovietTupolev Tu-144 (1968) and Anglo-FrenchConcorde (1969). However political, environmental and economic obstacles and one fatal Concorde crash prevented them from being used to their full commercial potential.
Supersonic flight brings with it substantial technical challenges, as the aerodynamics of supersonic flight are dramatically different from those of subsonic flight (i.e., flight at speeds slower than that of sound). In particular,aerodynamic drag rises sharply as the aircraft passes the transonic regime, requiring much greater engine power and more streamlined airframes.
To optimize drag, wingspan must be limited, which also reduces aerodynamic efficiency during subsonic flight, including takeoff and landing.[5] Minimizing wave drag is a crucial aspect of wing design. Since a supersonic aircraft must also take off and land at a relatively slow speed, its aerodynamic design must be a compromise between the requirements for both ends of the speed range.
One approach to resolving this compromise is the use of avariable-geometry wing, commonly known as the "swing-wing," which spreads wide for low-speed flight and then sweeps sharply, usually backwards, for supersonic flight. However, swinging affects thelongitudinal trim of the aircraft and the swinging mechanism adds weight and cost. Use of adelta wing, such as those used on theAerospatiale-BAC Concorde generates avortex which energises the flow on the upper surface of the wing at high speeds and attack angles, delaying flow separation, and giving the aircraft a very highstall angle. It also solves the issue of fluidcompressibility at transonic and supersonic speeds. However, it is, of course, inefficient at lower speeds due to the requirement of a high angle of attack, and therefore need the use offlaps.
Another problem is the heat generated due to air compression as well as friction as the air flows over the aircraft. Most subsonic designs use aluminium alloys such asDuralumin, which are cheap and easy to work but lose their strength quickly at high temperatures. This limits maximum speed to around Mach 2.2.
Most supersonic aircraft, including many militaryfighter aircraft, are designed to spend most of their flight at subsonic speeds, and only to exceed the speed of sound for short periods such as when intercepting an enemy aircraft. A smaller number, such as theLockheed SR-71 Blackbird reconnaissance aircraft and the Concorde supersonic airliner, have been designed to cruise continuously at speeds above the speed of sound, and with these designs the problems of supersonic flight are more severe.
Some early supersonic aircraft, including the first, relied onrocket power to provide the necessary thrust, although rockets burn a lot of fuel and so flight times were short. Earlyturbojets were more fuel-efficient but did not have enough thrust and some experimental aircraft were fitted with both a turbojet for low-speed flight and a rocket engine for supersonic flight. The invention of theafterburner, in which extra fuel is burned in the jet exhaust, made these mixed powerplant types obsolete. Theturbofan engine passes additional cold air around the engine core, further increasing itsfuel efficiency, and supersonic aircraft today are powered by turbofans fitted with afterburners.
Supersonic aircraft usually uselow bypass turbofans as they have acceptable efficiency below the speed of sound as well as above; or ifsupercruise is neededturbojet engines may be desirable as they give lessnacelle drag at supersonic speeds. ThePratt & Whitney J58 engines of theLockheed SR-71 Blackbird operated in 2 ways, taking off and landing as turbojets with no bypass, but bypassing some of the compressor air to the afterburner at higher speeds. This allowed the Blackbird to fly at over Mach 3, faster than any other production aircraft. The heating effect of air friction at these speeds meant that a special fuel had to be developed which did not break down in the heat and clog the fuel pipes on its way to the burner.
Another high-speed powerplant is theramjet. This needs to be flying fairly fast before it will work at all.
Supersonicaerodynamics is simpler than subsonic aerodynamics because the airsheets at different points along the plane often cannot affect each other. Supersonic jets and rocket vehicles require several times greater thrust to push through the extraaerodynamic drag experienced within thetransonic region (around Mach 0.85–1.2). At these speedsaerospace engineers can gently guide air around thefuselage of the aircraft without producing newshock waves, but any change in cross area farther down the vehicle leads to shock waves along the body. Designers use theWhitcomb area rule to minimize sudden changes in size.
However, in practical applications, a supersonic aircraft must operate stably in both subsonic and supersonic profiles, hence aerodynamic design is more complex.
One problem with sustained supersonic flight is the generation of heat in flight. At high speedsaerodynamic heating can occur, so an aircraft must be designed to operate and function under very high temperatures.Duralumin, a material traditionally used in aircraft manufacturing, starts to lose strength and deform at relatively low temperatures, and is unsuitable for continuous use at speeds above Mach 2.2 to 2.4. Materials such astitanium andstainless steel allow operations at much higher temperatures. For example, theLockheed SR-71 Blackbird jet could fly continuously at Mach 3.1 which could lead to temperatures on some parts of the aircraft reaching above 315 °C (600 °F).
Another area of concern for sustained high-speed flight is engine operation. Jet engines create thrust by increasing the temperature of the air they ingest, and as the aircraft speeds up, the compression process in the intake causes a temperature rise before it reaches the engines. The maximum allowable temperature of the exhaust is determined by the materials in theturbine at the rear of the engine, so as the aircraft speeds up, the difference in intake and exhaust temperature that the engine can create, by burning fuel, decreases, as does the thrust. The higher thrust needed for supersonic speeds had to be regained by burning extra fuel in the exhaust.
Intake design was also a major issue. As much of the available energy in the incoming air has to be recovered, known as intake recovery, usingshock waves in the supersonic compression process in the intake. At supersonic speeds the intake has to make sure that the air slows down without excessive pressure loss. It has to use the correct type ofshock waves, oblique/plane, for the aircraft design speed to compress and slow the air to subsonic speed before it reaches the engine. The shock waves are positioned using a ramp or cone which may need to be adjustable depending on trade-offs between complexity and the required aircraft performance.
An aircraft able tooperate for extended periods at supersonic speeds has a potential range advantage over a similar design operating subsonically. Most of the drag an aircraft sees while speeding up to supersonic speeds occurs just below the speed of sound, due to an aerodynamic effect known aswave drag. An aircraft that can accelerate past this speed sees a significant drag decrease, and can fly supersonically with improved fuel economy. However, due to the way lift is generated supersonically, thelift-to-drag ratio of the aircraft as a whole drops, leading to lower range, offsetting or overturning this advantage.
The key to having low supersonic drag is to properly shape the overall aircraft to be long and thin, and close to a "perfect" shape, thevon Karman ogive orSears-Haack body. This has led to almost every supersonic cruising aircraft looking very similar to every other, with a very long and slender fuselage and large delta wings, cf.SR-71,Concorde, etc. Although not ideal for passenger aircraft, this shaping is quite adaptable for bomber use.
In the 1960s and 1970s, many design studies for supersonic airliners were done and eventually two types entered service, the SovietTupolev Tu-144 (1968) and Anglo-FrenchConcorde (1969). However political, environmental and economic obstacles and one fatal Concorde crash prevented them from being used to their full commercial potential.
Airflow can speed up or slow down locally at different points over an aircraft. In the region around Mach 1, some areas may experience supersonic flow while others are subsonic. This regime is called transonic flight. As the aircraft speed changes, pressure waves will form or move around. This can affect the trim, stability and controllability of the aircraft, and the aircraft will experience higher drag than subsonic or fully supersonic speeds. The designer needs to ensure that these effects are taken into account at all speeds.
Flight at speeds above about Mach 5 is often referred to as hypersonic. In this region the problems of drag and heating are even more acute. It is difficult to make materials which can stand the forces and temperatures generated by air resistance at these speeds.
A sonic boom is the sound associated with theshock waves created whenever an object traveling through the air travels faster than thespeed of sound. Sonic booms generate significant amounts ofsound energy, sounding similar to anexplosion or athunderclap to the human ear. The crack of a supersonicbullet passing overhead or the crack of abullwhip are examples of a sonic boom in miniature.[7]
Sonic booms due to large supersonic aircraft can be particularly loud and startling, tend to awaken people, and may cause minor damage to some structures. They led to prohibition of routine supersonic flight over land. Although they cannot be completely prevented, research suggests that with careful shaping of the vehicle the nuisance due to them may be reduced to the point that overland supersonic flight may become a practical option.
Supercruise is sustainedsupersonic flight of a supersonic aircraft with a useful cargo, passenger, or weapons load performed efficiently, which typically precludes the use of highly inefficientafterburners or "reheat". Many well known supersonicmilitary aircraft not capable of supercruise can only maintainMach 1+ flight in short bursts, typically with afterburners. Aircraft such as theSR-71 Blackbird are designed to cruise at supersonic speed with afterburners enabled.
One of the best known examples of an aircraft capable of supercruise wasConcorde. Due to its long service as a commercial airliner, Concorde holds the record for the most time spent in supercruise; more than all other aircraft combined.[8]
Asupersonic transport (SST) is acivilaircraft designed to transport passengers at speeds greater than thespeed of sound. The only supersonic civilian aircraft to see service were the Soviet producedTupolev Tu-144 which first flew in 1968 and last transported passengers in 1978, withNASA retiring it from any use in 1997; and the Franco-British producedConcorde, which first flew in 1969 and remained in service until 2003. Since 2003, there have been no supersonic civilian aircraft in service.
A key feature of these designs is the ability to maintain supersonic cruise for long periods, so low drag is essential to limit fuel consumption to a practical and economic level. As a consequence, these airframes are highly streamlined and the wings have a very short span. The requirement for low speeds when taking off and landing is met by usingvortex lift: as the aircraft slows, lift must be restored by raising the nose to increase theangle of attack of the wing. The sharply swept leading edge causes the air to twist as it flows over the wing, speeding up the airflow locally and maintaining lift.
Other SST projects have included:
Supersonic business jets (SSBJ) are a proposed class of small supersonic aircraft. None have yet flown.
Typically intended to transport about ten passengers, SSBJs are about the same size as traditional subsonic business jets.
Projects for both large-scale andbusiness jet (see lower) passenger supersonic and hypersonic airliners (Aerion SBJ,Spike S-512,HyperMach SonicStar,Next Generation Supersonic Transport,Tupolev Tu-444,Gulfstream X-54,LAPCAT,Reaction Engines LAPCAT A2,Zero Emission Hyper Sonic Transport,SpaceLiner, etc.) were proposed and now are under development.
Astrategic bomber must carry a large bomb load over long distances. Consequently, it is a large aircraft typically with an empty weight exceeding 25,000 kg. Some have also been designed for related roles such as strategic reconnaissance and anti-shipping strike.
Typically the aircraft will cruise subsonically for most of its flight to conserve fuel, before accelerating to supersonic speed for its bombing run.[9]
Few supersonic strategic bombers have entered service. The earliest type, theConvair B-58 Hustler, first flew in 1956 and the most recent, theRockwell B-1B Lancer, in 1983. Although this and a few other types are still in service today, none remains in production.
Types to have flown include:
Some supersonic strategic bombers, such as theSukhoi T-4 are also capable of the reconnaissance role (although the Sukhoi remained a prototype).
TheLockheed SR-71 Blackbird was specifically designed for the role, and was a larger development of theLockheed A-12 reconnaissance aircraft which first flew in 1962.
Supersonic fighters and related aircraft are sometimes called fast jets. They make up the overwhelming majority of supersonic aircraft and some, such as theMikoyan-Gurevich MiG-21,Lockheed F-104 Starfighter andDassault Mirage III, have been produced in large numbers.
Many military supersonicfighters and similar aircraft offourth- andfifth- generations are under development in several countries, including Russia, China, Japan, South Korea, India, Iran and the United States.
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