A supersonic aircraft is an aircraft capable of supersonic flight, that is, flying faster than the speed 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; however, to date, only two supersonic aircraft, the Tupolev Tu-144 (first flown on December 31, 1968) and the Concorde (first flown on March 2, 1969), have ever entered civilian service, both commercially used as supersonic passenger airliners. Fighter jets are the most common example of supersonic aircraft.
The aerodynamics of supersonic flight is called compressible flow because of the compression associated with the shock waves or "sonic boom" created by any object traveling faster than the speed of sound.
Aircraft flying at speeds above Mach 5 are called hypersonic aircraft. Supersonic speed is defined relative to air speed; higher speeds can be achieved in terms of ground speed when flying in the same direction as fast-moving winds such as the jet stream.
History
The first aircraft to achieve supersonic speed during its flight was the American Bell X-1 experimental plane, which was powered by a 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 experimental Bell X-1 research rocket plane piloted by Chuck Yeager. Moreover, the first aircraft to break the sound barrier with a female pilot was an F-86 Canadair Sabre with Jacqueline Cochran at the controls."Jacqueline Cochran and the Women's Airforce Service Pilots." National Archives and Records Administration: The Dwight D. Eisenhower Presidential Library, Museum, and Boyhood Home. Retrieved: July 10, 2013. According to David Masters, the DFS 346 prototype captured in Germany by the Soviets, after being released from a Boeing B-29 Superfortress heavy bomber 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, a Douglas DC-8-43 (registration N9604Z) exceeded Mach 1 in a controlled dive during a test flight at Edwards Air Force Base. The crew was composed of the following members: William Magruder (pilot), Paul Patten (copilot), Joseph Tomich (flight engineer), and Richard H. Edwards (flight test engineer). This was the first intentional supersonic flight by a civilian airliner, and the only one ever performed by a civilian airliner other than the Concorde or Tu-144.
In the 1960s and 1970s, multiple design studies for supersonic airliners were conducted and eventually two types entered service, the Soviet Tupolev Tu-144 (1968) and Anglo-French Concorde (1969). However, political, environmental, and economic obstacles, alongside one fatal Concorde crash, eventually prevented them from being utilized to their full commercial potential.
Design principles Supersonic flight has always presented substantial technical challenges to engineers, 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.
Wings
To optimize drag, wingspan must be limited, which also reduces aerodynamic efficiency during subsonic flight, including takeoff and landing. 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 a variable-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 the longitudinal trim of the aircraft, and the swinging mechanism adds weight and cost. Use of a delta wing, such as those used on the Concorde, generates a vortex 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 high stall angle. It also solves the issue of fluid compressibility 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 requires the use of flaps.
Heating
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 as Duralumin, which are cheap and easy to work with but lose their strength quickly at high temperatures. This limits the maximum speed to approximately Mach 2.2.
Most supersonic aircraft, including many military fighter 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 the Lockheed 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.
Engines Some early supersonic aircraft, including the first, relied on rocket power to provide the necessary thrust, although rockets burn a lot of fuel and so flight times were short. Early turbojets 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 the afterburner, in which extra fuel is burned in the jet exhaust, made these mixed powerplant types obsolete. The turbofan engine passes additional cold air around the engine core, further increasing its fuel efficiency, and supersonic aircraft today are powered by turbofans fitted with afterburners.
Supersonic aircraft usually use low-bypass turbofans as they have acceptable efficiency below the speed of sound as well as above; or if supercruise is needed turbojet engines may be desirable as they give less nacelle drag at supersonic speeds. The Pratt & Whitney J58 engines of the Lockheed 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 the ramjet. This needs to be flying fairly fast before it will work at all.
Supersonic flight Subsonic aerodynamics is simpler than Supersonic aerodynamics because the airsheets at different points along the plane often cannot affect each other in subsonic flight. Supersonic jets and rocket vehicles require several times greater thrust to push through the extra aerodynamic drag experienced within the transonic region (around Mach 0.85–1.2). At these speeds aerospace engineers can gently guide air around the fuselage of the aircraft without producing new shock waves, but any change in cross area farther down the vehicle leads to shock waves along the body. Designers use the Whitcomb 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 speeds aerodynamic 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 as titanium and stainless steel allow operations at much higher temperatures. For example, the Lockheed 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 the turbine 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, using shock 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 of shock 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 to operate for extended periods at supersonic speeds has a potential range advantage over a similar design operating subsonically. Additionally, 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 as wave 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, the lift-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, the von Karman ogive or Sears-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.
Transonic flight
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.
Hypersonic flight