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# Supersonic speed

U.S. Navy F/A-18 approaching the sound barrier. The white cloud forms as a result of the supersonic expansion fans dropping the air temperature below the dew point.[1][2]

Supersonic speed is the speed of an object that exceeds the speed of sound (Mach 1). For objects traveling in dry air of a temperature of 20 °C (68 °F) at sea level, this speed is approximately 343.2 m/s (1,126 ft/s; 768 mph; 667.1 kn; 1,236 km/h). Speeds greater than five times the speed of sound (Mach 5) are often referred to as hypersonic. Flights during which only some parts of the air surrounding an object, such as the ends of rotor blades, reach supersonic speeds are called transonic. This occurs typically somewhere between Mach 0.8 and Mach 1.2.

Sounds are traveling vibrations in the form of pressure waves in an elastic medium. In gases, sound travels longitudinally at different speeds, mostly depending on the molecular mass and temperature of the gas, and pressure has little effect. Since air temperature and composition varies significantly with altitude, Mach numbers for aircraft may change despite a constant travel speed. In water at room temperature supersonic speed can be considered as any speed greater than 1,440 m/s (4,724 ft/s). In solids, sound waves can be polarized longitudinally or transversely and have even higher velocities.

Supersonic fracture is crack motion faster than the speed of sound in a brittle material.

## Early meaning

At the beginning of the 20th century, the term "supersonic" was used as an adjective to describe sound whose frequency is above the range of normal human hearing. The modern term for this meaning is "ultrasonic".

Etymology: The word supersonic comes from two Latin derived words; 1) super: above and 2) sonus: sound, which together mean above sound or in other words faster than sound.

## Supersonic objects

British Airways Concorde in early BA livery at London-Heathrow Airport, in the early 1980s

The tip of a bullwhip is thought to be the first man-made object to break the sound barrier, resulting in the telltale "crack" (actually a small sonic boom). The wave motion traveling through the bullwhip is what makes it capable of achieving supersonic speeds.[3][4]

Most modern fighter aircraft are supersonic aircraft, but there have been supersonic passenger aircraft, namely Concorde and the Tupolev Tu-144. Both these passenger aircraft and some modern fighters are also capable of supercruise, a condition of sustained supersonic flight without the use of an afterburner. Due to its ability to supercruise for several hours and the relatively high frequency of flight over several decades, Concorde spent more time flying supersonically than all other aircraft combined by a considerable margin. Since Concorde's final retirement flight on November 26, 2003, there are no supersonic passenger aircraft left in service. Some large bombers, such as the Tupolev Tu-160 and Rockwell B-1 Lancer are also supersonic-capable.

Most modern firearm bullets are supersonic, with rifle projectiles often travelling at speeds approaching and in some cases[5] well exceeding Mach 3.

Most spacecraft, most notably the Space Shuttle are supersonic at least during portions of their reentry, though the effects on the spacecraft are reduced by low air densities. During ascent, launch vehicles generally avoid going supersonic below 30 km (~98,400 feet) to reduce air drag.

Note that the speed of sound decreases somewhat with altitude, due to lower temperatures found there (typically up to 25 km). At even higher altitudes the temperature starts increasing, with the corresponding increase in the speed of sound.[6]

When an inflated balloon is burst, the torn pieces of latex contract at supersonic speed, which contributes to the sharp and loud popping noise.

## Supersonic land vehicles

To date, only one land vehicle has officially travelled at supersonic speed. It is ThrustSSC, driven by Andy Green, which holds the world land speed record, having achieved an average speed on its bi-directional run of 1,228 km/h (763 mph) in the Black Rock Desert on 15 October 1997.

The Bloodhound LSR project is planning an attempt on the record in 2020 at Hakskeen Pan in South Africa with a combination jet and hybrid rocket propelled car. The aim is to break the existing record, then make further attempts during which the team hope to reach speeds of up to 1,600 km/h (1,000 mph). The effort was originally run by Richard Noble who was the leader of the ThrustSSC project, however following funding issues in 2018, the team was bought by Ian Warhurst and renamed Bloodhound LSR. The new project retains many of the original Bloodhound SSC engineering staff, and Andy Green is still the driver for record attempt, with high speed trials expected to start in October 2019.

## Supersonic flight

Supersonic aerodynamics 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 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 Supersonic area rule and the Whitcomb area rule to minimize sudden changes in size.

The sound source has now broken through the sound speed barrier, and is traveling at 1.4 times the speed of sound, c (Mach 1.4). Because the source is moving faster than the sound waves it creates, it actually leads the advancing wavefront. The sound source will pass by a stationary observer before the observer actually hears the sound it creates.

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, when the air slows down, it does so 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. 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.

### History of supersonic flight

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 Charles "Chuck" Yeager. The first production plane to break the sound barrier was an F-86 Canadair Sabre with the first 'supersonic' woman pilot, Jacqueline Cochran, at the controls.[7] According to David Masters,[8] the DFS 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 1945, 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 were William Magruder (pilot), Paul Patten (copilot), Joseph Tomich (flight engineer), and Richard H. Edwards (flight test engineer).[9] This was the first supersonic flight by a civilian airliner other than the Concorde or Tu-144.[9]