Supersonic

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For other uses, see Supersonic (disambiguation)

A US Navy F/A-18E/F Super Hornet in transonic flight.

Any speed over the speed of sound (Mach 1), which is approximately 343 m/s, 1,087 ft/s, 761 mph or 1,235 km/h in air at sea level, is said to be supersonic. Speeds greater than 5 times the speed of sound are sometimes referred to as hypersonic. Speeds where only some parts of the air around an object (such as the ends of rotor blades) reach supersonic speeds are labelled transonic (typically somewhere between Mach 0.8 and Mach 1.3).

Sounds are travelling vibrations (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; (pressure has a little effect). Since air temperature and composition varies significantly with altitude, Mach numbers for aircraft can change without airspeed varying. In water at room temperature supersonic can be considered as any speed greater than 1,440 m/s or 4,724 ft/s. In solids, sound waves can be longitudinal or transverse and have even higher velocities.

Supersonic fracture is crack motion faster than the speed of sound in a brittle material. This phenomenon was first discovered by scientists from the Max Planck Institute for Metals Research in Stuttgart (Markus J. Buehler and Huajian Gao) and IBM Almaden Research Center in San Jose, California (Farid F. Abraham).

1 Supersonic objects
2 Breaking the sound barrier
3 See also
- 3.1 Other Flow Regimes
4 References
5 External links

[edit] Supersonic objects

Supersonic technology has been around for centuries. Many modern fighter aircraft are supersonic, but Concorde and the Tupolev Tu-144 were the only supersonic passenger aircraft. 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/Boeing B-1B are also supersonic-capable. The F-22 is among the first fighters to be able to sustain supersonic flight for prolonged periods of time without the use of afterburners.

Most modern firearm munitions are supersonic, with rifle projectiles often traveling at speeds approaching 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 pressures. During ascent launch vehicles generally avoid going supersonic below 30km (~90 thousand 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.^[1]

[edit] Breaking the sound barrier

Main article: Sound barrier

Supersonic aerodynamics are simpler than subsonic because the airsheets at different points along the plane often can't affect each other. Supersonic jets and rocket vehicles require several times greater thrust to push through the extra drag experienced within the transonic region (around Mach 0.85-1.5). 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 sectional area further down the vehicle leads to shock waves along the body. Designers use the Whitcomb area rule and minimize sudden changes in size.

It should be kept in mind, however, that the aerodynamic principles behind a supersonic aircraft are often more complex than described above due to the fact that such an aircraft must be efficient and stable at supersonic, transonic and subsonic flight.

At high speeds aerodynamic heating can occur, so an aircraft must be designed to operate and function under very high temperatures. For example, the SR-71 Blackbird jet could fly continuously at Mach 3.1 while some parts were above 315°C (600°F).

A cage around the engine reflects any shock waves.A spike behind the engine converts them into thrust.

To generate lift a supersonic aircraft has to produce at least two shock waves: One over-pressure downwards wave, and one under-pressure upwards wave. Withcomb's area rule states, we can reuse air displacement without generating additional shock waves. In this case the fuselage reuses some displacement of the wings.

Why real planes produce under-pressure shockwaves on the ground.

NASA has recently shown that a bow shock wave widens and flattens before it reaches the ground, while multiple shocks produce N-waves with a lot of energy in the audio range. As internal supersonic compression can unstart, designers want external compression. As these produce also external shocks, they have to be located at the nose.