The term supersonic is used to define a speed that is over the speed of sound (Mach 1). At a typical temperature like 21 °C (70 °F), the threshold value required for an object to be traveling at a supersonic speed is approximately 344 m/s, (1,129 ft/s, 770 mph or 1,238 km/h). Speeds greater than 5 times the speed of sound are often 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 labeled transonic (typically somewhere between Mach 0.8 and Mach 1.2).
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 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 (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.
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 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.
It should be kept in mind, however, that the aerodynamic principles behind a supersonic aircraft are often more complex than described above because 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).
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