Supersonic Flight: Supersonic Speed, Sonic Boom, Vapor Cone
Before we talk about supersonic flights, let’s understand what sound is. We define “sound” the sensation that we perceive as the consequence of a vibrating body. The body transmits its movement to the particles close to it, which start to vibrate causing a variation in pressure, that our hear perceives.
Sound is a wave, so it has a particular speed of propagation, equal to 1193,4 km/h at 0 °C. As far as propagation of sound in fluids is concerned, if a body is faster than sound we say it has a supersonic speed; if it is slower, it has a subsonic speed.
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.
A vapor cone, also known as shock collar or shock egg, is a visible cloud of condensed water that can sometimes form around an object moving at high speed through moist air, for example, an aircraft flying at transonic speeds. When the localized air pressure around the object drops, so does the air temperature. If the temperature drops below the saturation temperature, a cloud forms.
In the case of aircraft, the cloud is caused by expansion fans decreasing the air pressure, density and temperature below the dew point. Then pressure, density and temperature suddenly increase across the stern shock wave associated with a return to subsonic flow behind the aircraft. Since the local Mach number is not uniform over the aircraft, parts of the aircraft may be supersonic while others remain subsonic—a flight regime called transonic flight.
In addition to making the shock waves themselves visible, water condensation can also occur in the trough between two crests of the shock waves produced by the passing of the object. However, this effect does not necessarily coincide with the acceleration of an aircraft through the speed of sound or Mach 1.
The sound barrier or sonic barrier is the large increase in aerodynamic drag and other undesirable effects experienced by an aircraft or other object when it approaches the speed of sound. When aircraft first approaches the speed of sound, these effects are seen as constituting a barrier, making faster speeds very difficult or impossible. The term sound barrier is still sometimes used today to refer to aircraft approaching supersonic flight in this high drag regime. Flying faster than sound produces a sonic boom.
Supersonic speed produces a shockwave, a wave of propagation faster than the transonic wave; crashing together they produce a very fast and large variation in pressure and temperature, that we perceive as a sound (bOOm): the sonic boom.
A sonic boom is a sound associated with shock waves created when an object travels through the air faster than the speed of sound. Sonic booms generate enormous amounts of sound energy, sounding similar to an explosion or a thunderclap to the human ear. A decibel is the primary unit measurement of sound. “A thunderclap is incredibly loud, producing levels between 100 and 120 dBA (decibels A)- the equivalent of standing near a jet during take-off.”
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. This led to prohibition of routine supersonic flight overland. Although they cannot be completely prevented, research suggests that with careful shaping of the vehicle, the nuisance due to the sonic booms may be reduced to the point that overland supersonic flight may become a feasible option.
A sonic boom doesn’t occur the moment an object crosses the sound barrier; and neither is it heard in all directions emanating from the supersonic object. Rather the boom is a continuous effect that occurs while the object is travelling at supersonic speeds. But it affects only observers that are positioned at a point that intersects a region in the shape of a geometrical cone behind the object. As the object moves, this conical region also moves behind it and when the cone passes over the observer, they will briefly experience the boom.