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Supersonic Aerodynamics So far, we've only discussed aerodynamic concepts in subsonic flight. Once the speed of sound is exceeded, there are all new types of phenomena to explore. For example, you can't hear a plane traveling at supersonic speed until it's passed you by. The "Sonic Barrier" Early planes designed to travel faster than the speed of sound mostly ended in tragedy. Because of this fact, engineers believed that airplanes could not travel faster than the speed of sound due to a wall of wind that prevented such flight. Now supersonic flight has become more mainstream, we even have supersonic transoceanic flights. Lets take a look at some of the phenomena that occur when a plane travels faster than the speed of sound. Shock Waves and Wave Drag Any body moving through a fluid creates pressure disturbances, since the air has to move so that a body can pass through. Sound itself is a pressure disturbance. At subsonic speeds, the pressure disturbances are capable of moving faster than the plane is. You can think of it this way. The air that is being displaced can travel faster than the plane in all directions, including out in front of it. However, as the plane reaches mach numbers greater than one, the plane's velocity exceeds that of the pressure disturbances, and the high pressure air packs up at the nose and creates drag for the plane. This is known as wave drag, also when shock waves are generated. Two shock waves form at the nose and the end of the plane. Each shock wave forms a cone shape with the tip at the origin of the air displacement. These cones of abnormal pressure air travel downwards and backwards in relation to the plane. Air at the nose of the plane is being pushed and shoved away. At the tail, the plane is quickly moving away, faster than the air can replace the void. This creates a situation where the air pressure at the nose is high, and the air pressure at the tail is low. This does two things:
The drag being produced is simply because there is a buildup of high pressure air at the nose of the plane much like the bow wave of a moving boat. In addition, the low pressure region behind the plane exerts a pulling force on the flying aircraft. This type of drag is considerably strong, therefore very powerful engines are needed to sustain supersonic flight. Mach Numbers First of all, at room temperature at sea level, speed of sound is about 760 miles per hour or 1220 kilometers per hour. It's an incredible engineering feat that planes can travel faster then that. However, the speed of sound in air varies depending on altitude and temperature. The speeds of supersonic planes are measured in mach numbers, which is easily calculated by dividing the plane's airspeed by the local sonic speed. Since each day can have a different temperature and different planes fly at different altitudes, the speed of the plane is calculated in reference to the speed of sound at the current altitude and temperature. 
Sonic Booms Remember that these shock waves travel in a conical geometry at the speed of sound. The circular edges of these shock waves are where the audible sonic booms occur. Some sonic booms are strong enough to structurally damage buildings and shatter panes of glass. The intensity of the sonic booms all depend on four things:
The pressure difference created by a faster plane is greater because air is being displaced with greater force and frequency. A louder sonic boom is attributed to a greater difference in pressure. The larger the plane, the more air it needs to displace. It is because of this that larger planes create louder sonic booms. Intuitively, the sonic booms at each of the shock waves can be heard on the ground at different times. However, most airplanes are short enough that both shock waves arrive at approximately the same time, so that it sounds like one sonic boom. Only with very large aircraft, like the Space Shuttle at re-entry, can you hear two distinct sonic booms. As you move further away from the source of a sound, its intensity becomes weaker. It is no different with sonic booms. Therefore, the higher a supersonic aircraft is, the less intense its sonic booms are at sea level. If you’ve ever seen a Concorde, its nose is very sharp compared with other subsonic planes. The reason for this sharp nose is to decrease the intensity of the sonic boom created at the nose of the plane. If the nose were blunt, more high pressure air would gather at the nose and the shock waves would be accordingly stronger. Reducing Supersonic Inefficiency Stronger shock waves produce more wave drag for an airplane. In order to lessen the buildup of pressurized air at the nose, engineers create supersonic craft with sharp noses. The fuselage is also thinned out so that the cross-sectional area literally ripping through the air is not as large. According to the Area Rule, the cross-sectional area along the fuselage must be the same, if it is to minimize the amount of counterproductive forces. If the cross-sectional area changed often, every time the cross-sectional area increased and decreased, there would be that much more wave drag and shock waves. 
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