Sonic Bang

Just as wave drag due to lift is inescapable, so is the sonic bang. Adolf Busemann liked to illus­trate this by depicting the conical shock wave system and its reflection from the ground as the crow – bar that supported the weight of the aircraft (17) Ironically, w hile the weight of the aircraf t is to be found m the integral of the pressure signature over the ground, it is not to be found in the first-order pressure field there 118). In the U S. we call the sonic "hang" the sonic "Ычил " The "bang" in the some boom derives from the abrupt pressure increases through the two. and some­times more, shock waves emanating from a supersonic aircraft Wc call the integral of the posi­tive phase of the pressure w ith respect to time the "impulse" The bang is directly related to the outdoor annoyance of animals and humans; the impulse is related to structural damage and. to some degree, to indoor annoyance.

The increasing acoustic impedance (i. e., the product of the density and (he sound speed) below the aircraft in a real atmosphere freezes the shape of the pressure signature before it reaches the ground. In the approximation of an isothermal atmosphere this occurs in ЖҐІ atmospheric scale heights, or about 40.000 feet. This knowledge set me and my colleague A1 George to tackle the minimization of various parameters of the sonic boom signature, including its bang and its boom, or any weighted average you might use of the parameters. Indeed, for the cruise characteristics of the Mach 27 Boeing 2707 at 60.000 feet lifting 600.000 pounds, an air­craft 527 feet long need not have a sonic bang at all. i. e.. the pressure field below the aircraft need not steepen into shock waves (19J. But as we noted then, reducing or eliminating the "bang” in the sonic boom increases the impulse, or total pressure loading, for obvious reasons: the bang part of the boom, that is the shock waves, dissipates the energy in the signature. Conse­quently. reducing or eliminating the shock waves makes the impulse worse.

Very considerable studies by the NASA over the past decade have explored whether or not such shaping of the sonic boom signature would lead to an acceptable sonic boom The NASA’s conclusion reinforces ours of two decades ago. Unless a supersonic aircraft is very – light. but long, its sonic boom cannot be reshaped to be acceptable [201. Very small supersonic aircraft, such as a corporate supersonic transport, may have an acceptable, indeed nearly inaudi­ble, sonic boom. This stems, in рал. from a thickening of the shock waves as their strength is reduced.

SSTs will be constrained to subsonic operation over populated areas, and perhaps to supersonic operation over the oceans alone The penetration of the pressure field of sonic booms into water, versus their reflection from it, is now well understood (21). For aircraft traveling less than the speed of sound in sea water, this is simply a travelling source of acoustic radiation. Commercial transport at supersonic speeds over the oceans, and perhaps over unpopulated areas, is likely to continue to be acceptable. Rights over land areas with significant popula­tions of wildlife may not be allowed. Through constraints on aircraft routes wc can avoid the problems caused by sonic booms, but in doing so wc reduce the market for a second generation SST

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