This week's Planetary Radio is the second of two with Robert Zubrin. On "Questions and Answers" I answered my own question:When Space Shuttle Discovery landed at Edwards Air Force Base, I stood outside to listen for its boom. I was surprised to hear a boom-boom. Why, I wondered, does the Shuttle make a double sonic boom?
I found explanations of the answer online here and here, as well as a neat Youtube video of a cracking double-boom, accompanied by NASA TV footage, here (embedded below).
The reason for there being any sonic boom is because the Shuttle is outrunning its own noise. Sound travels through the air as pressure waves. When an aircraft moves faster than the expansion speed of those waves, the wavefronts pile up in a cone shape that expands aftward, much like the wake of a speedboat. The overlapping wavefronts reinforce each other and manifest as a big, sharp pressure wave, resulting in the boom sound.
Supersonic aircraft actually create many bow waves from all of their little protuberances, but all the little waves usually merge to form two big ones. One sonic boom is created at the nose of the aircraft. The second boom is created behind the aircraft, where air rushes in to fill the void left by the aircraft's passage. The air can only flow in so fast, though. The faster the aircraft is traveling, the farther behind it the second boom is. If the aircraft is moving relatively slowly, just over the speed of sound, we usually hear only one bang. But if the aircraft is moving more quickly and the two sonic booms are separated by more than a tenth of a second, we'll hear it as two separate booms.