|MadSci Network: Engineering|
The distinctive double sonic boom heard when the space shuttle lands results because the shuttle is large (at least relative to the other aircraft allowed to travel at supersonic speeds over land.) The text below is taken directly from information available through NASA's Spacelink system (http:// spacelink.nasa.gov) and enter "sonic boom" in the Search box.
"These shock waves form two cones, at the nose as well as at the tail of the vehicle. The shock waves move outward and rearward in all directions and usually extend to the ground. As the shock cones spread across the landscape along the flightpath, they create a continuous sonic boom along the full width of the cone's base. The sharp release of pressure, after the buildup by the shock wave, is heard as the sonic boom."
"The nose and tail shock waves are usually of similar strength. The time interval between the nose and tail shock waves is primarily dependent on the size of the aircraft and its altitude. Most people on the ground cannot distinguish between the two and they are usually heard as a single sonic boom. As the time interval increases, two booms are heard. A small fighter-type aircraft about 50 ft long will generate nose and tail shock waves of less than a tenth of a second (0.1 sec). The ear usually detects these as a single sonic boom."
"The interval between nose and tail shock waves on the Space Shuttles, which are 122 ft long, is about one-half of a second (0.50 sec), making the double boom very distinguishable."
You can access the entire document and much more at NASA Spacelink
John Haberman, Space Scientist NASA/Goddard Space Flight Center
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