Dual Sonic Booms

I have searched the Archives and posted to Cecil but have yet to receive and answer to this one. Hopefully one of the Engineers out there amongst the Teeming Millions will be able to answer this one.

When the Space Shuttle lands there are two distinct Sonic Booms. Since a Sonic Boom occurs when the speed breaks the Sound Barrier, how do two occur? We know that the Shuttle is not breaking the sound Barrier twice since there is only one Sound Barrier. Where does the second one come from?

(BTW I am not a kid doing a high school Physics assignment, I am 50 years old with a Masters from the University of Arizona!)

I am sure its in the archive somewhere but 2 sonic booms is common with the concorde as well. One from the front, one from the tail.

I am just guessing but could it have to do with the fact that the speed of sound changes in relation to density? Very high in the atmosphere the air is less dense, meaning the speed of sound would be slower. As the shuttle accelerates towards earth perhaps it does indeed break two sound barriers.

uhm, sonic booms do not come from breaking the sound barrier. They are produced continuously while the plane is exceeding the speed of sound.

Anyway, here is what NASA says http://www.dfrc.nasa.gov/EAO/FactSheets/SonicBoomsFACTS.html

Here’s a picture of an F/A-18 Hornet breaking the sound barrier. Water vapor condensation lets you see two shock waves; a large one at the tail and a smaller one at the cockpit. The shockwaves are what produce the sound you hear as a sonic boom. As Ned says, the shuttle’s shockwaves come mainly from the front and tail of the craft.

To clarify, the sonic boom is not an event, it is a cone shaped shock wave that is always moving. When the shockwave crosses your location you hear it as a boom. The nose and tail shock waves can be distinctly heard from fighter aircraft when they are close enough. I’ve heard it several times with F-14s not more than a few hundred feet away on a carrier.

BTW what you see in the F-18 pic is not the shock wave but water vapor condensing in the air from the pressure change. Video of the same scene might show a vapor cloud that almost appears to be flickering.

The nose produces a compression shock wave from pushing air ahead of it, and that creates a rarefaction wave behind it.
Each wave produces a boom as the air molecules rearrange themselves in response to the imposed pressure gradients - air in the nose compression wave suddenly spreads out, and ambient air suddenly backfills the rarefaction wave after the tail passes by.

If the wingtips or other parts extend outside the nose-induced shock cone, they’ll produce small booms of their own. The high drag losses from that are the reason planes designed for supersonic cruise have sharply-swept delta wings - that way, the entire aircraft stays inside the cone.