Does sonic boom happen only at...

>> What you hear as a sonic boom is caused by the pressure jump at a shock
>> wave trailed behind the object going faster than sound. The shock wave
>> is there as long as the object is going fast enough, but you are correct
>> that a fixed observer will only hear the shockwave once as it passes him
>> (or her).
>> 
>> By the way, some aircraft geometries permit more than one shockwave.
>> The shuttle (I think) produces two distinct booms, from two shock
>> waves. I suspect one wave is off the nose of the fuselage, and the
>> other is off the wing leading edges.
> 
> Actually, the second "boom" is from the _expansion fan_ that diverges
> from the Shuttle's tail. In fact, _ALL_ supersonic aircraft produce
> a "double boom" --- the classic "N-shaped" pressure-wave profile,"
> <http://www.vibrationdata.com/space/sonic.htm>:
> 
>    Pressure
>       !         |\
>       !         | \
>       +---------+  \  +---------> Time
>       !             \ |
>       !              \|
> 
> For short aircraft moving at modestly supersonic velocities and low
> altitudes, the two "booms" arrive so close together in time that they
> cannot be separately distinguished by the human ear. However, the Shuttle
> is so long, flies so high, and moves so fast that the time interval
> between the passage of the compression shock and the expansion fan are
> quite clearly and separately perceptible, arriving roughly half a second
> apart.
> 

Length of the vehicle is not the important factor in the time delay between 
the to sonic booms. The second boom is a Wake Shock, and is more a function 
of the frontal area of the vehicle not it's length. It's a function of how 
big a hole in the sky that the vehicle has to create to fly through.

At Mach 4, if the two booms are seperated by .5 seconds for the shuttle, 
the length of the vehicle would only account for .05 seconds 10% of the 
time delay.

The best description of the two booms would be.

The first boom is the sound made by the vehicle as it plows a hole in the 
air. Made by the bow of the vehicle.

The second boom is the sound made by the air as the hole collapses. 
Made by the wake of the vehicle. That's why the time delta is somewhat 
indepentant of the vehicles velocity. 

>>From the ground, at Mach 2 the second boom would appear to be 900 feet 
behind the vehicle. At Mach 4 the second boom appears to be 1900 feet 
behind the vehicle. And, at Mach 8 it appears to be 3900 feet back.

I was reading of a study using seismic sensors to detect sonic booms, and 
for one Shuttle landing at Edwards the two booms were 0.7 seconds apart. 
I'll bet the vehicle was traveling faster at the time and creating a bigger 
hole (larger diameter) in the air. So, in reality the two booms get further 
apart as Mach number increases, not shorter.

Slender body vehicles would create a much smaller hole in the air, and 
therefore it takes much less time for the hole to collapse. Taking it to 
it's limit as the frontal area goes to zero, so does the time delay between 
the bow shock wave and the wake shock wave.

Craig Fink