Re: Why does an aircraft cause 2 sonic booms?

Russ

Thanks for submitting this very intersting question. I'll admit to not being
able to answer this directly, My main speciality is in Astrophysics/Cosmology.
Fortunately there was a thread on a usenet group about sonic booms and one of
the participants was an aerospace engineer who has a decent knowledge of the
mechanics behind sonic booms. The answer I have is quoted verbatim from a
E-Mail fom this person, Rob. 

>I'd love to be able to answer your question, but I don't think I can with 
>any certainty.  I've never witnessed the phenomenon first hand, so I 
>don't know exactly what you're talking about.
>
>I DO have a guess, however.  This would be easier with pictures, but 
>imagine a cone moving supersonically through air.  From the leading edge 
>of the cone, a shock wave is produced that has a certain angle relative 
>to the direction of travel.  This angle is dependent on several things, 
>but one of them is the speed of the cone.  At higher speeds, this angle 
>is lessened, the shock wave angles back, closer to the cone.  The angle 
>also varies with the speed of sound, which in turn varies with 
>temperature (which varies with altitude).  The speed with which the shock 
>wave propogates is also a function of the speed of sound.
>
>Now imagine the Shuttle coming in at some huge mach number.  It sheds a 
>shock wave at a very slight angle, which then propagates towards the 
>earth through a variety of layers of air.  Over a fairly short period of 
>time, however, its velocity decreases.  It also begins to move through 
>lower parts of the atmosphere, which have a different temperature and 
>hence, a different speed of sound.  
>
>Unimportant, But Interesting Note: It also changes direction: Towards the 
>end, the shuttle goes into what is called the "Heading Alignment Circle" 
>(HAC).  This is sort of an upside-down, truncated cone, with its top edge 
>tangent to the landing point (so, it's not really a circle at all...).  
>The idea is that the shuttle comes in with an uncertainty to it's exact 
>heading, altitude and airspeed.  The software onboard guides the shuttle 
>to hit the HAC on a tangent at whatever alititude, speed and heading it 
>finds itself.  Then, it steers a path that goes along the edge of the 
>HAC, but spirals downward.  At the bottom of the HAC, the shuttle emerges 
>lined up with the runway.  The point of all this is that the shuttle goes 
>through a complex turn and altitude loss on every landing.  The thing I 
>don't know is whether the shuttle hits the HAC at a supersonic speed or not.
>
>It's also interesting, in a passing way, to mention that the onboard 
>computers calculate all this stuff, a human pilot couldn't do it a 
>million years. (some deletia)
>
>So, back to our picture of the shuttle.  As it comes down, it's giving 
>off shock waves at different angles, it's moving through air with 
>different speeds of sound and it's moving with different headings.  It 
>seems to me quite likely that an observer on the ground might hear two 
>shock waves whose timing has little to do with the airspeed of the 
>shuttle.  One could have been shed when the shuttle was going very fast, 
>the other when it was barely sonic (and the shock wave was more normal to 
>its direction of travel).  Also, a shock wave generated at high altitude 
>will pass through many layers of air with different sonic speeds and, 
>hence, different propogation speeds for the shock wave.
>
>So, there are too many variables here for me to figure out exactly what 
>is happening, but I can certainly imagine that folks on the ground could 
>hear sonic booms that seemed to have a funny timing.
>
>I hope this helps.

I would like to thank Rob  for an informative and
enlightening description of the mechanics behind the genration of sonic booms
from aircraft.

Yours

Dave Barlow