Space Shuttle Entry Double Sonic Boom

What causes the double sonic boom when the Space Shuttle lands? 
 
The first boom is the Bow Shock, created by the nose of the Orbiter as it 
plows a hole in the air for the Orbiter to fly through during supersonic 
flight. 
 
The second boom is the Wake Shock, it's the shock wave formed by the air 
filling in the hole created by the Orbiter. To the observer on the ground, 
it passes them approximately a half second behind the boom from the Bow 
Shock. 
 
>From the ground, the two shock waves would look like two parallel cones one 
following the other. The first cone would look like it's vertex originated 
just in front of the Orbiter. But, the vertex of the Wake Shock would 
originate much further back from the tail. At Mach 2 the vertex of the Wake 
Shock would be around 900 feet behind the tail of the Orbiter. And, at Mach 
4 the vertex would be 1900 feet behind the tail. 
 
A simple incompressible fluid analogy might go something like this. 
 
If the air were an incompressible fluid, and the Orbiter a simple pointed 
rod, the Bow Shock would originate at the tip of rod. Where the air is 
first separated to form a hole in the air. A hole that the rod is traveling 
in. The hole in the air being a vacuum. Some amount of energy (E) from the 
rod will have been expended pushing the air aside. The air, still having 
mass, and now having some radial velocity creates a circular hole that 
grows large with time. But, the surrounding air would act like a spring 
that slows down the expansion. At some point the hole stops expanding and 
then begins to collapse, accelerated by the pressure of the surrounding 
air. Collapsing faster and faster, back towards the center of the circle 
where it came from. When the hole in the air finally collapses, it creates 
the Wake Shock with the same energy (E) that it took to create the hole. At 
Mach 2 the rod and Bow Shock would now be a 1000 feet ahead of the Wake 
Shock, at the other end of the hole. 
 
The Orbiter is probably the most blunt supersonic vehicle in the world. 
Much different than most supersonic aircraft and missiles (slender bodies) 
which don't have to create as big a hole in the atmosphere to fly through. 
 
Also, air is compressible, so when you look at the flow field closer to the 
vehicle it's a little more complex. Since the Orbiter has a blunt nose, it 
has a Bow Shock wave, which has a cone angle that is decreasing with time. 
It's exponentially approaches an equilibrium cone from the inside. An 
equilibrium cone with a vertex that is just in front of the Bow Shock. 
 
The Wake Shock, on the other hand, begins just aft of the Orbiter, but with 
a smaller shock cone angle. A shaper cone, with a cone angle that is 
increasing with time, exponentially approaching the shape of a similar 
equilibrium cone to what the Bow Shock is approaching. But, the Wake Shock 
is approaching the cone from the outside, while the Bow Shock is 
approaching it's cone from the inside. Also, the vertex of the equilibrium 
Wake cone is much further back from the Orbiter, than the vertex of the 
equilibrium Bow cone is in front of the Orbiter. So the Wake Shock is 
formed over a much broader region behind the the Orbiter. 
 
It takes just an instant to create a hole in the air for the Orbiter to fly 
through, but it takes much longer for the air to fill it back in. 
 
So, on ascent, there should be only one sonic boom, as the hole behind the 
Stack is being filled with hot gasses from the engines. That is, until all 
the water condenses. 
 
Craig Fink