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

More on the Double Boom,

Since the time difference is a function of the size hole the Orbiter is
creating in the sky, it will be a strong function of frontal area and
velocity, not the length of the vehicle. So, at higher Mach numbers the
time delta should be larger. Also, at higher angles of attack the time
delta should be larger. The Orbiter spends much the time during entry at 40
degress angle of attack.

This seems to be true, as a study using seismic sensors to detect sonic
booms was performed back in the early 90s for a Shuttle landing at Edwards.
The two booms were seperated by 0.7 seconds at around Mach 5. The largest
seperation time was over 1 second.

Craig Fink