Why Does a Projectile Create a Trailing Mach Cone?

[dEdt]

I understand why the leading edge of a projectile creates a Mach cone in front of which the air is undisturbed. But apparently the trailing edge of the projectile also creates its own Mach cone behind which the air is undisturbed. I don't understand why this is the case.

 

[Baluncore]

The air inside the trailing edge mach cone is not undisturbed, it is the sub-sonic wake disturbance.

As displaced air returns to close behind the supersonic object it meets the air coming in from the other side so it's energy is reflected at the axis of symmetry. The point of energy reflection is traveling at the speed of the object. The reflected energy radiates outwards at the speed of sound forming the trailing edge mach cone. The apex of the trailing edge cone can be generated some distance behind the trailing edge if the object's trailing edge profile has not been optimised to reduce drag.

The leading edge mach cone is generated initially as a shock wave that quickly decays to a sonic wave as it's area increases, so it gets a flying start over the trailing edge mach cone that is initiated by air returning to the axis at only the speed of sound.