I saw a documentary about the Space Program. It claimed that as the Shuttle glides in to land, there are two sonic booms heard. How is this possible? Why two?
A sonic boom occurs when crossing the sound barrier. I can kinda see an odd number of sonic booms, as one attempts to slow down…but an even number on re-entry? 
Maybe someone can explain what’s happening.
“They’re coming to take me away ha-ha, ho-ho, hee-hee, to the funny farm where life is beautiful all the time… :)” - Napoleon IV
The boom is not a single event when “breaking” the sound barrier. The boom is the intersection of a conical pressure wave and your ear. The pressure wave is ongoing while flying faster than sound, much like the wake of a boat. A single aircraft has more than one pressure wave: nose, tail and possibly depending on configuration of wings and other bits. I’ve witnessed F-14 supersonic flybys at less than 100 yards and there are distinct but close booms. This is probably the case with the shuttle. At distance the separate booms may nod be distinct.
I’ve heard a double sonic boom. Maybe more than one, but it was definite the one time. (I thought they broke our windows for sure that time. I’ll never forget it.) It sounds kind of like the BOOM stuttered. BA-BOOM!!! The stealth planes used to fly over our house sometimes. (I know this because the news would report about the BOOMS it when it happened. You got to love living in a smaller city with less crime to report.) I heard a lot of those BOOMS around the time that the forces were gearing up for the Gulf War.
My understanding is that a sonic boom happens when the relative air speed is exactly at sound speed, so that the sound (pressure wave) trying to travel ahead of the plane pile up. This creates a huge pressure which makes a loud noise. This happens once at the nose of the aircraft. However, the air that hits the wings is affected by the nose of the craft and so is travelling with the plane at some speed. So the relative air/wing speed hits sound speed when the plane is flying somewhat faster than the sound.
Maybe you would like to check this thread and also this picture or this page as well.
Might be useful.
Saludos.
