...does that mean that a vacuum within the payload bay is brought down to Earth? Or is the pressure somehow equalised (i.e. is air pumped in) on the way down? I've always wondered how this was done with shuttle flights.
share"does that mean that a vacuum within the payload bay is brought down to Earth?"
No.
"Or is the pressure somehow equalised (i.e. is air pumped in) on the way down?"
Yes. There are vents in the side which are opened to let the air pressure equalise on the way up and on the way down.
Ah, see: http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts_coord.html
"There are 18 active vents in the orbiter fuselage, nine on each side. Each vent has a door that can be positioned for a specific purpose at various phases of flight. For identification, each door is numbered, starting at the nose of the orbiter. Each compartment has a dedicated vent on the left and right side of the orbiter for redundancy.
Internal vents are used to vent compartments that have no vent doors of their own, such as the nose wheel well, the two main wheel wells and the vertical tail section. Passive vents are used to back up vent 7 of the forward wing compartment, which responds to a delta pressure to open a check valve (passive vent) during ascent to vent the wing to the midbody if vent 7 fails; or, on descent, the midfuselage pressurizes the wing if vent 7 fails at a delta pressure of 0.72 to 1 psid. The aft bulkhead (X o 1307) has 14 one-way check valves that vent the payload bay into the aft fuselage at a delta pressure of 0.004 to 0.04 psid. Vent 8 vents the OMS/RCS pods, which are joined by a duct that enables the pod to vent through the opposite side of the vehicle if vent 8 fails to open.
In preparation for entry, the onboard operational sequence software (OPS 3) closes all vent doors. The doors remain closed until the velocity of the orbiter reaches 2,400 feet per second, when all vents are opened by the onboard GPCs."
You did your homework :)
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