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Answers Your Questions

From: Terry Callaghan, of Geelong, Australia
To: Tony Ceccacci, flight director for the orbit 2 team

Question: I was wondering how well sealed the payload bay is? And how the air inside is released as the shuttle heads into space, and how it refills during de-orbit?

Answer: The payload bay, or midbody, is designed to "leak," as are the aft and forward compartments of the orbiter, but not the crew compartment! Only the crew compartment is designed to withstand the pressure difference between its interior and outside. The remainder of the orbiter is designed to take no more than about 0.5 psi (34.5 mbar) differential pressure. Therefore, during ascent, the orbiter must vent these compartments to vacuum, and then during entry allow air back in to prevent damage to the vehicle's structure. This is achieved using a system of seven motor-driven vent doors along each side (14 total). The vents are closed during certain periods. Prior to launch, this is done to prevent outside air from contaminating the payloads and to prevent a possible buildup of combustible gases. However, some of the vents are open, but the compartments are purged with dry nitrogen gas to keep outside air from getting in. Just prior to liftoff, all of the vents are opened. The vents stay open throughout the mission, and are then closed for entry to keep out the hot plasma generated by the orbiter's plunge into the atmosphere. For the payload bay, there are three vents doors along each side (six total), which also vent the inside of the wings. The payload bay doors have both a pressure seal and a thermal barrier to make sure that venting, in both directions, takes place through the vent doors.

Kevin McCluney
Ascent/Entry Mechanical Systems

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Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 06/20/2002
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