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We know that the payload bay of the Space Shuttle Orbiter carried satellites, pieces of equipment for repairing satellites and components of the International Space Station.

Was the payload bay of the Space Shuttle depressurized before launch, or did they just open the door in orbit without worrying about anything?

How did they manage to depressurize once closing its doors? Were the doors strong enough to withstand the pressure loads? Was there any kind of air-valves?

If possible, kindly support your answer with an image.

Organic Marble
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Vishnu
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1 Answers1

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Not before launch, during launch.

The structure of the payload bay (and the other Orbiter compartments except for the crew compartment) was not strong enough to withstand either crush loads from a vacuum in the bay / atmosphere outside situation, or burst loads from a atmosphere inside / vacuum outside situation. Accordingly, the bay and other volumes were vented through a system of doors and ducts. This system was called the Active Vent System (AVS).

The doors were opened and closed by electric motors commanded by the onboard computers.

enter image description here

The doors were either closed or slightly open ("purge position") prelaunch to prevent intrusion of any propellant vapors into the bay - some compartments were being purged by dry air supplied from the ground. Before launch, all doors were commanded fully open.

The flight sequence of operations was as follows:

The vent door opening sequence is automatically initiated at T - 28 seconds. The vent doors are commanded open in a staggered sequence at approximately 2.5-second intervals. At T - 7 seconds, the Redundant Set Launch Sequence (RSLS) checks that all vent doors are open. If any door is out of configuration, a launch hold will be issued.

As the vehicle ascended and the ambient pressure dropped, the atmosphere in the payload bay flowed out through the doors.

The doors remained open during ascent and orbit operations.

The doors were closed by the crew using commands input into the computers prior to the deorbit burn. Some of the aft doors then re-open to vent any vapors ingested during the burn. All doors automatically re-closed prior to Entry Interface.

During the initial part of entry, the doors remained closed to prevent plasma intrusion. When the vehicle reached Mach 2.4 (at approximately 85,000 ft altitude), the doors opened automatically to equalize the pressure by allowing the ambient air to flow into the payload bay.

The door sequence on ascent aborts was completely automatic.

All information about the doors comes from the Mechanical Systems Training Workbook. I urge you to consult this document for more information on the system.

This photograph shows some of the open doors.

enter image description here

Image source, annotations mine

Organic Marble
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    That Mechanical Systems Training Workbook is amazing! – zeta-band Oct 23 '19 at 23:34
  • Can you explain the purpose of closing the doors? It seems like the end goal was to ensure that pressure was always equalised between the bay and the exterior so why could they not just have been permanently open? – Lars Oct 24 '19 at 10:31
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    @Lars my answer states the reason for closure of the doors each time it was done. Which time are you asking about? – Organic Marble Oct 24 '19 at 12:01
  • "When the vehicle reached Mach 2.4" does it mean the vehicle slowed down to Mach 2.4 ? – Uwe Oct 24 '19 at 19:19
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    @Uwe yes, I'm describing entry. The vehicle starts out at ~ Mach 25. I'll edit in the altitude, though, thanks. – Organic Marble Oct 24 '19 at 19:21
  • If the doors were closed prior to the deorbit burn, how would they have ingested anything during said burn requiring them to be reopened for venting afterwards? – Vikki Feb 25 '21 at 23:19
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    @Vikki-formerlySean The way it's written is a bit confusing. What happens, was, prior to the burn, the crew entered the command to close all the doors. All the doors close, but then 4 of the aft doors re-open immediately. So 4 aft doors are open during the burn, to vent associated vapors. I always thought it was odd that they close and then reopen; I assume it was some quirk of the software. – Organic Marble Feb 25 '21 at 23:37
  • @zeta-band Try googling "site:www.nasa.gov/centers/johnson type:pdf". You can add additional keywords like "Workbook", "Manual", "Training", "Checklist" etc. Have a field day :) – busdriver Oct 20 '21 at 08:21
  • @Organic Marble Little related to this question. I saw on animations that Shuttle airlock was placed in payload bay and used both for docking with ISS and for astronauts spacewalks. All while Orbiter was turned upside down to protect crew against debris strike and payload bay was open to allow radiators to radiate heat. But I also read that before Challenger airlock was placed inside the Crew Cabin (next to entry to payload bay). Is it true ? And how they could then make spacewalks on missions whit spacelab, which must be connected with crew cabin ? – David Cage Oct 23 '21 at 11:30
  • @DavidCage this may answer that https://space.stackexchange.com/a/36662/6944 there is a picture there that shows how EVAs were done with pressurized modules in the bay. if not, please ask a new question! – Organic Marble Oct 23 '21 at 12:24
  • I saw that picture before. Spacelab was always connected with Crew cabin with some tube and between tube and cabin was also airlock for docking with ISS and spacewalks. But I also saw in Jared Owen animations, that for early missions this airlock was always hidden inside Crew cabin (right next to entry to payload bay) and they made spacewalks through this entry to the payload bay. So if it is true, how they could make spacewalks in those early missions with spacelab, if spacelab must be connected with some tube to crew cabin entry with airlock ? – David Cage Oct 23 '21 at 12:45
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    @DavidCage it was basically the same as in the answer I linked to. There was an EVA hatch in the tunnel. https://lh5.ggpht.com/_1wtadqGaaPs/TF6EG3y7VlI/AAAAAAAAPAk/zBp7Hd2AvtU/tmp3D23_thumb1_thumb1.jpg?imgmax=800 https://lh4.ggpht.com/_1wtadqGaaPs/TF6EbBJtIUI/AAAAAAAAPA0/T4DcDRmYH40/tmp3D25_thumb_thumb.jpg?imgmax=800 – Organic Marble Oct 23 '21 at 13:18
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    @DavidCage I found an actual picture https://imgur.com/Gt4sXqS . It's from Spacelab, An International Success Story https://ntrs.nasa.gov/citations/19880009991 – Organic Marble Oct 23 '21 at 13:33
  • @Organic Marble This mean, that they never used internal airlock (hidden inside crew cabin) for missions with spacelabs, even on early missions. Why would they need it, when spacelab must have EVA airlock already mounted on tunnel adapter to Orbiter. Internal airlocks where probably used only on launching comsats. or other cargo missions, where astronauts could made spacewalk from internal airlock and entry to payload bay. – David Cage Oct 23 '21 at 13:36
  • @DavidCage If you want a lot of extra information, please ask a new question, so a proper answer can be written, instead of putting it in comments. – Organic Marble Oct 23 '21 at 13:45