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The (retired) NASA Space Shuttles has two decks. The flight deck, and the mid-deck.

Why is the mid-deck called "mid-deck"? It is not the middle of three decks. Shouldn't it, strictly speaking, be called the "lower deck" (following naval traditions of decks below the main deck)?

What is the naming convention used for the decks on the Space Shuttles?

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Actually, it is the middle of three decks, the lower one just didn't get much press. It was called the Lower Equipment Bay (LEB) or ECLSS bay. It was a pressurized compartment containing mostly life support equipment.

Here's a picture of me in Endeavour's LEB taken from the middeck.

enter image description here

Here's a schematic from the 1988 Press Handbook.

enter image description here

Finally, here is a layout of some of the life support equipment in the LEB. I've highlighted the humidity separators, they are the two white devices to the left of my left knee. This is from the ECLSS Training Manual.

enter image description here

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    $\begingroup$ Awesome answer. I guess this is as straight from the horse's mouth as you can get :) Maybe you could clear up some other things as well? What constitutes a deck? What makes a "bay" classify as a deck? If I dig a small hole in my lower floor, do I suddenly have a "lower lower floor" even if I can't really use it for anything more than storage? I hope I'm not coming off splitting hairs, but I'm genuinely curious about the definitions and to get clarifications :) $\endgroup$ – GTHvidsten Apr 24 at 20:51
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    $\begingroup$ @GTHvidsten welcome to space stack exchange! All of those names had been set up long before I showed up. A "bay" definitely wasn't always a "deck", there were aft avionics bays back near the main engines that were open to vacuum and were never referred to as decks. If I had to guess, I'd say it was because all 3 levels in the cabin were enclosed in the pressurized volume and were accessible in flight (although going in the LEB in flight was not all that common, it did happen). So they counted as 3. I'd also guess the bottom floor was a "bay" not a "deck" because you couldn't stand up in it. $\endgroup$ – Organic Marble Apr 24 at 21:25
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    $\begingroup$ @RockPaperLz-MaskitorCasket the 'bunny suit' was intended to keep the shuttle clean. I don't know why gloves were not required. The cleanliness guardians taped over my wedding ring with Kapton tape. $\endgroup$ – Organic Marble Apr 26 at 12:42
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    $\begingroup$ @Michael The 'bunny suit' was intended to keep the shuttle clean. I only had to wear the mask because I had facial hair. My clean-shaven co-workers did not have to wear a mask. $\endgroup$ – Organic Marble Apr 26 at 12:43
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    $\begingroup$ @OrganicMarble, my guess to the Kapon tape on your wedding ring wasn't for cleanliness, but rather to prevent any possibility of direct electrical contact with anything with live electric you might be reaching into, or touching. Don't want to create any sort of high-amperage shorts that might either damage equipment, or cause the ring to heat up so much that your finger gets burned. Even with lower voltages, in the right (or wrong, depending on perspective) situation, high amperage could turn a ring almost instantaneously into molten metal. $\endgroup$ – Milwrdfan Apr 26 at 21:03

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