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enter image description here

There are three of these on the ISS. One connects the Russian modules to the rest. One was used for docking the shuttles. One... kinda just sits there, with modules that were to be berthed canceled. (Edit: PMA-2 and PMA-3 are getting International Docking Adapters (IDA-1 and IDA-3, IDA-2 was lost in the CRS-7 loss of mission) attached for Dragon 2 and CST-100 docking ports.)

Any particular reason for the peculiar shape? One would think having the exits line up semi-concentrically, or having one of the edges line up would be the intuitive approach. So why the heavy slant?

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  • $\begingroup$ I was told it was to "push" more of the shuttle's payload bay out from "under" the ISS module to allow for more unobstructed ops. Not posting as answer because no ref, and this never made total sense to me, because why would all 3 be offset? $\endgroup$ – Organic Marble Dec 25 '16 at 17:22
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    $\begingroup$ @OrganicMarble: "Why would all 3" - because you don't want to redesign a part that just works, just to remove a harmless quirk - the slant is mostly harmless in the rest of applications. Although I'm finding the "unobstructed ops" explanation really weird too. This is definitely not a shape that supports unobstructed operations, where long straight unobstructed segments are optimal for moving payload, with crew members needed only at the "corners". Such a bend is definitely counter-productive. $\endgroup$ – SF. Dec 27 '16 at 16:42
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    $\begingroup$ No, it refers to more open external access to the orbiter payload bay, not for moving through the PMA. $\endgroup$ – Organic Marble Dec 27 '16 at 17:30
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The answer marked is incorrect, that is not an accurate design drawing of the PMA design and thus the conclusions flawed. In this document https://www.bis-space.com/belgium/wp-content/uploads/2015/10/frommir-2.pdf the ISS redesign history is discussed and the PMA design existed at least as early as 1993 in fig.10 the Russians propose a first design of ISS combining Russian and US modules, the PMA designed by Boeing can be seen on the 2nd US module and the 1st Unity module (rotated 90° to how it was finally installed in fig.12), in fig.12 the Russians still recommended a straight connector. We can therefore only conclude the oddly shaped PMA was to provide greater cargo bay clearance for the Shuttle (no definitive source but then again nothing else provides a plausible alternative) and had nothing to do with Russian hatches opening. Its additional use to connect the Unity and Russian modules was probably a practicality of having the PMA already designed and that it could play this duel role PMA Design Unity model with PMA 1 & 2 fitted and in Shuttle cargo bay prior to liftoff

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  • $\begingroup$ I believe that you are correct and that is indeed for docked shuttle cargo bay clearance, but I've never been able to find a reference that explicitly states so. $\endgroup$ – Organic Marble Jun 3 at 11:13
  • $\begingroup$ @OrganicMarble Found a comment in a book on the Shuttle link $\endgroup$ – Johannes Hamman Jun 3 at 21:13
  • $\begingroup$ Yes, I've seen that sort of thing, but the way we docked with the offset towards the front of the Orbiter, it seems to make the clearance worse actually. I'd like to validate my belief that the offset is for cargo bay ops! $\endgroup$ – Organic Marble Jun 3 at 21:26
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    $\begingroup$ @OrganicMarble Looking at the book the author wasn't very clear on the use of the word "cabin" he might have been referring to the cargo bay, regardless I agree what is needed is an original reference to the requirement from NASA to Boeing but can't seem to find anything except the reference SSP 42097 and SSP 42121, also early 90s it was called a PDA "Pressurized Docking Adapter". $\endgroup$ – Johannes Hamman Jun 3 at 21:52
  • $\begingroup$ This picture shows how the clearance is worse the way we actually flew it vs. docking yawed 180 degrees. imgur.com/a/kkR7NYU $\endgroup$ – Organic Marble Jun 3 at 22:07
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The main 'throat' of the PMA is not slanted -- it is a 31.5 inch passageway that is straight as an arrow. However, the throat has to have a bit more open room on one side because the hatch of the Russian docking port opens outwards. If there was not a space for the hatch to open beyond 90 degrees, it would block the main passageway.

The NASA document Space Station Program Androgynous Peripheral Assembly System to Pressurized Mating Adapter Interface Control Document, Part 1 Core (APAS to PMA-2 & 3 (SSP 42120) has a good diagram on page 3-14:

SSP 42120 page 3-14, Fig. 3.2.1.2.4-1; APAS to PMA stay-out/passageway envelopes

As far as I can tell, the slant is primarily there to cover the 'lump' required for the Russian hatch to open fully, but this is probably more than an aesthetic choice: a smooth slant provides one less lump for a 29-ton shuttle to bump against, and it is probably also a lot easier to fit the exterior micrometeoroid/orbital debris shield to a flat slope than to an ungainly lump.

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  • $\begingroup$ Interesting! This implies therefore that the Russian side must dock at a specific attitude, or else the hatch won't open fully. $\endgroup$ – ChrisR Apr 17 '17 at 7:30
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    $\begingroup$ As far as I know, all spacecraft docking mechanisms outside of Kerbal Space Program require a specific relative roll attitude. If there are any power, data or consumable port connections, they have to line up; masses have to be balanced in an expected way (e.g. Apollo CSM/LM), and so on. The circular tunnel is convenient for the humans, but not indicative of real rotational freedom at the connection point. $\endgroup$ – Russell Borogove Apr 17 '17 at 19:48
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    $\begingroup$ So why isn't it a axially symmetrical cone shape? That would cover the 2 hatch size requirements. $\endgroup$ – Organic Marble Apr 17 '17 at 20:09
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    $\begingroup$ the interior is angled, see this picture from STS-88 commons.wikimedia.org/wiki/File:Sts088-322-035.jpg $\endgroup$ – drewcassidy Aug 25 '19 at 20:13

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