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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, 2016 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, 2016 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, 2016 at 17:30
  • $\begingroup$ Scott Manley has a youtube video about this. $\endgroup$
    – Innovine
    May 14, 2021 at 5:44
  • $\begingroup$ I've been told that the slant wasn't related to the Space Shuttle's Docking Adaptor. $\endgroup$
    – James Ervin
    May 14, 2021 at 18:12

4 Answers 4

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The drawing in user10795's answer is not an accurate design drawing of the PMA design and thus the conclusions flawed.

In this document 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 dual 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, 2020 at 11:13
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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, 2020 at 21:52
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    $\begingroup$ @OrganicMarble: I always thought the clearance is meant for the biggest of cargo. $\endgroup$
    – ymb1
    May 14, 2021 at 1:15
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    $\begingroup$ @OrganicMarble: I like to think it's confirmed now :-) $\endgroup$
    – ymb1
    May 17, 2021 at 1:14
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    $\begingroup$ @ymb1 indeed! Excellent. $\endgroup$
    – Organic Marble
    May 17, 2021 at 2:06
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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, 2017 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, 2017 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, 2017 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, 2019 at 20:13
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The slant is to allow big payloads to be taken out with adequate clearance:

Meanwhile, the ISS design had to accommodate the shuttle. It needed to provide a zigzag tunnel mechanism (the Pressurized Mating Adapter) to optimize the clearance to remove payloads from the bay after the shuttle had docked.

— Hale, Wayne, and Helen Woods Lane, eds. Wings in Orbit: Scientific and Engineering Legacies of the Space Shuttle 1971–2010. Government Printing Office, 2010. p. 138.
(Links: homepage - text - PDF - ntrs.nasa.gov)

Note: that book isn't just any book; see this part of its forward:

We are grateful to all the institutions and people that worked on the book. (See appendix for complete list.) Each NASA field center and Headquarters contributed to it, along with many NASA retirees and industry/academic experts. There are a few who made exceptional contributions (...)

Example:

enter image description here
— Space Shuttle Flight 111 (STS-112) Post Flight Presentation (YouTube)

A finding worth mentioning: that PMA's basic design is from Space Station Freedom (SSF) Level 2 Program Office, a 1993 derivative of which was considered for Soyuzes docking to SSF as Assured Crew Return Vehicles (ACRV) after reaching the Permanently Crewed Capability (PCC) construction stage.

enter image description here
— Cruz, Jonathan, Marston J. Gould, and Eric Dahlstrom. Soyuz/ACRV accommodation study. National Aeronautics and Space Administration, Langley Research Center, 1993. p. 147. https://core.ac.uk/display/42789859

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    $\begingroup$ This is very awesome! Written confirmation of what was kinda the common knowledge! Great job. $\endgroup$
    – Organic Marble
    May 17, 2021 at 2:05
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Maybe they say it here on page 81 : https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110013394.pdf

“Pressurized Mating Adapters The Pressurized Mating Adapter was a transition structure originally designed to provide the interface between the Space Shuttle Orbiter and the Space Station Freedom. Its unique shape was developed to preclude contact between the Node or Lab endcone and the Orbiter Forward Fuselage during docking with the Shuttle External Airlock. It was constructed from four Aluminum 2219 forgings,” […]

(from https://www.forumastronautico.it/t/forma-dei-pressurized-mating-adapter-pma/34213/22)

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  • $\begingroup$ Unfortunately it actually makes the clearance worse imgur.com/a/kkR7NYU I'm personally convinced it was for cargo bay clearance for payload ops, but I have never found a reference. However, you may be on to something if the Freedom design was different. $\endgroup$
    – Organic Marble
    May 13, 2021 at 14:42

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