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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?

  • $\begingroup$ 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? $\endgroup$ 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. $\endgroup$ Dec 27 '16 at 17:30
  • $\begingroup$ Scott Manley has a youtube video about this. $\endgroup$
    – Innovine
    May 14 '21 at 5:44
  • $\begingroup$ I've been told that the slant wasn't related to the Space Shuttle's Docking Adaptor. $\endgroup$ May 14 '21 at 18:12

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

  • $\begingroup$ 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. $\endgroup$ Jun 3 '20 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". $\endgroup$ Jun 3 '20 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 '21 at 1:15
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    $\begingroup$ @OrganicMarble: I like to think it's confirmed now :-) $\endgroup$
    – ymb1
    May 17 '21 at 1:14
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    $\begingroup$ @ymb1 indeed! Excellent. $\endgroup$ May 17 '21 at 2:06

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.; 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.

  • $\begingroup$ 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. $\endgroup$ 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. $\endgroup$ 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 $\endgroup$ Aug 25 '19 at 20:13

Answer is:

Forget ISS and the Russians. Its the legacy design, inherited from Boeing's Pressurized Docking Adapter on Space Station Freedom. Originally, one of a pair next to each other on the end of Hab and Science modules (or nodes in earlier design), to fulfil the spec that two Orbiter-spec docks be available.

Okay, long story short:

The fact that concept artwork showed what turned out to be the docking adapter that was fitted inside the shuttle bay in real life, was, in these depictions, actually part of the docking adapter on the station, bugged me.

And the adapters depiction is not consistent. Unless you see that when NASA render it, its not usually present or is simplified (to short, straight etc). This also goes for any contractor not Boeing: all used simplified straight tubes.

It is important to remember that they were not making the docking adapter. When Boeing (and subcontractor McDonnell Douglas) render it (be it 3D, physical model or artwork) it shows the strange adapter. When you look at the artwork and the animations, then you see the origins of the PMA.

(Boeings design work actually began in 1988, with Work Package 4 on the U.S. Space Station Freedom)


They were PDA's and are angled opposite each other.

This matches with articles I read that the design, until the 1993 redesign, always specified 2 race track / parallel running modules (science and hab), and always must have two docking ports, to allow Space Shuttles be docked at either one.

Two ports are available at all times for Orbiter berthing, as specified in the system requirements.

(Conceptual Design and Evaluation of Selected Space Station Concepts, 1983)

And the ports must be shirt-sleeve walk through (ie. like moving from shuttle cab into spacelab). (there were drawings of spacelab being directly attached to the nodes of Freedom too).

And that spec went all the way back to the dual keel design. But that design had 'long boom' docking adapters, described as actively controlled load alleviation device.

The booms were both oriented to the left, as if keeping the shuttle slightly away from structures to the right side of the truss. In that design there are still the nodes, notably with the Cupola for overseeing operations, one on zenith, one on nadir.

When the redesign got rid of the nodes, the keel, and the long booms, the shorter PDA design appeared. A redesign later, the PDA orientation changed too, to being opposite each other, whilst retaining the nose up and nose down attitude of the docked Orbiters. It looked like they were changing how the shuttles were docked and keeping them as far as part as possible without making the adapter go beyond the extent of the shuttle bay it would fit when launched, and at the same time not be structurally weak by not being too long.

The PDA's and therefore the whole docking assembly being rotated away from each other gave enough clearance to satisfy the specification that the station be capable of supporting Orbiters at either dock.

It also brought a benefit that the slight outwards alignment meant that the manipulator arm of the shuttle payload bay had more room to move modules out and on to the station. Given that the shuttle arm was only on one side, this makes sense to have the side with the arm on the outside.

Around 1992, construction of the station (Space Station Freedom Media Handbook) refers to an unpressurized and then pressurized system taking over. This system was called the Pressurized Berthing Adapter, which had a sliding latch for lining up, and then had the Pressurized Docking Adapter (kinked) as part of its assembly. This was based on the standard pallet integration that consisted of trunnions that engaged with the PRLA and keel of the Orbiter's Payload Bay, giving a hard dock.

The unpressurized berthing adapter extension, locking and attach systems extend the unpressurized berthing adapter and latch and structurally join it to the mobile transporter element, thus providing the initial berthing interface between the orbiter and the space station freedom. This sequence is reversed when the pressurized berthing adapter is attached to the space station freedom and the unpressurized adapter extension can be removed.

Collectively known as Pressurized Berthing Adapter.

Canada's Arm (Automatic Control in Aerospace 1992) was now part of the docking process, capturing and pulling in the Shuttle to mate with the PBA.

the scenario and initial conditions for a simulation of automatic berthing of the Shuttle Orbiter to the Space Station Pressurized Berthing Adapter. The motion is relatively slow in order to avoid excessive excitation of oscillations in the SSF /SSRMS/Shuttle dynamic system which has a natural frequency below 0.01 Hz. It takes about 15 minutes before the Shuttle is positioned within the capture envelope.

After the redesign dumped the twin module and twin docking adapters, the PDA's became PMA's and went solo, as depicted in later artwork, all the way to 1997.

Why still kinked?

My understanding is that they had already been designed and built by then. So why build new docking adapters when you can still use the ones you made. (Node 1 and PMA-1 and 2 were undergoing structural tests post fabrication, in 1996. Documents referred to the station as Alpha, a name from 1993, the PMA's in their current form already existed on the baseline SSF in early 1993)

PMA was a known structural article when Node 2 (before that was replaced by Node 1) was being tested in 1995.

Departments of veteran affairs and housing and urban development, and independent agencies appropriations for 1997

Today, there is more than 80,000 pounds of flight hardware that has been manufactured. 32 percent of flight hardware complete. Node 1, PMA-1, PMA-2... ...Complete mechanical equipment installation into node structural test article (1st Qtr 1996) Production of flight hardware continued in earnest during FY1995. Boeing completed the primary structure for... node 1 and US laboratory. ..assembled the pressurized mating adapter.

(incidentally, they look like they could have fitted spacelab and attached to the internal airlock of the shuttle cab, as in a shorter, smoother tunnel than was used).

So, in short, when everyone has been trying to understand the kink, they have been looking at it from the wrong angle.

Turn the adapter 90 degrees and you see how it was originally designed to fit, along with a 'phantom twin' next to it. Nothing to do with loads too big, or space behind the shuttle cab, it was just Boeings concept of having two available docks at the same time. Payload Bay and arm clearance was a side benefit.

Nothing to do with hatch clearance either, these designs were from long before Russia got involved in 1993.

(As a side note, a study was done before the Russians signed on in 1993, for Soyuz to be used as a CRV. Multiple options were looked at, each using a PMA for the Soyuz to be docked to. The study for this exists based on a pre-existing base-line SSF configuration, that already had slanted PMA's attached, so this was never the origin for the PMA's, they were already designed that way) https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940012599.pdf

Slide 1


Slide 2


Slide 3



Dual Orbiters docked simultaneously was considered and tested for, but the Station ultimately proved structurally incapable. What is visible is the clearance designed for future craft docking given by rotating the PDA's 90 degrees, perhaps eventually simultaneously. https://core.ac.uk/download/pdf/42813354.pdf


From Space Station On-Orbit Assembly and Operation, 1991, you can see how the Space shuttle is berthed using the Space Station Remote Manipulator System, and when docked using the combination of Pressurized Berthing Adapter and Pressurized Docking Adapter.

Note that the PDA is rotated to the side.


For a short time in between Space station Freedom and International Space Station, there was another name.

In 1993, President William Clinton called for the station to be redesigned once again to reduce costs and include more international involvement. The White House selected the option dubbed Alpha.

In its new form, the station uses 75 percent of the hardware designs originally intended for the Freedom program.

Anyway, that's my take on it until someone from Boeing/MDc says "No..." ...


space station ref artwork:

option 93

1997 image showing slanted PMA

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    $\begingroup$ Any proof of this "My understanding is that they had already been built by then. So why build new docking adapters when you can still use the ones you made." ? Where'd your "understanding" come from? This seems to be a key point. Good point on the 90 degrees though outward bump though. $\endgroup$ May 14 '21 at 13:20
  • $\begingroup$ Absolutely, I was trying to find something to quote, but mostly I got it from the PMA test with the node1, i think in 96, one of the doc logos is "Space Station Alpha", I think a transition name between Freedom and ISS, around 1992-93. I think one of the congress articles or the astronautix article mentions a line like "In 1988, fabrication for the station was given a go ahead". I really have to go over my notes, but there was something about once Russia was involved, fabrication of the ISS parts that were previously for Freedom were ready for putting together (whereas Russia was not) $\endgroup$ May 14 '21 at 13:31
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    $\begingroup$ I also wonder if the original idea was for a port/starboard shift for arm clearance, why not use them for that on the ISS? This is all great background info though. $\endgroup$ May 14 '21 at 13:34

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 (...)


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. $\endgroup$ May 17 '21 at 2:05
  • $\begingroup$ @OrganicMarble: many thanks :D $\endgroup$
    – ymb1
    May 18 '21 at 5:06

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)

  • $\begingroup$ 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. $\endgroup$ May 13 '21 at 14:42

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