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There are two basic types of spacecraft docking systems:

  • Non-androgynous docking systems (examples: every U.S./Russian docking system currently in use for docking spacecraft) have two separate, distinct ports: one (the male/active/top) does all the actual work, and the other (the female/passive/bottom) just sits there doing nothing and waits to be docked to. A male port can only dock to a female port, and a female port can only dock to a male port (no gay docking ports, people).
  • In contrast, androgynous docking systems (examples: APAS, Clamp-O-Tron) have only one type of port, and any port can dock to any other port.

Androgynous systems have the obvious advantage that any spacecraft equipped with a docking port can dock (external protrusions, or lack thereof, permitting) to any other spacecraft using the same docking system; there’s no need to split your fleet and equip half with male ports and half with female ports,1 or to send up an adapter any time a spacecraft equipped only with male ports has to dock to another male-port-only spacecraft or a female-port-only spacecraft has to dock to another female-port-only spacecraft. You can just grab a spacecraft out of the bucket and toss it into the right orbit, or grab a spacecraft that’s already in orbit and manhandle it into position, without having to worry about whether its docking port is compatible with the one on your other spacecraft.

Yet, despite this advantage, androgynous docking systems seem to have fallen out of favour; all current and planned U.S. and Russian docking systems are non-androgynous,2 with the only androgynous system in use today being the Chinese Docking System, used for docking the Shenzhou and Tiangong spacecraft.

Why is this?


1: Example: the Soyuz 7K-OK (the first Soyuz variant to be flown manned) was equipped to dock with another Soyuz 7K-OK, but used a non-androgynous docking system; as a result, the fleet was divided into “active” and “passive” halves, depending on whether each spacecraft was equipped with a male or a female port, and docking missions required that one active and one passive spacecraft be launched - you couldn’t just grab a couple out of the barrel at random and stick them up there.

2: For that matter, NASA seems to have shied away from docking ports of any sort, favouring berthing, even though berthing a spacecraft requires physically grabbing it with an arm on the target spacecraft (thus making it useless for attaching to a dead, incommunicado, or malfunctioning spacecraft), and unberthing takes ages and requires a person on the target spacecraft to close valves and stuff after the hatch has been shut, as detaching without having done so would depressurise the target spacecraft (thus making it useless for abandoning ship in an emergency, unless you’ve got a crewmember you really don’t like handy).

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    $\begingroup$ NASA seems to be working on an androgynous docking system now. en.wikipedia.org/wiki/NASA_Docking_System I found it by clicking on a link on the APAS "See also" section. See also en.wikipedia.org/wiki/International_Docking_System_Standard $\endgroup$
    – RonJohn
    Jul 5, 2019 at 5:06
  • $\begingroup$ Are you mixing grapple fixtures with docking ports? Because they're not the same thing. $\endgroup$
    – DrSheldon
    Jul 5, 2019 at 12:32
  • $\begingroup$ Hi Sean, you have several questions open with good answers that are worthy of being accepted. Please reward the effort people have put into answering your questions and accept some. Thank you! $\endgroup$
    – DarkDust
    Jul 6, 2019 at 11:38

2 Answers 2

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Currently all future NASA plans for spacecraft use androgynous docking ports.

In the commercial cargo to ISS program, the SpaceX Dragon craft uses the NDS (NASA docking system) which is androgynous and follows international docking standards.

NASA's next generation of crewed spaceflight, in the form of the commercial crew program has both Boeing's CST Starliner and the SpaceX Crew Dragon using the androgynous NDS port. In the future, NASA's Orion spacecraft and the SNC Dream Chaser will also use the same NDS port.

As for why the Russians still use non-androgynous docking ports is mostly for historical and financial reasons. Russia is currently not seriously working on any upgrade to their manned or unmanned spacecraft and believe firmly in "if it ain't broke, don't fix it."

Also, there really isn't that big of a distinction between berthing and docking when it comes to port requirements. SpaceX has said on multiple occasions that the Dragon craft could dock itself but NASA wants to play it safe and minimize risk so they grab it with the Canadarm instead.

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    $\begingroup$ Important to note that the IDAs on ISS are not truly androgynous -- they are strictly passive. $\endgroup$
    – Tristan
    Jul 5, 2019 at 18:11
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Quick addition, APAS, in its androgynous form, fell out of favor in Russian circles simply because of mass. APAS is of a much greater mass than SSVP leading to the situation where only 2 cosmonauts could be flown instead of the usual 3. The mass of APAS attached led to too many sacrifices elsewhere on payload, so SSVP was carried on.

The only time APAS/ASTP-Soyuz actually flew and docked with another spacecraft was Soyuz 19 (ASTP to Apollo) in 1975. Soyuz 22 started out with APAS but had it replaced with Earth Observing gear instead, but retained the bulked forward shape. Since then, Soyuz TM-16, was the only Soyuz to fly with APAS instead of SSVP.

Where APAS was ok was when it was part of the Buran space shuttle program and the Mir-Shuttle programs (APAS-89).

In January 1993, Soyuz TM-16 was the first Soyuz without a probe and drogue SSVP docking system since 1976. It carried an APAS-89 docking collar to dock with an androgynous docking port on the Kristall module on the Mir space station. This was a test of the docking system in preparation for dockings by the Space Shuttles.

Only two cosmonauts could be carried to keep it within launch weight parameters. This was due to the extra mass caused by the APAS-89 installation.

Later APAS-95, as on the ISS, was no longer androgynous.

Chinese have adopted APAS simply due to their adaptation of former Soviet technology. They do not have the shackles of Soyuz and since they generally do not get the go ahead on projects unless 80% is already technology proven and available, APAS was a much better future-proofing option than SSVP though aperture size remains the same. Shenzhou, heavier and slightly larger than Soyuz allows it to have the heavier docking collar, and does not have the same launcher parameter constraints as Soyuz does. Given its China, it would be very hard to find sources for this.

https://en.wikipedia.org/wiki/Soyuz_TM-16

http://www.spacefacts.de/mission/english/soyuz-tm16.htm

https://space.skyrocket.de/doc_sdat/soyuz-tm-apas.htm

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