I read about the mechanism of "docking" of Command Modules & Lunar Modules of the Apollo missions (the probe & drouge). It seems it was far too complex & had too many parts. I felt it is in principle similar to that of the mechanism for "Opening & Closing" of an umbrella. Can anyone explain the need for so many parts used on the actual docking mechanism on LMs & CMs? Could it have been as simple as that of an umbrella - may be with multiple latches, unlike the umbrella which typically has only one latch?

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    $\begingroup$ If (and when) that single latch on your umbrella fails, as they inevitably eventually do, it is no big deal at all.. If your docking port fails, your whole mission and possibly your life is FUBAR. Also, an umbrella just needs to keep a few grams of water from passing its membrane, a docking port needs to secure multiple tons of metal at wildly varying temperatures, hurtling though space at thousands of km per hour, and establish a reliable airtight seal against vacuum. One would expect the mechanism for the docking port to be a bit more complex, and expensive, than for the umbrella. $\endgroup$ Jan 8, 2022 at 9:09
  • $\begingroup$ @CuteKItty_pleaseStopBArking What's the significance of "hurtling though space at thousands of km per hour"? It's relative velocity between docking partners that goes into calculations. How quickly ground whizzes by can be ignored. $\endgroup$
    – Jens
    Jan 8, 2022 at 20:24
  • $\begingroup$ @Jens the sheer amount of violence one has to apply to get it up to that speed. Launch vibration and g-forces are a nontrivial engineering obstacle. $\endgroup$ Jan 8, 2022 at 20:43

3 Answers 3


There were issues with the docking system so it possibly it needed to be more complex. Docking is always a trade off of mechanism simplicity VS event simplicity. Just using pipe flange with bolt holes would be mechanically simple but makes each event complicated with a space walk, spanner and lots of free floating fasteners, or you can make it involve a set of robot arms.

The docking process needs to handle several conflicting objectives:

  • Be airtight when separated
  • Handle collisions during docking process, at least without leaking
  • Allow soft dock with as much misalignment as possible (target may be rotating)
  • For Apollo, bring 40 tonnes to a stop in a couple of cm
  • Mechanically rotate so latches align
  • Bring into lateral alignment
  • Pull the two parts together and hold
  • Mechanically lock the outer rings (need a latch every 10cm or so)
  • Test and then equalise pressure
  • Allow the hatch hardware to be dismounted and moved out of the way.
  • Be as light as possible.
  • Support undocking.
  • And pretty much any failure mode prevents the mission and/or kills the crew

There are many ways to solve that combined problem, and notably different methods have been used for other craft but none of them are simple.

So yes the Apollo docking probably could have been simpler, it was however the first of it's kind, mission critical and coming after some lessons learned on Gemini docking. It certainly did need to be more complex than an umbrella (see mechanical alignment requirements above)

One thing to note when comparing Apollo hardware part count with current is that it was mostly hand built so there was a tendency to make multiple parts and bolt them together to final form, and deal with tolerances through adjustable elements, where current tendency would be to CNC an element from a single block and just re-make it if out of tolerance or needing a change.

  • $\begingroup$ I felt that the Russian-designed docking mechanism used on shuttle was extraordinarily complex. $\endgroup$ Jan 7, 2022 at 0:51
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    $\begingroup$ Another set of conflicting objectives: even in the event of a complete system failure, the two spacecraft need to stay securely docked and sealed airtight and they must not separate. Also, even in the event of a complete system failure, it must be possible to undock and separate the two spacecraft. $\endgroup$ Jan 7, 2022 at 7:44
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    $\begingroup$ @GremlinWrangger: The requirements from a docking arrangement, as stated by you are absolutely correct. I still feel all these requirements could have been met with by far simpler (may not be as simple as that of the umbrella latch) arrangements. Anyway, they designed it the way, they felt it appropriate. However many times, on many issues, I felt the original designers tended to 'OVER-DO" things, to make everything "Failsafe", by imagining the un-imaginable. Thanks for your response though. $\endgroup$
    – Niranjan
    Jan 7, 2022 at 16:11
  • $\begingroup$ @Niranjan would take it as a given that by today's standards it is over-engineered, we have 70 years more experience, finite element analysis and CNC. In particular the Apollo docking adapter was designed to handle much higher loads during docking than it ever saw - they assumed astrounauts would be worse at docking than they turned out to be, and that they might have an injured crew member trying to dock. Current docking systems generally assume if someone is hurt you just go home. $\endgroup$ Jan 8, 2022 at 3:45

The Soviets designed a super-simple docking mechanism for their failed lunar program.

enter image description here

Of course, the price was that you had to EVA-transfer from craft to craft.

It was never used.

Source: https://space.stackexchange.com/a/37914/6944

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    $\begingroup$ Guessing there would also be limits on Apollo style maneuvering while docked if you had coupled too far off center $\endgroup$ Jan 7, 2022 at 1:44
  • $\begingroup$ What is EVA in EVA-transfer ? $\endgroup$ Jan 7, 2022 at 7:11
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    $\begingroup$ @Itération122442: EVA = Extra-Vehicular Activity, aka "spacewalk". IOW: the Kontakt docking system purely provides mechanical coupling of the two spacecraft, but no hatch – the only way to get from one spacecraft to the other is to go outside. $\endgroup$ Jan 7, 2022 at 7:42


A standard umbrella requires a very complicated arm and hand mechanism to close it. This will not work in space.

In addition, an umbrella is not airtight, does not allow crewmembers to pass through it, and does not align and hold together two vehicles with a mass of many tens of tons.


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