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I was watching the docking process of Dragon to the ISS including both the "soft dock" process, which was then followed by the hard dock involving 2 sets of 6 locking mechanisms.

I believe I'm correct in saying that the commentary said that they would confirm an airtight seal had been created.

What would happen if this seal wasn't created? Would they release and re-attempt at the same port, or move to the secondary port?

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    $\begingroup$ I wouldn't be astonished if both of your suggestions are in various branches of that massive procedure document we heard them calling out during the time from dock to transfer into the ISS. $\endgroup$ Jun 1 '20 at 11:21
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I don't have anything Dragon-specific, but a general rundown would be something like this:

  1. Remain hard-docked initially while some basic troubleshooting is performed. If there is a leak, it has to come from somewhere -- try to find out where. Start with the docking mechanism itself:

    • Review the high resolution approach imagery of the visiting vehicle -- visually inspect all the seals for defects.

    • Review the SSRMS imagery of the IDA sealing surface that was taken in the few days before the launch. If any defects are found (debris, MMOD craters, etc.), determine if any of those defects coincide with the seal locations. Note that seals on the visiting vehicle are redundant.

    • Review all telemetry signatures coming from the docking mechanism to see if any hooks may have had trouble closing.

  2. If there are no indications that the docking mechanism or its sealing surfaces are the source of the trouble, determine if the unpressurized portion of the PMA/IDA combo itself is leaking. This could come from one of two sources:

    • A leak in the unpressurized portion of the PMA or the IDA tunnel, due, say, to an MMOD strike -- this can be investigated with robotic and crew imagery of the hardware to look for an entry hole.

    • A leak in the seal between the IDA and the PMA. This would be harder to track down. It's unlikely that this would be the case, since these hardware items were inspected immediately prior to installation and they have never been separated since. That said, it's impossible to actually pressure-test that combination without a vehicle there, so if it does leak, you won't know until you dock something there.

  3. (My informed speculation) If the leak rate is low enough as indicated by telemetry, it may be possible to open the hatch on the ISS side of the vestibule, which would allow crew access to the interior. Crew could then use the handheld ultrasonic leak detector to find where it's coming from.

  4. If the leak rate is too high to allow interior inspection, I would expect the call would be made to undock and move to the other docking port. I would imagine the crew on the visiting vehicle would be called upon to take copious amounts of hi-res imagery of the docking port they are leaving.

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    $\begingroup$ Old school: Shuttle / ISS Post Docking Leak Check procedure 2.101 and subs nasa.gov/centers/johnson/pdf/… $\endgroup$ Jun 3 '20 at 17:36
  • $\begingroup$ "Review the high resolution approach imagery of the visiting vehicle -- visually inspect all the seals for defects." I don't believe high resolution images of the whole docking port allow finding of seal defects smaller than 1 mm. Only very large defects of several mm size may be found this way. $\endgroup$
    – Uwe
    Jun 3 '20 at 20:00
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    $\begingroup$ @Uwe I don't have public data handy to prove it, but what you said is incorrect. They take photos for exactly this purpose, and I've been involved in projects to simplify the process of examining these images to compare them with historical imagery. Also -- the seals are large enough that sub-millimeter defects are unlikely to impair sealing ability. $\endgroup$
    – Tristan
    Jun 4 '20 at 16:41

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