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What procedures are followed during opening of the hatches of both ISS and Crew Dragon, and why does it take so long?

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    $\begingroup$ I was actually more puzzled by how long it took to get the hatch open following the launch abort on Wednesday. I kept thinking to myself, "Didn't they learn anything from Apollo 1?" I understand that they were being careful not to damage anything prior to the next launch window on Saturday, but still ... Were they doing some kind of testing or verification of the hardware or procedures? $\endgroup$ – Dave Tweed Jun 1 at 14:52
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    $\begingroup$ @DaveTweed You just pull the handle and "you're outa there" :-). But what we saw was (presumably) the time taken to "safe" the launcher by draining fuel. During that period the capsule stays sealed allowing for instant pad-abort if something goes wrong enough. See || " ... if the launch gets called off after the rocket is fueled, the normal process is to keep the astronauts in the capsule until the fuel is drained. Then they can come down the tower the same way they went up. But ..." $\endgroup$ – Russell McMahon Jun 2 at 2:35
  • $\begingroup$ @RussellMcMahon: I understand all that. The support crew didn't even arrive at the tower until the vehicle had been drained. But it still took them something like 20 minutes to get the hatch open after getting out on the gantry. Why was that? $\endgroup$ – Dave Tweed Jun 2 at 4:05
  • $\begingroup$ @DaveTweed according to the commentary I was listening to, they were doing air tests to ensure there was no propellant that had escaped, and they were forming an air-tight seal around the hatch to prevent any Foreign Object Debris entering the capsule. $\endgroup$ – crazyloonybin Jun 2 at 13:00
  • $\begingroup$ @crazyloonybin: Yes, I watched those things, neither of which took a significant fraction of the 20 minutes. They seemed to be spending a lot of time with the latch mechanism itself for some reason. The video wasn't very clear, but one of the technicians seemed to be doing something with a tool that looked like some kind of wrench. Did something get stuck? $\endgroup$ – Dave Tweed Jun 2 at 14:05
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As I understand the livestream, the most important thing was pressure equalisation and subsequent leak checking.

I guess this could be done faster, but it's just not worth taking any risks on it. Imagine there's some problem with the docking securing. As long as there's no pressure between the hatches, this wouldn't be seen (no mechanical load). Now as you add pressure, this works to... well, press both spacecraft apart. Maybe even the faulty joint would briefly withstand this, and at first only leak some air. But if it then breaks after the hatches are open you have a huge disaster. If it breaks with the hatches still closed... well, you'll probably have a mission abort and the capsule has to return to Earth. Annoying but not catastrophic.

Anyway the crews generally have lots of stuff to do at that point, like getting the audio connections up running, ...cleaning the toilet...

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    $\begingroup$ As on a sailing ship, you work at "sea speed", not "land speed". $\endgroup$ – David Tonhofer Jun 1 at 12:40
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    $\begingroup$ Pressure also has to do with heat settling. Since there is a vacuum, things that heat up, like the outside of the capsule, can only get rid of it through radiating it. When you put air next to it when pressurising the airlock, the heat from the capsule will heat up the air between the airlocks. You can only accurately measure if you are losing air when the temperature stays constant, and this takes time. $\endgroup$ – Sumurai8 Jun 1 at 12:41
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    $\begingroup$ @Sumurai8 if you measure the temperature, the mass of air can be easily calculated from that and the pressure. $\endgroup$ – Organic Marble Jun 1 at 13:07
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    $\begingroup$ @OrganicMarble but if the temperature doesn't stay constant this means you have a temperature gradient, so in order to "measure the temperature" you have to make a large amount of measurements to take care of the temperature gradient and then also repeat these in order to take care of temperature change, and still end up with way worse accuracy. $\endgroup$ – Jens Schauder Jun 1 at 15:33
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    $\begingroup$ This assumes that the whole volume has the same temperature, which isn't the case, otherwise the temperature wouldn't change. And the temperature differences can be huge. A quick search says it might well be 250Kelvin difference between the sunny and the shady side. $\endgroup$ – Jens Schauder Jun 1 at 15:45
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The main concerns, in the order they occur (but in increasing order of slowness of waiting)

  • pressurization and leak checking, then
  • temperature differences (from gasses changing pressure as it's filled in) leading to possible additional leaks.

Then one side is opened and:

  • dangerous gases air out - in zero-g they don't mix very quickly, and tend to accumulate in pockets, such as carbon dioxide, carbon monoxide, or contaminants such as HCl (hand-held gas detectors are used).

For the Demo 2 docking yesterday, the Node-2 hatch was opened hours in advance, to allow it to air out (but this wait was a big delay in the Demo 1 docking); but the docking hatch cant be opened until docking is sealed, on the ISS side was opened and aired out for, I believe it was 1 hour, before the dragon hatch was opened and people moved through it.

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