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Cargo ships to the ISS have a pressurized part, directly connected to the space station.

  • The ATV had 48 m3
  • The Cygnus have 27m3
  • The HTV have 14 m3
  • The Dragon have 10m3
  • The Progress have 7.6m3 (?)

Is this air recovered before the craft leave the station ? What about the Dragon, which has a return capacity ?

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    $\begingroup$ I don't have time tonight to look up the references, but no, the air isn't recovered. There is no mechanism to pump the air from the visiting vehicles into the ISS. $\endgroup$
    – Organic Marble
    Apr 13, 2018 at 2:21
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    $\begingroup$ At least for the dragon I'm sure it's no. It's mission capability requires it to be pressurised on return as it's likely that being depressurized would destroy return samples. $\endgroup$
    – Dragongeek
    Apr 13, 2018 at 9:04

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This is not done, but I couldn't find a definite reference for you. NASA has chosen not to publish recent ISS checklists. It's also hard to prove a negative, but for what it's worth, here is a fairly detailed sequence of events concerning a Cygnus departure from here.

Approaching the end of its stay at ISS, the S.S. Gene Cernan was buttoned up by the crew – going through the usual procedure of closing out the interior of the spacecraft before closing its hatch, then outfitting the vestibule between ISS and Cygnus with bolt drive assemblies and removing power and data connectors before closing the Unity module hatch to permit the standard leak check to be performed. Sixteen bolts were released to free the Cygnus spacecraft from Unity and transfer control over to Canadarm2 that had grappled the spacecraft on November 30.

Incidentally, some of the air is scavenged from the ISS airlock when it's depressurized for a spacewalk. The EVA OPS Checklist procedures 1.215, step 23 shows how a depress pump is used to pump the airlock down to 10.2 psi, sending the air back into the main body of the ISS. Further steps in the CREWLOCK DEPRESS/REPRESS CUE CARD part of the checklist show how the pump is then used to pump down the airlock to 2 psi. The final 2 psi is wasted into space.

I quote all this EVA stuff just to show that pumping down a volume into the ISS is time-consuming and requires special equipment that just doesn't exist at the CBM/hatch interface for the visiting vehicles. Furthermore, since the visiting vehicles arrive full of air, there is no loss incurred by letting them leave full of air. There, could, of course, be a gain if what you suggest was done.

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  • $\begingroup$ Cygnus isn't depressurized, otherwise how could they do the burning experiments that it has been doing after separation? $\endgroup$
    – PearsonArtPhoto ♦
    Apr 13, 2018 at 12:21
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    $\begingroup$ I'm just adding a proof that Cygnus isn't depressurized. If it was, they couldn't do the burning experiments, unless they had a separate system just to oxygenate the experiment, which seems odd, and not as useful. $\endgroup$
    – PearsonArtPhoto ♦
    Apr 13, 2018 at 12:24
  • $\begingroup$ @PearsonArtPhoto I believe the experiment was sealed? So probably bringing its own oxidizer? $\endgroup$
    – jkavalik
    Apr 13, 2018 at 12:40
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    $\begingroup$ They could’ve foregone the scavenging just for one mission:) $\endgroup$
    – Antzi
    Apr 13, 2018 at 15:21
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    $\begingroup$ I similarly lack reference, but a further expansion on why this is not done: avionics are far easier to keep functioning in a pressurized environment than they are in a vacuum environment. Vacuum environments present unique cooling and arcing/corona problems that must be addressed. By keeping even disposable vehicles pressurized through their entire mission, critical avionics can be kept in the pressurized portion, which greatly simplifies the design. $\endgroup$
    – Tristan
    Apr 13, 2018 at 16:19

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