Do they have a system that pumps the air out or does it just let it out?


1 Answer 1


I haven't found a reference giving the number, but we can make a back of the envelope approximation.

Page 49 of The International Space Station - Operating an Outpost in the New Frontier reminds us that the crewlock on the ISS is the same volume as the Shuttle Orbiter airlock.

From work experience as a Shuttle Systems instructor, I know

  • Volume of empty airlock ~ 185 cubic feet enter image description here

  • Volume of suited crewmember ~ 10 cubic feet enter image description here

  • So the air volume in airlock with 2 suited crewmembers present ~ 165 cubic feet

(If you are wondering about the 228 cubic feet in the table vs the 185 cubic feet in the diagram, the difference is the 43 cubic feet of the tunnel extension, which the ISS crewlock doesn't have)

enter image description here

Assuming normal conditions of 1 atm and 72 deg F the ideal gas law gives us ~ 12 lbm or 5.4 kg of air in the airlock at the start of the depress.

This answer How Is The Quest Joint Airlock Operated? tells us that the ISS airlock depress pump scavenges ~ 10 psi of the air. So ~ (5/15) * 5.4 kg is lost or in round numbers, 2 kg.

(Source of images: SMS Systems Console Handbook)

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    $\begingroup$ It is surprising and lamentable that official spaceflight documents use cubic feet. $\endgroup$ Commented Nov 21, 2022 at 12:48
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    $\begingroup$ @Peter-ReinstateMonica the shuttle was designed in the 1970s. The only space program to land people on the Moon to date used English units. I'm kind of tired of people complaining about it. Use some historical perspective. At this point it's just units-virtue-signaling, we get it. $\endgroup$ Commented Nov 21, 2022 at 12:51
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    $\begingroup$ Point taken ;-). $\endgroup$ Commented Nov 21, 2022 at 12:53
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    $\begingroup$ @SaturnV the scavenge pump is turned back on for the depress from 5 psi for some more time, I didn't account for that in this estimation. Among other things. If you found a real source, that's much better than this. I encourage you to write an answer and reference it. $\endgroup$ Commented Nov 21, 2022 at 13:03
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    $\begingroup$ I didn't 'work on' the systems but I helped train the crew and flight controllers on the shuttle ECLSS. The flight controllers monitored the systems and advised the crew on how to configure them. The techs at KSC turned the wrenches on the systems in the hangar. Everything was designed and built before my time. $\endgroup$ Commented Nov 21, 2022 at 13:15

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