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Suppose there is a fire inside a large spacecraft, and there are crewmembers trapped inside. Is it possible to extinguish the fire by venting and then repressurizing the cabin, without killing the people?

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  • $\begingroup$ Possibly as many as five could survive given enough time to put on spacesuits first. $\endgroup$
    – called2voyage ♦
    Sep 20, 2016 at 13:37
  • $\begingroup$ For information, the infamous MIR fire was extinguished using foam and water. Link here. Also this page seems to implies they use either foam or carbon dioxide on the ISS. $\endgroup$
    – Andy
    Sep 21, 2016 at 18:06
  • $\begingroup$ Repressuring the cabin would be so slow that the people can't survive that without a space suit. $\endgroup$
    – Uwe
    Jan 11, 2017 at 15:45

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Depressurization causes little if any harm so long as it's neither too fast (if the air in your lungs exits too fast it will do severe damage), nor too long (due to a lack of oxygen.) So long as the space involved isn't too big these requirements can be met.

However, if it's a solid material that is burning (and how many flammable liquids or gases would be on a space station anyway?) you would be facing the definite possibility that the fire would simply reignite when you put the air back in the room. In vacuum there would be little cooling of the hot materials, they very well might remain above ignition temperature.

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    $\begingroup$ There would be adiabatic cooling during the venting, but that might not be enough $\endgroup$
    – Demi
    Sep 22, 2016 at 2:27
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    $\begingroup$ @Demi: I'm afraid if the decompression is slow enough not to cause damage to people, it won't be quick enough to produce a noticeable effect by adiabatic cooling. Anyway, I agree with you that it doesn't seem to be enough, even if venting were fast. $\endgroup$
    – Pere
    Sep 23, 2016 at 18:40
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    $\begingroup$ what sort of decompression speed are we talking about here? Jim LeBlanc's suit pressure dropped from 3.8psi to 0.1psi in 10 seconds and he was fine (well, no serious damage to his lungs anyway) $\endgroup$
    – Innovine
    Sep 23, 2016 at 19:50
  • $\begingroup$ @Innovine It's been way too long since I've seen it, I don't remember. What I was reading was able to express it in terms of the hole size vs compartment size. $\endgroup$
    – Loren Pechtel
    Sep 23, 2016 at 23:35
  • $\begingroup$ Judging from the list of examples in the Wikipedia article on uncontrolled decompression, many people seem to be capable of withstanding a fall of over 10 psi in a fraction of a second without permanent damage. $\endgroup$
    – Vikki
    Oct 3, 2019 at 1:36
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Scientific American author Anna Goslin reported 2008 on NASA experiences with depressurization in animal tests and also in accidents involving humans. According to this article , surviving a near-vacuum for a short time is possible.

I doubt however that this would be a viable approach to extinguish an onboard fire. You would need to vent quickly, then repressurize with nitrogen or any other non-oxidizing gas that may be available. The crew would need to use oxygen masks like in aircraft. Venting and repressurizing a compartment of reasonable size within 60 seconds might be too challanging.

EDIT after additional thinking: Oxygen masks would probably not prevent the crew from going unconscious, as the gas-exchange in their lungs will not work at all at low ambient pressure. The emergency venting valves would need to be designed to not create too much thrust. You must prevent that they suck in any objects and get stuck. The chilling effect of adiabatic expansion of the inert gas would be enormous.

2nd EDIT: Read this Aviation Week article to get an impression of gas exchange problems under low pressure conditions.

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    $\begingroup$ Sounds like Halon flood systems are the best option then $\endgroup$
    – Demi
    Sep 20, 2016 at 16:12
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    $\begingroup$ @Demi Halon flood will asphyxiate anyone in it. That's why there are big Halon Abort buttons in Halon-protected rooms--if you're in there you're better off with the fire than with the Halon. $\endgroup$
    – Loren Pechtel
    Sep 22, 2016 at 2:15
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    $\begingroup$ When I worked in a government simulator facility the first thing they showed me was the Halon release kill switch. "If the fire alarm goes off, don't run outside, run for the kill switch!" $\endgroup$
    – Organic Marble
    Sep 22, 2016 at 3:15
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    $\begingroup$ @Andreas: I don't say this is a good method to extinguish a fire. I just say your answer improves it by addressing one of the concerns expressed in Loren Petchel's answer because you prevent the fire to reignite. That's why I upvoted it. $\endgroup$
    – Pere
    Sep 23, 2016 at 19:01
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    $\begingroup$ @LorenPechtel: Halon kills fire by disrupting the chemical reactions of the combustion process, not by displacing oxygen, allowing it to kill a fire without also killing the crew; the space shuttle's fire extinguishers used halon, for exactly this reason, as do fire extinguishers on commercial airliners. $\endgroup$
    – Vikki
    Oct 3, 2019 at 1:38

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