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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?

Fred
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Demi
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2 Answers2

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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.

Loren Pechtel
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    There would be adiabatic cooling during the venting, but that might not be enough – Demi Sep 22 '16 at 02:27
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    @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. – Pere Sep 23 '16 at 18:40
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    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) – Innovine Sep 23 '16 at 19:50
  • @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. – Loren Pechtel Sep 23 '16 at 23:35
  • 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. – Vikki Oct 03 '19 at 01:36
  • What about adding nitrogen or argon to raise the total pressure of air in the space, thus increasing the amount of energy a fire would spend heating the added inert gas? Long-term exposure to an elevated pressure of either would be harmful, but in the short term someone could breathe about as easily with a 10% oxygen atmosphere at twice standard pressure as a 20% oxygen mix at standard pressure, but fire would burn less readily. – supercat Oct 18 '21 at 17:45
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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.

Andreas
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    Sounds like Halon flood systems are the best option then – Demi Sep 20 '16 at 16:12
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    @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. – Loren Pechtel Sep 22 '16 at 02:15
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    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!" – Organic Marble Sep 22 '16 at 03:15
  • @Pere Imagine how that would chill down the crew! First depressurize below vapor pressure, then expand high pressure nitrogen into the compartment, which will again be very cold. I would not want to be in there. – Andreas Sep 23 '16 at 18:54
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    @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. – Pere Sep 23 '16 at 19:01
  • @Pere You made me think on this again, I added some thoughts. Upvoted as helpful comment. – Andreas Sep 23 '16 at 19:19
  • How are you supposed to kill a fire with Halon in a totally sealed, pressurized environment? Won't you need to evacuate the air to get the Halon in, or do you think Halon works if you just raise the halon partial pressure? There are also distribution concerns with Halon, since on earth it relies on being heavier than air, and so sinks to the floor and fills the room up, drowning the fire. Not so in zero-g, it'll just swirl around and follow convection currents. I think Halon is probably a pretty poor fire extinguisher in zero-g. – Innovine Sep 24 '16 at 08:13
  • Regarding your edit, there are oxygen masks that provide positive pressure to lungs and allow breathing in environments where ambient pressure is too low to breathe. I don't know for sure that these work in vacuum, but they work at 50-60,000 feet so maybe shouldn't be discounted without more research. – nexus_2006 Sep 25 '16 at 15:29
  • @nexus_2006 My concern was more about the hemoglobin losing its ability to bind oxygen, see for example this answer in BIO.SE. This answer suggests that the limit may be around 121mbar when breathing pure oxygen. I was not able to find information on what level of differential pressure to ambient a human lung will support without taking damage. – Andreas Sep 25 '16 at 17:36
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    @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. – Vikki Oct 03 '19 at 01:38