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My child is learning about the human body and frequently wants to know whether his favorite things work the same way. He asked me if spaceships fart (we've talked about Firetrucks and tailpipes, to his hysterical amusement). I was going to say that spaceships do not, but I'm not actually qualified. I need some expert sources.

When the pressure inside gets too high, do spaceships release the excess air? If they have waste gases that aren't useful or breathable do they vent those into the vacuum?

We all gotta do it sometimes. I mean it's only natural right?

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    $\begingroup$ Before the spaceship would burst it is better to relase some gas to keep the pressure within limits. But even needed gases are released from an airlock before an EVA. $\endgroup$ – Uwe Mar 7 '18 at 15:13
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    $\begingroup$ Astronauts do as well, quite an annoyance on the early Apollo missions $\endgroup$ – PlasmaHH Mar 7 '18 at 16:00
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    $\begingroup$ Besides dumping unneeded gases consider what a truster really is. They often expel hot gas in order to provide trust. In other words you could tell your child that space ships "fart" in order to move around. Could also be a good moment to briefly introduce Newton's laws of motion if he asks why this is effective. $\endgroup$ – Thijser Mar 7 '18 at 16:26
  • $\begingroup$ Even regardless of internal pressure constraints, all self-propelled vessels must expel some kind of matter, usually gaseous, because of conservation of momentum. Otherwise, you're not going anywhere! $\endgroup$ – Kevin Mar 7 '18 at 23:37
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    $\begingroup$ In addition to specific venting events, many everyday materials slowly "outgas" in vacuum. Those chosen for spacecraft usage may often be those that do this minimally, but it is an issue considered in the design. $\endgroup$ – Chris Stratton Mar 8 '18 at 5:56
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You bet. Not only gases, but astronaut pee as well! Which could result in spectacular light shows.

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This happens for several reasons:

  1. Spacecraft and the tanks inside them are built to be as light as possible, which means no stronger than necessary. If the pressure inside them gets too high, it must be released to keep the hull from rupturing. The device that does this is called a "positive pressure relief valve".
  2. There is a finite amount of storage available for waste. If the storage gets too full, it must be dumped into space or sent back to Earth if it is not recycled. The space station (ISS) does a good job on recycling waste water. The space shuttle didn't recycle waste water (urine) and it was dumped into space when the storage started getting full.
  3. Some gases are hazardous to the crew. The ISS vents hydrogen overboard to keep the explosive gas from accumulating.
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Section 30.4 of this NASA document describes passivation of spacecraft at end of life. The objective is to remove all sources of stored energy including pressurized gases and the way to do it is to vent them to space.

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There was a proposal by Mars One for a mission to Mars. It suggested using hydroponics to produce oxygen.

But a MIT study reviewed it and showed that this model could not work, because the plants actually produced too much oxygen. Thus they would need to vent it, and that would take with it nitrogen. Which if they did not keep sufficiently in the atmosphere of the vessel the slightest spark could start a fire, ala Apollo 1 capsule fire.

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  • $\begingroup$ That does not seem to make much sense, plants do not produce oxygen out of nothing, but from the water and then consume the co2 out of the air. No co2, no O2. $\endgroup$ – PlasmaHH Mar 7 '18 at 16:07
  • $\begingroup$ Right, but they had too much oxygen, and limited nitrogen. So they would have to reduce the oxygen content. (And I guess their plan had been to vent it, and easier to vent air (not seperated O2) which would take Nitrogen with it). $\endgroup$ – geoffc Mar 7 '18 at 16:10
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    $\begingroup$ @Uwe Agreed, but I think the point was that the Mars One plan did not include them, which was what the study was considering. $\endgroup$ – geoffc Mar 7 '18 at 17:48
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    $\begingroup$ The hydroponics should be able to produce more oxygen than the minimal demand of the astronauts. Human demand of oxygen depends very much on activity level. But it should be possible to control the amount of oxygen produced by the hydroponics by varying the intensity and daily duration of ilumination. Without light, no oxygen production is possible. But plants consume carbon dioxide ehaled by the humans to generate oxygen. Thus the amount of oxygen depends on the amount of carbon dioxide available. $\endgroup$ – Uwe Mar 7 '18 at 18:07
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    $\begingroup$ Heinlein solved that in the 40s. Burn plant clippings to consume the excess oxygen. I think the novel was Space Cadet. $\endgroup$ – EvilTeach Mar 8 '18 at 15:38
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The materials that spacecraft are made of can release gas into space. It's called outgassing, and is a problem that crops up from time to time with satellites and spacecraft.

Source : https://en.wikipedia.org/wiki/Outgassing

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I was looking for the quote from Chris Hadfield, but I can't seem to find it - but I believe that when he went blind in space, NASA had him vent some of the gasses in his suit. Not exactly a spaceship, but not so different. And of course, to cool their space suits they vent water to space.

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When the pressure inside gets too high, do spaceships release the excess air?

Typically no, because astronauts need all the air they can get. Thus, cabin air pressure is regulated and CO2 is chemically absorbed by (in the Space Shuttle) metal oxides. Moisture of exhalation is also absorbed. As those gasses are removed, more oxygen is added. (There's always a backup, though...)

OTOH, as the fuel and oxidizer warm up, it must be vented out of those tanks, both on the ground and in space.

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