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What was the air mix in the Apollo spacecraft?

I know there was a change from pure oxy after the One fire, but what did they replace it with?

I read that the original reason they didn't want nitrogen was because of fears that blood bubbles would form with the pressurization, depressurization, etc. of the whole craft (the LM, anyway), inflating and deflating suits, etc., causing "the bends." But, of course, as they learned, at one atmosphere of pure oxygen, Velcro literally explodes from a single spark.

So did they end up using nitrogen like in a regular air mix anyway? Someone else told me they used helium -- but if that's so, why didn't Neil Armstrong sound like Donald Duck when he screwed up his scripted line at the foot of the ladder?

What air mix was used, and were there different mixes in the CM and the LM?


What's used today in Soyuzes, the ISS, and the privates?

4 Answers

  • 8 years ago
    Favorite Answer

    Pure oxygen is only dangerous at high pressure. Earth normal sea level pressure of 14.7 psi is too high for safety of pure oxygen, but the Apollo 1 fire happened during a leak check, where the pressure in the space craft was increased above normal to check for leaks. In hindsight this seems suicidal.

    After the fire the Apollo cabin atmosphere at launch was changed to 60% oxygen and 40% nitrogen at sea-level pressure (14.7 psi or 1013 millibars). During ascent the cabin rapidly vented down to 5 psi (345 millibars), releasing approximately 2/3 of the gas originally present at launch. The vent then closed and the environmental control system maintained a nominal cabin pressure of 5 psi as the spacecraft continued into vacuum. The cabin was then very slowly purged (vented to space and simultaneously replaced with 100% oxygen), so the nitrogen concentration fell asymptotically to zero over the next day. Although the new cabin launch atmosphere was significantly safer than 100% oxygen, it still contained almost three times the amount of oxygen present in ordinary sea level air (20.9% oxygen). This was necessary to ensure a sufficient partial pressure of oxygen when the astronauts removed their helmets after reaching orbit. (60% of 5 psi is 3 psi, compared to 20.9% of 14.7 psi, or 3.07 psi in sea-level air.)

    This all has to do with the concept of partial pressure. Natural Earth air at sea level has a pressure of 14.7 psi and is composed of 21% oxygen, 78% nitrogen, and 1% Argon and other gasses. This means that 21% of the pressure comes from oxygen and 79% of the pressure comes from the other gasses (mostly nitrogen). 21% of 14.7 psi is 3 psi. This means that pure oxygen at 3psi is just as safe as regular air. So the ideal atmosphere for a space craft is pure oxygen at 3psi. The problem with that is how to change from oxygen/nitrogen at 14.7 to pure oxygen at 3 in the 10 minutes it takes to launch. The above described design was a compromise.

    The Soyuz, Shuttle, and all new vehicles are 14.7 psi and 21% oxygen. The down side is that this pressure makes space suits too stiff, so they still use 100% oxygen at 3 psi to go outside in a suit, meaning they need to decompress slowly, like a diver coming up from a dive while breathing regular air at high pressure.

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  • 8 years ago

    Hard to believe anyone was dumb enough to use pure oxygen. You sure about that? That would get someone very high very fast. Remember some years ago when trendy bars in New York were called Oxygen Bars because they upped the oxygen inside? Seems like kind of a safety hazard.

    Fascinating question though!

    EDIT: Thanks Campbelp... for a great answer!

    But about Dr Yes's "fart factor": I recall in the 60s American TV talked about eating food out of tubes. I'm not sure what the astronauts ate but all those pressure changes and strange food must have involved a huge fart factor. Not to mention the stress and sweating poisons into the air. I'll bet the air inside smelled pretty bad after a few days. And they didn't have toilets in space yet, did they? Diapers as I recall. Must have been nasty.

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  • Anonymous
    8 years ago

    What I hears while at space camp it is about 80% nitrogen and 20% oxygen.

    So an astronaut going on a space walk needs to decompress in the airlock first.

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  • I want to know what percentage of the air on the space station is fart.

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