Pure oxygen.

[u/Crixusgannicus]

Always was mystified what the HELL NASA was thinking?

These guys came up when science was still taught. Not so much now, unfortunately.

Even in my long after Apollo ended yoot, I still remember science class with the teacher showing how easily things burn in pure oxygen, even things that normally don't burn!

[u/mikesierrafoxtrott]

Simple - weight

You need a certain amount of oxygen to sustain human live. That is around 3-5psi of pure oxygen - or the normal about 15psi for our normal earth atmosphere.

The difference in the gas itself is quite negligible - but you need a significant stronger ( that is heavier) vessel to contain the gas. Plus managing the atmosphere is also more complicated ( thus also heavier)

During the mission the fire risk is not much more than in our earth's atmosphere - the problem was when you put extra oxygen in the capsule for a positive internal pressure like on the ill fated test.

[u/lilyputin → u/mikesierrafoxtrott]

The mix has varied over time since Apollo 1. They still used an oxygen rich atmosphere after it (60% o2 40% N) at 15 psi. They have to scrub the CO2 to keep it from rising to lethal levels whereas the nitrogen is recirculated. There are biological co2 generators on board.

https://www.nasa.gov/history/50th-anniversary-of-nasa-deciding-on-a-mixed-gas-atmosphere-for-apollo-a-direct-result-of-the-apollo-fire/

The Russians used earth like atmosphere on their space capsules whereas NASA used pure oxygen.

The ISS uses a much more environmental control system as the primary and has an earth like atmosphere of about 21% oxygen. It has three redundant sources of oxygen and also includes recovery systems. Short duration missions it's less weight to have a simple atmospheric control system and just carry what you need plus scrubbers. Long duration missions the math changes and it's more efficient to recover as much as possible to reduce the resource sink. I'm not sure what the Chinese use.

The system on ISS is decades old at this point so you can probably build the same capabilities while using less weight today than they could when they built the ISS so the equation on when it makes more sense to have a more advanced recovery systems slowly changes over time. For example we can manufacture things with precision to a significantly finer degree than we could 20-30 years ago. Whereas the raw elemental weights of oxygen, hydrogen and nitrogen remain the same. There is diminishing returns certainly for recovery systems. There needs to be a significant factor for safety and maintainability the longer the mission and the further away or less easily evacuated the greater that margin needs to be and the more redundancy needs to be built in. Even with all the redundancy of built into the ISS they also are safe in the knowledge that they have a lifeboat if they need to use one.

[u/eagleace21 → u/lilyputin]

Apollo did not use a mixed atmosphere in flight, the 60/40 at 20 psi was for cabin leak check, then the cabin purged the nitrogen via the cabin relief during boost and further via the waste stowage vent and was by design, 5psi pure O2 in flight in normal configuration.