Questions about atmosphere maintenance | Life Support Workgroup | Forum

I'm trying to clarify my understanding of what needs to be onboard the CM to facilitate 24 hours of breathable atmosphere for the astronaut, I'm going to run through how I think it works here, if anybody can point out anything I've missed, I'd appreciate.

So, first off, pressurising the CM with 100% oxygen is bad – fires will start more easily, spread more quickly and burn more hotly, as learned from Apollo 1.  So we pressurise the CM with something like 20% oxygen 80% nitrogen, which is roughly the composition of regular old Earth air.

So at the very beginning of the mission, the cabin contains 80% nitrogen, 20% oxygen.

With each breath the astronaut takes, their lungs fill with 80% nitrogen, 20% oxygen.  The nitrogen doesn't interact with the body at all, it just gets breathed straight back out, right?  But some of the oxygen is absorbed through the lungs and also some carbon dioxide is expelled.  So, as the mission progresses, the nitrogen level of cabin remains entirely constant.  Oxygen levels slowly go down and at the same time carbon dioxide levels slowly go up.  Is this accurate?

If this is how it works, then, all we need to do is inject fresh O2 to replace what has been used and remove CO2 before it builds up too much.

Until we do an EVA, where we vent the entire cabin.  After EVA, in order to repressurise, we need another 80% of the cabin volume of nitrogen and 20% of that volume of oxygen.

So, in the cylinders behind the pressure vessel we need to store nitrogen and oxygen, and in order to know how much we have to rate the cabin for not only a number of hours of occupancy but also a number of EVAs.  24 hours seems like a good occupancy rating, what about EVAs?  Just the one?

Even if we go with just the one EVA, I suspect we'll end up having to store a lot more nitrogen than oxygen.  There are 3 tanks behind the pressure vessel in the concept diagrams, although of course that is merely conceptual.  Maybe one oxygen tank in the middle and two nitrogen tanks either side would let us store what we need and maintain symmetry?  Or does it make more sense to store tanks of our actual 80/20 mix to use for repressurising after EVA, and also a tank of straight O2 for injection?