When watching the Vintage Space video Apollo’s Flammable Oxygen Atmosphere Explained, I started to think: was there ever any spacecraft proposed with nitrogen-oxygen atmosphere, but without a system for nitrogen replenishment?

Nitrogen is not used by humans for breathing, so, theoretically, the content of nitrogen in the cabin should remain constant, unless there is a leak or you need to depressurize the spacecraft (i.e. for EVA).

Thus, it seems that such atmosphere would combine reduced flammability with the engineering simplicity of oxygen-only life-support system, at cost of having a (big?) problem in case of depressurization event.
Has anyone ever seriously proposed such solution for any manned spacecraft?

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    $\begingroup$ there are always leaks $\endgroup$ – JCRM Feb 22 at 20:56
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    $\begingroup$ @JCRM Yes, but the spaceflights are usually not infinite in duration - thus I imagine that a few-day flight could be possible if leaks would stay within tolerance limits $\endgroup$ – Ijon Feb 22 at 21:09
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    $\begingroup$ Apollo was pure O2, Skylab and shuttle were both O2/N2. Both Skylab and shuttle had N2 tanks for atmosphere makeup. $\endgroup$ – Organic Marble Feb 22 at 21:29
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    $\begingroup$ @OrganicMarble Yes, I am pretty sure that no spacecraft with life-support system as described by me was ever flown (reasonably so, it seems quite risky for me) - but I wanted to know, if anyone has considered this idea, especially in times when safety standards were lower than today $\endgroup$ – Ijon Feb 22 at 21:39
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    $\begingroup$ @OrganicMarble in reference to your comment I've just asked How much nitrogen did Skylab and the Space Shuttle carry for atmosphere makeup? $\endgroup$ – uhoh Feb 23 at 4:40

I'm going to say "no".

Even the original request for proposal (scanned archive here) for the Apollo spacecraft, which at that time specified an oxygen-nitrogen mixture for the cabin atmosphere, stated that:

Oxygen and nitrogen are stored supercritically. The gas supply is sized for two complete cabin repressurizations in addition to leakage.

So even then they included allowances for nitrogen replenishment above and beyond what leakage would make necessary. They later changed to a pure oxygen system due to the greatly-increased simplicity of having to only manage a single-gas environment (simple pressure sensor) rather than a dual-gas one (custom sensor system).

I wasn't able to find any primary references for this, but as far as non-U.S. missions go, the Soyuz spacecrafts also had/have mixed-gas systems and presumably included storage containers to replenish cabin supply.

Now, it's very hard to prove there does not exist a case where a reserves-free nitrogen system was ever proposed, at least not without examining every proposal for every spacecraft design. However, if the first designs for dual-gas spacecraft included these reserves, and (as you mentioned in a comment) if safety standards were strictly lower for earlier programs, it is reasonable to say no such design proposal has ever been documented.

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