What are the reasons for deciding to use a 100% $ O_2 $ cabin atmosphere in the early US space programme?
Was danger of nitrogen narcossis a factor?
Did all missions prior to Apollo 1 use a 100% $ O_2 $ atmosphere?
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Despite a higher risk of a fire, pure oxygen also has some advantages.
First, the internal pressure of the vessel is only a fifth of a normal breathing mix, allowing less structural load on the hull of the spacecraft. The resupply system is also simplified, because a system including nitrogen must have an extra tank for the nitrogen. (If you had them mixed, you end up with a higher and higher nitrogen pressure over time). A small mass saving is therefore achieved. For a minimal spacecraft where you simply open the hatch and vent the cabin air when performing an EVA, pure oxygen simply means less air wasted. Nitrogen narcosis seems to not be an issue, as I find it difficult to imagine an accident of increase in the pressure.
As for the decision making process in the early US space programme, the slightly higher complexity of a nitrogen system must have felt a little redundant. The early US Mercury and Gemini also used pure oxygen, but the early Soviet spacecraft, like Vostok, used a normal atmospheric breathing mix. Note that modern EVA suits do still use pure oxygen.
It is very difficult to make a soft or hybrid space suit manouverable with higher pressure (over 5 or 6 psi for example), because they are effectively just big balloons which just get stiffer and stiffer with more pressure. With such a low pressures, using pure oxygen is mandatory to get a human breathable partial pressure for oxygen. For this reason, all western space suits use pure oxygen at lower pressure.
Using higher pressures for cabin atmospheres requires adding nitrogen in to the mixture to prevent oxygen toxicity from higher oxygen partial pressure. This means that decompression sickness (the bends), which results from nitrogen inside the bloodstream being released as bubbles inside the body when moving to a lower pressure, is a real problem and requires hours of preparation to avoid it for an EVA.
Using pure oxygen also for the cabin atmosphere is an optimal choice with almost no downsides - except for the fire hazard. Fires burn hotter based on the proportion of gas that is oxygen, not based on partial pressure, so 100% oxygen atmosphere is very flammable. Unfortunately the fire hazard is so great that it is the limiting factor why 100% oxygen atmospheres are no longer used in cabins.