We didn't go with that here in part because I don't have a publicly available document confirming/explaining that.
There's this paper, which explains something of the general concept of an intermediate pressure "exploration atmosphere", but confirmation of its use or specific implementation details don't seem to be in any of the publicly released RFQ documents I'd been able to get ahold of. We'll have to see.
The abstract suggests 100 percent of the thinking is about lower pressure nitrogen-oxygen mixes transitioning to pure low pressure oxygen.
What ever became of the idea of using an alternative neutral gas to replace nitrogen? Practically speaking the only substances I can think of that would qualify as biologically "neutral" would be noble gases. And of them, helium tends to lead the pack by far on almost every practical metric, mainly because helium is produced by radioactive decay and so accumulates in some natural gas deposits, and all the others (except radon, obviously not a candidate!) are overwhelmingly legacy traces from Earth's formation and found pretty much exclusively in the atmosphere, so obtaining argon or neon can only be obtained by fractional liquefaction of air, which makes them much more expensive than helium which itself is quite pricey. The only advantage of a heavier noble gas would be getting one with atomic mass close to 28 so mass density, sonic speeds and voice timbre are about the same as in natural sea level air. Oh, and it would be less leaky than helium though more so than nitrogen.
Clearly if we have an 80 percent helium or argon atmosphere, those noble gases would dissolve in blood and tissues just as nitrogen does, though in somewhat different concentrations I guess. And so a sudden pressure drop would run a similar risk of bubbles forming and thus decompression sickness. I believe the reason helium specifically is the go-to gas is not just a matter of price but also that precisely because one of helium's liabilities is that it is leaky, between being a small atom and one with essentially zero tendency to chemically interact, this is an asset in this case--you still give the astronauts involved the bends if you drop the pressure on them instantly, but gradual decompression can proceed faster than with nitrogen because the surplus helium quickly finds its way out of lung tissue and into the breathed mix, and the molecular concentration drops with less impediment in the blood. Perhaps argon or whatever (I haven't bothered to check the atomic masses of higher order noble gases nor do I remember which one is which in sequence) would have very little advantage over just sticking with nitrogen.
If so practically speaking I am talking about people breathing about 80 percent helium, 20 percent oxygen, more or less (the lighter molar mass of the helium might mean the optimum mix has somewhat different proportions I suppose) in molar terms at standard pressure, and higher percentages of oxygen at lower pressures. Their voices get all Mickey Mouse squeaky, food tastes weird, odors behave strangely, and the helium is gradually seeping out into space and must be renewed.
I expect that for very long term human habitation, complete substitution of nitrogen with helium would be bad for health, that actually while human biology doesn't make a lot of chemical use of atmospheric nitrogen directly, letting concentrations drop to zero and stay there will throw this and that process off over time. So a tri-gas mix would involve having quite a bit of nitrogen, only partially displaced by helium, if one proposed this for a very long term habitat.
OTOH it seems clear that people can operate for hours, indeed days and perhaps weeks or months, breathing oxy-helium with no nitrogen. I would not suggest a mixed tri-gas then.
As for why SSE and the OTL Orbiter program and thus modern OTL ISS, along with the entire Soviet/Russian crewed space program from Vostok on, rely on sea level nitrogen-oxygen mix, that seemed obvious enough once I thought through the first posts. Americans could get away with pure oxygen atmosphere low pressure capsules (usually, barring the Apollo 1 tragedy) because the craft were disposable. With Orbiter intended to return to Earth and be launched again, not by accident our launch sites are at sea level, and anyway a Shuttle might be diverted to an emergency field, some at high altitude--most near sea level. And then it gets launched from sea level. It made sense for the vehicle to be designed around SL pressures, and sustain them rather than having relative pressures fluctuate.
But to prepare for EVA, especially once we have such a spacious structure as SS Enterprise, what if instead of either setting aside a small section the prebreathing crews need to "camp out" in for a very long time, they adopted oxy-helium mix masks and continued to be free to move around most of the station, having a transition chamber with the air being gradually changed in it matched to their general measured or estimated degree of transition where they could remove masks to eat or just get a rest from the things, and then shift over to lowering the pressure and raising the oxygen percentage more rapidly, spending a lot less time confined in this? And I suspect there is a pressure intermediate between 100 percent oxygen partial pressure and standard SL 100 kPascal pressure, where a moderate amount of helium reduces fire risk considerably versus pure low pressure oxygen, and yet a sudden drop from that to the lower pure oxygen pressure would not risk decompression sickness--either that, or anyway this drop could proceed a lot more quickly than with nitrogen being the neutral element.
I do realize that until we are talking about a huge "Space Station V" or bigger structure with lots of industrial infrastructure, we can't just adjust air mixes at will, reversibly back and forth; to even approximate that without shipping up massive amounts of disposable gases we'd need to separate gases which I think we can only do with compression-cooling-liquefaction. (Well, we could chemically absorb oxygen in various ways, but the real prize here is separating nitrogen and helium--which we could do by liquefaction, first the oxygen (after water vapor and CO2 traces I mean) would condense out, then the nitrogen; what is left is practically pure helium).
Anyway this is something I thought we could do with the hydrogen tank--using inflatable structures, wall off the tail end of the ET, where we have a built in standard inspection hatch. One segment, immediately adjoining the oxy-nitrogen pressurized volume in the hydrogen tank (if any) has nearly pure oxy-helium at standard pressure (or as much below SL standard as we can suddenly have a nitrogen-saturated crew member pop into through a simple airlock without risk of decompression sickness, the lower the pressure the better) where EVA crew and techs supporting them are able to gradually outgas their nitrogen saturation without thereby having to transition straight to pure oxygen. Crew would not have to enter and leave a transitional pressure change chamber in batches; individuals pop in and either with direct metering of their blood chemistry ongoing or periodic, or using timers based on medical time tables, know about when it would become safe for them to suit up to enter a low pressure pure oxygen only section. We clearly would want to minimize the number of items and tasks to be performed in this chamber; I gather that fire risk is only moderately greater than in sea level air mix, but a risk remains, nor would crew want to hang out here unnecessarily, so this chamber could in fact be an airlock, or an antechamber to an inflatable airlock attached to the outside of the inspection hatch. At a low enough pressure helium-oxygen mix I would think it could be safe to just suit up, switch to breathing pure oxygen in the suit, and drop the pressure pretty rapidly--for a crew member whose blood nitrogen is largely gone and replaced by helium anyway; someone else who came in just before and tried the same trick would suffer badly for it. EVA crew returning to the station can actually come in anywhere (where an airlock exists of course) and pressurize direct to SL pressure and nitrogen mix; this would not be pleasant for them but not I think medically dangerous, so emergency entry airlocks can be spotted all over the exterior for quick ingress. But it is dangerous to go out without being prepared.
So what if we only have this one EVA prep hangar-cabin in the hydrogen tank, but the EVA work should be done at the nose end of the station? Should we make the EVA teams spacewalk all the way from the tail end to the nose outside the hull?
I'd think it would work just as well to have them don a suit with a pure oxygen breathing feed, and go back out the lock leading to the main station atmosphere. It means they have to bump their internal suit pressurization up from the intermediate helium-oxygen EVA team shack standard to full SL pressure with pure oxygen (except for the traces of helium they are exhaling) and move through the station in their EVA suit (or alternatively a simpler internal use pressure suit, if we have a pure-oxygen atmosphere changing station such as the airlock itself for them to switch into a better suit for exterior work) to the more desirably close auxiliary airlock, and then carefully but I'd think fairly rapidly lower the pressure to the proper low pure oxygen pressure, and then vent the air in the airlock and exit. The reason this works is that the important thing is to get the nitrogen out of their fluids, and any levels of helium saturation which would bubble out. This is what the EVA operations shack is for, as noted crew just rotate in as needed and individually qualify for EVA when their blood is safe enough. Then they can exit anywhere, if they can keep the nitrogen out (it should work to wear a high full sea level pure oxygen mask proceeding in shirt sleeves through the main station atmosphere to a suitable EVA suit at any airlock; high pressure LOX might be harmful long term and poses a fire hazard but I suppose healthy astronauts can handle it transiting from one end of the station to the other), and as noted reentry into the station can be done anywhere. Though it would be most comfortable and safe to enter a low pressure pure oxygen lock and gradually have the nitrogen let in, or proceed from a low pressure oxygen atmosphere "back porch" to the oxy-helium "shack" and thence into full pressure nitrogen-oxygen station air.
Now I don't see any discussion of using helium either here or generally in modern astronautics I have noticed. Other things being equal we ought to just stick to oxy-nitrogen equivalent either to SL or some moderate altitude such as 3 km/10,000 feet, around 70-75 percent SL pressure. It is only if we have either concerns about sudden unexpected decompression or are planning EVAs that helium ought to come into play.
But aside from any advantages it offers to EVA and so on, helium is lighter than nitrogen; item cost has not mattered much to NASA operations; in fact I was under the impression until recently that prior to STS anyway NASA routinely used helium in the capsule atmospheres, though now my impression is more that the US standard prior to Skylab was pure oxygen at low pressure.
Perhaps there are drawbacks to helium that have eclipsed it, and by means such as indicated in the abstracted article we can accomplish pretty much what helium offers well enough with lower pressure nitrogen-oxygen mixes?
Aside from astronautics I was also under the impression deep sea diving uses oxy-helium mixes too; I have to wonder from the silence on the subject in space travel whether I was informed by materials written in the 1940s-early 70s and that later generations have been disillusioned or anyway less gung ho about helium solving the problems.
