Nitrogen in Apollo atmosphere

[Dalhousie]

This question came up on another board.  Quick Googling did not come up with a coherent answer, so I thought I would try the brains trust here.

What was the N2 content of the Apollo atmosphere over time?

Clearly when the astronauts entered the CM it was a 20:80 mix of O2 and N2. Several sources say that the CM was initially pressurised with a 40:60 mix.  How was this change achieved?

It is generally stated that the pressure was bled down to 5% psi during during ascent, after which time it was topped up with O2.   Bleeding a 20:80 atmosphere down to 5 psi would result in an atmosphere with an O2 partial pressure of 1 psi.  Doing this with a 40:60 mix would give a 2 pisi partial pressure of O2, which is barely adequate. Topping it up with O2 would increase the pressure above 5 psi.  So what was the actual procedure?

Some sources mention the entire atmosphere was purged and replaced with O2 prior to the astronauts removing their helmets?  Is this correct and if so, when was it done? Was there any residual N2?

A single authoritative source that explains this would also be appreciated.

Thanks

[Dalhousie]

One more question:

What is the longest time people can breathe pure oxygen at reduced pressure?  The literature seems coy about this.  Presumably >2 weeks, but perhaps <4 weeks.

[Dalhousie]

Nobody?  Anybody?

[Peter B]

Have you read the Apollo Flight Journal? The page covering the launch of Apollo 8 provides a bit of a description of the process.

[Dalhousie]

Thanks, will follow this up

[Dalhousie]

One more question:

What is the longest time people can breathe pure oxygen at reduced pressure?  The literature seems coy about this.  Presumably >2 weeks, but perhaps <4 weeks.

Unfortunately adds more smoke and no light.

In some places states that O2:N2 is 35:65, elsewhere 40:60.  Which is correct?

So now there are three different atmospheres: normal air, O2 enriched, and pure O2, and two changes, from normal air to enriched, and from enriched to O2.

How was the enriching achieved?  Simple adding of more O2 boosting internal pressure by either 15 or 20%?  or something else?

Why was an O2 enriched atmosphere used, rather than normal atmosphere?


Still nothing on how and when the atmosphere was purged, or what the final composition was.

So now there are three different atmospheres

[onebigmonkey]

All I can find is that the pressure was adjusted using the CM's cabin relief valves Chariots for Apollo states that pure O2 at full pressure helped checked for leaks on the pad, and specifically mentions the 60:40 figure is the one where the fire risk becomes less. The pure O2 mix at sea level pressure obviously turned out to be a mistake.

I'd always assumed that part of the reason for the reduced pressure 02 was to help reduce mass - it's one less gas tank to worry about and you don't have to engineer the structures to allow for it in space.

Here's what CfA says:

The command module could withstand only about half [atmospheric] pressure in space, and the lunar module even less. Moreover, a mixed atmosphere in space would complicate the environmental system—Faget said the system "would get confused and would put too much nitrogen in the cabin, a very insidious thing because there was no way to detect [it]. The astronauts would just get sleepy—and die. Another complication was that a switch back and forth from the two-gas system in the cabin and the 100 percent oxygen in the hoses connected to the suits might give the crew aeroembolism, or the bends.

So the question was twofold: How much nitrogen was needed on the pad to prevent fire? And how much oxygen was needed during launch while the cabin pressure relief valve was venting? Tests revealed that a 60-percent oxygen and 40-percent-nitrogen mixture at a pressure of 11.2 newtons per square centimeter (16.2 pounds per square inch) on the pad would result
in 1.4 newtons (2 psi) in orbit after venting, which would give a partial pressure of oxygen compatible with the oxygen atmosphere and pressure in the suits. The cabin pressure would be lower at first, but the mixture would be breathable and it would sustain life. In fact, by the time the craft reached orbit, Faget said, the cabin mixture would actually be about 80 percent oxygen. And there was a bonus in this arrangement beyond the safety factor: no structural changes were needed in the spacecraft to accommodate this combination of oxygen and nitrogen.

David Woods' excellent 'How Apollo Flew to the Moon' says this:

The reason for sealing the crew inside their suits so early was linked to the fire in the Apollo 1 spacecraft. In its aftermath, it was decided that the cabin would have a mixed atmosphere of nitrogen and oxygen prior to launch, to make the interior much less flammable. After launch, as the vehicle ascended and the outside air pressure diminished, the cabin
atmosphere was allowed to vent overboard and be replaced by pure oxygen from the spacecraft's tanks. As it did so, automatic systems ensured that adequate cabin pressure was maintained, never going below about one-third of the atmospheric pressure at sea level. In the space of several minutes, the pressure in the suits also dropped by two-thirds, and without preparation this could cause nitrogen in a man's bloodstream to come out of solution and give him the 'bends' - a problem also faced by divers who rise too rapidly through a column of water. To avoid this condition, nitrogen was flushed
out of the crew's bodies by having them breathe pure oxygen for several hours prior to lift-off while sealed in their suits.

and

The rationale for the cabin atmosphere to use in space was simple enough. On Earth, we experience air pressure at about 1,000 millibars. Since about 20 per cent of that air is oxygen, we say that the partial pressure of oxygen is about 200 millibars.

To simplify the design of the Apollo spacecraft and to save weight, NASA decided to use a single gas for all stages of the flight. By having pure oxygen, there was no need to engineer the spacecraft's hull to hold sea-level pressure against the vacuum of space, or to carry apparatus to store nitrogen and to regulate the gas mixture.

Instead, the spacecraft designers set the cabin pressure so that the concentration of oxygen molecules presented within the lung, where gases are exchanged to and from the blood, was similar to what would be found on Earth. This was achieved by regulating the oxygen atmosphere within the cabin at around 350 millibars. By adopting this lower pressure, the hull could be lighter, since it only had to hold two fifths of sea-level pressure at most.

In the light of [the Apollo 1] tragedy, the Block II spacecraft was redesigned to have a two gas atmosphere while on the ground with a mix of oxygen and nitrogen at a 60/40 ratio at a pressure of 1,000 millibars. Although this ratio was relatively rich in oxygen when compared to normal air, it suppressed flammability while minimising the time required to flush nitrogen out of the cabin after launch. During ascent, the cabin was maintained at sea-level pressure until the outside pressure had dropped by 400 millibars, then the pressure relief valve began to bleed the nitrogen/oxygen air out of the spacecraft to maintain a 400-millibar difference across the hull. During this time, the crew were sealed in their suits breathing only oxygen from the suit circuit. The pressure in their suits was kept slightly high so that the excess gas would help to flush the nitrogen out of the cabin air. Like passengers in an aeroplane, they could feel the drop in pressure make their ears pop.

So in a nutshell, reduced pressure means less engineering, less mass, less health risk and less fire risk. Cabin pressure relief valves (which could be worked manually as well as automatically) sorted out the pressure and the O2 was sourced from the CSM tanks.

I'm sure there are much better technical answers than that

[JayUtah]

Yesterday I went through both the G-, H-, and J-mission flight plans and the Apollo Operations Handbook looking for the precise sequence of steps that got the cabin to the point where the crew could doff helmets and gloves right after SECO and not suffocate.  Some commentators have claimed there was a purge to completely replace the atmosphere with pure oxygen at 5 pisa, but I find no reference to such a step.

Others have correctly pointed out the two pressure gradients -- suit-to-cabin and cabin-to-ambient -- but there is still some confusion in how the mixed cabin atmosphere at ~16 psia arrives at a breathable atmosphere with 5 psi partial-pressure of O2 via venting and feed.  I didn't post anything yesterday because I hadn't run the numbers or verified my hypothesis.  So today's post contains a fair amount of non-rigorous handwaving.  I think part of the confusion could be an unreferenced hysteresis in these valves.  A relief valve typically trips at a certain pressure, but then often doesn't close again until the pressure has dropped to a much lower value than the trip pressure.  Since this is basic control-system theory, it may not be called out in every document that applies to operating the ECS.  The CM cabin vent valve doesn't open until 6 psig.  But I'm betting it closes again at something considerably lower, such as 4 psig.

On the other side of the equation, you can have the O2 feed valve open at, say, 4.5 psig and adjust the feed rate so that it doesn't fight the venting.  That is, it feeds oxygen at a slower rate than the cabin vent rate, so the cabin relief valve will eventually close -- just not as soon as it would have if there weren't gas also being fed into the cabin.  The O2 feed valve could then close at something like 5.5 psig:  enough to have filled the cabin substantially with more oxygen, but not so much that the relief valve trips again.  The flight plan calls for the to manually pressurize the cabin to 5.7 psia at various points where suit elements are being donned or doffed, and this happens with pure oxygen.  So I can see how you could arrange hystereses and feed rates to maintain the overall pressure within a certain range while also gradually increasing the oxygen concentration such that you can take your helmet off at the end of the process without passing out.  It just may not be spelled out in so many words in the procedures.  That would be something you'd look for in the detailed design specs of the ECS, which I don't have handy at the momen.

[Dalhousie]

Thanks Jay. Fascinating stuff.

So, as I understand it at lift off we have a 40:60 mix. By the time it drops to 5 psi it starts getting topped up,  while still being vented to maintain pressure at 5psi.  By the time the crew take off their helmets the atmosphere must be about 80% O2 and 20% N2. Presumably the suits are gradually vented during ascent as well, whilst being pure O2. Was this done by an automnatic pressure release valve?

Interestingly this composition is close to the Skylab atmosphere of 74% O2 and 26% N2. Since Skylab would have had an Earth normal atmosphere on the hab, there must have been a similar procedure during its ascent.  Was this mix the result?

This means that the LM would have had a mixed gas atmosphere until the first EVA, replaced by pure O2.  Likewise the CM.

Would this procedure have the same for Mercury and Gemini?

[JayUtah]

So, as I understand it at lift off we have a 40:60 mix. By the time it drops to 5 psi it starts getting topped up,  while still being vented to maintain pressure at 5psi.

Yes.  It's a little like starting off with a glass that's 40% scotch and 60% soda by volume.  Pour off half the glass, then fill it up to the top again with only scotch.  Repeat.  You'll eventually have a glass that's mostly scotch, with only traces of soda remaining.

Presumably the suits are gradually vented during ascent as well, whilst being pure O2.

Yes.

Was this done by an automnatic pressure release valve?

I recall yes.  But as with most such designs for Apollo, I recall it could be configured to operate in several ways, including fully manual.

Since Skylab would have had an Earth normal atmosphere on the hab, there must have been a similar procedure during its ascent.  Was this mix the result?

Yes, but the procedure wasn't exactly the same.  Skylab went to orbit with a sea-level ambient atmosphere, but the pressure wasn't relieved gradually during the ascent.  On orbit, the habitat was vented to a very low total pressure in a single step, then repressurized with pure oxygen.  That's easier to engineer when you aren't yet containing humans that have to be kept alive throughout the whole process.

This means that the LM would have had a mixed gas atmosphere until the first EVA, replaced by pure O2.  Likewise the CM.

Right, although the CM and LM cabin atmospheres were allowed to mix before the LM was isolated for the translunar coast.  But if you're saying that you're never going to get rid off all the nitrogen by the vent-repress cycle I described, then yes I agree.

Would this procedure have the same for Mercury and Gemini?

Mercury and Gemini used pure oxygen environments, but I don't know the details of the ground purge and fill procedures.  And yes, I believe Gemini launched with a 16 psia pure-oxygen cabin pressure. *shudder*

[Dalhousie]

Thanks Jay

Regarding Skylab, I recall that the air was vented several times before the 1st crew because mission control was concerned about possible toxic outgassing from overheating. 

To maintain the 74% O2 and 26% N2 mix was this mix pre stored or were there separate N2 and O2 tanks??

What was the justification for this gas ratio?

[JayUtah]

Regarding Skylab, I recall that the air was vented several times before the 1st crew because mission control was concerned about possible toxic outgassing from overheating. 

I don't know, but that sounds correct and prudent.

To maintain the 74% O2 and 26% N2 mix was this mix pre stored or were there separate N2 and O2 tanks??

Separate tanks.

What was the justification for this gas ratio?

The justification for the nitrogen at all was the same as for Apollo.  The nitrogen is a diluent that absorbs ignition energy without resulting in combustion.

I don't know the exact design parameters offhand for Skylab, but those decisions are generally made as compromises of lots of competing concerns.  Overall design pressure, for one.  The degree of dilution desired.  The need to maintain a narrow range of O2 partial pressure.  Consumables budgeting.  Emergency contingencies.

Whenever you see an oddly non-round number presented for something, you can usually assume it's the product of pages of what look like overblown equations.  It's not at all difficult math, it's just usually not something one keeps in his head longer than necessary.

[Dalhousie]

Mercury and Gemini used pure oxygen environments, but I don't know the details of the ground purge and fill procedures.  And yes, I believe Gemini launched with a 16 psia pure-oxygen cabin pressure. *shudder*

At leastthey had ejector seats to get out in  a hurry