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Nitrogen in Apollo life support system
by
Dalhousie
on 18 Apr, 2020 01:43
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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 the atmospheric 20:80 mix of O2 and N2. Several sources say that the CM was initially pressurised with a 40:60 mix. If so, was this change fromatmsopheric to oxygen enriched air 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?
An authoritative source would be appreciated.
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. A source would be appreciated.
Thanks!
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#1
by
MATTBLAK
on 18 Apr, 2020 01:59
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A month of pure O2 at 5psi for about a month should be fine. Also look up Aquanauts breathing Helium/Oxygen under high pressure for weeks - there certainly must be some published literature on that.
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#2
by
Dalhousie
on 18 Apr, 2020 02:16
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A month of pure O2 at 5psi for about a month should be fine. Also look up Aquanauts breathing Helium/Oxygen under high pressure for weeks - there certainly must be some published literature on that.
That's what I suspect too. But I can't find a source for it.
Not sure of the relevance of Helox mixtures though.
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#3
by
MATTBLAK
on 18 Apr, 2020 02:22
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I only mentioned it because NASA was once greatly interested in the gas mixtures that Aquanauts breathed. Space Medicine specialists were consulted by the U.S. Navy for 'Sealab' I believe. The one and only chance I got to meet and interview Scott Carpenter back in the 1980s: I had more than half an hour with the man and forgot to ask him about Sealab!! I've been kicking myself ever since.
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#4
by
Dalhousie
on 18 Apr, 2020 02:52
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MOL was supposed to use Heliox for some reason, never understood why. It does not clearly add to safety and the communications issues would be huge. Appaently also briefly considered for Apollo after Apollo 1 fire.
Heliox is wildly used by the commercial diving industry, as is trimix (He+N2+O2). Occasional hydrox for extremely deep diving.
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#5
by
Johnnyhinbos
on 18 Apr, 2020 03:17
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Oxygen is a corrosive and leads to pulmonary toxicity over time. This is a function of the partial pressure of O2, so of course the lower the ppO2 (partial pressure oxygen) the lesser the effect (and I’ll ignore the more dangerous central nervous system O2 toxicity that can lead to the death of a diver without warning - that’s another conversation).
So - breathing ambient pressure air at sea level - i.e 1 atmosphere (1 ATA). Use 14.7 psia (vs psig). Oxygen is 21%, meaning it’s 3.087 psia. 5 psia O2 is about 34% O2 at 1 ATA. Now, I know the Apollo CM wasn’t held at 1 ATA but the ppO2 is still elevated to 5 psia, so I would think there would be chronic pulmonary toxicity issues.
In rebreather diving (tech diving in general but it really factors in for rebreathers) this type of toxicity is tracked by Oxygen Toxicity Units (OTU). 1 OTU is earned breathing pure O2 at 1ATA for 1 minute. 300 OTU / day is the calculated limit and the OTU clock adds up over time (dives, days, etc). You can really feel this tox building up after a week long multi-dive activity where you do decompression diving with high ppO2 final deco stops. Shortness of breath, burning lungs, and “the coughs”.
I’ve had them. And don’t even ask what it’s like after 23 hours in a decompression chamber (over five days) at 60 feet sea water simulated pressure in a 100% pure oxygen environment- not pleasant.
For some supplemental reading:
https://www.rebreather.org/blog/pulmonary-o2-tox-calculator/
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#6
by
Dalhousie
on 18 Apr, 2020 03:27
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Oxygen is a corrosive and leads to pulmonary toxicity over time. This is a function of the partial pressure of O2, so of course the lower the ppO2 (partial pressure oxygen) the lesser the effect (and I’ll ignore the more dangerous central nervous system O2 toxicity that can lead to the death of a diver without warning - that’s another conversation).
So - breathing ambient pressure air at sea level - i.e 1 atmosphere (1 ATA). Use 14.7 psia (vs psig). Oxygen is 21%, meaning it’s 3.087 psia. 5 psia O2 is about 34% O2 at 1 ATA. Now, I know the Apollo CM wasn’t held at 1 ATA but the ppO2 is still elevated to 5 psia, so I would think there would be chronic pulmonary toxicity issues.
In rebreather diving (tech diving in general but it really factors in for rebreathers) this type of toxicity is tracked by Oxygen Toxicity Units (OTU). 1 OTU is earned breathing pure O2 at 1ATA for 1 minute. 300 OTU / day is the calculated limit and the OTU clock adds up over time (dives, days, etc). You can really feel this tox building up after a week long multi-dive activity where you do decompression diving with high ppO2 final deco stops. Shortness of breath, burning lungs, and “the coughs”.
I’ve had them. And don’t even ask what it’s like after 23 hours in a decompression chamber (over five days) at 60 feet sea water simulated pressure in a 100% pure oxygen environment- not pleasant.
For some supplemental reading:
https://www.rebreather.org/blog/pulmonary-o2-tox-calculator/
However this is not about breathing O2 at high pressure, where the risks have been well known for almost a century. It is about breathing it at low pressure, about 400 HPa or less. Where the risks are much lower.
In Apollo this was done for periods of almost two weeks, without ill effect.There is some arm waving ab=bout this being not being good for long periods, but I have yet to find a definitive source or any upper limit.
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#7
by
Johnnyhinbos
on 18 Apr, 2020 03:39
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Oxygen is a corrosive and leads to pulmonary toxicity over time. This is a function of the partial pressure of O2, so of course the lower the ppO2 (partial pressure oxygen) the lesser the effect (and I’ll ignore the more dangerous central nervous system O2 toxicity that can lead to the death of a diver without warning - that’s another conversation).
So - breathing ambient pressure air at sea level - i.e 1 atmosphere (1 ATA). Use 14.7 psia (vs psig). Oxygen is 21%, meaning it’s 3.087 psia. 5 psia O2 is about 34% O2 at 1 ATA. Now, I know the Apollo CM wasn’t held at 1 ATA but the ppO2 is still elevated to 5 psia, so I would think there would be chronic pulmonary toxicity issues.
In rebreather diving (tech diving in general but it really factors in for rebreathers) this type of toxicity is tracked by Oxygen Toxicity Units (OTU). 1 OTU is earned breathing pure O2 at 1ATA for 1 minute. 300 OTU / day is the calculated limit and the OTU clock adds up over time (dives, days, etc). You can really feel this tox building up after a week long multi-dive activity where you do decompression diving with high ppO2 final deco stops. Shortness of breath, burning lungs, and “the coughs”.
I’ve had them. And don’t even ask what it’s like after 23 hours in a decompression chamber (over five days) at 60 feet sea water simulated pressure in a 100% pure oxygen environment- not pleasant.
For some supplemental reading:
https://www.rebreather.org/blog/pulmonary-o2-tox-calculator/
However this is not about breathing O2 at high pressure, where the risks have been well known for almost a century. It is about breathing it at low pressure, about 400 HPa or less. Where the risks are much lower.
In Apollo this was done for periods of almost two weeks, without ill effect.There is some arm waving ab=bout this being not being good for long periods, but I have yet to find a definitive source or any upper limit.
No - I specifically said NOT high pressure - that’s CNS toxicity. I’m talking about pulmonary toxicity. Did you even bother to read my post?
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#8
by
Dalhousie
on 18 Apr, 2020 05:59
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Oxygen is a corrosive and leads to pulmonary toxicity over time. This is a function of the partial pressure of O2, so of course the lower the ppO2 (partial pressure oxygen) the lesser the effect (and I’ll ignore the more dangerous central nervous system O2 toxicity that can lead to the death of a diver without warning - that’s another conversation).
So - breathing ambient pressure air at sea level - i.e 1 atmosphere (1 ATA). Use 14.7 psia (vs psig). Oxygen is 21%, meaning it’s 3.087 psia. 5 psia O2 is about 34% O2 at 1 ATA. Now, I know the Apollo CM wasn’t held at 1 ATA but the ppO2 is still elevated to 5 psia, so I would think there would be chronic pulmonary toxicity issues.
In rebreather diving (tech diving in general but it really factors in for rebreathers) this type of toxicity is tracked by Oxygen Toxicity Units (OTU). 1 OTU is earned breathing pure O2 at 1ATA for 1 minute. 300 OTU / day is the calculated limit and the OTU clock adds up over time (dives, days, etc). You can really feel this tox building up after a week long multi-dive activity where you do decompression diving with high ppO2 final deco stops. Shortness of breath, burning lungs, and “the coughs”.
I’ve had them. And don’t even ask what it’s like after 23 hours in a decompression chamber (over five days) at 60 feet sea water simulated pressure in a 100% pure oxygen environment- not pleasant.
For some supplemental reading:
https://www.rebreather.org/blog/pulmonary-o2-tox-calculator/
However this is not about breathing O2 at high pressure, where the risks have been well known for almost a century. It is about breathing it at low pressure, about 400 HPa or less. Where the risks are much lower.
In Apollo this was done for periods of almost two weeks, without ill effect.There is some arm waving ab=bout this being not being good for long periods, but I have yet to find a definitive source or any upper limit.
No - I specifically said NOT high pressure - that’s CNS toxicity. I’m talking about pulmonary toxicity. Did you even bother to read my post?
Yes I did. I am not interested in CNS toxicity, which can occur above 160 KPa. Equally not interest in pulmonary toxicity, which can be an issue above 50 KPa.
I reiterate, I am only interested in long term issues associated with breathing O2 at Apollo pressures, 0.33 KPa
Breathing pure oxygen at one atmosphere is high pressure. And the partial pressure of O2 in Apollo during normal operations was only 0.333 of an ATM
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#9
by
MATTBLAK
on 18 Apr, 2020 06:20
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We know that pure oxygen at low partial pressures has a good experience baseline with Gemini and Apollo. We could reasonably assume a doubling or slightly more than that duration. Beyond this; someone with contacts should ask a Space Medicine expert or Aerospace Doctor if they can answer our question for us. Is there an 'Ask an Expert' resource anywhere for relatively esoteric things like this?
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#10
by
gparker
on 18 Apr, 2020 09:25
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What was the N2 content of the Apollo atmosphere over time?
Clearly when the astronauts entered the CM it was the atmospheric 20:80 mix of O2 and N2. Several sources say that the CM was initially pressurised with a 40:60 mix. If so, was this change fromatmsopheric to oxygen enriched air 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?
The sequence went something like this:
Before launch: The astronauts in their suits breathe 100% oxygen at slightly above atmospheric pressure. (The slight overpressure prevents outside N2 from leaking in.)
Before launch: The command module is filled with 60:40 O2:N2 at atmospheric pressure. (Note 60% O2, not the 40% you used for your calculations.)
During ascent: The CM cabin is vented down to 5 or 6 psi. O2 is 60% or more, so the oxygen pressure is at least 3 psi which is roughly sea-level equivalent for humans. The astronauts are still breathing 100% O2 in their suits, and the suit pressure decreases along with the cabin pressure. Even after reaching 5 psi the environmental system continues to pump O2 into the cabin. That extra pressure forces cabin air overboard through the cabin pressure relief valve, taking more nitrogen with it.
After ascent: The extra oxygen injection is stopped. There isn't much nitrogen left. The astronauts take their helmets off about 25 minutes after launch. Another cabin vent is left open for most of the first twelve hours, which slowly purges cabin air to be replaced by oxygen. This reduces the N2 content further. By the time of the first EVAs there isn't any risk of decompression sickness.
Sources:
Samonski & Tucker,
"Apollo Experience Report: Command and Service Module Environmental Control System"see pages 13-14
Apollo Flight JournalA16 -001:30:00 GET: description of cabin atmosphere
A16 000:23:05: crew remove their helmets
A13 003:05:39, 004:14:39, 011:30:40, 012:34:41: crew close the slow cabin vent during TLI but otherwise leave it open for the first twelve hours.
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#11
by
John Santos
on 18 Apr, 2020 10:13
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Skylab? 80 days. Was the crew at Apollo's atmosphere for the entire time?
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#12
by
MATTBLAK
on 18 Apr, 2020 11:24
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No - Skylab atmospheric mix was 5 psi, but a 74% oxygen, 26% nitrogen mix. I don't know what the desired minimum ppm of C02 was.
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#13
by
Hog
on 18 Apr, 2020 13:32
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A month of pure O2 at 5psi for about a month should be fine. Also look up Aquanauts breathing Helium/Oxygen under high pressure for weeks - there certainly must be some published literature on that.
The heliox and Trimix(oxygen, helium and nitrogen-the breathing gases-not the ED fix) is to prevent nitrogen narcosis aka "rapture of the deep" aka "the martini effect". Every 50 feet breathing "air" is akin to drinking a martini on an empty stomach. At 300+ feet you might as well be sucking on a surgical anesthesia machine and you will lose consciousness.
There are videos of people losing their lives during bouts of Inert gas narcosis, they just seem utterly confused and some pop their regulators out of their mouths.
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#14
by
Dalhousie
on 22 Apr, 2020 03:45
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What was the N2 content of the Apollo atmosphere over time?
Clearly when the astronauts entered the CM it was the atmospheric 20:80 mix of O2 and N2. Several sources say that the CM was initially pressurised with a 40:60 mix. If so, was this change fromatmsopheric to oxygen enriched air 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?
The sequence went something like this:
Before launch: The astronauts in their suits breathe 100% oxygen at slightly above atmospheric pressure. (The slight overpressure prevents outside N2 from leaking in.)
Before launch: The command module is filled with 60:40 O2:N2 at atmospheric pressure. (Note 60% O2, not the 40% you used for your calculations.)
During ascent: The CM cabin is vented down to 5 or 6 psi. O2 is 60% or more, so the oxygen pressure is at least 3 psi which is roughly sea-level equivalent for humans. The astronauts are still breathing 100% O2 in their suits, and the suit pressure decreases along with the cabin pressure. Even after reaching 5 psi the environmental system continues to pump O2 into the cabin. That extra pressure forces cabin air overboard through the cabin pressure relief valve, taking more nitrogen with it.
After ascent: The extra oxygen injection is stopped. There isn't much nitrogen left. The astronauts take their helmets off about 25 minutes after launch. Another cabin vent is left open for most of the first twelve hours, which slowly purges cabin air to be replaced by oxygen. This reduces the N2 content further. By the time of the first EVAs there isn't any risk of decompression sickness.
Sources:
Samonski & Tucker, "Apollo Experience Report: Command and Service Module Environmental Control System"
see pages 13-14
Apollo Flight Journal
A16 -001:30:00 GET: description of cabin atmosphere
A16 000:23:05: crew remove their helmets
A13 003:05:39, 004:14:39, 011:30:40, 012:34:41: crew close the slow cabin vent during TLI but otherwise leave it open for the first twelve hours.
Thanks, that is most helpful, and answers my question.
That leaves by other question: what is the current longest period pure O2 (at Apollo pressures) has been used for? I assume there is some ground test data for this.
Plus a supplementary question: was at atmosphere with minor N2 chosen for Skylab
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#15
by
gparker
on 22 Apr, 2020 12:53
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That leaves by other question: what is the current longest period pure O2 (at Apollo pressures) has been used for? I assume there is some ground test data for this.
Apollo-era tests extended to at least 30 days ("Physiologic Response to Increased Oxygen Partial Pressure" parts i, ii, iii). I suspect that the tests stopped there; after the spaceflight folks switched to sea-level pressure there might have been nobody since who cared enough to fund a study.
Plus a supplementary question: was at atmosphere with minor N2 chosen for Skylab
My guess: they needed to stay at 5 psi to stay compatible with Apollo hardware like the CM, and the 75:25 mix was the most nitrogen they could use at that pressure to reduce the potential health and fire risks.
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#16
by
Dalhousie
on 01 May, 2020 08:19
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That leaves by other question: what is the current longest period pure O2 (at Apollo pressures) has been used for? I assume there is some ground test data for this.
Apollo-era tests extended to at least 30 days ("Physiologic Response to Increased Oxygen Partial Pressure" parts i, ii, iii). I suspect that the tests stopped there; after the spaceflight folks switched to sea-level pressure there might have been nobody since who cared enough to fund a study.
Plus a supplementary question: was at atmosphere with minor N2 chosen for Skylab
My guess: they needed to stay at 5 psi to stay compatible with Apollo hardware like the CM, and the 75:25 mix was the most nitrogen they could use at that pressure to reduce the potential health and fire risks.
Thanks for the references. They are a great help.