Would an astronaut survive burial on the Moon?

[ka9q]

Followup -- I don't think it's quite as bad as 1%. It's the partial pressures that matter, and in a single-gas system without a diluent you could withstand a greater percentage of CO₂ in the total.

1% CO₂ in sea level air is about 1 kPa of partial pressure. If the O₂ is at 21 kPa, this is more like 5% of the total, by volume, so the vents would purge about 20x as much O₂ as CO₂, not 99x.

So the efficiency figures would be:

Rebreather: 100%
Scuba: 25%
Cabin purging and venting: 5%

A typical adult crewmember consumes 840g/day, so this would increase it to 16.8 kg/day per crewman. Stored as LOX (density 1141 kg/m³, that would occupy 14.7 L. This might be entirely tolerable in a PLSS or pressurized rover if it eliminated the need for cooling water and LiOH cartridges.

On the other hand it would eliminate the option of recovering the exhaled CO₂ and especially the H₂O for possible recycling. Given the scarcity of water on the moon, this might be a problem.

[Donnie B.]

How about once-through scuba-style for an EVA suit, where simplicity and endurance matter most, and CO2 scrubbing for a rover and any fixed facilities? 

I don't think use of a mouthpiece would be too objectionable in a suit, though you'd have to be sure you could recover it if it fell out of your mouth.

[ka9q]

CO₂ scrubbing at a base certainly makes sense. The Shuttle and ISS already use regenerative alternatives to non-renewable LiOH for CO₂ scrubbing, but the hardware is more complex than LiOH and probably too much to put into a PLSS.

The Apollo astronauts suited up several hours before launch and breathed pure O₂ from that point to begin flushing the N₂ from their systems to avoid possible decompression sickness during launch as the cabin pressure rapidly dropped. You may remember them carrying portable O₂ supplies on their way to the pad. They contained a supply of LOX, and maybe that's how they also cooled themselves. So there's precedent for a simple once-through LOX-based system.

Because oxygen will be abundant in any lunar base, scrubbing of exhaled air will probably be motivated not by conserving O₂ but by the need to conserve C and especially H₂, two elements that are rare on the moon.

[ka9q]

I have to admit that the first thing that popped into my head, after reading the thread title, was the old misdirection joke "a plane crashes right on an international border, where do they bury the survivors?".

Oh I dunno,  wherever they happen to be when they eventually die?

[Noldi400]

Like many of the discussions here, this got me thinking. Could abundant oxygen production on the moon allow the use of simpler but less efficient life support systems?

I see a range of numbers, but a typical human breathing air exhales 4-5% CO₂ and 15-16% O₂. Since air is about 21% O₂, this means that in a once-through scuba-type mode your oxygen efficiency would be about  25%. Assuming these figures would stay the same for pure O₂ at reduced total pressure (i.e., same O₂ partial pressure), you'd have to carry about 4x as much O₂ as in a conventional rebreather-type life support system where the CO₂, H₂O and trace gases are scrubbed so the unbreathed O₂ can be breathed again.

That's actually not bad considering how much you can get rid of: no CO₂ absorption system, no water condenser/separator, no charcoal filter. Depending on the workload you might also be able to get rid of the cooling system too by relying on gas cooling supplemented by the heat of vaporization of LOX. (Gas cooling was the original plan for Apollo, but water cooling was added when studies showed that at very high workloads it would take less power to drive a water pump than a suit blower at the necessary speed.)

But it would require you to use a scuba-type mouthpiece or a small face mask to limit mixing between exhaled and inhaled air. If instead you simply purged the cabin continuously through a vent, assuming good ventilation keeping exhaled air thoroughly mixed in the cabin, your vent rate would depend on the maximum tolerable CO₂ concentration. That appears to be about 1%, so to maintain that level you'd have to continuously vent CO₂ as fast as its produced, and you'd have to vent roughly 99 times as much O₂ along with it. (The actual figure is a bit better because CO₂ has a higher molecular weight than O₂.)

It's probably more a matter of partial pressure than percentage. The partial pressure of CO₂ (_pCO₂) in the venous blood returning to the lungs is about 6.7 kPa; if the _pCO₂ of inhaled air approaches that level, the CO₂ in the blood will not diffuse out and be exhaled. CO₂ builds up in the blood and causes all kinds of nasty effects (even biology is physics, huh?).  It's just an educated guess, but I would think that a   _pCO₂ around 5.0 kPa would be a safe margin.

[ka9q]

5 kPa corresponds to 5% at normal pressure. That's definitely too much CO₂. The sources I see say you can breathe up to 1% indefinitely with no noticeable symptoms; that's 1 kPa at normal pressure. This is still quite a bit higher than the standards for indoor air quality, which limit CO₂ to about 0.25%. See the Wikipedia article on carbon dioxide.

I realized after posting my article that it is indeed the partial pressure that matters, so you're right about that. In a single gas system with lower total pressure, the O₂ efficiency would be better than 1%; it would be more like 5%. The same would be true with air except that in addition to venting 95% of your oxygen, you'd also vent quite a bit of diluent gas. I'm assuming nitrogen is extremely scarce on the moon so you wouldn't want to do this.

[ka9q]

I've also been thinking about the cooling problem; if a PLSS stored LOX, could you use it for astronaut cooling?

My physiology references say that a typical crew member consumes 840g O₂/day. Each liter (presumably at normal temperature and pressure) represents 5 kcal of metabolism assuming a mixed diet (carbohydrates and fats have different requirements). 1 L O₂ @ NTP is 1.331 g, so that's 5 kcal/1.331 g or 15.73 MJ/kg. 840 g of daily O₂ consumption represents 13.2 MJ or 3156 kcal, which seems about right for an active astronaut.

The heat of vaporization of O₂ is 6.82 kJ/mol; the molecular weight is 32, so that's 213 kJ/kg. It then has to be warmed (310-90) = 220 K after boiling, so for a specific heat of about 0.91 kJ/kg-K. that's 213 + 200.2 = 413.2 kJ/kg to convert LOX to breathable O₂. Your 840 g/day of O₂ consumption would therefore require 347 kJ/day or 4 W to warm, but you put out 13.2 MJ/day or 153 W, so there isn't anywhere near enough heat capacity in the LOX you breathe to absorb your metabolic heat.

But that's in a rebreather with 100% efficiency. Again we're assuming we can squander LOX because it will be plentiful on the moon, so you'd have to boil about 39x as much as you'd actually breathe. But this is not far from the 20x metabolic consumption you'd need in a simple purge system that maintains CO₂ at 1%. You could just reduce the CO₂ level to about 0.5%

So the bottom line is that if you're prepared to use LOX really inefficiently you could provide metabolic cooling and eliminate the need for CO₂ scrubbing.

But wait, there's more. The significant temperature difference between the human body and LOX (220K) means you could (in theory, anyway) run a heat engine off the difference and produce useful work with it, possibly eliminating the battery in your PLSS as well! The Carnot efficiency of a heat engine operating between 310 K (body temperature) and 90 K (LOX boiling point) is 1 - (90/310) = 71%. My figures assume a metabolic rate of 153W, so this could (again at 100% of Carnot efficiency, which is too optimistic) produce 109 W of electricity. But the Apollo PLSS drew a nominal 41W (8.4 W for the water pump, 21.8 W for the fan and 10.9 W for the communications system, which could certainly be improved with newer technology) so you'd need a heat engine with only 38% of Carnot efficiency. So it's entirely plausible that the whole PLSS could be powered off the astronaut's own metabolic heat.

[Noldi400]

5 kPa corresponds to 5% at normal pressure. That's definitely too much CO₂. The sources I see say you can breathe up to 1% indefinitely with no noticeable symptoms; that's 1 kPa at normal pressure. This is still quite a bit higher than the standards for indoor air quality, which limit CO₂ to about 0.25%. See the Wikipedia article on carbon dioxide.

I realized after posting my article that it is indeed the partial pressure that matters, so you're right about that. In a single gas system with lower total pressure, the O₂ efficiency would be better than 1%; it would be more like 5%. The same would be true with air except that in addition to venting 95% of your oxygen, you'd also vent quite a bit of diluent gas. I'm assuming nitrogen is extremely scarce on the moon so you wouldn't want to do this.

I guess that educated guess was only half-educated.  I neglected to consider that alveolar air has a considerably higher level of CO₂ than inhaled air because of anatomic dead space - alveolae don't empty completely at exhalation, so there's always a blending.  I was partly right in that the alveolar pCO₂ needs to stay below around 5 kPa to assure adequate CO₂ exhalation.  The empirical evidence seems to be that, as you said, 1 kPa in inhaled air is about the maximum that is tolerable - higher and that and it eventually builds up and becomes toxic.

BTW, I happened on an interesting bit of trivia. According to my A&P text, in microgravity the gas exchange capacity of the alveoli is 28% greater than in 1 gee. (Spacelab)

[Allan F]

How would that energy be extracted? Pleiter-elements?

[ka9q]

How would that energy be extracted? Pleiter-elements?

You mean my heat engine working off body heat vs LOX? It could be anything capable of working between those two temperatures; maybe a Stirling engine. Thermocouples and Peltier devices would probably be too inefficient. NASA Glenn has been working on a Stirling engine for converting Pu-238 RTG heat to electricity that's much (4-5x) more efficient than the thermocouples used on current RTGs. Since it has moving parts and is considerably more complex than the thermocouples, they're only doing this because of the critical shortage of Pu-238.

The Carnot limit is a theoretical limit on conversion efficiency that applies to any heat engine, set only by the ratio of the hot side and cold side temperatures. As always it's difficult to approach the theoretical limits in practice.

It's easy to see intuitively how the Carnot limit works. Kelvin is an absolute temperature scale so there's no heat energy at all in something at 0K. So if you start with heat at, say 310K and dump waste heat at 90K, you are not extracting 90/310 = 29% of the heat input, leaving 71% that you can extract.

An interesting point about my heat engine idea is that if you really could produce a heat engine with 71% efficiency, only 29% of the astronaut's body heat would reach the LOX, the rest having been converted to useful work. That would reduce the LOX boil rate by over a factor of 3. So you could reduce your LOX consumption rate back down to that required to maintain a 1% CO₂ concentration or even higher.

Edited to add: Actually it seems more complicated than that. Any electricity generated from a heat engine working between body temperature and LOX temperature would be used to power systems within the PLSS like pumps, fans and radios. They would in turn generate additional waste heat that would have to be removed, e.g., by boiling LOX, but some of that waste heat could again be used to generate more elecricity! The only energy that wouldn't have to be removed is that radiated by radio transmitters, which would be a small fraction of the total.

But you'd still eliminate the need for a battery and all the extra waste heat the PLSS systems would generate from that battery power.

[ka9q]

I neglected to consider that alveolar air has a considerably higher level of CO₂ than inhaled air because of anatomic dead space - alveolae don't empty completely at exhalation, so there's always a blending.  I was partly right in that the alveolar pCO₂ needs to stay below around 5 kPa to assure adequate CO₂ exhalation.

I found similar information in the physiology literature. The partial pressures of CO₂ and O₂ remain remarkably constant in the alveolae as you breathe, and as you say you only exchange a fraction of that air on every breath assuming you're breathing normally.

There always have to be partial pressure gradients between the blood and the alveolae, and between the alveolae and the ambient air so that the gases continue to flow in the direction you want. It's like needing a voltage drop, however small, across a conductor to get it to carry the current you want.

[Noldi400]

I neglected to consider that alveolar air has a considerably higher level of CO₂ than inhaled air because of anatomic dead space - alveolae don't empty completely at exhalation, so there's always a blending.  I was partly right in that the alveolar pCO₂ needs to stay below around 5 kPa to assure adequate CO₂ exhalation.

I found similar information in the physiology literature. The partial pressures of CO₂ and O₂ remain remarkably constant in the alveolae as you breathe, and as you say you only exchange a fraction of that air on every breath assuming you're breathing normally.

There always have to be partial pressure gradients between the blood and the alveolae, and between the alveolae and the ambient air so that the gases continue to flow in the direction you want. It's like needing a voltage drop, however small, across a conductor to get it to carry the current you want.

More so with the alveolar-capillary interface, since there's an actual physical barrier there and the osmotic pressure is required to move the molecules across it. The ambient-alveolar differential is just a result of the fact that the alveola are anatomical dead ends, so there's always a little "stale air" in them.  The body does have mechanisms to cope with that, though - if CO₂ levels begin to rise, you automatically breathe deeper, which ventilates the alveola more completely.  According to the literature I looked at, the body can evidently compensate for quite a long time for slightly elevated ambient CO₂ levels.

The body also tries to maintain a certain range of pCO₂ in the blood - it's an important component of pH balance, which is supremely important.  I need to pull up some of the NASA Biomedical research - it would be interesting to see what data they were able to gather on physiological changes while living for 10 - 12 days in microgravity and a low pressure O₂ atmosphere.

[ka9q]

Speaking of blood pH and overventilation, just the other day I was wondering how dogs can pant without going into hypocapnia. They pant to lose heat since they don't sweat through their fur, and that wouldn't always be directly associated with metabolic rate, e.g., on a hot day. My wife, a nurse, didn't seem to know the answer offhand.

Perhaps they've evolved some different mechanisms for controlling blood pH. Maybe we humans lost those mechanisms since we don't need to pant to lose heat.

[ka9q]

need to pull up some of the NASA Biomedical research - it would be interesting to see what data they were able to gather on physiological changes while living for 10 - 12 days in microgravity and a low pressure O₂ atmosphere.

Check out "Biomedical Results of the Apollo Program" or something like that. That's your bible on the topic. If you can't get it because of the NTRS shutdown let me know and I'll dig up my copy.

[Noldi400]

Speaking of blood pH and overventilation, just the other day I was wondering how dogs can pant without going into hypocapnia. They pant to lose heat since they don't sweat through their fur, and that wouldn't always be directly associated with metabolic rate, e.g., on a hot day. My wife, a nurse, didn't seem to know the answer offhand.

Perhaps they've evolved some different mechanisms for controlling blood pH. Maybe we humans lost those mechanisms since we don't need to pant to lose heat.

As it happens, I asked a vet about this once.  "Thermic panting" is shallow enough that air movement is confined almost entirely to the oronasopharynx, trachea, and broncheal tree, so it really doesn't increase gas exchange very much. There is also some vascular shunting that goes on, but it's mostly just the shallowness of the breathing.