Life support
This is going to be looking at things from a combination of contemporary tech (and reasonable extensions thereof) and future tech based mainly on the Traveller RPG. It's probably easily adaptable to other backgrounds.
Humans need a few basic items. I'm going to list them in order of importance. That is, how quickly you'll be in trouble without them.
They are:
air
temperature
water
food
gravity
First is air. This includes pressure and composition. In a vacuum, you've got less than 30 seconds of useful consciousness. This has been determined by both experiments (on humans and chimps) and accidents both in space and large vacuum chambers on earth.
The reason you pass out is because if you aren't holding your breath (*really* bad idea) the pressure in your lungs drops to the evaporation pressure of water at body temp. Which means that as blood passes through your lungs, both the oxygen *and* the CO2 come out of the blood and into the air passages and leave your body.
CO2 isn't a biggie here. Low CO2 "turns off" your breathing reflex which won't be a problem for a bit. But low oxygen means that you'll have a slug of "totally" de-oxygenated blood heading straight to your brain. There it'll strip the existing oxygen out with the result that your brain cells quit working. Oops.
Longer term, your body will take other damage from vacuum, but you've got *minutes* for that.
If you have at least a few PSI of pressure, you don't have to worry about any of the above. To function normally you need at least 3 PSI of oxygen. Native of the Andes or Himalayas can get by on considerably less.
You don't want to run the partial pressure of oxygen higher than that. That's because at higher pressures it becomes toxic. You can also run into nasty fire hazards. At normal pressure somewhere between 20% & 25% oxygen you hit the point where *wet* organic material will burn. This includes *you*.
Nitrogen is good as a filler gas to make up the rest of the pressure. It's also quite common and is needed by plants if you are going to grow food. It does, however mean that you will get the bends if you go to lower pressure too fast.
That's a problem with lower tech spacesuits, as you want to run them at 3 psi of pure oxygen to reduce a number of problems. Higher tech suits can let you use normal air.
Finally, you want a certain level of humidity to avoid damage to the nose, mouth & lungs. As well as to prevent rapid dehydration.
Now, we've covered what should be in the air. Nitrogen should mostly stay put. Oxygen can be replaced from tanks even on moderately long missions (weeks). Humidity, we'll get to that in a moment.
Now, there are things you *don't* want in air. Any of a *large* number of compounds are unpleasant at quite low concentrations, and actually toxic at higher ones. As I recall, there are even a few where the toxic concentration is *below* the "unpleasant" one.
So we have to remove these. The two biggies are CO2 and water. Yes, water is a problem
The basic way we get energy is by eating food and combining it with oxygen. Most of this is provided by carbohydrates, though lipids (fats and oils) and protein contribute as well.
Carbohydrates have a "generic" formula of Cx(H2O)y
So the reaction is:
X O2 + Y Cx(H2O)y -> X CO2 + Y H2O
Thus you wind up with a *lot* more water than you start with.
A fair bit of that goes into the air from respiration and from perspiration.
You also get all that CO2.
Fortunately, you can deal with both the same way. By passing the air thru cartridges containing lithium hydroxide. Other hydroxides will work in a pinch.
The CO2 reacts to make lithium carbonate, and the water is absorbed by the hydroxide or the carbonate as well.
Activated charcoal will deal with the rest of the gases well enough. You have to replace the cartridges of lithium hydroxide and activated charcoal every so often. They can be "regenerated" elsewhere.
That works fine for smaller ships. Larger ships and stations will need more complex methods.
Temperature is next, because being too cold or too hot will incapacitate you a lot faster than lack of food or water. On a spaceship or space station, being too cold is unlikely to be a problem. That's because they are essentially giant thermos flasks, and the people and equipment inside will generate a *lot* of heat.
In space there are only two ways to get rid of heat. You can't conduct it away, because you are surrounded by a vacuum. On an asteroid or planetary base, that might be an option.
You can't use convection, because, again, no air. If you are on a body with air or water (or other liquid) you can use that.
As an *extremely* temporary measure you can allow water to evaporate into the vacuum. This carries away a lot of heat, but uses up water a lot faster than you'd expect. It's used to cool spacesuits but isn't practical for anything else.
So that leaves radiation. You pump a working fluid into radiator fins and let the waste heat radiate out into space. Trouble here is that the amount of heat you can get rid of (measured in watts) depends *solely* on the temperature of the radiator and on its surface area. Note that we are talking about the *absolute* temperature (degrees above absolute zero) and that the energy radiated goes as the *4th* power of the temperature. So something twice as hot radiates away *16* times as much heat (in terms of watts per square meter)
Oh yeah, if any part of a radiator "fin" can be "seen" from another, they will be throwing heat at each other and reducing the *effective* area. So you are basically stuck with a pair of big flat fins on on each side of the ship, sticking straight out.
For systems that work at a high temp (like a fusion reactor) radiators can be at a very high temp. The energy you can get out of a heat engine (and even reactors are heat engines in the end) depends on the *difference* in absolute temperature between the input and the output.. So a few million degrees in and a few thousand degrees out and the radiator fins at a few thousand K are radiating heat like crazy.
But the parts of the ship *people* live in need to be kept around room temp. Call it 300K. Waste heat doesn't radiate that well at those temps. So you need *huge* radiators.
The first thing the Space Shuttle did on reaching orbit was open the cargo bay doors. Not because they needed the power from the solar cells on the inner surfaces of the doors, but because they needed the *heat* radiators on the inner surface of the doors to keep things livable.
That's the major failing of every single SF ship and station I've ever seen. Not enough radiator area. The crews would cook in their own juices. :-(
Note that this *does* make it possible to force a ship to surrender without blowing holes in it. Just aim a nice, powerful (but slightly defocused) laser at their radiators. They'll have to cut power use to a minimum to avoid cooking, and eventually they'll have to surrender or fry.
Ok, next we have food and water. Easy to carry enough of both for a long time. But a lot of the uses of water produce *dirty* water. And food produces waste, both scraps and "sewage".
So you want to divide the wastewater system into two categories: graywater and blackwater.
Graywater is stuff like what goes down drain after you wash your hands or body. It can be re-used for things like flushing toilets.
Blackwater is sewage. Both urine and feces, as well as water contaminated in other ways (like chemicals from cleaning floors or equipment).
You can just dump the blackwater into a holding tank and pump it out lat the end off the trip. But on anything but the smallest ship, you'd probably want to do better.
Given decent power sources, you can recycle the water in various ways. You can also use a dehumidifier before running the air through the lithium hydroxide and activated charcoal. That's give you a fair bit of water that needs very little treatment before being added back to the potable water supply. It'll also extend the life of the lithium hydroxide cartridges.
You can also dry out the stuff that'd go to the holding tank. If you aren't worried about saving water you can just vent to vacuum and let it evaporate away. That'd leave you with some somewhat damp "solids" which will take up a lot less room.
Or you can boil things and capture the water vapor. This will sterilize the solids, though the condensed vapor should probably be distilled or electrolyzed to purify it as other things will boil off as well.
You can heat the leftover solids to further dry & sterilize them. If your reactor can supply a fair bit of gamma (but not neutrons) you can sterilize the solids that way as an extra step. Then compress them into blocks and seal them up.
The blocks of waste, and your extra water will be valuable to many places you stop. They'd likely pay you for the organics, even if they had to further treat them to remove various chemicals.
Oh yes, you can also electrolyze the water to hydrogen & oxygen, and just put the oxygen back in the tanks, and either vent the hydrogen or dump it back in the fuel tanks if you use hydrogen for fuel.
Large installations can refurbished the activated charcoal and lithium hydroxide cartridges. In both cases you do it be heating them to drive off the absorbed/adsorbed materials.
Medium sized ones could flash heat the air which would cause most of the contaminants to oxidize. Run the results thru a few chemical fliters (to remove various chlorine and sulfur compounds as well as break down nitrogen oxides. Then run the air thru the dehumidifiers and CO2 scrubbers.
The CO2 scrubbers may chill things enough to freeze it out, or may heat things hot enough to break it down (with fusion reactors this would be fairly simple). Of course with the heat method, some poor soul gets to chip the carbon buildup out of the system...
The really large places can use biological "scrubbing" algae or even plants to process the CO2 back into oxygen. A lot of the unwanted volatiles can be handled by some plants as well.
On planets or large asteroid bases, you can even go as far as the artificial marsh treatment system. You sterilize the sewage, then run it through a series of marshes. The nasty chemicals get taken up by the various grasses. Which can be burned and things like heavy metals recovered from the ashes.
After going through several of these marshes the water in the final stage is potable and fish such as tilapia are raised the ponds.
On an asteroid you'd need to create some *big* caverns for the marshes, and have some sort of gravity. On a non-habitable planet you can blast big caverns underground, or possibly dome over areas.
Food production deserves an article of its own.
Finally, gravity. People can function for very long times in zero g. But it still appears that long term we need something approaching 1 g. Spin gravity requires a very large radius of spin to avoid folks getting nauseous as they move around due to Coriolis forces acting on their inner ears.
Traveller and other universes have various sorts of artificial gravity to handle this. But it does need to be considered.
This entry was originally posted at http://kengr.dreamwidth.org/1035239.html. Please comment there using OpenID.
Humans need a few basic items. I'm going to list them in order of importance. That is, how quickly you'll be in trouble without them.
They are:
air
temperature
water
food
gravity
First is air. This includes pressure and composition. In a vacuum, you've got less than 30 seconds of useful consciousness. This has been determined by both experiments (on humans and chimps) and accidents both in space and large vacuum chambers on earth.
The reason you pass out is because if you aren't holding your breath (*really* bad idea) the pressure in your lungs drops to the evaporation pressure of water at body temp. Which means that as blood passes through your lungs, both the oxygen *and* the CO2 come out of the blood and into the air passages and leave your body.
CO2 isn't a biggie here. Low CO2 "turns off" your breathing reflex which won't be a problem for a bit. But low oxygen means that you'll have a slug of "totally" de-oxygenated blood heading straight to your brain. There it'll strip the existing oxygen out with the result that your brain cells quit working. Oops.
Longer term, your body will take other damage from vacuum, but you've got *minutes* for that.
If you have at least a few PSI of pressure, you don't have to worry about any of the above. To function normally you need at least 3 PSI of oxygen. Native of the Andes or Himalayas can get by on considerably less.
You don't want to run the partial pressure of oxygen higher than that. That's because at higher pressures it becomes toxic. You can also run into nasty fire hazards. At normal pressure somewhere between 20% & 25% oxygen you hit the point where *wet* organic material will burn. This includes *you*.
Nitrogen is good as a filler gas to make up the rest of the pressure. It's also quite common and is needed by plants if you are going to grow food. It does, however mean that you will get the bends if you go to lower pressure too fast.
That's a problem with lower tech spacesuits, as you want to run them at 3 psi of pure oxygen to reduce a number of problems. Higher tech suits can let you use normal air.
Finally, you want a certain level of humidity to avoid damage to the nose, mouth & lungs. As well as to prevent rapid dehydration.
Now, we've covered what should be in the air. Nitrogen should mostly stay put. Oxygen can be replaced from tanks even on moderately long missions (weeks). Humidity, we'll get to that in a moment.
Now, there are things you *don't* want in air. Any of a *large* number of compounds are unpleasant at quite low concentrations, and actually toxic at higher ones. As I recall, there are even a few where the toxic concentration is *below* the "unpleasant" one.
So we have to remove these. The two biggies are CO2 and water. Yes, water is a problem
The basic way we get energy is by eating food and combining it with oxygen. Most of this is provided by carbohydrates, though lipids (fats and oils) and protein contribute as well.
Carbohydrates have a "generic" formula of Cx(H2O)y
So the reaction is:
X O2 + Y Cx(H2O)y -> X CO2 + Y H2O
Thus you wind up with a *lot* more water than you start with.
A fair bit of that goes into the air from respiration and from perspiration.
You also get all that CO2.
Fortunately, you can deal with both the same way. By passing the air thru cartridges containing lithium hydroxide. Other hydroxides will work in a pinch.
The CO2 reacts to make lithium carbonate, and the water is absorbed by the hydroxide or the carbonate as well.
Activated charcoal will deal with the rest of the gases well enough. You have to replace the cartridges of lithium hydroxide and activated charcoal every so often. They can be "regenerated" elsewhere.
That works fine for smaller ships. Larger ships and stations will need more complex methods.
Temperature is next, because being too cold or too hot will incapacitate you a lot faster than lack of food or water. On a spaceship or space station, being too cold is unlikely to be a problem. That's because they are essentially giant thermos flasks, and the people and equipment inside will generate a *lot* of heat.
In space there are only two ways to get rid of heat. You can't conduct it away, because you are surrounded by a vacuum. On an asteroid or planetary base, that might be an option.
You can't use convection, because, again, no air. If you are on a body with air or water (or other liquid) you can use that.
As an *extremely* temporary measure you can allow water to evaporate into the vacuum. This carries away a lot of heat, but uses up water a lot faster than you'd expect. It's used to cool spacesuits but isn't practical for anything else.
So that leaves radiation. You pump a working fluid into radiator fins and let the waste heat radiate out into space. Trouble here is that the amount of heat you can get rid of (measured in watts) depends *solely* on the temperature of the radiator and on its surface area. Note that we are talking about the *absolute* temperature (degrees above absolute zero) and that the energy radiated goes as the *4th* power of the temperature. So something twice as hot radiates away *16* times as much heat (in terms of watts per square meter)
Oh yeah, if any part of a radiator "fin" can be "seen" from another, they will be throwing heat at each other and reducing the *effective* area. So you are basically stuck with a pair of big flat fins on on each side of the ship, sticking straight out.
For systems that work at a high temp (like a fusion reactor) radiators can be at a very high temp. The energy you can get out of a heat engine (and even reactors are heat engines in the end) depends on the *difference* in absolute temperature between the input and the output.. So a few million degrees in and a few thousand degrees out and the radiator fins at a few thousand K are radiating heat like crazy.
But the parts of the ship *people* live in need to be kept around room temp. Call it 300K. Waste heat doesn't radiate that well at those temps. So you need *huge* radiators.
The first thing the Space Shuttle did on reaching orbit was open the cargo bay doors. Not because they needed the power from the solar cells on the inner surfaces of the doors, but because they needed the *heat* radiators on the inner surface of the doors to keep things livable.
That's the major failing of every single SF ship and station I've ever seen. Not enough radiator area. The crews would cook in their own juices. :-(
Note that this *does* make it possible to force a ship to surrender without blowing holes in it. Just aim a nice, powerful (but slightly defocused) laser at their radiators. They'll have to cut power use to a minimum to avoid cooking, and eventually they'll have to surrender or fry.
Ok, next we have food and water. Easy to carry enough of both for a long time. But a lot of the uses of water produce *dirty* water. And food produces waste, both scraps and "sewage".
So you want to divide the wastewater system into two categories: graywater and blackwater.
Graywater is stuff like what goes down drain after you wash your hands or body. It can be re-used for things like flushing toilets.
Blackwater is sewage. Both urine and feces, as well as water contaminated in other ways (like chemicals from cleaning floors or equipment).
You can just dump the blackwater into a holding tank and pump it out lat the end off the trip. But on anything but the smallest ship, you'd probably want to do better.
Given decent power sources, you can recycle the water in various ways. You can also use a dehumidifier before running the air through the lithium hydroxide and activated charcoal. That's give you a fair bit of water that needs very little treatment before being added back to the potable water supply. It'll also extend the life of the lithium hydroxide cartridges.
You can also dry out the stuff that'd go to the holding tank. If you aren't worried about saving water you can just vent to vacuum and let it evaporate away. That'd leave you with some somewhat damp "solids" which will take up a lot less room.
Or you can boil things and capture the water vapor. This will sterilize the solids, though the condensed vapor should probably be distilled or electrolyzed to purify it as other things will boil off as well.
You can heat the leftover solids to further dry & sterilize them. If your reactor can supply a fair bit of gamma (but not neutrons) you can sterilize the solids that way as an extra step. Then compress them into blocks and seal them up.
The blocks of waste, and your extra water will be valuable to many places you stop. They'd likely pay you for the organics, even if they had to further treat them to remove various chemicals.
Oh yes, you can also electrolyze the water to hydrogen & oxygen, and just put the oxygen back in the tanks, and either vent the hydrogen or dump it back in the fuel tanks if you use hydrogen for fuel.
Large installations can refurbished the activated charcoal and lithium hydroxide cartridges. In both cases you do it be heating them to drive off the absorbed/adsorbed materials.
Medium sized ones could flash heat the air which would cause most of the contaminants to oxidize. Run the results thru a few chemical fliters (to remove various chlorine and sulfur compounds as well as break down nitrogen oxides. Then run the air thru the dehumidifiers and CO2 scrubbers.
The CO2 scrubbers may chill things enough to freeze it out, or may heat things hot enough to break it down (with fusion reactors this would be fairly simple). Of course with the heat method, some poor soul gets to chip the carbon buildup out of the system...
The really large places can use biological "scrubbing" algae or even plants to process the CO2 back into oxygen. A lot of the unwanted volatiles can be handled by some plants as well.
On planets or large asteroid bases, you can even go as far as the artificial marsh treatment system. You sterilize the sewage, then run it through a series of marshes. The nasty chemicals get taken up by the various grasses. Which can be burned and things like heavy metals recovered from the ashes.
After going through several of these marshes the water in the final stage is potable and fish such as tilapia are raised the ponds.
On an asteroid you'd need to create some *big* caverns for the marshes, and have some sort of gravity. On a non-habitable planet you can blast big caverns underground, or possibly dome over areas.
Food production deserves an article of its own.
Finally, gravity. People can function for very long times in zero g. But it still appears that long term we need something approaching 1 g. Spin gravity requires a very large radius of spin to avoid folks getting nauseous as they move around due to Coriolis forces acting on their inner ears.
Traveller and other universes have various sorts of artificial gravity to handle this. But it does need to be considered.
This entry was originally posted at http://kengr.dreamwidth.org/1035239.html. Please comment there using OpenID.