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SpaceNut,
Dook has actually posed an interesting problem for us to solve. He's insistent on using a RTG, rather than a fission reactor. The question is, "How do we squeeze every last watt of power out of the RTG?" To my knowledge, that's a problem NASA has never seriously considered because they're using RTG's to provide electrical power.
All current technology thermoelectric generators are ridiculously inefficient at converting heat to electricity, but RTG's are still generating a lot of heat. The new graphene thermoelectric converters may eventually manage to generate substantial electrical power, but the simplest solution is a heat engine.
Edit: I feel I should qualify my remarks about NASA trying to solve the electrical power problem using RTG's. To NASA's credit, they did develop the ASRG, which used a Stirling engine, before they subsequently cancelled that program. However, that was still a relatively low efficiency design. The 26% efficiency was substantially better than the 1% to 2% efficient RTG's currently used. There are better methods to utilize the heat produced by taking advantage of resources you have to take with you to use on Mars. The ASRG would've required less Pu238 for the same electrical output as the MMRTG, but it was only suitable for power requirements typical of robotic exploration missions.
Last edited by kbd512 (2016-11-01 23:53:56)
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NASA's SAFE-400 has a much more efficient power converter than SP-100. They both produce 100 kWe, but SP-100 requires 2000 kW thermal to do it while SAFE-400 requires 400 kW thermal. It uses Brayton cycle. The catch is that requires relatively high temperature to work. And high differential between the hot side and cold. That sounds about the same as you quoted for Sterling.
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The greenhouse can supply oxygen? I think you're idea is that the greenhouse is connected to the rest of the habitat so the oxygen that the greenhouse makes is used in breathing. My idea is to have the greenhouse built over the buried hab, the hab would have a regular pressurized atmosphere while the greenhouse is at 2 psi.
I may still be thinking too far ahead, but my reasoning is this: You will at some point want to use your greenhouse for food and oxygen. In a preliminary base, you're probably going to be field testing technologies to use in a future colony. Therefore you would want to demonstrate food and oxygen production (as well as climate control etc.) in your early greenhouse.
Otherwise, again, why do you have one at all?
Agricultural labor is more productive in a breathable atmosphere? Yes, but I think your idea of a first greenhouse is something much larger than the one I was thinking of. A small greenhouse wouldn't need much labor at all, water the trees once a day, run a dehumidifier once in a while, check the pressure inside and turn on the pump if it needs a boost. You can do that in a Mars Suit. I don't know about oxygenating the greenhouse to a level that would allow humans to breathe inside of it, kind of seems like a waste of oxygen. I'll have to think about that one.
20 kPa of Oxygen at 300 K has a density of about 0.25 kg/m^2 so you're not really "wasting" much oxygen at all.
I didn't consider that fruit would be damaged by moving it from a low pressure greenhouse to a higher pressure habitat. The fruit would probably get distorted but you can still eat it.
It's not going to make or break the greenhouse idea but most people would prefer pristine fruits to sort-of-squished fruits. There comes a point where you're better off eating rehydrated food than grown food, although I'm not sure where that point is. In mass and development cost (not zero) one imported greenhouse is worth a whole lot of imported, dehydrated food.
The inflatable greenhouse seems fine? For how long? It's plastic bag thin. Mars wind at 6 mph will move it around and temperature changes will have an effect on it over time. What if someone is using a shovel near the sidewall? If you're okay with it lasting a year then that's fine but why not just build something a little stronger?
How about thicker material? The greenhouse will have to be affixed to the ground somehow. I tend to favor aeroponics or hydroponics (more efficient use of volume) so there won't be much digging, but you do make a good point about resilience. I'm not sure what the answer is.
The thing about plants is that they can get fooled by temperature changes. They will go dormant and drop their leaves. Fruit dies if the temperature hits freezing. So if you're going to heat your greenhouse, are you going to use waste heat from a nuclear plant or RTG?
For any facility (not just a greenhouse) there's going to be a specification for the allowable diurnal temperature swing. Swings of 10-15 C in one day are not exceedingly rare on Earth. Your specification for the allowable temperature swing determines the thermal mass you need to couple to your greenhouse to remain within acceptable temperature ranges.
Depending how sophisticated your greenhouse is, you can modify the airflow between the layers to change the rate of heat transfer, specifically to cool the greenhouse. As I said you want to run hot (By this I mean that the equilibrium temperature is towards the upper end of your specification so that when you turn something on it's usually a cooling device) whenever possible because Mars is a cold planet and it's therefore easier to cool than to heat. IF you can make the air between the layers move faster, the heat transfer between them will increase.
I would like to reiterate that a multilayer, airtight greenhouse will have a tendency to run hot because most materials that are transparent to visible light absorb infrared and radiation is the dominant mode of heat loss. You could also coat the outer side of each layer to be IR reflective/non-emissive. This will not affect photosynthesis (IR is too low energy for plants to use) but it will reduce the outbound heat flux even further.
Putting an RTG near people is a bad idea. It's not as radioactive as a fission reactor but it still generates plenty of radioactive byproducts, not to mention X-rays and gamma rays. Such things should be strictly kept outside of enclosed spaces. If you want to pipe heat in that's fine (although as I just said it probably won't be necessary). Safety first and all.
I like the mirror heating idea. I wonder if there could be some kind of heat sink we could develop inside the greenhouse? Maybe a storage tank full of water that the mirror heats during the day? Not sure if it would give you very much.
The great thing about water is that it's dense and has a high heat capacity. One kg of water will absorb about 4 kJ for each degree celsius. One cubic meter of water will absorb 4 MJ per degree celsius, 60 MJ per 15 degrees celsius. On a volumetric basis, water holds 3200 times more heat per unit volume than air does. You might be able to get a little bit more out of some kind of wax that melts near room temperature and slowly freezes overnight but water is really useful and pretty thermally stable.
If there was a sudden depressurization loss inside your habitat, either with normal nitrogen atmosphere or just oxygen, you should be able to get to a Mars Suit breather or to an emergency Spare Air cannister and then slip on a Mars Suit. But if they are using a normal nitrogen atmosphere they will have to get the spare air and run to the Mars Hab lander or maybe the pressurized rover and slowly decompress or die a painful death. Without nitrogen, once you get your Mars Suit on you can work the problem. I don't know if a Mars Suit would work as a decompression device?
I think you're overestimating the effects of decompression sickness, which is usually relatively mild, but in any case it's less of an issue with the lower-nitrogen atmosphere I'm proposing..
-Josh
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SpaceNut,
Dook has actually posed an interesting problem for us to solve. He's insistent on using a RTG, rather than a fission reactor. The question is, "How do we squeeze every last watt of power out of the RTG?" To my knowledge, that's a problem NASA has never seriously considered because they're using RTG's to provide electrical power.
All current technology thermoelectric generators are ridiculously inefficient at converting heat to electricity, but RTG's are still generating a lot of heat. The new graphene thermoelectric converters may eventually manage to generate substantial electrical power, but the simplest solution is a heat engine.
Edit: I feel I should qualify my remarks about NASA trying to solve the electrical power problem using RTG's. To NASA's credit, they did develop the ASRG, which used a Stirling engine, before they subsequently cancelled that program. However, that was still a relatively low efficiency design. The 26% efficiency was substantially better than the 1% to 2% efficient RTG's currently used. There are better methods to utilize the heat produced by taking advantage of resources you have to take with you to use on Mars. The ASRG would've required less Pu238 for the same electrical output as the MMRTG, but it was only suitable for power requirements typical of robotic exploration missions.
I'm not insistent on using an RTG over a nuclear reactor. Whatever powers the ERV, whether it's an RTG or nuclear reactor, is already going to be on Mars when settlers get there, so, why not just drive over to it, put it on a Mars Cart, and bring it back to the base?
If it's an RTG, it's doable. If it's a very heavy nuclear reactor, I don't know how you pick it up and put it on a Mars Cart and tow it over sand and rocks with a solar/battery powered rover.
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I actually think RTGs - particularly ones using stronium-90 - will be ideal for powering bases if a much more efficient system for transforming thermal energy to electricity is used. Strontium titanate gets ~250 W/kg (pure strontium is more than double that), and produces beta radiation, with very little gamma radiation, so we shouldn't need much shielding at all. Perhaps we can get 25 W/kg for the overall system? That would mean a 1 tonne nuclear battery would generate 25 kWe. Enough for a rover, perhaps? Using pure strontium would double that. The half life is 29 years, so that leaves us with plenty of time to start using in situ energy resources.
Re. greenhouses, I favour low pressure ones. It puts a lot less strain on the structure, so they should be easier to fabricate and lighter, but there'll still be enough pressure for humans using lightweight oxygen masks to do the gardening. Yes, it means an airlock, but we're probably going to be overproducing oxygen if it's providing all the food so we'll need to actively manage the habs atmosphere somehow.
Use what is abundant and build to last
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The greenhouse can supply oxygen? I think you're idea is that the greenhouse is connected to the rest of the habitat so the oxygen that the greenhouse makes is used in breathing. My idea is to have the greenhouse built over the buried hab, the hab would have a regular pressurized atmosphere while the greenhouse is at 2 psi.
To me, the first settlement isn't really so much of a test bed. The exploration teams can set up small inflatable greenhouses and try and grow plants in them, if they stay for 550 days that Zubrin wanted instead of the 30 days that NASA wanted.
The greenhouse is going to produce some oxygen but we're talking very low amounts. Even a very large greenhouse wouldn't produce enough oxygen to sustain a single person. The greenhouse is mainly to grow food.
You're not wasting oxygen by using it to provide a breathable atmosphere in your greenhouse? Okay, but the plants will grow better in a high CO2 atmosphere and there is going to be some leakage of the atmosphere in the greenhouse to the outside. I just don't like the idea of using large MOXIE's to make oxygen and then using some of it to fill a greenhouse where it slowly leaks out.
The plastic bag greenhouse, to me, is not something we should use in a permanent settlement. The exploration teams should use them to test growing on Mars and see what the temperature changes are inside from day to night but if we're talking about permanent settlement, they need to have something sturdy. Typically, plastic bags have micro holes in them that allow air to leak out, that's why balloons don't float forever. So a thin plastic hab is going to need to be pumped up at times, if one loses too much pressure it falls to the surface and if the wind moves it, it tears, and you lose your all your plants.
You think an airtight greenhouse will tend to run hot on Mars? If it's an assembled greenhouse, it's not going to be perfectly air tight. If it's a plastic bag greenhouse, I don't know, I think it's still going to leak some, not as much as the assembled greenhouse. And, to pump up the pressure inside you're going to be taking in outside cold CO2, so, I'm not so sure your greenhouse is going to run hot.
Putting an RTG inside your greenhouse is a bad idea? Okay. We may need some kind of heat sink inside. The French take rocks from the river and place them around their grape vines to keep the temperature more stable.
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I wish that people posting things about radiation hazards had some passing familiarity with the actual threat posed, the magnitude of the dosages received, and how those dosages compare to the overall radiation environment on Mars or in space. The radiation received from GCR's and SPE's is more of an actual hazard to human health than the low energy gamma emitted from the RTG.
NASA and DoE personnel have been working in close proximity to RTG's for decades. The only protective equipment worn is insulating gloves to prevent thermal burns from hot RTG casings. If the casing of the RTG is intact, the radiation dosage received is negligible. Back in the 60's and 70's when we were still trying new things and weren't afraid of failures, we actually performed long term testing of RTG's to determine what the actual radiation emissions were as the Pu238 decays.
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How long can a Mars Suit oxygen supply last? I looked up a rebreather and it says the limit is set at 3 hours but the 3 liter cylinder will give up to 10 hours of diving.
Would the Mars Suit use a rebreather or a regular oxygen bottle?
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How long can a Mars Suit oxygen supply last? I looked up a rebreather and it says the limit is set at 3 hours but the 3 liter cylinder will give up to 10 hours of diving.
Would the Mars Suit use a rebreather or a regular oxygen bottle?
EMU style or MCP style?
What partial pressure O2?
What will the O2 cylinder be pressurized to?
What is the body mass of the astronaut and is the astronaut male or female?
What kind of physical activities would be undertaken?
We can't estimate how long something would provide enough O2 without knowing the answers to those questions.
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Anyone else who wants to have a serious discussion about these things and not just argue for the sake of arguing?
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I do think that the preparation of soil for crops will consume all available organic waste material and there wont be any left over for other purposes.
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I agree. Creating viable soils on Mars will be a key activity and a top priority I would think for the first few decades. Whilst it's true we can grow crops hydroponically and by other methods, soil growing (under natural sunlight if possible) is simply a very efficient method of crop production so I am sure we will want to move to that as soon as possible. It will also have valuable aesthetic and pyschological impacts - no one wants to see a tree growing with plastic water tubes attached. We want to see a tree putting its roots down in soil.
Here's a nice link to an article about soil constituents:
http://articles.extension.org/pages/544 … components
Basically it's 1. Minerals 2. Water 3. Organic matter. 4. Gases 5. Micro-organisms.
I do think that the preparation of soil for crops will consume all available organic waste material and there wont be any left over for other purposes.
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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You would have to have a very large greenhouse if all of your solid waste went towards fertilizer. Fertilizer is not something you add all the time. Human urine can be used as fertilizer too but it's really strong so it only takes something like 8 drops per gallon.
The only way to have a very large greenhouse quickly is with an inflatable. They're great at first but not something that will last. Also, how do you keep heat inside so your fruit trees don't go dormant and drop their leaves?
The most inside temperature variation that we could have without killing the fruit/vegetables would have to be 33 degrees F to maybe just over 100 F.
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Has anyone looked at how big of a domed greenhouse we could ship to Mars using hard plastic panels? If one whole Mars Direct launch was used just for the components of a large greenhouse? I think it could be huge, maybe something like a 75 foot diameter dome? It wouldn't need to be that high at the center, maybe 25' max, so the crew doesn't have to get up on high scaffolding.
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Also, we're going to put sand bags on top of the Mars Hab to protect against cosmic radiation but we're going to leave our plants out in a greenhouse exposed to this radiation. The vegetables don't live long enough for them to really be affected but the fruit trees would get more than fruit trees on the Earth.
Last edited by Dook (2016-11-04 09:07:28)
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Dook,
This is what I was saying before about the carbon cycle. Consider a tomato. The tomato, in growing, consumes CO2 and water. The tomato plant is made mostly from sugars, starches, and water (starches are just big sugar molecules). Photosynthesis creates these things from CO2 and water.
Humans consume the sugars and starches and turn them back into CO2 and water. It's a cycle. If you are using photosynthetic plants to grow all of the colony's food, they are necessarily also producing enough oxygen for the colonists to breathe. The problem, if anything, will be too much oxygen (food waste), but that's an easy problem to solve.
-Josh
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Dook,
This is what I was saying before about the carbon cycle. Consider a tomato. The tomato, in growing, consumes CO2 and water. The tomato plant is made mostly from sugars, starches, and water (starches are just big sugar molecules). Photosynthesis creates these things from CO2 and water.
Humans consume the sugars and starches and turn them back into CO2 and water. It's a cycle. If you are using photosynthetic plants to grow all of the colony's food, they are necessarily also producing enough oxygen for the colonists to breathe. The problem, if anything, will be too much oxygen (food waste), but that's an easy problem to solve.
I tried looking up how much oxygen a tree produces a day and the estimates are all over the place.
One website said that a giant Oak tree produces 12 tons of oxygen per day.
Another said a tree produces about 260 lbs of oxygen per year.
"A single mature tree can absorb carbon dioxide at a rate of 48 lbs./year and release enough oxygen back into the atmosphere to support 2 human beings."
- McAliney, Mike. Arguments for Land Conservation: Documentation and Information Sources for Land Resources Protection, Trust for Public Land, Sacramento, CA, December, 1993
"One acre of trees annually consumes the amount of carbon dioxide equivalent to that produced by driving an average car for 26,000 miles. That same acre of trees also produces enough oxygen for 18 people to breathe for a year."
- New York Times
" A 100-ft tree, 18" diameter at its base, produces 6,000 pounds of oxygen."
- Northwest Territories Forest Management
"On average, one tree produces nearly 260 pounds of oxygen each year. Two mature trees can provide enough oxygen for a family of four."
- Environment Canada, Canada's national environmental agency
"Mean net annual oxygen production (after accounting for decomposition) per hectare of trees (100% tree canopy) offsets oxygen consumption of 19 people per year (eight people per acre of tree cover), but ranges from nine people per hectare of canopy cover (four people/ac cover) in Minneapolis, Minnesota, to 28 people/ha cover (12 people/ac cover) in Calgary, Alberta."
- U.S. Forest Service and International Society of Arboriculture joint publication
The amount of oxygen produced per plant/tree can't be very much because the efficiency of photosynthesis is extremely low.
Also, fruit trees go dormant in winter so they drop their leaves and vegetables are short lived so you would have a big plant that would die and you would then have to regrow one from seed.
Last edited by Dook (2016-11-04 10:24:23)
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Dook
Do you know if the oxygen production estimates allow for that consumed in rotting the fallen trees? Obviously the Mars colonists aren't going to allow such a useful structural material to rot so the compost system only has to deal with twigs, bark and leaves.
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Some comments:
1. For the small early colony, energy production will not be a problem, whether we have nuclear reactors, RTGs or PV panels.
2. Given energy will not be an issue separating out hydrogen, oxygen and carbon from Mars water and atmosphere won't be an issue either.
3. Human waste will contain a range of plant nutrients that cannot easily be found or processed on Mars. Human and food waste will be vital if we are to develop Mars agricultural.
4. There is no need - and a good deal of risk involved in an attempt - to move straight to natural sunlight soil-based agriculture under glass or plastic domes. I am fairly convinced myself that our first agriculture on Mars will be indoors under artificial light. There will be little incentive to move away from that until the population grow substantially - maybe 1,000 plus. Naturally sunlit plant growth will probably first be required for the creation of human-friendly natural environments on Mars for our recreation and delight. So I can imagine domes and covered gorges being used to create pleasant small parks with trees and maybe some manageable non-plant life forms like butterflies.
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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Dook,
Because different types of plants have different growth rates, sizes, and photosynthetic efficiencies, looking at oxygen production per day is basically useless. One blade of grass produces virtually no oxygen. A giant bamboo stalk produces quite a lot.
You're better off looking at oxygen produced per kilogram of plant and then comparing the food production rates of various plants used in agriculture. When selecting plants there will definitely be interest in plants with high photosynthetic efficiency and high growth rates.
I think you'll find that it takes a lot of plants to feed a person, but if you do your math right that it also takes that same large amount of plants to provide oxygen for them to breathe.
-Josh
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Dook
Do you know if the oxygen production estimates allow for that consumed in rotting the fallen trees? Obviously the Mars colonists aren't going to allow such a useful structural material to rot so the compost system only has to deal with twigs, bark and leaves.
The oxygen production estimates for trees is all over the place. I was hoping someone here already had better information on it. I don't know if they factor in rotting trees or whether they factor in that trees consume some oxygen to produce oxygen.
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Dook,
Because different types of plants have different growth rates, sizes, and photosynthetic efficiencies, looking at oxygen production per day is basically useless. One blade of grass produces virtually no oxygen. A giant bamboo stalk produces quite a lot.
You're better off looking at oxygen produced per kilogram of plant and then comparing the food production rates of various plants used in agriculture. When selecting plants there will definitely be interest in plants with high photosynthetic efficiency and high growth rates.
I think you'll find that it takes a lot of plants to feed a person, but if you do your math right that it also takes that same large amount of plants to provide oxygen for them to breathe.
Okay, but I think we're talking about an absolute minimum number of settlers, like 3, and multiple very large greenhouses in order for the plants to provide enough food and oxygen to be sustainable.
So you would have to use large inflatables to get the vegetables and fruit trees going until you can build multiple large permanent greenhouses, then move the plants inside.
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NASA studies, and other studies, have shown a greenhouse to produce sufficient food for settlers actually produces too much oxygen. Far too much. So if compost consumes some, that's Ok.
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What size was the greenhouse? What plants were used?
The biosphere experiment had to be stopped both times because of low oxygen and low food production.
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http://www.nasa.gov/vision/earth/living … ouses.html
Low pressure makes plants act as if they're drying out.
In recent experiments, supported by NASA's Office of Biological and Physical research, Ferl's group exposed young growing plants to pressures of one-tenth Earth normal for about twenty-four hours. In such a low-pressure environment, water is pulled out through the leaves very quickly, and so extra water is needed to replenish it.
That's bad.
For example, if you store fruit at low pressure, it lasts much longer.
That's okay.
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