Author Topic: Mars ISRU for food crops and consumables.  (Read 96525 times)

Offline aero

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Mars ISRU for food crops and consumables.
« on: 09/02/2012 04:26 am »
The Mars One thread drifted into greenhouses and growing food on Mars. This thread is to continue that discussion.

Should this get a new thread?  Mars-One hasn't given any credible public presentation that they have considered a garden just yet.

Yeah, probably should. I dont want to start it because I might not have much more to add.

The discussion above is what I meant and has produced two approaches for putting bounds on the energy requirements:

* Energy to grow an acre of corn using sunlight (we can do at least this well)
* Fundamental efficiency of photosynthesis - possibly improvable with selective wavelengths (we can do at most this well)

(a wacky alternative could be if we could power life other than by photosynthesis, e.g. if it turned out more efficient to create a sugar or alcohol through non-biological means and fed this to some bacteria.. I have no idea if any such mechanism exists so Im not pushing it)


Some are concerned that sun light might not be strong enough on Mars to grow Earth food plants. Solar intensity at the top of Mars atmosphere ranges from 715 down to 492 W/m^2. That is about 1/2 to 1/3 of that of Earth. The wide variation is due to the eccentricity of Mars' orbit. The question is, "Can food plants be grown with only 1/3 to 1/2 the natural sunlight of Earth?" The answer is yes, because food plants grow on Earth in the band of latitudes between Munich and Siberia which only receives 1/2 to 1/3 the full solar intensity at Earth's equator, because of the high latitude. This is true even if the Mars atmosphere attenuates the solar radiation to the same extent that the Earth's atmosphere does, which is not proven.

To broaden this topic a bit though, there are other crops valuable to a new colony that are not edible. It's really to bad that duct tape doesn't grow on bushes, but rope is made from hemp, and clothing, blankets, draperies, rugs etc. from cotton. Bonsai (trees) might be nice, too.

If we colonize Mars, we must live off the land.
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Online Robotbeat

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Re: Mars ISRU for food crops and consumables.
« Reply #1 on: 09/02/2012 06:27 am »
Chemically converting carbon, hydrogen, oxygen, and nitrogen into fibers and even macronutrients using just air and water and electricity makes a lot of sense in my opinion, if it can be done effectively enough. And would be awesome on Earth as well. And should be more efficient than with plants and photosynthesis.
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Offline guckyfan

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Re: Mars ISRU for food crops and consumables.
« Reply #2 on: 09/02/2012 11:58 am »
The Mars One thread drifted into greenhouses and growing food on Mars. This thread is to continue that discussion.

Some are concerned that sun light might not be strong enough on Mars to grow Earth food plants. Solar intensity at the top of Mars atmosphere ranges from 715 down to 492 W/m^2. That is about 1/2 to 1/3 of that of Earth. The wide variation is due to the eccentricity of Mars' orbit. The question is, "Can food plants be grown with only 1/3 to 1/2 the natural sunlight of Earth?" The answer is yes, because food plants grow on Earth in the band of latitudes between Munich and Siberia which only receives 1/2 to 1/3 the full solar intensity at Earth's equator, because of the high latitude. This is true even if the Mars atmosphere attenuates the solar radiation to the same extent that the Earth's atmosphere does, which is not proven.

Great thread start, thanks a lot.

To broaden this topic a bit though, there are other crops valuable to a new colony that are not edible. It's really to bad that duct tape doesn't grow on bushes, but rope is made from hemp, and clothing, blankets, draperies, rugs etc. from cotton. Bonsai (trees) might be nice, too.

If we colonize Mars, we must live off the land.

Good point that gives us much to discuss.

Chemically converting carbon, hydrogen, oxygen, and nitrogen into fibers and even macronutrients using just air and water and electricity makes a lot of sense in my opinion, if it can be done effectively enough. And would be awesome on Earth as well. And should be more efficient than with plants and photosynthesis.

Is there any reference to that? A quick search yielded nothing for me.

But independently of that is is a valid point to discuss. It might have its value initially or much later if a cheap energy source is available, like fusion power.

My opinion is it will be the other way around. Algae are easy to grow and will provide food as well as raw material for a developing chemical industry.

Bioengineering will help to develop different kinds of algae which will utilize more of the spectrum and use the available light better.

That last part is speculation and will probably need a lot of advances in that field but research on growing algae to replace the waning fossil ressouces of earth are already on the way.


Offline guckyfan

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Re: Mars ISRU for food crops and consumables.
« Reply #3 on: 09/02/2012 12:04 pm »
Transferred my last post from the Mars One thread over here.


The discussion above is what I meant and has produced two approaches for putting bounds on the energy requirements:

* Energy to grow an acre of corn using sunlight (we can do at least this well)
* Fundamental efficiency of photosynthesis - possibly improvable with selective wavelengths (we can do at most this well)

Efficiency of artificial lighting could probably be further improved by supplying only the needed amount of light for the phase of growth. I believe plants need a lot less light while sprouting from seed and while developing foliage. They will certainly need a lot of energy while producing the nutrients. That curve of needs would be a research objective when developing green houses with artificial lighting for mars. Or maybe green house operators know already a lot about it.

(a wacky alternative could be if we could power life other than by photosynthesis, e.g. if it turned out more efficient to create a sugar or alcohol through non-biological means and fed this to some bacteria.. I have no idea if any such mechanism exists so Im not pushing it)

Somebody mentioned completely artificial production of food. That may be more energy efficient. I was not able to find anything on that using google. Any info?

BTW, nobody commented on my suggestion of producing algae yet. I still believe it would solve most of the problems without the need of artificial lighting and big pressurized structures.


Offline aero

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Re: Mars ISRU for food crops and consumables.
« Reply #4 on: 09/02/2012 02:46 pm »
Quote
BTW, nobody commented on my suggestion of producing algae yet. I still believe it would solve most of the problems without the need of artificial lighting and big pressurized structures.

There are probably some delicious recipes to be cooked up from algae, I just have never tried them and more, I don't know of any. Algae sounds distasteful but if we have food animals or pets on Mars, then maybe they would eat algae based food.

Perhaps a use for algae would be as a fertilizer to condition the soil by feeding the micro orginsms needed in the soil by the plants. Of course we could most likely produce most of the fertilizer, but the needed trace elements for plant growth may require decay of prior plant (algae) growth material.

Question. If we use long, narrow greenhouses, what is the most economical way to keep them warm at night? A removable thermal blanket?

Question. Do we know how much atmospheric attenuation of sunlight results from the Mars atmosphere?

Question. If plant life existed on Mars eons ago, will the soil already be conditioned for the reintroduction of plant life?
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Offline guckyfan

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Re: Mars ISRU for food crops and consumables.
« Reply #5 on: 09/02/2012 04:07 pm »
I wrote much of this on the other thread but think I should repeat it here.

There are probably some delicious recipes to be cooked up from algae, I just have never tried them and more, I don't know of any. Algae sounds distasteful but if we have food animals or pets on Mars, then maybe they would eat algae based food.

I doubt the delicious. There are plenty of recipes but they all use algae only as a healthy supplement to the more tasty common ingredients. Easiest way would be a drink that will provide all of the dietary requirements except good taste.

But with advancing capabilities oil, carbohydrates and protein can be separated and worked into more palatable dishes.

People will want more conventional dishes, vegetable and fruit raised in conventional greenhouses to supplement the staple algae.

Yes, algae will make perfect feed for fish, shrimp and chicken as starter meat sources. But that would not be one of the earlier concerns.

Perhaps a use for algae would be as a fertilizer to condition the soil by feeding the micro orginsms needed in the soil by the plants. Of course we could most likely produce most of the fertilizer, but the needed trace elements for plant growth may require decay of prior plant (algae) growth material.

Not really an requirement initially. Most plants will grow readily on anorganic fertilizers. Best example they do very well in aquaculture.

But if grown in soil, remnants like roots will remain after harvest. That is when soil life like bacteria, fungi will be required to recycle the organics. With time a healthy soil with lots of organic components will develop.

My biggest concern with martian soil are the minerals. I don't know if there are components that will oxidize and so consume the oxygen produced by the plants. If that happens it may take a long time, until the greenhouse becomes a source of oxygen. Worse, the lack of oxygen may make aerobic ground life impossible. And higher plants will not tolerate an anaerobic climate at their roots.

On earth geologic ages passed before life produced enough oxygen to oxidize all minerals and finally an atmosphere containing oxygen developed.

Again growing algae in bioreactors will not have any of these problems.

Question. If we use long, narrow greenhouses, what is the most economical way to keep them warm at night? A removable thermal blanket?

I proposed large domes of thin plastic sheets lifted up by very small pressure. These plastic sheets would have UV-reflecting or absorbing coating on the outside and infrared reflecting coating on the inside. Such coatings are in use already on earth and are quite effective.

Thin domes like this would not be feasible on earth as wind, rain, hail will destroy them. The mars atmosphere is extremely thin and a light dome from very thin plasticsheets will be sufficient.

If the sheets would be 1mm the weight would be app. 1kg/m allowing large structures shipped from earth.

Also such domes on earth would not provide adequate insulation as the earth atmosphere has strong thermal conduction. The mars atmosphere does not. So we deal mainly with infrared which can be reatained through the coating.
A second dome inside may be required to stop thermal condution even better. But this dome not exposed to martian wind could be even thinner than the outer shell, in the range of 1/10mm thickness.

This is my idea only and would need to be verified but I am confident, it will work that way.

Question. Do we know how much atmospheric attenuation of sunlight results from the Mars atmosphere?

No idea but less than earth.

Question. If plant life existed on Mars eons ago, will the soil already be conditioned for the reintroduction of plant life?

I am afraid not. I doubt that life on mars existed long enough to oxidize all minerals at the surface.  Certainly there is not enogh ground life to deal with organic waste. But what we know, is that all required minerals are available.

Offline JohnFornaro

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Re: Mars ISRU for food crops and consumables.
« Reply #6 on: 09/02/2012 04:44 pm »
From that other thread, pertinant to this one:

1) 100 watthour = 85.984 522 786 Calorie [nutritional]

So 100 wattday would be a 2064 Calorie a day diet, if you could eat the watts.

You could actually look at it another way, by comparing field crops on earth to Calories produced. I don't want to do the work to get it right, but one acre of corn receives about 4000 kwh (~4000 m^2 at 1 kw/m^2)sunlight per hour of sun shine. 2) Discounting the energy needed to produce fertilizer, assuming 6 hour (very short) sunshine days and an 8 week (very short) growing season. Multiply that out and get

4000 kWhr * 6 hr/day * 7 days/week * 8 weeks/growing season = 1.3 GWhr

to produce 150 bushels of corn.

3) Hope you like feed corn. I'd rather eat potatoes myself.

1) You have 9 insignificant digits in your figure.  I'm lazy too, but am willing to accept for the moment, that 100 watthours roughly equals 86 calories.

2) I agree about discounting fertilizer for the moment.  In my soil, there is none needed.  However, that's a massive amount of "seed" soil that would have to be launched, if the idea of "seed" soil is accepted as one possible strategy.  In my case, 50'x70'x1' deep by 150 pounds/ft^3 equals about 260 tons of soil.  To be launched and landed.  Remember, other amendment processes are possible.  This approach has the advantage of simplicity, as well as automatically providing a good starting soil for that garden.

The soil could be used as radiation shielding for the trip up.  Being damp, it would have some good qualities.  Assuming an MAV, and ISRU methane production, the soil could be landed over time, from the orbiting mothership, itself equipped for the possible journey back to LEO.

If non-poisonous martian soil can be assumed, it's possible to suggest a martian soil ammendment process.  Till 130 tons of seed soil into 650 tons of martian soil to a depth of one foot.  Plant your plants in one quart paper containers at the appropriate spacing, roughly at 12" centers.  Over time, in theory, the martian soil is amended, and the process can continue over the years.  Very high initial costs, but the amortized costs, if successful, almost negligible after, say, fifty years.  Remember, other amendment processes are possible.

I planted my corn in the wrong place, and only got about 6 hours of Earthly sunshine, resulting in roughly 50% expected cob size. You really need 8 to 10 hours, for roughtly   And that after more like 10-12 weeks.  So your gigawatt number is way low.

3) This is the argument for variety.  Clearly, genetically modified plants may prove useful.  Algae and aquaculture should be considered.  Fortunately, seeds themselves are relatively massless; each colonist could bring eight or so ounces of her favorite varieties, including flowers, I'd think.

But anyhowwww.....

The discussion above is what I meant and has produced two approaches for putting bounds on the energy requirements...

Same here.  Obviously, we're going to have to discuss these bounds here.  Mars-One ain't gonna.

The question is, "Can food plants be grown with only 1/3 to 1/2 the natural sunlight of Earth?" The answer is yes, because food plants grow on Earth in the band of latitudes between Munich and Siberia which only receives 1/2 to 1/3 the full solar intensity at Earth's equator, because of the high latitude.

Well, yes and no.  You forget that no plants are grown in these latitudes during the winter; only the summer, when the Sun is much higher in the sky, and the solar insolation is much higher than the relatively constant 1/2 to 1/3 available on Mars.

Some kind of solar concentration is necessary.  I'm thinking a hybrid of reflective panels, and active LED lighting designed to augment certain wavelengths.  If there is a hybrid lighting system, it could probably be multi level.

Also, the SkyLab experiments seemed to confirm for me that plants in lower gees would be taller and more spindly than here on Earth.  My corn is maybe 6'-7' hi; lower than the 8'+ I've had in previous gardens.  I'm thinking that the corn plants on Mars will be 12'-16' high.  The appearance of the garden will be quite different from earthly ones.

Plants for other purposes would be useful.  Wood is very handy for making furniture; the tools required for wood working are low in technology and mass, when compared to the tools required for producing and manufacturing plastics.  But then, your greenhouse may need to be 300' tall in spots.

More later.

Quote
...rope is made from hemp...

Hemp, of course, has many, many uses.
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Offline SpacemanInSPACE

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Re: Mars ISRU for food crops and consumables.
« Reply #7 on: 09/02/2012 06:09 pm »
Chemically converting carbon, hydrogen, oxygen, and nitrogen into fibers and even macronutrients using just air and water and electricity makes a lot of sense in my opinion, if it can be done effectively enough. And would be awesome on Earth as well. And should be more efficient than with plants and photosynthesis.

I'll agree that artificial food would be more efficient, but desirable? Hardly. Do people really find swallowing compacted nutrients as a daily supplement for food appealing? I don't believe Mars Colonization will work if we think we can survive like robots.

Edit: Clarity
« Last Edit: 09/02/2012 06:53 pm by SpacemanInSPACE »
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Offline guckyfan

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Re: Mars ISRU for food crops and consumables.
« Reply #8 on: 09/02/2012 07:40 pm »
The main problem with growing food during winter is the temperature, not so much the light. Also in northern latitudes the days become quite short. Even with reduced light intensity on mars near the equator the days would not be so short I believe. As long as the domes can keep the temperature in a range the plants need, gowth will be less but not stop, especially with added reflectors increasing the lighting.

Multi level with augmented light might be possible for some vegetable crops, but probably not for the staple foods.

Probably a mix of different methods will be used.

Offline guckyfan

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Re: Mars ISRU for food crops and consumables.
« Reply #9 on: 09/02/2012 07:50 pm »
An interesting research project will be, how low pressure can be for growing plants under different conditions. Lower pressure will mean larger structures become possible, or structures with less strength.

I think it might be possible to go as low as 100 or even 50mBar pressure, as long as temperatures favorable for plants are maintained.

One obstacle for very low pressure might be the life in the ground, not so much the plants themselves. Bacteria and fungi will need a sufficient level of oxygen partial pressure. If bacteria start operating in an anaerobic mode the crop plants cannot thrive. With aquaculture it may be possible to get lower.


Offline neilh

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Re: Mars ISRU for food crops and consumables.
« Reply #10 on: 09/02/2012 08:46 pm »
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Offline JohnFornaro

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Re: Mars ISRU for food crops and consumables.
« Reply #11 on: 09/02/2012 11:25 pm »
I'm curious about the energy/maintainability/space tradeoffs of growing plants for materials vs, say, producing plastic from methane

Interesting links, albeit long on forward looking statements and short on specifics. 

The comparison of a methane production plant, and its associated methane conversion plant, also associated with its plastic to chair manufacturing plant, supported by its need for PV electricity or fission based electricity, would have to be compared with the incremental costs of adding forestry acreage and passive solar reflection to the biosphere, and the costs of launching a bag of carpenter's tools, including a skill saw and drill.  So there's that.

I want to say that it will be easier to grow trees on Mars, but that can't be honestly asserted without a better comparison of the two strategies.  The way I put it above, makes it sound like growing trees would be easier, given a three hundred foot tall dome.  After all, colonists could live in the forest, in the additional structure required, turning the forest into a dual use system, which the methane machine system could never be.

I'm seeing a different approach to a colonization effort.  There will be the high tech skin and life support system, but the food and furniture will be simpler, and grown in place, awaiting transformation by relatively low tech solutions.
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Offline aero

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Re: Mars ISRU for food crops and consumables.
« Reply #12 on: 09/03/2012 12:09 am »
Although I don't know that Mars One intends a permanent long term settlement, it seems highly likely given the one way trips. If you are planning the second center of human life as insurance aginst catastrophe here on Earth, then the Mars colony must be independently self sustaining without any support from Earth. That means, without information or even communications, let alone any tech materials delivery. A telescope with which to view Earth would be good, but what you could see would be all the information you could count on.

Given the above, it is not prudent to depend on technology for which you cannot make the replacement parts or program the computers yourself on Mars. Starting with the use of advanced technology is one thing, and necessary, but an objective should be to become self sufficient as soon as possible. To me, that means making our own green houses from locally made materials and growing our own food, finding our own fuel and developing away from dependence on the biannual transport from Earth.

Of course we would have a huge digital library and endless debate as to the need to transfer the library into paper books, but that's farther into the future. We'd also have the problem of the other colonies that are not so foreward thinking who relied on Earth with perhaps an easier lifestyle as a result.
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Offline JohnFornaro

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Re: Mars ISRU for food crops and consumables.
« Reply #13 on: 09/03/2012 01:46 pm »
1) Although I don't know that Mars One intends a permanent long term settlement, it seems highly likely given the one way trips.

2) If you are planning the second center of human life as insurance aginst catastrophe here on Earth, then the Mars colony must be independently self sustaining without any support from Earth.

3) That means, without information or even communications, let alone any tech materials delivery. A telescope with which to view Earth would be good, but what you could see would be all the information you could count on.

4) Given the above, it is not prudent to depend on technology for which you cannot make the replacement parts or program the computers yourself on Mars. Starting with the use of advanced technology is one thing, and necessary, but an objective should be

5) to become self sufficient as soon as possible. To me, that means making our own green houses from locally made materials and growing our own food, finding our own fuel and developing away from dependence on the biannual transport from Earth.

6) Of course we would have a huge digital library and endless debate as to the need to transfer the library into paper books, but that's farther into the future. We'd also have the problem of the other colonies that are not so foreward thinking who relied on Earth with perhaps an easier lifestyle as a result.

1)  They say they do.

2)  Which is only one rationale.  The main rationale, is that expanding intelligent life in the Solar System is what we chose to do.

3)  Focus on the beginning game, not the end game.  Their plan only works if there is, at the beginning, resupply at least every two years.

4)  Therefore, the "above" can't be assumed.

5)  Even so, that would be a desirable goal.  To that end, this thread focuses on the garden.

6)  Which is so far into the future as to be discounted from the current discussion.
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Offline guckyfan

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Re: Mars ISRU for food crops and consumables.
« Reply #14 on: 09/03/2012 01:58 pm »
The comparison of a methane production plant, and its associated methane conversion plant, also associated with its plastic to chair manufacturing plant, supported by its need for PV electricity or fission based electricity, would have to be compared with the incremental costs of adding forestry acreage and passive solar reflection to the biosphere, and the costs of launching a bag of carpenter's tools, including a skill saw and drill.  So there's that.


I think I should get a few more arguments and infos over from the Mars One thread. Building large structures will be extremely difficult to impossible due to the effect of internal pressure. I quote:
One acre = 43,560 sq ft, = 4046.856 sq m. Atmospheric pressure = 101,235.01 N/sq m = 2116.22 lbs/sq ft.
The total lifting force of the enclosed atmosphere = 410,047,767 N, or 92,182,405 Lbs.
Circumference of the dome = 225.509 m, or 739.8588 ft.
Dividing total lifting force by the circumference gives 1,818,321.28 N/m, or 124,594.59 lb/ft.
Edit: We've been through this before on another thread. The dome could be held down if you put a lot of water depth on top of it. Then gravity would work to restrain the forces. But remember, the force is the same as it would be on earth, but the weight of water is not. Or you could make a heavy dome out of Mars iron, that would give the strength needed, and smaller windows would not need to withstand so much total force. I think we should consider finding a deep cave and sealing it against air leaks to live in, initially. I don't know what to do for the greenhouses, except to make them small.
 


So forces become huge. To make a comparison that can be intuitively understood: If you make a dome of cast iron with a wall strength of 1,5m or more than 4 feet, it would still be lifted off its foundation by airpressure on earth sea level.
Or another example the pressure on earth sea level is equivalent to a 10m water column, so the weight on 1m would be 10mt on earth. With lower gravity on mars it becomes ~ 30m of water.
To reduce the forces you can reduce the pressure.
Atmospheric pressure = 101,235.01 N/sq m = 2116.22 lbs/sq ft.
Sufficient for humans  ~     65,000     N/sq m ~ 1300      lbs/sq ft.
At Mt. Everest level      ~    31,400     N/sq m ~    656      lbs/sq ft.
At Mt. Everest level humans can work with simple oxygen masks and can survive and reach a shelter without. Plants can probably thrive without any problems at these pressures so working greenhouses at this pressure is easy. The forces are only 30% of sea level pressure and structures can become larger but still need to be very strong.
That pretty much rules out habitats humans can live in with trees.
But for plants you can probably go lower. Humans would have to work in mars suits or in pressurized cabins.

Temp.(C)  Vapor pressure water   N/m
==============================================
---------------------------
  0               6,10            610
  5               8,7              870
 10              12,3           1230
 15              17,0
 20              23,4             234   

The highest pressure on mars would be inside crater Hellas Planitia with ~ 1200 N/m. That crater is huge and you can place the initial settlement there and have a lot of space to expand.
If you can make plants grow at that pressure and below 7C, you still need a dome to keep the temperature up and to contain the water vapor. But the dome would not need to be pressurized and can be made from light material with any desired size, even an acre. If I were to start a colony I would have the tolerance of plants for low pressure tested. Maybe you can adapt hardy northern trees to that climate.
My biggest concern as alredy stated is the ground life needed to support the plants and needs oxygen. Maybe bacteria and fungi can be supported by driving pipes in the ground and circulate air through them.

Offline JohnFornaro

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Re: Mars ISRU for food crops and consumables.
« Reply #15 on: 09/03/2012 02:11 pm »
So forces become huge. To make a comparison that can be intuitively understood: If you make a dome of cast iron with a wall strength of 1,5m or more than 4 feet, it would still be lifted off its foundation by airpressure on earth sea level.

This is well known in principle.  I'll get  back to ya with a sketch.

Quote
The highest pressure on mars would be inside crater Hellas Planitia with ~ 1200 N/m.

Thanks for the site suggestion.  But... Why are you talking force per area?  Pressure would be in kg/m^2.  That confuses me.
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Offline guckyfan

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Re: Mars ISRU for food crops and consumables.
« Reply #16 on: 09/03/2012 04:25 pm »
Thanks for the site suggestion.  But... Why are you talking force per area?  Pressure would be in kg/m^2.  That confuses me.

Because the forces to deal with increase with area. Double the area and the material for a dome must double in strength.

The way around that would be a longstretched greenhouse or habitat with the maximum width the material can handle.

The round cylindrical habitats shown by Zubrin are also a good design to withstand pressure for habitats but not for greenhouses with sunlight for the plants.


Offline aero

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Re: Mars ISRU for food crops and consumables.
« Reply #17 on: 09/03/2012 06:46 pm »
Let's try to configure a long, narrow greenhouse for Mars. It needs an airlock to start with and it might be ok to make the greenhouse go around in a circle so that once everything has been tended, the "farmer" would have circled back to the airlock. That's not my point though.

My point is to design a clear plastic (Mylar) structure that will withstand roughly 14 lb/sq inch.  (14 psi) The tensile strength of Mylar (a polyethylene plastic) is 28,000 psi in one direction, and 34,000 psi in the cross-ways direction. I'm going to allow a safety margin, so I'll use 15,000 psi. A comonly used thickness for Mylar is 20 mils (0.020 inches) so the area of the end of a one inch wide strip would be 0.020 square inches giving a total strength of 0.02 * 15,000 = 300 pounds. At 14 psi on 1 inch of length, Mylar will withstand the force on 21 inches, (21 in^2). That would be 21 inches up, with another 21 inches down and anchored on the other side. That's pretty small. I suggest that the mimimum usable height of the greenhouse should be 7 feet in the center aisle, but let me calculate for about 8 feet height.

The cross section of this greenhouse will be dome shaped, a half circle in end view, like a tunnel. I propose to use a layered approach with the inner greenhouse contained within slightly larger, but lower pressure tunnel of Mylar, with more, slightly larger tunnels at lower pressure until the outer tunnel can stand against vaccum or Mars atmospheric pressure. Let me guess that the radius of each tunnel will be 6 inches larger than the next inner tunnel. The first tunnel (the greenhouse) has an 8 ft, or 96 inch radius, so its circumference (half circle) is pi * d/2 or about 300 inches. Twenty mil Mylar can withstand 300 pounds as above, which can support a pressure drop of 2 psi. (300 pounds for each anchor, 2 anchors)

The second innermost Mylar tunnel will be 96 + 6 or 102 inches in radius giving a circumference of 320 inches. This tunnel will support a slightly smaller pressure drop of 1.87 psi. let's fast forward to the 9th or outermost Mylar tunnel. The tunnel radii are: 1-8', 2-8.5', 3-9', 4-9.5', 5-10', 6-10.5', 7-11', 8-11.5', 9-12' with the outer (largest) enclosing tunnel supporting ~1.33 psi. But by adding up all of the contributions, the total facility can support 14.57 psi within the greenhouse itself.

That is one way to make a greenhouse on Mars. Another way would be to use a single sheet of ~180 mil Mylar. There are several advantages to using multiple layers, but 9 layers may be excessive. Three layers of ~60 mil, or four layers of ~45 mil would be my preference but safety from leaks is an absolute driving requirement and with multiple layers, a safety margin which allows a leak failure in one layer is feasible.

How would this greenhouse be assembled? Put up the outer tunnel first, pressurize it to 1.33 psi, then roll out the next innermost tunnel and pressurize it to 2.7 psi, and soforth, until all tunnels were pressurized and the greenhouse is at 14.5 psi. This could be delivered from earth with each tunnel already rolled within the next outer tunnel so that on Mars the only task would be to anchor the edges and the airlock, then inflate the greenhouse.
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Offline aero

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Re: Mars ISRU for food crops and consumables.
« Reply #18 on: 09/03/2012 07:10 pm »
It takes more math than I am comfortable with, but by constructing the greenhouse using the layered approach discribed above, it might be possible to fill one of the intermediate layers with water, instead of gas. This could give very good temperature stabilization for the structure. The thickness of the water layer would be much less than 6 inches due to water weight but the its my gut feeling that something workable could be designed.
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Offline guckyfan

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Re: Mars ISRU for food crops and consumables.
« Reply #19 on: 09/03/2012 09:20 pm »
I like that multi-layer design. But why do you calculate full earth pressure? We can go a lot lower. I would not go above Mt. Everest pressure for greenhouses to enable larger structures allowing a three times larger structure with the same material strength. Or much less material for the same size.

4 or 5 layers may not be exzessive. More layers means less loss of total margin, if one layer fails.

Maybe it would be good to have water in every gap. A thin layer of very clean water would probably reduce loss of light which occurs on every surface of those layers. It would need to be calculated and tested. The weight of the water would be of very little concern, as the pressure can hold up a column of several meters depending on internal pressure. Not to thick a layer of water to minimize losses.

With water between the layers you can just put the desired pressure into the greenhouse. If I am not wrong, the pressure should be distributed evenly between the layers all by itself.

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