Indoor farming update

RobertDyck · 2014-11-11 22:28:02

PV to ambient: current triple junction photovoltaic cells for space have an efficiency of 29.5%. From SpectroLab, it goes "Triple Junction", then "Improved Triple Junction" (ITJ), then "Ultra Triple Junction" (UTJ), and now "NeXt Triple Junction" (XTJ). Current white LED at 8.7 watt screw base lamp and 120 volt AC has an efficiency of 10.1–13.6%. All together, conversion from sunlight to electricity and back to sunlight is 29.5% * 10.1% = 2.9795%

Ambient light requires a spectrally selective coating. The plastic film I selected is PCTFE, which has light transmission greater than 95%. The spectrally selective coating will reduce that a bit, to between 80% and 85% light transmission for blue and green light, less for red. It tapers from about 85% transmisison to 30% at the boundary between red and IR. But even at 30%, it's much more efficient than a PV/LED combination. And that doesn't takein to account any power loss for DC to AC conversion, voltage change, or loss during storage in a battery.

SpaceNut · 2014-11-11 22:41:06

The Effect of Array Tilt Angle on Energy Output

RobertDyck · 2014-11-11 23:01:09

Yea, tilt will. But plants have evolved to deal with it themselves. Some plants actually tilt their leaves during the day to track the Sun. Good for them. Let them deal with it.

I mentioned a long narrow greenhouse with a mirror on either side. The greenhouse will be half as high as it is wide. Each mirror will be as long as the greenhouse. Mirrors tilted to reflect sunlight into the sides of the greenhouse. That means as much light from the mirrors as from directly overhead. With a greenhouse perfectly at the equator, and at the summer solstice, each mirror will be tilted 45°. Mirror width will actually be greater than greenhouse height, but sized so with the mirror tilted, its height above ground will equal the top of the greenhouse. And of course bottom of the mirror will be ground level. You can see how this works at high noon on the summer solstice.

At dawn, sunlight will reflect from the east edge of the mirror, some distance westward into the greenhouse. As the Sun rises, the angle will change. At high noon, sunlight will reflect straight into the greenhouse. At dusk, sunlight will reflect from the west edge of the mirror, some distance eastward into the greenhouse. It doesn't matter how far sunlight shines east or west from the point of reflection, as long as it gets into the greenhouse somewhere. To ensure maximum Sun, you could extend the mirror east and west beyond the ends of the greenhouse. The extension would be half the width of the greenhouse. That assumes the bottom/ground edge of the mirror is right against the greenhouse.

This means mirrors do not have to track the Sun's daily movement. The greenhouse will still get double sunlight anyway. Exact angle and which patch of mirror illuminates a particular leaf will change through the day. But mirror angle will have to change to track change of seasons. When angle of the sun changes 2°, mirror angle will have to change 1°. This means the mirrors will have to change 1° every second week. One reason for that is Mars long year, about twice an Earth year. I could give you the long math to calculate that if you wish.

This works very well at the equator, or tropical latitudes. It doesn't really work at high latitudes. But a human base would be built at or close to the equator where it's relatively warm.

GW Johnson · 2014-11-12 18:38:15

If you erect a clear-walled structure with a solid, regolith-covered roof in the middle of a small crater (or other depression), you can lay mirrors on the north side rim from east to west, so that sunlight bounces into the building through the walls.

You'll get quite a geometric concentration, because each mirror panel can be quite a bit larger than the projected area of the clear receiving wall. Mirrors can be nothing but aluminum panels.

Blow the dust off now and then with compressed Martian air. The vertical clear walls on the pressurized building will not generally collect dust. Can be 3 layers of glass, for good thermal insulation.

To compress Martian air, freeze it, put the dry ice in a closed vessel, and warm it. Sublimation in a confined volume is self-compressing.

This should work just fine at almost any latitude. Further from the equator requires more reflection panels to get the needed insolation into the building.

Better plan on auxiliary power to get you through a long dust storm: weeks to months, according to Mariner 9.

GW

Last edited by GW Johnson (2014-11-12 18:40:30)

RobertDyck · 2014-11-12 20:06:26

Ah. Radiation shielding for plants. Organisms that withstand more radiation than humans, more radiation than exist on Mars. As Dr. Zubrin said "There be Dragons here!"

Impaler · 2014-11-12 21:49:23

RobertDyck wrote:

Impaler wrote:
How many times do I have to repeat myself
Please stop. It doesn't matter how many times you repeat yourself. That doesn't make it so.

Please START reading what I am writing, I am not saying that diffuse light can't be absorbed by a Panel, I said repeatedly that it can and will be and your insultingly repeating a point I'm both aware of and not in disagreement with while completely ignoring what I am actually writing. I said that directional light can not be EQUATED to diffuse because angle of light to the ground and the tilt of a panel impacts performance for direct light so your need to be aware of what kind of light you have and how it may or may not effect performance characteristics.

And btw Diffuse light from the sky is a half-hemisphere, not a plane.

Yes a lone sunflower in a field can and will tilt itself and maintain a near zero tilt of it's leaves to the suns light. But a field of plants collectively can not do this, they shade each other and effectively act as a flat panel horizontal to the ground, and thus total light collection drops due to the angle of the sun.

PV efficiency for Triple Junctions are well above 29% for a lab system, but I assumed thin-films which are actually below they are currently mid 20's in the lab and just 20% in the marketplace so your number of 29% is actually generous or very forward looking for a deploy able solar system on Mars. But your LED efficiency is way off, your using luminous efficiency of a white LED being run off AC. First off were going to run a DC system cause our electric source, electric storage and electric demand are all DC so were not going to have the huge inefficiency in converting AC-DC. Second we would use narrow spectrum red and blue, not blue with a phosphor over it (which is how white is made) as phosphors cause we don't want white light and phosphors cause us to lose efficiency. Third because this is not a device for the home market (aka not cheap) the voltage will be lowered which increases efficiency but raises the number of individual LED's needed (this will gradually happen in the home market as manufacturing costs come down). Lastly we will use the metric of 'wall-plug-efficiency' because 'lumins' are a measure of the human eye's repose to light and a white light spectrum will calculate out at far lower then it's actual efficiency of electric-energy-in to light-energy-out. Current catalog LED's are up around 50% http://www.cree.com/LED-Components-and- … -XTE-Royal (datasheet on page) and new ones just hitting the market are even higher at 75% http://www.prnewswire.com/news-releases … 77491.html My earlier 90% figure might have been the lab level efficiency but it's not that far off and we should eventually see consumer market plant growing LED sets with near that kind of efficiency.

In fact if Solar and LED efficiencies continue on their current pace it may be possible for the combination to actually come out ahead of sunlight in the photosynthetically important frequencies, but this is unlikely to be cost aka mass effective for a colony because Multi-junction solar is heavy compared to thin-film. The point still stands that a modest solar field and LED's could replace a greenhouse of between 1/3 and 1/2 the area. And after all area is cheap on mars so who cares how big a solar system we need, mass is the metric of merit.

RobertDyck · 2014-11-12 22:47:58

Impaler wrote:

And btw Diffuse light from the sky is a half-hemisphere, not a plane.

Doesn't change the fact light flux per unit area is the same for direct vs diffuse light. Only occlusion would reduce that. But I'm tired of arguing with you.

Impaler wrote:

PV efficiency for Triple Junctions are well above 29% for a lab system, but I assumed thin-films which are actually below they are currently mid 20's in the lab and just 20% in the marketplace so your number of 29% is actually generous or very forward looking for a deploy able solar system on Mars.

The 29.5% figure that I quoted is directly from SpectroLab, specifically their XTJ solar cells. Currently available for satellites. Solar panels sold by Canadian Tire to recharge a car battery have 6% efficiency. Solar panels sold by Home Depot for cottages have 14% efficiency. But solar panels currently sold for satellites are *MUCH* more expensive.

I included links for my sources. When text is in blue, that means you can click on it. The blue "SpectroLab" in a previous post will take you directly to their web page, where they sell solar cells and panels for satellites. By they way, SpectroLab was bought by Boeing Satellite Systems. They're now a wholly owned subsidiary. They're one of two manufacturers in the US of solar panels for satellites. I believe the panels for Spirit and Opportunity were manufactured by the other one.

Actually, I read a paper published in the fall of year 2000. The Los Alamos National Laboratory discovered a photovoltaic chemistry that theoretically almost exactly matches the Sun. So should be extremely efficient. They looked for someone who could build one, so they could test it. The materials lab at the University of California in Berkeley did it for them. They did the materials characterization, and discovered this chemistry is transparent to colours of light that it doesn't convert to electricity. For a triple junction cell, the top two junctions are as well. But with this new one, they could adjust the absorption spectrum by adjusting the concentration of nitrogen. I'm talking about gallium-indium-nitride. So you could make a multi-junction cell where each junction has the same chemistry, just difference concentration of nitrogen. UC Berkeley calculated that a two junction cell would convert 56% of incident sunlight, a three junction cell 64%, and a 36 junction cell 72%. Another scientist later optimized the junctions, he calculated for 2, 3, 4, 5, 6, 7 and 8 junctions. He found 2 and 3 junction cells would have the same efficiency that UC Berkeley calculated, but an 8 junction cell would have 70.2% efficiency. Ok, so my interpretation is that 36 junctions for 72.0% and 8 for 70.2%? That means 8 is optimal for this chemistry.

To my frustration, no one has done any serious work to put this into production. Manufacturers of terrestrial cells want the guys who make solar panels for satellites to pay for development; they want to rip-off the technology. But the two manufacturers for space photovoltaics have a multi-decade plan to slowly increase efficiency from 24% to 45%, gouging their customers with each fractional percentage increase. They have no motivation to jump directly to 70%. So no one is working on this.

Furthermore, since every junction is the same chemistry, just different concentration, it's less expensive to manufacture. Instead of gallium-indium-phosphate / gallium-arsenide / germanium, you just have gallium-indium-nitride. That means fewer chemicals, fewer rare metals, and instead of many different steps for manufacture, you have the same steps repeated for each junction. So less expensive per unit area vs current space PV cells. And notice the metals: gallium and indium, currently used for the top junction of current cells. More power and less expensive? But again, manufacturers don't want to do it. They want to gouge their customers.

That means the best currently available is 29.5% conversion efficiency. Beginning of life. That degrades in space after years exposure to radiation. PV would last substantially longer on Mars surface vs Earth orbit, because radiation is so much lower. But you get the idea.

Impaler · 2014-11-13 00:10:25

Are you seriously trolling me now? I never said flux per unit area was different, that is not the central point I was arguing. Pointing out Hemisphere vs Planar was a bloody FYI correcting a small error on your part, an error of NO CONSEQUENCE as I intentionally separated it in it's own line and did not reference it in any way in the main body. How about responding the full argument I made rather then trying to hide behind an off hand correction pretending it is relevant to the point at hand.

Multi-junction cells being make Nth layers deep with super high efficiency is very nice and all but unless they can put these layers down in a film the weight of bulk silicon just kills the power/weight ratio for these things when they have to be taken to Mars (not space). Were discussing Martian surface solar tech here, thin-films just crush any bulk silicon here both because of their low mass but also because they can just rolled out so easily by humans or machines. Everyone who has looked at the Mars surface solar question has come down on the side of thin-film. The 14% cruddy Home Depot panels are likely old thin-films, as I said labs versions are up at 20% and that will hit the market eventually, given that were decades away from getting to Mars this is not unreasonable.

But I have to believe that this whole mess about panel efficiency is just another dodge on your part because you failed to respond to my correction of your erroneous LED efficiency numbers which is what invalidated your light-electric-light <3% round trip efficiency statement at the top of the page. This is clearly false, Solar Cell + LED can effectively do the job of bringing light for photosynthesis indoors without relying on transparent roofing.

Last edited by Impaler (2014-11-13 00:13:51)

RobertDyck · 2014-11-13 05:55:24

I should check more often. I got the efficiency for Home Depot solar panels when I talked to my friend when looked at them for his cottage. Today they are slightly better. Now 18% instead of 14%.
390-Watt Monocrystalline Grid Tied PV Solar Panel

And no. Laboratory versions of triple junction cells were at 30% many years ago. But it's taken many years for the two manufacturers to go from 24% to 29.5% for space PV. The fact it's taken so many years, is highly suspicious. And since Boeing has bought SpectroLab, I believe they're experiencing the same problems that engineers have reported for the Shuttle program. That is, senior corporate executives overrule enginners because they don't want anything that reduces cost.

And why do we keep having the same argument? I keep using the greenhouse design as the Case For Mars studies, published before the founding of the Mars Society. I updated it by finding a transparent film that can actually withstand the environment on Mars. At a Mars Society convention, I talked to the person who wrote the "Case For Mars" paper. And I brought a sample of the film, with data sheet from the manufacturer. All real hardware. So why do so many people on this forum keep arguing for really stupid ideas?

Using a PV panel to convert sunlight to electricty, then convert back to light for plants? That will always be far less efficient that just using ambient light. And no, radiation is not a worry. Plants can withstand radiation far worse than Mars. Plants can also withstand lower pressure, and even brief exposure to Mars ambient conditions. As I've posted elsewhere on this forum before, I actually attended the Canadian Space Exploration Workshop 4 and 5; hosted by the Canadian Space Agency. At #5, researchers from Guelph University presented their research. They used a hypobaric chamber to grow plants at reduced pressure. And actually measured plant growth. One experiment they reduced pressure for spinach all the way to Mars ambient, and held it there for one hour before restoring pressure. The plant completely wilted, but as soon as they restored pressure, it perked back up and continued growing. This demonstrated plants can withstand complete pressure loss on Mars. As long as the greenhouse is repaired and pressure restored, it won't kill the plants. But their research was primarily about plant growth. They measured plant growth in terms of carbon fixation. They found plant growth rate did not decline with reduced pressure, all the way down to 10 kPa. That's 100 mbar. Humans cannot tollerate pressure that low, but plants can. They found plants adapted by increasing the rate of water transpiration through leaves. As long as plenty of water was available to plant roots, growth rate was not affected. In a sealed greenhouse, water is not consumed, it goes in a big circle. Water is absorbed by roots, transported to leaves, transpires into the air through leaves, condenses on walls/ceiling, and drips down back into the soil. A big circle. The lower the pressure, the faster water goes through this circle.

So no, we don't need radiation shielding for a greenhouse. No, we don't need 14.7 psi pressure. No, we don't need artificial lighting. We do need artificial lighting as an emergency backup in case of prolonged dust storms, but not for normal operation. And any Mars settlement will require power for ISRU, manufacturing, or science. In fact, using power for artificial light in the greenhouse means you have to completely cut off all mining, refining, or manufacturing operations.

And no, thin film is not the magical solution that makes PV practical. Using thin film PV cells with 12% efficiency, which is what you get from amorphous silicon, will not compare with 29.5% of current triple junction cells, or higher efficiencies of cells we already know how to make. By the way, the two manufacturers of space cells have already done tests in the lab using 4 junction cells. So the higher efficiencies they're talking about have already been demonstrated.

And Impaler, please stop saying what I'm talking about is "probably this" or "probably that". I've provided links to my sources. Please look at the sources. I provided actual sources or real hardware.

I don't know why you're obsessed with avoiding transparent roofing. This is not an issue. You have to clean dust from PV panels, just as you would for windows. If you're trying to avoid cleaning, that just won't do it. If the issue is radiation, then we can repeat another discussion we've had on this forum many times. Radiation has already been characterized. It was measured in Mars orbit by the MARIE instrument on Mars Odyssey, with calculations for the Mars surface. It was supposed to be confirmed, "ground truth", by instruments on the Mars 2001 lander. But unfortunately that lander was cancelled. Now "ground truth" has been provided by the Curiosity rover. It has confirmed what we in the Mars Society have known since the Odyssey team first published their data. That Mars is safe. Surface radiation is on average half the intensity of the International Space Station. The exact character of radiation, that is exactly which types of radiation get through, is very important. The most dangerous radiation is heavy ion galactic cosmic radiation. Mars atmosphere blocks 90% of that at a high altitude location like Meridiani Planum, or 98% at a low altitude location like Utopia Planetia, or Elysium Planetia. I prefer Elysium because that's flat and relatively smooth, so safe to land. It's low altitude, so lots of atmosphere for good radiation shielding. It's at the equator, so warm for Mars. And has been show to have subsurface water. That's rare near the equator. Remember, the "frozen pack ice" is in Elysium Planetia.

Utopia Planetia also has subsurface water, probably permafrost, but it's at a moderate latitude. It's a vast area, but one point is the same latitude as Winnipeg, Canada, where I live. I can tell you first hand the seasonal differences in length of day. That would strongly affect greenhouse and PV operation. And Mars is cold. You don't want a high altitude location where cold is worse.

This idea of underground greenhouses is a hold-over from lunar designs. You have to do that on the Moon. The Moon has micrometeoroids, and hard radiation. The Moon is outside Earth's magnetosphere, so radiation is worse than ISS. The fact underground greenhouses are required on the Moon, is one reason not to settle there. Please don't bring Lunar problems to Mars.

GW Johnson · 2014-11-13 17:46:25

In my post (#54 above), the radiation shielding is for the gardeners, not the plants. The specific worry is the extreme solar flare event. Those are lethal outside the Van Allen belts here, and Mars has no such belts. The data we have obtained at Mars does not, I repeat not, cover that event.

Like Robert Dyck, I do not think we need to build underground greenhouses. And most of the time at "reasonably low" elevations, cosmic ray radiation isn't much of a problem. For really temporary stuff, some sort of clear plastic inflatable should work. Maybe a few months. For something usable long term, you need "real construction" of some kind. Plastics just "die" when exposed to UV light and to vacuum.

The only problem with a clear roof is the solar flare radiation event. This is by far more a problem for the gardeners than it is the garden. But, if your roof is a shield, then your greenhouse becomes just another place to shelter from the flare event. "Suspenders-and-belt".

GW

Impaler · 2014-11-13 18:26:06

Low pressure greenhouses would make the structural and material costs of a greenhouse significantly lower, but claiming that their will be no impacts to plant food production are far from proven. Studies so far show that plants are experiencing a significant number of different gene expressions vs normal pressure and their look to be mixed results, some plants get sturdier stems but shorter etc, and different plants show different responses. We would need to see a full growth and food harvest cycle under low pressure to get metrics on how much food production is going to be effected, which critically includes testing the nutritional value of any food produced.

Also we can not simply assume that any mass reduction in the pressure vessel translate directly into 'system' mass reduction. Their are going to be water systems, trays, air circulation fans (plants will get too spindly without some air circulation), and possibly some kind of growing medium that needs to be brought from Earth (vermiculite?). As far as I know no one has even tried to do the math on how much this is going to mass and how thouse systems compare to the pressure vessel mass, are they insignificant or very significant. High intensity artificially lit systems have big savings potentials here as well because they can utilize all that stuff in a more rapid harvest cycle.

It looks like most of these transparent low pressure greenhouse ideas go back to some 1981 British Interplanetary Society paper. In 81 the enclosed alternative was an aluminum can being lit with maybe High pressure Sodium lights at best. We now have access to Bigelow expandable which give excellent mass/volume ratios, LED lighting and thin-film solar. Technologies which completely turn the tables on these old assumptions. As the OP link shows we even have a developing industry to do enclosed growing on Earth which is and will develop the full system integration and knowledge base for the eventual development of a Mars surface growing system.

Lastly low pressure is going to exclude humans from the greenhouse, 10kPa is far below what humans can tolerate even with breathing masks, they would need some kind of suit so going into the greenhouse is effectively an EVA which entails hours of logistics. Not something that's going to be practical for picking some fresh veggies for a salad. Astronauts would not be able to get into the greenhouse but maybe once a week (most Exploration missions only assume EVA on the Martian surface is done every few days at most and that is the core science activity with the highest priority) which means poor maintenance of the equipment and poor care for plants. And worst the lose of the pleasurable nature of being in close contact with growing plants as demonstrated in the Antarctic.

Last edited by Impaler (2014-11-13 20:16:42)

RobertDyck · 2014-11-13 21:43:45

So that I am clear, I am recommended the greenhouse have the same pressure as the habitat. That way the greenhouse becomes an extension of liveable space. But you don't need 1 atmosphere pressure. Apollo and Skylab used 5.0 psi, with 60% oxygen, 40% nitrogen, by volume. Some guys in the Mars Homestead project wanted to maximize pressure, so I calculated a "good" pressure mix. Start with zero prebreathe time to go outside in a spacesuit. So 3.0 psi pure oxygen in the suit, 2.7 psi partial pressure O2 in the habitat. That way a leak producing 10% pressure loss in the suit would result in the same partial pressure O2 that astronauts are accustomed to. Then add N2 and Ar in the same proportion as Mars ambient. That way you can just filter out dust, use a freezer then sorbent to remove CO2, a bit more processing, then you have diluent gas the for habitat. This is for the first base / settlement. You could just use oxygen/nitrogen for a science mission. There is a ratio of total pressure for the lower pressure environment (suit) to partial pressure of nitrogen in the higher pressure environment (habitat). Partial pressure of N2 must be at or below that to permit zero time prebreathing O2 before decompression in an airlock.

There has been much discussion of hydroponics vs soil agriculture. I argue for soil. The reason is hydroponics requires nutrient solutions. Those are extremely heavy; far too heavy to transport from Earth. Making them on Mars means extracting nutrients such as potassium and phosphate from soil. That's energy intensive, and requires equipment. Since source material is soil anyway, it's far easier to just plant seeds in soil and let plant roots do the work. So this means bringing empty plastic trays from Earth, and a shovel. Fill trays with Mars soil.

Mars soil will require some basic treatment. Start by soaking with water. That will neutralize superoxides, and release a little oxygen into the greenhouse. Expect soil to fizz a little as it does this. And since typical Mars soil is slightly alkali, treat it was carbonic acid. That means carbonated water. The simplest way of making that is Mars atmosphere pressurized in a bottle of water. Leave overnight. The CO2 will dissolve in water. When you initially soak the soil, use that. It will add a bit of carbon as well as reducing soil alkali.

Most Mars soil is high in iron, and low in potassium. The best solution is pick a good patch of soil. But you will have to create nitrogen fertilizer. Mars soil has no nitrogen within detection limits of instruments sent so far. I could describe how to make ammonium nitrate fertilizer, but since the Oklahoma bombing it's not a good idea to post that on the internet. You could use nitrogen fixing organic systems; legumes naturally have a bacterium that grows in symbiosis. Sprinkle spores of the bacterium on seeds of a legume, or permeate the soil with spores. The legumes will have nodules in their roots where this bacterium grows. The plant provides water and carbohydrates to the bacterium, and the bacterium converts water and nitrogen from air into nitrate. Since the nitrate is produced right in plant roots, the plant has so much that it actually leaks out into the soil. This is why legumes are so high in protein. Beans, peas, alfalfa, clover, etc. But plants will take a while. For a science mission, you will probably have to make chemical fertilizer to do it quickly enough.

Plastic soil tray:

RobertDyck · 2014-11-13 21:47:53

GW Johnson wrote:

For really temporary stuff, some sort of clear plastic inflatable should work. Maybe a few months. For something usable long term, you need "real construction" of some kind. Plastics just "die" when exposed to UV light and to vacuum.

PCTFE is extremely durable. It withstands vacuum, cold, and is highly resistant to UV. I really recommend it for a science mission. It will last much longer than other plastic inflatables. But sand storms will sand-blast it. It's just a question of how long. I have said for a permanent settlement, the easiest material to make in-situ for greenhouse windows would be glass. Just normal, good old glass.

Impaler · 2014-11-14 04:38:58

If your using Apollo/Skylab pressure then their is practically no point to not going full normal cause thouse systems were only around 1/3 atmospheric, not the 1/10 you were siting earlier for plant growth that would allow super minimalistic plastic balloon greenhouses. NASA has moved away from that kind of pressure now for >20 years, I don't see them going back because of the fire hazard. The EVA pre-breath is a bitch I agree, but I think were more likely to move towards higher pressure hard suits or Mechanical counter pressure suits then back to low habitat pressures.

Growing medium CAN be water for certain plants, generally things like lettuce/spinach, but not for other plants such as Tomatoes. If were assuming water ISRU then you will defiantly Hydo any plant that you possibly can, the advantages in saving the mass and especially the volume of any growing medium is huge, just tubs/tanks/hoses etc and your good to go. The question though is if the FULL diet can be supplied with Hydroponic plants, I'm doubtful it can be which frankly makes the whole greenhouse thing a non starter as a primary food source.

Their is no soil on Mars, it's regolith, the stuff is going to be toxic, not least of which is the fact it's 1% salt. It is going to more then 'fizz' when you put water on it, it's likely that the gypsum compounds will harden it into an alkaline brick. And even if you could actually make something non-toxic out of it it's not going to do anything more then a vermiculite mix would do aka hold water/air for the roots of plants that can't handle being fully submerged in water. I'm doubtful the work/equipment/water etc needed to process the stuff will be worth it and we will use a growing medium (not a soil) brought from earth. IF we get very lucky and find the right kind of mica mineral (which is volcanic) on Mars we could make vermiculite from the bulk ore (just bake in an electric furnace), thus avoiding the witches brew that is the mixed regolith.

RobertDyck · 2014-11-14 06:30:56

OMG! Not fire! We've gone over this time and again.

I read about the Case for Mars conferences in the book "The Case for Mars". Yea, same name. That was the spring of 1998. I really wanted to go because this is what I've been interested in since I was a pre-schooler; but I didn't have enough money. That "Case for Mars" conference turned out to be the inaugural convention of the Mars Society. If I went, I could have called myself a founding member. But I didn't actually join until 1999. At one convention, Robert Zubrin did introduce me as a founding member. Unfortunately I had to correct him. I didn't join until a year after its founding. But I have been doing this that long.

No. Fire is not an issue. The rate of combustion is determined by partial pressure of oxygen. So sea level pressure with the same proportion as Earth's atmosphere will have 3.0 psi partial pressure of oxygen. Pure oxygen at 3.0 psi total pressure still has 3.0 psi partial pressure oxygen. That means the rate of combustion is exactly the same. A match will burn just as fast. A strip of Velcro will burn just as fast. No difference what so ever. And no, nitrogen does not moderate the rate of combustion. Nitrogen is irrelevant. Water will, but nitrogen won't.

RobertDyck · 2014-11-14 06:32:00

Thank you for mentioning Mechanical Counter Pressure. I am also a fan of MCP. Actually, I learned about MCP through the Mars Society forum. The first one, that started in right after the Society's founding. When that first webmaster quit the Mars Society, he took the website down. We didn't have a forum for over a year. Then New Mars was founded to replace it. But someone posted about MCP on that original forum. My initial response was that's just science fiction, it doesn't exist. Then a member who is a practicing medical doctor, specializing in decompression sickness, provided me with a copy of Dr. Paul Webb's original paper. Wow! It works! And he did it in 1966, paper published in 1967. Amazing! Why isn't this being used now? Of course the reason is when Richard Nixon cancelled Apollo, many related programs were cancelled. MCP was one of them. The MCP suit was designed for the surface of the Moon, but wasn't ready in time for Apollo 11, so they upgraded existing suits. An Air Force high altitude suit was adapted for the first Mercury mission. Which was incrementally improved until the end of Mercury. Then that was further improved for Gemini. Which went through more improvements. Then the first Apollo suit was an incremental improvement on the last Gemini suit. Which went through various iterations to become the white EMU suit we have on ISS today. The guys working on that want to keep their job, so they oppose the new design. Of course they could always move to the team for the new suit, but that's not what's happening. Frustrating.

RobertDyck · 2014-11-14 06:41:34

Mars regolith: Yes, technically it's regolith. But if you've ever talked with any of the researchers who actually built instruments for Mars landers/rovers, or researchers operating them and studying Mars, you would learn they've given up on that term. The problem is "regolith" refers to anything on the surface with no organic content. That includes bedrock, boulders, rocks, stones, gravel, sand, and soil. So what term to you use to distinguish them? For years now rocks are called rocks, sand is called sand, and soil is called soil. This was reinforced when they got data from the TES instrument on MGS. It found clay. Researchers weren't expecting clay. That's the result of millions of years of flowing water acting on igneous rocks. That means rivers and streams, and rain, on the surface of Mars. But there it is. In 2001, MGS found clay. With clay in there, the loose regolith with the consistency of soil, may as well be called soil.

And did you see results from Mars Phoenix? It analyzed Mars soil on the edge of the north polar ice cap. Phoenix found Mars soil is so good that you could grow asparagus. The soil is slightly alkali, but asparagus likes alkali soil. Yea, it would need some treatment. You need to soak the soil in water to neutralize superoxides and peroxides. Salt is rather high. But asparagus could take it.

Producing soil for other crops now becomes an issue of soil remediation. Any agriculturalist with a degree in soil analysis could work with it.

A number of years ago, after Mars Pathfinder and Sojourner but before Spirit and Opportunity, I tried to design a Mars soil simulant. There were a number of people who wanted to conduct experiments for ISRU, others wanted greenhouse experiments. I thought with everything sent to Mars that we knew enough to make a highly realistic Mars regolith simulant. So I looked up detailed soil analysis, and found NASA doesn't (didn't) actually know that much. I spoke on the phone directly with primary investigators for the APXS instrument on Sojourner. Got their data directly from them. I read on NASA's website about something called a CIPW analysis. But the data on their website was sketchy, vague, imprecise. So I looked up what "CIPW" is. I learned how to do it, and tried to get precise results using specific data samples from Sojourner. I couldn't get the numbers to balance. I mentioned this at a Mars Society convention, so Dr. Carol Stoker suggested I talk to the guy who did the analysis for NASA. So I looked him up, and asked him. He said he had the same problem, and gave up. He gave me his spreadsheet. Interesting! His first page had details I would never have thought of. But I found an erroneous assumption. I corrected his error, based on data from MGS. But that didn't help, the numbers still didn't balance. The problem is the proton mode of the Alpha-Proton-Xray-Spectrometer didn't work. Each mode was supposed to measure different elements, with overlap. The proton mode was for light elements. Using the alpha mode gives imprecise data for light elements, and nothing at all for hydrogen. Using data from APXS, there's no way to distinguish feldspar from clay without hydrogen. There's a big difference between feldspar and clay. So I wasn't able to devise a mineral blend to simulate Mars regolith.

One of my points is you can learn a lot from other Mars Society members. I certainly have. I see you have yet to learn some of the lessons that I learned from 1999 through 2001.

Impaler · 2014-11-14 17:08:39

RobertDyck wrote:

OMG! Not fire! We've gone over this time and again.
I read about the Case for Mars conferences in the book "The Case for Mars". Yea, same name. That was the spring of 1998. I really wanted to go because this is what I've been interested in since I was a pre-schooler; but I didn't have enough money. That "Case for Mars" conference turned out to be the inaugural convention of the Mars Society. If I went, I could have called myself a founding member. But I didn't actually join until 1999. At one convention, Robert Zubrin did introduce me as a founding member. Unfortunately I had to correct him. I didn't join until a year after its founding. But I have been doing this that long.
No. Fire is not an issue. The rate of combustion is determined by partial pressure of oxygen. So sea level pressure with the same proportion as Earth's atmosphere will have 3.0 psi partial pressure of oxygen. Pure oxygen at 3.0 psi total pressure still has 3.0 psi partial pressure oxygen. That means the rate of combustion is exactly the same. A match will burn just as fast. A strip of Velcro will burn just as fast. No difference what so ever. And no, nitrogen does not moderate the rate of combustion. Nitrogen is irrelevant. Water will, but nitrogen won't.

NO, your badly mistaken, material flammability is driven almost entirely by % of Oxygen with slight additional increases in flammability due to higher total pressure, the inert gasses have a significant quenching effect on fire. See http://ntrs.nasa.gov/archive/nasa/casi. … 005041.pdf or here (bottom of page 11) http://spaceflightsystems.grc.nasa.gov/ … 213689.pdf

Jacobs Sverdrup ESC Group wrote:

By contrast with human respiration that depends primarily on oxygen partial pressure in the atmosphere, materials flammability depends strongly on oxygen concentration (volume percent) and to a lesser extent on total pressure.

I'll stick with doing my own searches of published research rather then just take the word or 'society members' thank you very much.

Last edited by Impaler (2014-11-14 17:09:58)

Terraformer · 2014-11-14 17:13:53

Hence why people in undersea habitats can't smoke cigarettes.

SpaceNut · 2014-11-14 20:47:08

When Mars is at its hottest – still cold enough to freeze water – the atmospheric dust can absorb the energy of the sunlight, which causes warm pockets of air to rapidly move towards colder, low-pressure regions, generating winds up to 45 metres per second (162 kilometres per hour or 100 miles per hour) that begin to pick up dust particles from the ground, adding to the atmospheric dust content and increasing heating, pushing the winds harder and faster until the atmosphere is filled by dust.

http://www.nasa.gov/mission_pages/mars/ … 21127.html

RobertDyck · 2014-11-14 21:50:44

Impaler wrote:

NO, your badly mistaken, material flammability is driven almost entirely by % of Oxygen with slight additional increases in flammability due to higher total pressure, the inert gasses have a significant quenching effect on fire. See http://ntrs.nasa.gov/archive/nasa/casi. … 005041.pdf or here (bottom of page 11) http://spaceflightsystems.grc.nasa.gov/ … 213689.pdf
Jacobs Sverdrup ESC Group wrote:
By contrast with human respiration that depends primarily on oxygen partial pressure in the atmosphere, materials flammability depends strongly on oxygen concentration (volume percent) and to a lesser extent on total pressure.
I'll stick with doing my own searches of published research rather then just take the word or 'society members' thank you very much.

Actual NASA technical papers. Very good. And contrary to studies done before and after the Apollo 1 fire. And I've had people tell me there was no such thing as Apollo 1, that there was a technical name for that test. There was, but it was renamed Apollo 1 postumously to honour the dead. Studies done by North American, manufacturer of the Apollo Command Module, showed that Velcro does not sustain a flame in 100% oxygen at 3.0 psia. It smoulders, but does not burn. Same response in open air at sea level. However, in 100% oxygen at 17.7 psia, it burns like the head of a match. North American wrote a memo to NASA telling them not to proceed with a pressure test using pure oxygen at 17.7 psia. NASA did that because the pressure differential across the hull would be the same as in space. But North American said it wasn't safe. But the memo was ignored, people died. Similar to the memo from ATK engineers saying not to launch Challenger in 1987 because outside temperature was too cold. It was ignored, people died.

One problem we currently have is suit designs use too much pressure. This restricts movement, too much counterforce for joints. And the paranoid obsession re fire results in such things as a fire proof exterior for a suit that's only worn in the vacuum of space, never inside the spacecraft. Exterior of the EMU suit is Orthofabric, a double layer fabric with 400 denier Goretex facing (sometimes spelt Gore-Tex), Nomex backing, and every 3/8" two thread of Normex are replaced by Kevlar. The Kevlar provides rip-stop, the total assembly provides micrometeoroid protection. One reason for Nomex is fireproofing. It's the same material as fire fighter's jacket and pants. Mars has a CO2 atmosphere, so fire proofing is not required, or useful. And Mars surface has no micrometeoroids, that protection isn't useful either. Tennara architectural fabric would be better for a Mars suit. That fabric is entirely 400 denier Goretex, with a twill weave (like jeans) instead of a plain weave, and no backing.

One great operational problem with ISS is its pressure. Astronauts can't just put on a spacesuit, cycle through the airlock, and go outside. They have to go through hours of oxygen prebreathe to purge nitrogen. That is dangerous, and an obstacle to EVAs. A well designed habitat and suit system does not require any oxygen prebreathe at all. And suit pressure must be designed for maximum operation of the suit. It doesn't have to copy Earth.

The papers you linked are very interesting. I will study them. And quickly looking at the number from the first paper you linked, the characterization you quoted is still not accurate. Although total pressure is more of a factor than I thought, flammability is still predominantly partial pressure.

But also remember the suit design I advocate is MCP, so will operate in the CO2 atmosphere of Mars. Only the helmet and a counter-lung vest will be inflated with breathing air. Boots will also be inflated, with an air dam at the ankle. The most practical gas to fill those boots would be the same air / gas-mix as the habitat. MCP fabric means Mars atmosphere (~95% CO2) all the way to skin. The MCP layer next to the skin is elastomer, but the outermost layer is Goretex, which is PTFE. Also note the greenhouse film I keep talking about is sold by Honeywell under the brand names Aclar or Clarus, chemical name is PCTFE, but until 1995 was sold by 3M by the name Kel-F. Note how non-flammable PTFE and Kel-F are.

Last edited by RobertDyck (2014-11-15 09:54:24)

Impaler · 2014-11-15 01:34:54

RobertDyck wrote:

The papers you linked are very interesting. I will study them. And quickly looking at the number from the first paper you linked, the characterization you quoted is still not accurate. Although total pressure is more of a factor than I thought, flammability is still predominantly partial pressure.

No, I think your reading the graph wrong. On the first of the two papers, Table 2 you can clearly see the minimum concentration of oxygen to sustain combustion hardly changes from full to half normal pressure for most materials. While it dose rise it is on the order of 1-2% for most materials though it is greatest for the most fire resistant materials (Kel-F 81 particularly). And again for most materials the minimum partial pressure of O2 to sustain combustion is dropping rapidly as overall pressure drops, most materials need near half the original partial pressure when total pressure is halved, in other words these materials would burn just as well at high altitude as at low.

Only for Kel-F 81 do the tow factors get close to parity, with both Oxygen concentration and Partial pressure changing by comparable magnitudes (44% higher oxygen percentage, 44% lower partial pressure). Extrapolating this materials properties (very tentatively as we only have 3 data points) we would expect it to become incombustible even in pure oxygen of 1 or 2 bars and it dose seem to have the mechanical properties necessary to make a greenhouse (strength, impermeable etc), though Wikipedia says it will degrade under radiation.

RobertDyck wrote:

But also remember the suit design I advocate is MCP, so will operate in the CO2 atmosphere of Mars. Only the helmet and a counter-lung vest will be inflated with breathing air. Boots will also be inflated, with an air dam at the ankle. The most practical gas to fill those boots would be the same air / gas-mix as the habitat. MCP fabric means Mars atmosphere (~95% CO2) all the way to skin. The MCP layer next to the skin is elastomer, but the outermost layer is Goretex, which is PTFE. Also note the greenhouse film I keep talking about is sold by Honeywell under the brand names Aclar or Clarus, chemical name is PCTFE, but until 1995 was sold by 3M by the name Kel-F. Note how non-flammable PTFE and Kel-F are.

I'm all for developing these suits but I'm inclined to think that the donning time will be so long that they may be impractical, not to mention the decompression/pre-breath times because they still deliver a very low pressure to the body. Russian Orlan, one-piece rear entry suits have very fast entry, no bre-breathing and their much more compatible with dust mitigation systems (hooking the rear hatch into a wall portal so the suit never enters the habitat), Russian pragmatism usually beats American technical over-reach, our future suits will more likely follow their patterns.

Last edited by Impaler (2014-11-15 01:37:53)

RobertDyck · 2014-11-15 09:17:37

Impaler wrote:

I'm all for developing these suits but I'm inclined to think that the donning time will be so long that they may be impractical, not to mention the decompression/pre-breath times because they still deliver a very low pressure to the body.

That's why habitat pressure must be low. Eliminate prebreathe entirely. I already calculated pressures. Since lower pressure reduces counter force for joint movement, you want to reduce suit pressure. Apollo suits used 3.3 psi pure oxygen; Apollo capsule and Skylab used 3.0 psi oxygen + 2.0 psi nitrogen for a total of 5.0 psi. That way, if a suit leaked, an astronaut could endure 10% pressure loss before suit pressure dropped to partial pressure he was used to in the capsule. Following that principle I suggest dropping suit pressure to 3.0 psi pure oxygen, and habitat O2 partial pressure to 2.7 psi. That still allows for 10% pressure loss. And suit pressure is still greater than partial pressure of oxygen in Boulder, Colorado. Then using the rule developed by the US navy and diving industry, for zero prebreathe time total suit pressure to partial pressure of nitrogen is a ratio of 1:1.2; so with suit pressure at 3.0 psi that makes habitat N2 3.6 psi. And Mars atmosphere measured by Viking 2 lander has 2.7% nitrogen, 1.6% argon, so keeping the N2:Ar ratio the same, that gives the habitat 2.133 psi partial pressure argon. Argon also has a limit, but this is below that limit. Adding that up, habitat will have 2.7 psi O2 + 3.6 psi N2 + 2.133 psi Ar = 8.433 psi total.

And of course it's most practical to ensure the greenhouse has the same pressure and gas mix as the habitat. This makes the greenhouse an extension of living space, and the greenhouse provides oxygen recycling for the habitat.

Of course, with no radiation shielding what so ever, time in the greenhouse will have to be limited. But I calculated that as well, with the assistance of a student from MIT. Assuming a permanent base or settlement on Mars is burried in regolith, with light pipes to allow sunlight to be piped into the habitat. But at least 2 metres depth of Mars regolith soaked with water, freezing to become permafrost. That provides so much radiation shielding that the habitat will have none. Then the goal was to keep annual radiation exposer to what a US nuclear reactor worker receives. That requires limiting the time an astronaut spends outdoors in a spacesuit to 40 hours per week. And with a plastic film greenhouse, that has no more radiation shielding that a spacesuit, time in the greenhouse counts toward time outdoors in a spacesuit. But again, that's 40 hours per week. That's a work week. Not at all a problem.

Also remember, I was part of the Mars Homestead Project, phase 1, hillside settlement. The burried habitat will have a 2 story atrium that looks like this...

louis · 2014-11-15 20:07:31

I think the idea that the first Mars colonists will be popping in and out of suits is ridiculous. There's no need. People will stay in an artificially pressurised environment nearly the whole time. When move between pressurised habs they will travel in pressurised vehicles. It's so much easier than any other approach, that I can't see any other approach being adopted.

Of course there will be walking on Mars but it will be rare to begin with. It will probably only really take off with tourism when people will want that walking and "fingerpads on stone" feel to their experience.

But for the first colonists there is simply no need to get into that dangerous territory of moving in and out of suits.

Impaler · 2014-11-16 02:14:59

That atmosphere is wholly unsafe and isn't going to happen, it is 42% oxygen and NASA won't go above 30%. Standard 100 mBar on ISS is the future, were not going to be rolling back the clock 40 years on Atmospheres and taking fire-risks (the most horrible way to die in space or on the ground too for that matter) just to indulge in wearing skin-tight space suits.

The Homestead Project strikes me as borderline science fiction (I read it years ago), I think your arguments consistently stem more from attachment to a romanticized vision then from practical engineering analysis and siting that project just reinforces it as it is one of the more outlandish things ever made, particularly in the raw materials processing.

Last edited by Impaler (2014-11-16 02:16:25)

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