New Mars Forums

Hmm.

I don't discount self-pressurizing propellants out of hand.  However, I think something similar to what you are proposing could be employed to pressurize cryogenic fuels.  The trick would lie in not blowing the whole works by introducing hot, uncombusted reactants into the propellant tanks along with the pressurizer exhaust.  In the worst case scenario, the pressurizer would provide a flow path directly between the fuel and oxidizer tanks.

You'd better hope not. 

A big part of many proposed mars expeditions (Mars Direct, etc.) is an autonomous oxygen production plant for in situ production of life support supplies and propellant.  If what you've just claimed is true, you may kiss autonomous ISRU goodbye. 

Living plants are desirable because of their ability to produce food.  That is the only thing they do that can't be imitated by mechanical systems.

PCTFE would definitely have acceptable properties for the windows of a pressurized greenhouse.  Back when it was sold under the brand name Kel-F, was also extensively used in satellites.  According to the 1965 edition of the Lockheed Space Materials Handbook (sorry, no link), PCTFE suffers greater percent loss of tensile strength than Teflon under UV radiation (15% vs. 6% under arbitrarily high – read “more than earth orbit” – UV illumination) and also more outgasing in vacuum conditions.  It’s still pretty good, though – an excellent choice for windows in a pressurized greenhouse.  You should also consider that its tensile strength increases remarkably as the temperature decreases, more than doubling at a lowest design temperature of -140 C.

Regarding strength, Robert, you keep expressing the strength of materials in terms of psi internal pressure.  I don’t think that’s sufficiently informative.  The burst pressure of a greenhouse window is going to depend on its size.  I can get a window of Saran Wrap to withstand 19.4 psi if I just make it small enough.  I think you should consider expressing strengths in terms of yield strength, not internal pressure, for a better comparison with other materials.

That’s pretty good!  I’ve already put it in my favorites.

I also tried my hand at greenhouse design with a set of articles for the New Mars Wiki.  John Creighton did a great one, too.  Reading back over them, I’ve found a few mistakes that I intend to go back in and edit out when I get time.  (The lighting level is underestimated, the suggested pressure is too high, etc.)  (I’d love help with it, BTW.)  But the section on greenhouse structure does address size considerations. 

Attaining the necessary window size for a solar illuminated greenhouse is going to be a problem for almost any transparent material, especially anything with layers thin enough to be inflatable.  I don’t think layered PCTFE alone will prove to be a sufficient building material.  We’re going to need to reinforce it with something.

I think that article hit it, SpaceNut.

I am curious: a reasonable amount of data has started to accumulated from these depressurized growth experiments, enough to begin making educated estimates, but what I've seen to focuses mostly on growth rates and other metabolic indicators.  Has anyone ever actually sown, raised, and harvested a crop from one of these things, then eaten it?  Are there any measures of actual productivity under these conditions?

I've mentioned a few times that I think we should have a dedicated crewman to operate the greenhouse, but that's really a bare minimum. The eventual establishment of a farm on Mars is so important to permanent settlement that I think we should go beyond that.

I think we should have a dedicated expedition to start the greenhouse.

An old reference I found for growing plants under lights recommends about 300W/m^2 lamp power, or only 190 kW for a 440 square meter greenhouse whose power system runs at 70% efficiency.  Since the recommended fluorescent + incandescent combination is only 40-50% efficient, that 190kW would include at least double the amount of heat required, necessitating some sort of cooling system to shed the rest.  So, make it an even 200 kW, easy, to run both greenhouses.  That can be reduced to a little under 150 kW by keeping one half dark, but the dark half will still need heat even when there are no lights. 

The minimum illumination to get a crop from flowering plants is about 130 W/m^2.  (Some make do with less, some need more.)  Martian sunlight can provide this, but not during the thickest dust storms, and not at latitudes above 70 degrees north or south.  There’s also the pressure wall to consider, which is going to have to block a lot of light in order to provide a thick enough wall.  Reducing pressure allows a thinner wall and/or bigger windows, but research indicates that you can only take the pressure so low before the corresponding decrease in absolute humidity and boiling point catches up to your crop.  I believe transmission of only up to 85% of incident light is attainable through the greenhouse windows.  (A greenhouse constructed of 100% transparent material is unattainable, IMHO.)  That would increase the mass of the greenhouse (due to the accompanying thicker walls) and further restricts its useful range to about 60 degrees north or south.  Still, windows don’t add as much mass as a 200kW dedicated powerplant, even with extra collector area to provide heat.  That makes solar greenhouses competitive, even with these limitations.   

The drop in light during an exceptionally thick dust storm isn’t 100%, so all that backup lighting would need to do is make that up.  100W/m^2 lamp power should be sufficient to keep up productivity during dust storms at the equator or keep the greenhouse going at higher latitudes – one third what’s necessary for growing under lights exclusively.  Those lamps need not sit idle when not in use, either – artificial lighting is better than window light for many agricultural applications, because you can adjust it.  Or, since dust storms tend to be seasonal, colonists can simply plan to adapt seasonally by laying in enough supplies to ride it out.

Nitrogen conservation will be more useful than trying to recycle it.  Grey water and black water waste will be well worth recycling, but not just for the water.  The solid and solute content may prove an excellent source of nutrients for plant growth, once properly composted.

Shipping a dozen kilos of hydroponic nutrients might be worthwhile as well.  The actual amount necessary for a given crop is remarkably small.  Augmenting that with compost (you can use compost for hydroponics, too) would stretch that supply out for years.

Setting up a functioning greenhouse of any size larger than a room in the hab is going to require a lot more work than merely inflating a balloon and setting out some seeds in a tray.  Greenhouse design is going to be tricky.   IMHO, even the Mars Homestead project hasn’t given it all proper consideration, and they’d made more design progress than most. 

It's definitely the sort of thing we should not wait to start doing.  Likewise, we shouldn't throw all our seeds in the pot at once before we get some experience with the local growing conditions, either.  So, starting small and building larger as we go is an excellent idea.  By the end of the first martian year, we should know enough to start a full crop, but I would not count on it before then.

Amen!  Which, I suppose, is the number one reason that we should refuse to go "on short notice" without some compensation far more impressive than mere prestige.

I take issue with the claim that the only attainable trajectory for premature return is >18 months transit, but in the end that's an issue of design philosophy, not mission philosophy.  The real issue is whether to go with minimal preparation.

What's wrong with that? 

That scheme would not preserve the wastes for potential future recycling - you'd need to vent the holding tank/boiler at the top and skip the "deposited on the Martian soil" step.  But yes, that's the basic idea.  Store (not strew) the effluent nearby, and come back to it later if we decide we care.

By my estimate, the equipment needed to grow food for a crew of six would take about as much mass as a second hab.  It should be sent seperately, and preferrably wait until well into the first mission.  And its setup and operation will take more than  one person's man-hours of work per day - it should be spread between two or more people.

If you're just scouting, it's best to leave the greenhouse on Earth.

Yes. 

Almost anything worth recycling will keep at -60C temperature, and all that's necessary to store it for centuries is a properly secured plastic cover.  If we like the location, equipment for 100% recycling can come later.  If we don't like the location, the trash midden will be the least valuable thing left behind.

As far as I've read, there are no known instances of a GRB source repeating, so they're probably not a constant source that is rotating.  One current model suggests that they are supernova-like events, and probably not frequent in any given galaxy. 

I believe I've figured out where that 1/20000 figure comes from - it's the ratio of the subtended area of two 1 degree wide jets versus the surface area of a sphere.

Nickname, getting the projected black hole mass up to 50% of the universe takes a lot of fudging.  The most likely value with our current assumptions is 0.5% to 5%, with 50% being the crazed outer limit of a wild-ass guess.  Realistically, I can only get you to 5%.

Thanks, Nickname.  Upon consideration, 10 events per day and 1/20000 detection probability may be within the realm of the possible, provided their model is correct.  I would tend to doubt that, except that none of these has ever repeated to my knowlege (i.e., it's not something rotating) and there have been several cases of associated visible supernovas.  So, the model is probably about right, too.

There've probably been about 1 quintillion of these things since the beginning of the universe - each leaving behind around 3 to 10 solar masses.

That's cool.   8)

I still have doubts about the ability of this to explain gravitational deviations on the intergalactic scale, though.

The SWIFT observatory has identified events with redshifts as high as z=3.2.  That's near the edge of the visible universe - about 10 billion light years away.  At that distance, "brighter" events would be more visible, but that's still one heck of a horizon.  There are a lot of stars in that space - probably at least 1000 for every black hole formed in a GRB, and that's a highly conservative estimate (10000 or more would not surprise me - I need a better estimate for the number of stars in a 10 BLY sphere   :? ).  Most of those won't be solar mass, so the actual mass of black holes will be more favorable.  But it's still only a few percent at best - a start, nothing more. 

This does not account for the 90% difference in observable mass predicted by "Dark Matter" theories. 

Of course, these guesses are quite wild.  SWIFT results might only be linear to three billion light years or so.  My guesstimate of the stellar density might still be too high, although it's as low as I can take it with a straight face.  A combination of either might decrease my calculated stellar mass by as much as a factor of 20, making GRB black holes able to explain as much as - maybe - 50% of the mass in the universe. 

Where's the rest?