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Thread: Centrifugal habitat on the Moon?

  1. #1 Centrifugal habitat on the Moon? 
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    I would like to think that some day, many humans will call the Moon home. Of course, we don't know for certain this would even be possible, never mind feasible or desirable. We already know that a microgravity environment can degrade a human's muscles and skeleton significantly in just a few days. Maybe 16.6% of Earth's gravity would cause long term, even irreversible, problems for the health of those living in it. So, without much regard to the costs involved, I propose an underground centrifugal habitat in order to mimic an Earth environment. I hope to get feedback with regard to the basic physics involved in such an undertaking.

    This would be a huge cylinder with vertical walls (which, on the scale I propose, means the interior would be a cone not a cylinder) which is rounded off at the bottom. The vertical portion would have a full Earth g, 9.8 m/s2 which means the ground in the vertical portion would be perceived to slope downhill at arctan(0.166)=about 9.4 degrees. There could be soil, vegetation, and rivers which flow down to the bottom. The rounded bottom will have regions of decreasing gravity, down to Lunar gravity at the bottom, which will be a body of water. Some means of pumping the water and silt back to the top would be needed. This might be accomplished with a tube down the center, which also provides light.

    The height of this "cylinder" may depend on how deep a hole could be dug before the rock gets too hot or otherwise unusable. But as long as we're thinking big, maybe we could cool the Moon down over hundreds (thousands?) of years, with infrared or microwave radiation beamed away, which draws its power from the temperature differential between the upper crust and the magma interior. At any rate, in terms of providing an Earth-like habitat, bigger would be better.

    The radius should also be very large. My notion is to fill the interior with air, so that some sort of weather could take place. "Naturally" occurring rain and wind would be desirable. I don't know if there have been any long term studies of the effect of Coriolis forces on living beings. Some proponents of spinning habitats in space say that one rotation in 45 seconds would be acceptable. Of course, any spinning ring in space would have to hold itself together by its own structural integrity, which places a severe upper limit on the radius. But deep underground on the airless Moon, huge electromagnets embedded in the walls and floor of the containing shell could allow a much greater radius. One thing the Moon has plenty of is solar energy, so these magnets could be strong enough to provide the immense centripetal forces needed. They would also keep the centrifuge spinning.

    Suppose, for example, it turns out that humans need a rotation period of at least 5 minutes to avoid any long term health problems. Centripetal force is proportional to the radius, and proportional to the square of the angular rotation. A 9.8 m radius cylinder rotating at one radian per second, or 2*pi seconds per revolution, would provide 1 g of acceleration. Is that right? So a cylinder spinning at 300 seconds per revolution is slower by a factor F=300/(2*pi), and the radius would have to increase by a factor of F2 to maintain 1 g. That puts the radius at about 22.4 km, and the interior diameter of the cylinder would be about 44.7 km. Since the structure is really a cone, the perceived gravity would actually decrease a bit as you go lower relative to the Moon. Let's say the vertical wall section is 50 km high. Let's also say this structure is located on either the South or North Lunar pole. The polar radius is 1736 km. If we have 1 g at the top , the bottom of this 50 km strip would have about 97% of 1 g. The ground at the bottom of the strip would appear to slope at about 9.2 degrees, as opposed to 9.4 degrees at the top.

    An advantage of locating this habitat on a lunar pole is, there may be ice there for water, although it's not likely to be enough. Also, there would be fewer precession issues the magnets would have to deal with as the moon orbits the Earth. That's not to say that a similar habitat elsewhere on the Moon would not be possible, though.

    Ingress and egress would occur through the top center. There could be electromagnetic catapults to launch and possibly also to land incoming vessels.

    There are many basic issues here that need to be addressed. What sort of "natural" weather would the inhabitants get? Would tornadoes or excessive lightning be a problem? How would one safeguard the power supply? How delicate would the ecosystem be? Would a few spores of fungus on someone's clothing, or some unscreened disease, wreak havoc?

    You might ask, why bother? I would paraphrase Heinlein and answer, Earth is too fragile to keep all our eggs in one basket. I believe humans will live on Mars and elsewhere in the Solar system some day. The Moon is a necessary first step, so we can learn from and hopefully survive our mistakes. At any rate, my intended focus here is less on "why" and more on "how."

    Please, be nice, but show me my glaring errors here, or make any comment you like. This all seems possible to me with the science we already know. What did I miss? Thanks for your time & attention!


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    Quote Originally Posted by twixtfanatic View Post
    This all seems possible to me with the science we already know. What did I miss?
    Strength of materials primarily. Calculate the tensile strength you would need near the top - I think it's going to be insanely high.

    But in any case, until we know that there is a problem living in 1/6 gravity it's probably too soon to start designing city-sized centrifuges. Indeed, reduction in G may give a new life to people with heart problems, arthritis, osteoporosis etc so people may actively shun such an environment.

    And if there is some benefit in being in higher G's you are probably better off with small spinning structures where people work/sleep than massive structures such as the one above. It would sure suck to have one bad bearing cause the death of everyone in a city.

    One thing the Moon has plenty of is solar energy
    It doesn't have much more than Earth (1300 w/sq m instead of 1000 w/sq m.) Also the month-long day makes storage a much bigger problem. The only place you can avoid this issue is at the poles, where a very careful selection of location might give you month-long power.

    My notion is to fill the interior with air
    While you could get oxygen from lunar rocks, and might be able to get some water from polar ice, nitrogen is going to be very hard to come by.


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  4. #3  
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    I'm surprised you didn't use a cone; I know it occurred to you. Is the water feature worth the trouble of having everything built upon an unlevel surface?

    Structural issues are painful only if you insist on a clear empty interior. Allow wires spanning the "sky" and it's alright.

    Getting on and off. Consider how gondolas disengage the moving cable to pause for passengers. A similar system may be less heroic than launching from catapults. Of course the "car" does not really travel to a destination, you're more like crossing from one side of a train platform to the other.
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    Quote Originally Posted by billvon View Post
    Quote Originally Posted by twixtfanatic View Post
    This all seems possible to me with the science we already know. What did I miss?
    Strength of materials primarily. Calculate the tensile strength you would need near the top - I think it's going to be insanely high.
    Imagine that the containing shell for my habitat has a series of rings built into it, horizontal surfaces projecting out from the shell wall, which were all at the ends of very powerful electromagnets. Above these horizontal surfaces would be horizontal surfaces projecting from the outside of the habitat, with magnets that would repel vertically from the containing shell. (Maybe there is an upper limit on flux density and strength which would render my concept impossible.) Before the structure starts spinning, these surfaces would be in contact. In this manner, the weight of the habitat would be largely transferred to the shell. Once spinning, the entire habitat would float in a vacuum. It has been proven that a static equilibrium is impossible for floating permanent magnets, but if the floating object is spinning, as the "Levitron" toy demonstrates, an equilibrium is possible. So, the habitat itself would not be hanging from a support point at the top. Most of the weight would be transferred to the outer shell. The top of this shell would look like a trumpet shape pointing down. Would it be possible to transfer all the weight of kilometers of moon rock above the habitat, as well as the habitat itself, to the sides like that? Once the habitat is spinning at full speed, besides horizontal surfaces, vertical surfaces of magnets would be needed to keep the structure from flying apart. Again, the outer shell must be strong enough to withstand the immense forces. So the containing shell might turn out to be considerably larger than the habitat, to keep itself together and to guide the magnetic flux lines.

    But in any case, until we know that there is a problem living in 1/6 gravity it's probably too soon to start designing city-sized centrifuges. Indeed, reduction in G may give a new life to people with heart problems, arthritis, osteoporosis etc so people may actively shun such an environment.
    Perhaps. But if they ever want to return to Earth, they may need to get used to Earth gravity.

    And if there is some benefit in being in higher G's you are probably better off with small spinning structures where people work/sleep than massive structures such as the one above. It would sure suck to have one bad bearing cause the death of everyone in a city.
    If it turns out that a fast rotation like 45 seconds per revolution is tolerable in the long term, then a smaller structure would indeed make sense. Even that would be potentially very dangerous, though. My idea involves no physical bearings, but there are plenty of things that could go wrong. All these powerful electromagnets with their North ends in close proximity will repel from each other. A magnet might break loose from its moorings. Once it does, how will you repair it without shutting down the spin? What sort of magnetic flux turbulence might you encounter? What if there is a moonquake, or some other shockwave? What if the habitat becomes unbalanced, e.g. too many people gather on one side?

    Despite all that, what quality of life would you get living in a cubicle? It seems people would need to "get outside." so to speak, once in a while.

    One thing the Moon has plenty of is solar energy
    It doesn't have much more than Earth (1300 w/sq m instead of 1000 w/sq m.) Also the month-long day makes storage a much bigger problem. The only place you can avoid this issue is at the poles, where a very careful selection of location might give you month-long power.
    It depends on what sort of power network is established. It's always shining somewhere on the Moon (except during a Lunar eclipse of course.) Also it may be possible to store a lot of energy.

    My notion is to fill the interior with air
    While you could get oxygen from lunar rocks, and might be able to get some water from polar ice, nitrogen is going to be very hard to come by.
    So, the heavier isotopes of Nitrogen found on the moon would not be healthy to breathe? Hey, we could import it. All I'm asking for is about 1014 cubic meters at 1 atmosphere pressure.
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    Quote Originally Posted by twixtfanatic View Post
    Imagine that the containing shell for my habitat has a series of rings built into it, horizontal surfaces projecting out from the shell wall, which were all at the ends of very powerful electromagnets. Above these horizontal surfaces would be horizontal surfaces projecting from the outside of the habitat, with magnets that would repel vertically from the containing shell. (Maybe there is an upper limit on flux density and strength which would render my concept impossible.)
    1) If they were that strong you'd be hard-pressed to keep them from crushing the structure before you started it spinning.
    2) If you blow a fuse*, the field collapses and everyone dies. (* - or lose superconductivity through coolant leak)
    3) Spinning magnets introduce powerful eddy currents. You'd have to make your structure out of mostly non-conductive materials which would be tough.

    Perhaps. But if they ever want to return to Earth, they may need to get used to Earth gravity.
    Have an Earth hotel where such people go for a few months.

    If it turns out that a fast rotation like 45 seconds per revolution is tolerable in the long term, then a smaller structure would indeed make sense. Even that would be potentially very dangerous, though.
    Things like trains, boats, carousels etc all have a lot of engineering history on their side.

    Despite all that, what quality of life would you get living in a cubicle? It seems people would need to "get outside." so to speak, once in a while.
    Right - and you could. You could work and sleep in 1G then spend the rest of your time in 1/6G. Most people work/sleep in small rooms today.

    It depends on what sort of power network is established. It's always shining somewhere on the Moon (except during a Lunar eclipse of course.) Also it may be possible to store a lot of energy.
    We haven't managed to do either of them here on Earth despite a tremendous demand for that - so it's probably a long way off.

    So, the heavier isotopes of Nitrogen found on the moon would not be healthy to breathe?
    I have no idea. The problem is that there is almost zero nitrogen on the Moon.
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    I doubt that heavy gravity will be needed 24 hours a day. Even just one sixth gravity is not as bad as zero G. I suspect that a mini centrifuge to do (say) one hour of exercises per day would be sufficient, and would be a damn sight easier from an engineering viewpoint.
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    Though not original, I think there's good potential in many ideas like this.

    For example, imagine a train of sleeper cars that run on a simple circular train track at a banked angle and running at a speed to create a 1/2 G of acceleration; people spend about half the day there sleeping, getting a workout, eating one or two major meals, making love in the shower and other things easier to do with gravity. The basic technology of the cars running on the track aren't too much more complex than found in any amusement park (of course the life support are pretty complex). People work in low G environment, when done they go to a station, where they board a train that runs on an track and speeds up to match and moving into the living/sleeper cars; it also serves to remove trash, supplies etc.
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    Quote Originally Posted by Pong View Post
    I'm surprised you didn't use a cone; I know it occurred to you. Is the water feature worth the trouble of having everything built upon an unlevel surface?
    In order to create a surface normal to the perceived vertical at every point, a rounded shape like the interior of an egg would be necessary. In that case, the region with close to Earth normal gravity would occur only at the top. My concept would have this rounded portion at the bottom, and an extended region 50 km wide of close to Earth normal on the vertical walls. Of course the design would depend on what the inhabitants would want. Lower gravity regions would be useful; for example, the g/4 region could get humans used to Martian gravity. Compared to all the other hurdles, living on overall perceived 9 degree slope seems minor. There could be plenty of locally flat areas. In fact, the interior could be built as a series of steps, rings which are close to flat or only a slight slope, with steep portions that drop down to the next step. The steep portions could coincide with the horizontal surfaces and magnets that transfer weight of the structure to the containing shell, as I described in my previous post.

    Structural issues are painful only if you insist on a clear empty interior. Allow wires spanning the "sky" and it's alright.
    Maybe a lot of spanning cables could hold the structure together, but I have my doubts. These cables would need to be approximately 20 to 40 kilometers long, and would have to withstand immense forces. Also they could disrupt the weather patterns. If the center of my lake were heated, water vapor would rise in a column and then cycle down the sides. At least, that's my theory. Instead of a tube down the middle, as I suggested, light could be provided by lights on the interior surface which point at the opposite side. Maybe cables that do not go through this central column of rising air would help with the overall structural strength, and provide a means of transportation as you suggest.

    Getting on and off. Consider how gondolas disengage the moving cable to pause for passengers. A similar system may be less heroic than launching from catapults. Of course the "car" does not really travel to a destination, you're more like crossing from one side of a train platform to the other.
    I was referring catapults for getting on and off of the Moon itself. Cable cars may be possible on the interior, especially for going up and down the cliff regions. Some sort of subway system might also work.
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    Quote Originally Posted by billvon View Post
    Quote Originally Posted by twixtfanatic View Post
    Imagine that the containing shell for my habitat has a series of rings built into it, horizontal surfaces projecting out from the shell wall, which were all at the ends of very powerful electromagnets. Above these horizontal surfaces would be horizontal surfaces projecting from the outside of the habitat, with magnets that would repel vertically from the containing shell. (Maybe there is an upper limit on flux density and strength which would render my concept impossible.)
    1) If they were that strong you'd be hard-pressed to keep them from crushing the structure before you started it spinning.
    That is why I distinguished between two sets of magnets: horizontal surfaces to deal with lunar gravity, and vertical surfaces to deal with centripetal forces. The current running through them would be controllable, not only to get from stationary to spinning, but to compensate for imbalance or local failures, and if necessary to slow the whole thing to a stop again.

    2) If you blow a fuse*, the field collapses and everyone dies. (* - or lose superconductivity through coolant leak)
    Or a software glitch creates a power surge/dropout, or an atmosphere leak causes unexpected turbulence, or terrorists take over the control center, or the AI running the show decides we are not worthy. If humans can withstand or even prefer low gravity most of the time, certainly a simpler solution would be better.

    EDIT: My concern is for the long term status of a colony on the Moon. After several generations spending most of the time in 1/6 g, what would we look like? Would we be smaller? Would we become essentially a different sub-species? Such change is not necessarily bad, but I don't think we should hand this legacy to future generations just because the engineering is so much simpler.

    3) Spinning magnets introduce powerful eddy currents. You'd have to make your structure out of mostly non-conductive materials which would be tough.
    So, humans would be exposed to these powerful magnetic flux lines, like living next to an MRI machine? This cure might be worse than the disease. I was hoping to sheid the interior from most of the magnetism.
    Last edited by twixtfanatic; January 13th, 2014 at 02:43 PM.
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    Quote Originally Posted by Lynx_Fox View Post
    For example, imagine a train of sleeper cars that run on a simple circular train track at a banked angle and running at a speed to create a 1/2 G of acceleration . . .
    If anyone is interested, James Hogan wrote a book called "endgame enigma" that involved a subterranean "G wheel" of the sort discussed here. (For a very different purpose but same principle applies.)
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    Quote Originally Posted by twixtfanatic View Post
    Quote Originally Posted by Pong View Post
    I'm surprised you didn't use a cone
    There could be plenty of locally flat areas. In fact, the interior could be built as a series of steps.
    Nice. Your solution is much better.

    Quote Originally Posted by twixtfanatic View Post
    Quote Originally Posted by Pong View Post
    Structural issues are painful only if you insist on a clear empty interior. Allow wires spanning the "sky" and it's alright.
    need to be approximately 20 to 40 kilometers long
    I hadn't fathomed the scale of this thing.

    Now I'm wondering if the bearing can provide some support. Suppose the hole is firm smooth masonry, and nearly matches the cylinder shape. What if you fill that gap with a lubricant, like water? And seal the hole so it can take pressure. Then the outward "weight" of the habitat compresses against the lubricant. This allows a flimsy habitat shell (no thicker than a speedboat hull) because it's nicely supported. There will be drag from the fluid, but plainly no worse than ships on lakes.

    But with the radius you're talking about, the rim velocity will be pretty high. I guess this bearing would overheat.

    Quote Originally Posted by twixtfanatic View Post
    Quote Originally Posted by Pong View Post
    Getting on and off.
    Cable cars may be possible on the interior, especially for going up and down the cliff regions. Some sort of subway system might also work.
    You didn't get it. Here's an illustration: The cable car is outside the rim, stopped at the Luna Firma platform firmly planted on the Moon's surface and facing the colony. Meanwhile the spinning colony is nearly grazing the cablecar, and each revolution you see whiz past one colony cablecar platform with people waving across the gap. They're all standing sideways! If you could freeze this at the right moment you'd have an M. C. Escher train station with platforms on either side of the tracks. There are no tracks. Instead there're a pair of cables running around the circumference of the colony; one moving with the colony, one (not moving) with the Moon. The cablecar is slung securely over both. The car doesn't move with the whizzing cable because the conductor hasn't engaged the cable brake yet. But as she does, the cable begins to accelerate the car along the cable and away from the station. This is exactly how terrestrial cablecars work. In our application, she'll gradually brake against the cable so that the car pulls in to Colony Station, at which point the car's just hanging idly on that cable.
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