Author Topic: Elon Musk: glass geodesic domes  (Read 238518 times)

Offline meekGee

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Re: Elon Musk: glass geodesic domes
« Reply #260 on: 11/13/2016 11:38 pm »
So about this "how to actually build it" thing....

Let's consider triangles that are ~1 m on the side, with edges that have to carry, in a 50 m dome, ~100 tons or 1E6 N. (give or take).

How do we build this, mechanically?

Approach 1:  Build an edge-only dome using struts (and some pinned connectors), then attach non structural (but pressure bearing) panes.

This was my first guess, but the main problem is robustness. If you lose even one strut, the whole thing fails.

Second, by the time you are done with the 6-way pinned connections, it's very difficult to seal to.

Third, the panes weigh a lot, and all that mass is wasted.


Approach 2: Use structural panes.

Build the domes out of panes, not struts. The panes attach along their common edges, not just at the vertices.

Take simple acrylic, with a yield strength of 60 MPa.   If you have an entire 1 m edge to work with, you need an edge width of 16 cm to carry the 100 tons.

But Carbon Composite is 10x stronger, in an isotropic fabric...  So you can have 1.6 cm carry the load.  or, maybe 10 cm if you use a network that is 85% open and much lighter, and sandwich it between two thin acrylic sheets..

Doesn't matter exactly. There are a several ways to make a structural pane. The point is - you have a load bearing pane, with edge connectors, and no stresses near the vertices (important!)

One advantage is that each connection is only between two entities (along the common edges) and not 6 - much easier.

The other advantage is that you have built in redundancy.  You can lose a pane, and all you really have is a hole.  The entire thing doesn't collapse.  (I think!)

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Offline Robotbeat

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Re: Elon Musk: glass geodesic domes
« Reply #261 on: 11/14/2016 12:31 am »
The heavy part of the dome will be the glass panels. The structure won't be too bad, though sealing the triangles may be annoying. But anchors just need to be deep enough that their friction cones have enough surface area on the non-overlapping side. It's fine that they overlap, it's just that the overlapping area doesn't count so much. As long as the total weight of the volume of the soil in all the cones is equal to the pressure times bottom area of the dome, you're fine.

To ensure this, you just need long enough cables. Cables made out of the right material can have an extremely good strength to density ratio, like 4GPa/(g/cc). Much stronger than the Windows or the quasi-isotropic Carbon fiber composite of the frame, which would be around 0.4GPa/(g/cc) (much worse for the glass due to knock-downs for stress concentrations). Then the anchors themselves, but if the cables are long, the anchors will be a minority of the mass.

So the anchors and cables would be much less mass than a whole sphere. Also, realize that you still have to solve the problem of how to properly rest on the soil and walk around on the sphere.


And there's another thing: you could have a vertical cable in the center of the dome anchored deep underground. This could carry some of the load as well. The dome wouldn't have to be so high for a given area.
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Offline TripD

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Re: Elon Musk: glass geodesic domes
« Reply #262 on: 11/14/2016 01:25 am »
Would the use of structural panes lend themselves to single replacement after the dome is built?

Couldn't the overlap issue of friction cones be alleviated by staggering the depth of them?  I would be more concerned with how the failure of one effects the other, otherwise.
« Last Edit: 11/14/2016 02:17 am by TripD »

Offline LMT

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Re: Elon Musk: glass geodesic domes
« Reply #263 on: 11/14/2016 01:42 am »
Anchors Away

If you pick a commercial ground engineering system and run the numbers, you can really advance the NSF dome anchoring designs.  Just sayin'.
« Last Edit: 12/14/2016 06:37 pm by LMT »

Offline meekGee

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Re: Elon Musk: glass geodesic domes
« Reply #264 on: 11/14/2016 02:15 am »
The heavy part of the dome will be the glass panels. The structure won't be too bad, though sealing the triangles may be annoying. But anchors just need to be deep enough that their friction cones have enough surface area on the non-overlapping side. It's fine that they overlap, it's just that the overlapping area doesn't count so much. As long as the total weight of the volume of the soil in all the cones is equal to the pressure times bottom area of the dome, you're fine.

To ensure this, you just need long enough cables. Cables made out of the right material can have an extremely good strength to density ratio, like 4GPa/(g/cc). Much stronger than the Windows or the quasi-isotropic Carbon fiber composite of the frame, which would be around 0.4GPa/(g/cc) (much worse for the glass due to knock-downs for stress concentrations). Then the anchors themselves, but if the cables are long, the anchors will be a minority of the mass.

So the anchors and cables would be much less mass than a whole sphere. Also, realize that you still have to solve the problem of how to properly rest on the soil and walk around on the sphere.


And there's another thing: you could have a vertical cable in the center of the dome anchored deep underground. This could carry some of the load as well. The dome wouldn't have to be so high for a given area.

The panels will be heavy, but they don't look like they can offset the force from the pressure differential.

Looking at the depth of the anchors, don't think of them as cones.  Just figure on an inverted triangular prism running around the perimeter - we're talking about anchors that are 1 m apart, and some 15 m deep.

With a friction angle of 30 degrees, you get a 15 m wide triangle (ignoring for a second the tilt angle.

I'm guessing because this is so critical, they'll go even deeper - margin is your friend, and it's a lot cheaper to drill just a little bit deeper then it is to pick up the pieces afterwards if an anchor fails...   
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Offline Paul451

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Re: Elon Musk: glass geodesic domes
« Reply #265 on: 11/14/2016 02:34 am »
To ensure this, you just need long enough cables. Cables made out of the right material can have an extremely good strength to density ratio, like 4GPa/(g/cc). Much stronger than the Windows or the quasi-isotropic Carbon fiber composite of the frame, which would be around 0.4GPa/(g/cc)

The anchors, by definition, have to hold the same load as the frame. If the anchor-cables are so mass-efficient, why wouldn't you just use them as the tensile frame as well?

Offline meekGee

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Re: Elon Musk: glass geodesic domes
« Reply #266 on: 11/14/2016 03:21 am »
Anchors Away

If you pick a commercial ground engineering system and run the numbers, you can really advance the NSF dome anchoring designs.  Just sayin'.

That cheating.   Trying to have fun first
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Offline meekGee

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Re: Elon Musk: glass geodesic domes
« Reply #267 on: 11/14/2016 03:27 am »
Would the use of structural panes lend themselves to single replacement after the dome is built?

Couldn't the overlap issue of friction cones be alleviated by staggering the depth of them?  I would be more concerned with how the failure of one effects the other, otherwise.

I think you can make the tiles replaceable.  I depends on the edge-to-edge connector. 

I'm thinking about tiles that have a composite net core, in two layers.  Near the edge, to core is doubled over, and the tile has a thicker peripheral flange. 

So now you can grab the edge with a metal connector, and the tile doesn't lose strength near its edge.

The metal connector can be a one-piece slide-on, or a two part bolt-on. The latter type allows replacement.

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Offline KelvinZero

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Re: Elon Musk: glass geodesic domes
« Reply #268 on: 11/14/2016 07:37 am »
Borehole digging could be a relevant topic. I think this could be a good way to get water early on. (bore into glacier near equator until reach ice, somehow cycle hot water through or perhaps have a heater.

If you can dig really deep holes then anchoring becomes pretty plausible I imagine. Here is an interesting quote:
https://en.wikipedia.org/wiki/Borehole#History
"Borehole drilling has a long history. Han Dynasty China (202 BC – 220 AD) used deep borehole drilling for mining and other projects. Chinese borehole sites could reach as deep as 600 m"

This range also helps when we don't really know if we will hit the supposed glacier ice at ten meters or a bit deeper.

I have always heard that boring is really hard, at least robotically, and I still can't really imagine sending an ITS without real confidence of hitting water and that it contains no surprises that will mess up your ISRU equipment. But in any case one way or another we have to assume this is solved already for this discussion.

I guess we can find good numbers for weight and labour for equipment like this.

(note: I think we could demonstrate that if the depth of the anchor needed to be more than 0.5𝛑R then it would weigh a fair bit more than the lower half of a sphere.. as well as not providing that double volume. This is argued by conceptually cutting the lower sphere into strips and comparing these to anchor cables.)
« Last Edit: 11/14/2016 11:30 am by KelvinZero »

Offline mfck

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Re: Elon Musk: glass geodesic domes
« Reply #269 on: 11/14/2016 09:58 am »
Does the dome have to be sealed, as in 100% air tight pressure vessel or is it enough to have it at positive pressure(PP)? I mean, considering that ISRU fuel production is a hard requirement to land people (iirc), industrial scale O2 and N2 production and CO2 processing would already be there when they start to dome up.

Wouldn't a PP, open system, dome relax the structural requirements, particularly the anchoring PITA at hand?

Yes, you can't have short sleeve / breathable environment from that way, but you can have a much more benign one under the dome:

Less dust, thermal balance, attenuated radiation and pressure environment... Could probably grow stuff at low (1/2atm?) pressure and reduce the personal protection to rebreather and MCP suits....

Yes, it's wasteful, but I suggest to allow an abundance in some resource, else this whole enterprise is, probably, doomed if it has to squeeze every margin. Energy abundance would seem the most beneficial (I.e nuclear vs PV), allowing ISRU overproduction, which can't be bad, fuel and ECLSS wise.
« Last Edit: 11/14/2016 10:00 am by mfck »

Offline francesco nicoli

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Re: Elon Musk: glass geodesic domes
« Reply #270 on: 11/14/2016 10:29 am »
Does the dome have to be sealed, as in 100% air tight pressure vessel or is it enough to have it at positive pressure(PP)? I mean, considering that ISRU fuel production is a hard requirement to land people (iirc), industrial scale O2 and N2 production and CO2 processing would already be there when they start to dome up.

Wouldn't a PP, open system, dome relax the structural requirements, particularly the anchoring PITA at hand?

Yes, you can't have short sleeve / breathable environment from that way, but you can have a much more benign one under the dome:

Less dust, thermal balance, attenuated radiation and pressure environment... Could probably grow stuff at low (1/2atm?) pressure and reduce the personal protection to rebreather and MCP suits....

Yes, it's wasteful, but I suggest to allow an abundance in some resource, else this whole enterprise is, probably, doomed if it has to squeeze every margin. Energy abundance would seem the most beneficial (I.e nuclear vs PV), allowing ISRU overproduction, which can't be bad, fuel and ECLSS wise.

what's the point in building a gigantic glass dome ifyou cannot really going around it without a mask?

Offline mfck

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Re: Elon Musk: glass geodesic domes
« Reply #271 on: 11/14/2016 10:51 am »
Does the dome have to be sealed, as in 100% air tight pressure vessel or is it enough to have it at positive pressure(PP)? I mean, considering that ISRU fuel production is a hard requirement to land people (iirc), industrial scale O2 and N2 production and CO2 processing would already be there when they start to dome up.

Wouldn't a PP, open system, dome relax the structural requirements, particularly the anchoring PITA at hand?

Yes, you can't have short sleeve / breathable environment from that way, but you can have a much more benign one under the dome:

Less dust, thermal balance, attenuated radiation and pressure environment... Could probably grow stuff at low (1/2atm?) pressure and reduce the personal protection to rebreather and MCP suits....

Yes, it's wasteful, but I suggest to allow an abundance in some resource, else this whole enterprise is, probably, doomed if it has to squeeze every margin. Energy abundance would seem the most beneficial (I.e nuclear vs PV), allowing ISRU overproduction, which can't be bad, fuel and ECLSS wise.

what's the point in building a gigantic glass dome ifyou cannot really going around it without a mask?
It's in the quote... 

Offline Ionmars

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Re: Elon Musk: glass geodesic domes
« Reply #272 on: 11/14/2016 12:45 pm »
Semantics:
In some cases, the difference between an anchored dome and a sphere could be in the words chosen. Consider a dome with anchors that are extensions of the struts that contain the panes. Now bend those anchors back under the dome in a curve that mirrors the curve of the dome above. They continue until they meet each other in the middle of the structure and are joined together.  The anchors would be interlaced with struts, like the topside structure, but solid material would be employed between the anchors rather than window panes. As suggested earlier, the bottom 1/3 could be back-filled with regolith. We could then name the underside structure the self-connected anchors of a dome or the underside of a sphere.

This could also be accomplished with MeekGee's interlocking panes as structural elements.

Offline Ionmars

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Re: Elon Musk: glass geodesic domes
« Reply #273 on: 11/14/2016 01:24 pm »
Multipurpose:
Assume a dome/sphere structure like I described above. The first purpose os to provide viewing of the outside panorama.
As purpose number 2 we could addd features that would utilize this volume as a central park. Fill the bottom 1/3 with water to form a pond. Provide floating walkways that meander across the surface. Add a deck in the center large enough for a bandstand or village meeting place.

Purpose number 3 could be raising fish in the pond as a supplemental protein source.

Number 4. Build vertical tubes anchored in the bottom of the pond that rise high above the surface. Pump water from the bottom of the pond and sprinkle it into the air. This serves to aerate the water for the benefit of the fish and fountains for the people.

Number 5. Hang plants and vines up and down the tubes. Grapes and tomatoes for food supplements and flowers for aesthetics.

You could probably improve on this list. 

Offline envy887

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Re: Elon Musk: glass geodesic domes
« Reply #274 on: 11/14/2016 01:28 pm »
To ensure this, you just need long enough cables. Cables made out of the right material can have an extremely good strength to density ratio, like 4GPa/(g/cc). Much stronger than the Windows or the quasi-isotropic Carbon fiber composite of the frame, which would be around 0.4GPa/(g/cc)

The anchors, by definition, have to hold the same load as the frame. If the anchor-cables are so mass-efficient, why wouldn't you just use them as the tensile frame as well?

The frame elements can't be in pure uniaxial tension like a cable. They will be pulled equally in 2 directions at every point. And they also have to hold compression loads when the dome is depressurized. So fibers aren't an option unless they are in a composite, which reduces the overall and specific strength.

Offline meekGee

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Re: Elon Musk: glass geodesic domes
« Reply #275 on: 11/14/2016 02:36 pm »
Does the dome have to be sealed, as in 100% air tight pressure vessel or is it enough to have it at positive pressure(PP)? I mean, considering that ISRU fuel production is a hard requirement to land people (iirc), industrial scale O2 and N2 production and CO2 processing would already be there when they start to dome up.

Wouldn't a PP, open system, dome relax the structural requirements, particularly the anchoring PITA at hand?

Yes, you can't have short sleeve / breathable environment from that way, but you can have a much more benign one under the dome:

Less dust, thermal balance, attenuated radiation and pressure environment... Could probably grow stuff at low (1/2atm?) pressure and reduce the personal protection to rebreather and MCP suits....

Yes, it's wasteful, but I suggest to allow an abundance in some resource, else this whole enterprise is, probably, doomed if it has to squeeze every margin. Energy abundance would seem the most beneficial (I.e nuclear vs PV), allowing ISRU overproduction, which can't be bad, fuel and ECLSS wise.
I think sealing will not be a problem. An inner rubber skirt (non load bearing) would do it. You can even have a full rubber sheet, and only seal to the ground  if you do ground penetrations.

But I do think that the habitats have to be pressure vessels, and doors from the habitats to the dome should be airtight and normally closed.

A failed pane will not cause insta- depressurization.

I see the colonists first building the torus, and then building the dome while living in the torus.

After the dome is up, maybe the atmosphere in the dome is high in CO2 (how much is still ok for people) and maybe you have the ability to pump in CO2 to keep pressure if there a small leak. (Though that's a large pump).

All this talk about drilling, and pumping, and ISRU...  That's a lot of power.  Just sayin'.
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Offline guckyfan

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Re: Elon Musk: glass geodesic domes
« Reply #276 on: 11/14/2016 06:46 pm »
But I do think that the habitats have to be pressure vessels, and doors from the habitats to the dome should be airtight and normally closed.

I think, emergency shelters would be good enough. No need to build the whole habitats pressure resistant.

Offline TripD

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Re: Elon Musk: glass geodesic domes
« Reply #277 on: 11/14/2016 07:13 pm »
But I do think that the habitats have to be pressure vessels, and doors from the habitats to the dome should be airtight and normally closed.

I think, emergency shelters would be good enough. No need to build the whole habitats pressure resistant.

Except for the structural benefit to having a lower pressure in the outer dome .  As for emergency shelters, there will be access ways to other parts of the colony which would provide a safe place for evacuation.

Offline meekGee

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Re: Elon Musk: glass geodesic domes
« Reply #278 on: 11/14/2016 07:21 pm »
But I do think that the habitats have to be pressure vessels, and doors from the habitats to the dome should be airtight and normally closed.

I think, emergency shelters would be good enough. No need to build the whole habitats pressure resistant.

I'd wait till there are a few domes standing for a few years before being too confident.

If the habitats are a secondary pressurized volume, dome failures will have a lot lower cost in lives.

It doesn't mean you have to airlock in and out of them - just close the door behind you... 

And if you build the habitats first, and only afterwards erect the dome, then this will be a natural outcome of this strategy anyway.

At this point, emergency shelters are only for people that are, at the time, outside the habitats but inside the dome.  I think that's more prudent.
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Offline Ionmars

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Re: Elon Musk: glass geodesic domes
« Reply #279 on: 11/14/2016 08:43 pm »
...
...
...
After the dome is up, maybe the atmosphere in the dome is high in CO2 (how much is still ok for people) and maybe you have the ability to pump in CO2 to keep pressure if there a small leak. (Though that's a large pump).
...
According to Wikipedia “In concentrations up to 1% (10,000 ppm), it will make some people feel drowsy and give the lungs a stuffy feeling. Concentrations of 7% to 10% (70,000 to 100,000 ppm) may cause suffocation, even in the presence of sufficient oxygen, manifesting as dizziness, headache, visual and hearing dysfunction, and unconsciousness within a few minutes to an hour.”

Surprisingly, very high CO2 levels may also become detrimental to plants. According to HydroFarm (speaking about CO2 enrichment for plants) “Above 2,000 PPM, CO2 starts to become toxic to plants and above 4,000 PPM it becomes toxic to people.” It appears that CO2 levels should be controlled whenever living organisms are involved.
https://www.hydrofarm.com/resources/articles/co2_enrichment.php

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