Elon Musk: glass geodesic domes

[lamontagne]

In general a good half of all plutonic rocks will probably have quartz grains in them.
Here are some sources showing you the element contents and the resulting mineral composition.
https://en.wikipedia.org/wiki/QAPF_diagram
https://en.wikipedia.org/wiki/TAS_classification

Basically it all boils down to the ratio of alkali elements (Na K) to Si.
So I am not too worried about finding large deposits of granites or the likes.
The other source of very pure quartz is hydrothermal deposits. Basically hot, high pressure water on cracks that deposits its dissolve SiO2 when cooled.
Where I live in NH there is huge mountains made of quartzite. A white rock formed of mostly pure quartz in a very fine grain form. These deposits are hydrothermal.
And mars did have water at one point.
And it did have molten rock at one point. Olympus Mons tallest volcano in the Solar System.

True Mars did have water and molten rock at some early point, but that was a long time ago and in all ikelyhood there have been billions of years with very little water and a lot of very fine dust blowing all over the planet.

In my little research on glass, I came across the information that most wind blown dunes, such as the ones in Michigan where in fact too coarse for glass making and needed to be ground finer.  So who knows, really what we will find?  Wind separation, or centrifugal separation, is often used to differentiate out materials with different densities, so there may be spots where this happened on Mars.
Anyway, on the first ships to Mars, I expect we will include a complete process lab, with miny grinders, mini cyclones, mesh separator, reagents and all that good stuff to actually measure the conditions and determine what can be done.  Then they will then have a synod or so to react before they send the next ships, with appropriate process equipment.  I expect you can do quite a bit with 100 tonnes.

[Lar]

If the cyclones, grinders, separators, etc are made to be adjustable, they can get pilot production going with the first human shipload... this isn't something robots can handle though.

[Slarty1080]

In general a good half of all plutonic rocks will probably have quartz grains in them.
Here are some sources showing you the element contents and the resulting mineral composition.
https://en.wikipedia.org/wiki/QAPF_diagram
https://en.wikipedia.org/wiki/TAS_classification

Basically it all boils down to the ratio of alkali elements (Na K) to Si.
So I am not too worried about finding large deposits of granites or the likes.
The other source of very pure quartz is hydrothermal deposits. Basically hot, high pressure water on cracks that deposits its dissolve SiO2 when cooled.
Where I live in NH there is huge mountains made of quartzite. A white rock formed of mostly pure quartz in a very fine grain form. These deposits are hydrothermal.
And mars did have water at one point.
And it did have molten rock at one point. Olympus Mons tallest volcano in the Solar System.

True Mars did have water and molten rock at some early point, but that was a long time ago and in all ikelyhood there have been billions of years with very little water and a lot of very fine dust blowing all over the planet.

In my little research on glass, I came across the information that most wind blown dunes, such as the ones in Michigan where in fact too coarse for glass making and needed to be ground finer.  So who knows, really what we will find?  Wind separation, or centrifugal separation, is often used to differentiate out materials with different densities, so there may be spots where this happened on Mars.
Anyway, on the first ships to Mars, I expect we will include a complete process lab, with miny grinders, mini cyclones, mesh separator, reagents and all that good stuff to actually measure the conditions and determine what can be done.  Then they will then have a synod or so to react before they send the next ships, with appropriate process equipment.  I expect you can do quite a bit with 100 tonnes.

I look forward to it. I would have thought that our understanding will be able to improve quickly in multiple directions once boots are on the ground.

But note although a hundred tons sounds like a large amount bear in mind that there will be a lot of competition for this mass, including the fittings, floors, chairs, rovers, solar power, scientific kit, food, water, radiation protection, etc etc etc. So first mission kit may be modest.

[Lar]

This is a good point. What's a reasonable mass budget for this out of the first 100 tonnes?  500kg? that's actually respectable, a pilot plant is not out the question... 50kg? still can do some investigation.

[lamontagne]

Dome and airlock.
My guess is 60 to 70 tonnes for a 30m diameter dome.

[Paul451]

Dome and airlock.
My guess is 60 to 70 tonnes for a 30m diameter dome.

What keeps the ~14,000 tonnes of pressure in. (Half of it straight up. About 75 tonnes of force per linear metre around the rim.)

[Rocket Surgeon]

Dome and airlock.
My guess is 60 to 70 tonnes for a 30m diameter dome.

What keeps the ~14,000 tonnes of pressure in. (Half of it straight up. About 75 tonnes of force per linear metre around the rim.)

If you don't mind me asking, how did you get that number? If you've got a link and/or equation, that would be awesome!

[Lampyridae]

I've been thinking about this for a while now, but I believe we're going about glass domes all wrong. Glass is good in compression, as Paul451 has mentioned several times, but not good in tension. And domes are good for compression under passive Earth weight, but are fiddly when you try and pressurise them (they want to be spheres).

So, perhaps domes should be inverted.



You sacrifice some volume, but all domes really are on Mars is a way to gawk at the open sky. By sacrificing some volume, you also further lower the pressure on the panes. Build a large cylinder and stick the inverted dome in as a roof.

[guckyfan]

The dome is IMO not for being a skylight to look at the sky or let light in from above. It is to allow a free view to the landscape around it. An inverted dome would not serve this purpose. The forces will not be contained by the glass but by the structure that holds the glass panes. Those would be metal or carbon composite which are very good at tension forces.

[Paul451]

I've been thinking about this for a while now, but I believe we're going about glass domes all wrong. Glass is good in compression, as Paul451 has mentioned several times, but not good in tension. And domes are good for compression under passive Earth weight, but are fiddly when you try and pressurise them (they want to be spheres). So, perhaps domes should be inverted.

This doesn't solve either issue. The glass is still under the exact same tension in the frame. The frame is under compression, but making a tensile frame was never my objection. And you still have 75 tonnes of force per linear metre of rim, trying to rip the rest of that roof off.

If you have a structure capable of holding that force, and the glass can handle the air-pressure, then the structure in that image can also have an upward dome.

I just like to remind people of the forces involved whenever they plonk a dome on the surface, with no apparent sub-structure. I don't think it's an unsolvable problem, I just think people either don't know or really want to forget that it even exists as a problem. It's not a little house. It's a pressure vessel.

a 30m diameter dome.

What keeps the ~14,000 tonnes of pressure in. (Half of it straight up. About 75 tonnes of force per linear metre around the rim.)

If you don't mind me asking, how did you get that number?

It's just air pressure. At approximately sea-level pressure, you've got 100,000 Pa pushing outwards on the dome. That's creates a force equivalent to 10 tonnes per square metre. Force = Pressure X Area. (Using equivalent-mass-on-Earth as a pseudo-unit for force to make it easier to visualise. Since Force = Mass X Acceleration, we let Acc=1g.)

All of the force that isn't vertically aligned is taken up by the frame. So as long as the frame is strong enough... But the vertical component is unopposed, it's trying to lift the dome off the ground. Thankfully, working that out is simply the cross-sectional area of the dome (a circle radius 15m). The dome is, presumably, only attached at the rim (otherwise why bother with a dome shape), and so you divide the total unopposed force by the circumference (circle radius 15m) to work out the upward force pushing on every linear metre of rim.

(You can try to drop the pressure, but that opens up other issue.)

[Dalhousie]

Washing only removes easily separated contaminants, such as clays. 

It can't create the high purity silica you need in the first place. Which does not appear to occur on Mars in any quality.

Silicon is one of the most common elements. Are you saying
- Mars has no silicon? I'm dubious of that one
- Or that all silicon is bound up in other compounds than SiO2? If so which ones?
- Or that the SiO2 is mixed (alloyed) with other compounds in a  way that  no amount of crushing to get finer and finer grains will allow it to be mechanically separated? (that's the one I was afraid of, but see above, lamontagne is saying that the melts form pure crystals of various things. The crystals may be intermingled, but they are pure if you just crush fine enough and then separate)

Did I miss any possibilities?

Of course Mars has silicon as silicates. 

The martian crust is mostly mafic in composition - basalts, dolerites, gabbros, and the like.  This means that while they are up to 44% SiO2 by analysis, that is largely tied up in other silicates, olivine, pyroxenes, amphiboles, feldspars, etc. mafic rocks would generally at most contain only 5% free SiO2 (quartz, and usually none.

Felsic rocks - granites and their volcanic equivalents rhyolites and dacites - contain anywhere from 20 to 60% quartz.  But these rocks, as far as we know, are very rare on Mars.  Mostly due to the absence of plate tectonics.

Quartz is liberated by weathering of host rocks - hydration, carbonation, and oxidation.  Other minerals are transformed to clays.  As far as we know weathering on Mars is weak, and has been for 3.5 billion years. Without the weathering the quartz won't be liberated and the sedimentary sorting that allows pure deposits to form on rivers to beaches can't happen.  Martian sedimentary rocks appear lithic for the most part, composed of unweathered rock fragments.  Some forms of hydrothermal silica are also pure.  Homeplate is a good example, but it is only 98% SiO2.  Quartz veins and siliceous alteration is a possibility, but generally are associated with felsic rocks, which appear rare.

To make clear glass you need extremely pure silica, about 99.5% pure SiO2, AKA quartz.  I am not saying that such deposits are absent on Mars, but they are certainly rare, we have not seen them yet.  so let's not be too too quick to construct industries based on deposits that are not known and are probably quite rare.

Note that if we do find deposits of such purity they will be in demand for making metallic silicon for solar cells and computers. Assuming we can find enough strong reductants (on Earth coal, wood, or methane) to reduce SiO2 to Si.

[Lampyridae]

The dome is IMO not for being a skylight to look at the sky or let light in from above. It is to allow a free view to the landscape around it. An inverted dome would not serve this purpose. The forces will not be contained by the glass but by the structure that holds the glass panes. Those would be metal or carbon composite which are very good at tension forces.

You're thinking of the frame taking the weight of the glass. Pressure is distributed evenly across a pressure vessel. If you cut the dome into panels, the panels themselves still are subject to tension across them. The tension is proportional to radius: the larger the radius, the larger the tension. But there is a way to reduce the tension.

Parachutes don't tear because they use lobes to reduce the pressure across the fabric and instead the tension is transferred to the cords between them. Lobing reduces the radius and therefore the tension on the material. High-altitude balloons take advantage of this. But, I'm not sure how transferrable this is to rigid structures.

(note: for future reference in our discussions please assume that we account for the dome bottom somehow, making it a sphere and burying the bottom etc)

[guckyfan]

The dome is IMO not for being a skylight to look at the sky or let light in from above. It is to allow a free view to the landscape around it. An inverted dome would not serve this purpose. The forces will not be contained by the glass but by the structure that holds the glass panes. Those would be metal or carbon composite which are very good at tension forces.

You're thinking of the frame taking the weight of the glass.

Not at all. I am thinking of the atmospheric pressure that puts the frame into tension. The single glass pane only transfers the pressure on it to that frame. Only while the dome is not pressurized the structure needs to hold the weight of the glass under compression which is miniscule compared to the pressure forces causing tension.

BTW I regularly have argued that any structure would need to be stable both under pressure and without pressure. That's why using mass to counter pressure is problematic and needs to be designed carefully.

[lamontagne]

Dome and airlock.
My guess is 60 to 70 tonnes for a 30m diameter dome.

What keeps the ~14,000 tonnes of pressure in. (Half of it straight up. About 75 tonnes of force per linear metre around the rim.)

Steel rods going through the foundation walls and into a fairly thick bottom slab.
I'm presuming reasonable amounts of calcium  or magnesium carbonates are found, to be able to pour concrete in place.

The calculations for the dome, including the anchoring question, are in the joined spreadsheet.  The steel structure has to represent about 3% of the dome area to provide an adequate factor of safety.

The hoop stress is entirely taken up by the steel structure.  I have serious doubts that at this scale the geodesic shape is the best one.  My guess is that circular sections terminating at anchor points would be better.

The glass in the dome in under bending stress, so the panes need to be kept relatively small in order to reduce this to a reasonable amount.  It's also better to keep them small so when they fail, the depressurisation is not too severe.  You want to have time for your crack intervention team (a second job for the fire brigade) to bring in the patch!  The image shows panes about 1m accross, but perhaps 500mm would be better.

Steel is strong stuff.  Standard construction steel at 40 000 psi strength is 20 tonnes per square inch.  So you need a section of about 3 inches square for the tension, plus a safety factor of three would give us 10 square inches of steel per meter.    So a 1 inch section rod every 10 cm.  Sorry for the mixed units  :-)  my structural design days were imperial. 

A real design would  finish with some kind of tension spreader beam at the base of the dome, before the foundation wall, to conduct the forces from the steel hoops in the dome to the steel rods in the concrete/compressed regolith blocks.

[guckyfan]

I would love to be able to do good design pictures. My idea of a dome:

Build a central cylindrical building that can hold pressure if needed. Give it a massive thick roof that extends quite a lot beyond the perimeter of the building, heavy enough to hold the pressure. Build the geodesic dome around it. That would give some radiation shielding and would reduce the pressure force that needs to be anchored to the ground by a lot, depending on how big that mushroom hat is.

[Lampyridae]

The dome is IMO not for being a skylight to look at the sky or let light in from above. It is to allow a free view to the landscape around it. An inverted dome would not serve this purpose. The forces will not be contained by the glass but by the structure that holds the glass panes. Those would be metal or carbon composite which are very good at tension forces.

You're thinking of the frame taking the weight of the glass.

Not at all. I am thinking of the atmospheric pressure that puts the frame into tension. The single glass pane only transfers the pressure on it to that frame. Only while the dome is not pressurized the structure needs to hold the weight of the glass under compression which is miniscule compared to the pressure forces causing tension.

BTW I regularly have argued that any structure would need to be stable both under pressure and without pressure. That's why using mass to counter pressure is problematic and needs to be designed carefully.

Tension in a pressurised vessel doesn't work like that. It doesn't matter whether your truss structure is made of unobtanium. The pane must be able to withstand the pressure placed upon it: the outward component.

To use your example, you could replace the 60cm thick acrylic aquarium window at the Churaumi Aquarium in Okinawa with a titanium frame (because hey that's helluva strong)... and insert single sheets of 6mm window glass to hold back 7 500 tonnes of water. Water doesn't somehow see the frame and choose to push there and avoid the window glass, nor does the window glass teleport the 10 tonnes per square metre pressure to the frame. The truss does help, but not all that much.

Glass is weak. This is why the cupola windows on the ISS are 2x25mm thick, and the ISS modules are 2.5mm thick aluminium. To make the dome bigger, the glass must be thicker. But if you're producing Victorian-era quality glass, then you have a real problem. I kind of think acrylic will be easier in terms of ISRU.

[lamontagne]

So I made a little test for the glass, spreadsheet joined.  Turns out that for a span of 500mm you would need glass about 50mm thick (2 inches).  So glass slabs, not glass sheets.

This is for a glass beam.  Continuously supported plates would be under considerably less strain, but nevertheless, that's thick glass.
Polycarbonate is twice as strong as soda glass, so it would be about 35 mm thick.

There are formula for supported plates
https://en.wikipedia.org/wiki/Bending_of_plates#Simply-supported_plate_with_uniformly-distributed_load

Too much work for me but if someone want to tackle it would love to see the results!

So although the mass of steel remains the same the mass of glass goes up to 140 tonnes, so an overall mass of 170 tonnes.
I guess we should include the anchor mass,that should be at least as much as the dome, so the mass requirements for a dome are about 200 tonnes, plus the concrete for the slab.

Really need to want to look outside  :-)

[guckyfan]

Tension in a pressurised vessel doesn't work like that. It doesn't matter whether your truss structure is made of unobtanium. The pane must be able to withstand the pressure placed upon it: the outward component.

Yes, of course the pane needs to withstand the pressure. So it can not be too big as a single pane. I have built fish tanks and I am very much aware of this. But it is solely the structure that needs to take the whole load determined by the size of the dome.

[IncongruousGoat]

Washing only removes easily separated contaminants, such as clays. 

It can't create the high purity silica you need in the first place. Which does not appear to occur on Mars in any quality.

Silicon is one of the most common elements. Are you saying
- Mars has no silicon? I'm dubious of that one
- Or that all silicon is bound up in other compounds than SiO2? If so which ones?
- Or that the SiO2 is mixed (alloyed) with other compounds in a  way that  no amount of crushing to get finer and finer grains will allow it to be mechanically separated? (that's the one I was afraid of, but see above, lamontagne is saying that the melts form pure crystals of various things. The crystals may be intermingled, but they are pure if you just crush fine enough and then separate)

Did I miss any possibilities?

Of course Mars has silicon as silicates. 

The martian crust is mostly mafic in composition - basalts, dolerites, gabbros, and the like.  This means that while they are up to 44% SiO2 by analysis, that is largely tied up in other silicates, olivine, pyroxenes, amphiboles, feldspars, etc. mafic rocks would generally at most contain only 5% free SiO2 (quartz, and usually none.

Felsic rocks - granites and their volcanic equivalents rhyolites and dacites - contain anywhere from 20 to 60% quartz.  But these rocks, as far as we know, are very rare on Mars.  Mostly due to the absence of plate tectonics.

Quartz is liberated by weathering of host rocks - hydration, carbonation, and oxidation.  Other minerals are transformed to clays.  As far as we know weathering on Mars is weak, and has been for 3.5 billion years. Without the weathering the quartz won't be liberated and the sedimentary sorting that allows pure deposits to form on rivers to beaches can't happen.  Martian sedimentary rocks appear lithic for the most part, composed of unweathered rock fragments.  Some forms of hydrothermal silica are also pure.  Homeplate is a good example, but it is only 98% SiO2.  Quartz veins and siliceous alteration is a possibility, but generally are associated with felsic rocks, which appear rare.

To make clear glass you need extremely pure silica, about 99.5% pure SiO2, AKA quartz.  I am not saying that such deposits are absent on Mars, but they are certainly rare, we have not seen them yet.  so let's not be too too quick to construct industries based on deposits that are not known and are probably quite rare.

Note that if we do find deposits of such purity they will be in demand for making metallic silicon for solar cells and computers. Assuming we can find enough strong reductants (on Earth coal, wood, or methane) to reduce SiO2 to Si.

And I take it that there aren't any practical processes for getting SiO2 out of silicates?

[rsdavis9]

And I take it that there aren't any practical processes for getting SiO2 out of silicates?

Its more like nobody bothers because there is ample SiO2 in the wild. We have already seen hydrothermal SiO2 on mars. We will probably find the felsic minerals (granites etc). Probably need to dig a little. Mechanical separation is easier if these sources exist.