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#1 2019-05-20 21:48:20

JoshNH4H
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From: Pullman, WA
Registered: 2007-07-15
Posts: 2,513
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Covered Craters, Tented Towns

I've been rereading the Mars trilogy, and one of the aspects of the series that has really stood out to me this time around has been Robinson's description of tented towns.  I transcribed some good examples in this thread.  The idea of a domed crater or tented town was not new when Robinson wrote about it.  Indeed, descriptions and visualizations often grace any mention of permanent human settlement of outer space bodies, with a couple examples reproduced below:

images?q=tbn:ANd9GcQ0-CwYqb4NJtxli8HgsFD0w1zqtoVXMZjqy5lBwi6R24xsRPQASA

9d5bdf875f6c08fb515c48c27a82df18.jpg

I would say that the attraction of these domed craters is fairly obvious: They are visually open to the universe.  Robinson describes it well in the opening of Red Mars:

For the Mars veterans in the crowd it was giddy stuff: they were out on the surface, they were out of the trenches and mesas and craters, they could see forever! Hurrah! [...] The tent fabric itself was invisible, and so taken all in all, it appeared that they stood in the open air.  That was gold, that was.  Nicosia was going to be a popular city.

However, in this same passage Robinson seems to make what I would describe as the Fundamental Error of Space Construction:

All the buildings were set inside what was in effect an immense clear tent, supported by a nearly invisible frame [...] Four or five kilometers downslope the end of the city was marked by three slender skyscrapers, beyond which lay the low greenery of the farm.  The skyscrapers were part of the tent framework, which overhead was a network of sky-colored lines.

In this passage, and in others, Robinson discusses the enclosure as a fundamentally compressive building, where the biggest structural concern is the weight of the materials it is built from.  This impression is strengthened by his use of the word "tent", which by analogy to the construction of tents on Earth conveys to the reader a fundamentally wrong impression of the loading on such a structure.  Elsewhere he discusses the "foundations" of the tent and the idea of "floating" a dome on a cushion of air.

To the extent that his writing conveys this impression, it conveys the wrong impression.  The fundamental structural force on all pressurized structures is the internal pressure of the structure acting on its outer walls, and it's not close.

I don't mean to come down specifically on Robinson for this.  He is a fiction writer (a damn good one in my opinion), and the product of his work is a science fiction novel, not a blueprint for a building.  The misconception I am describing is widespread and this is one example, close at hand, of many.

To illustrate the great importance of internal pressure vs. the small importance of structural weight, I will use the example of a hemispherical steel dome 250 m across containing a pressure of 50 kPa (0.5 atm) against vacuum or near vacuum (Mars being close enough to vacuum that the difference is not important in this case) with a safety factor of 5, typical for this sort of structure.  I have not included the equations or calculations that I used, but rest assured that my conclusions are correct.  I can prove this if anyone cares to check.

Anyway, the thickness of the dome wall in this case is 5.2 cm, and its mass is 40,000 tonnes.  At Mars g of 3.7 N/kg, the gravitational force down on the hemisphere is 150 MN.  That seems like a lot, sure.  But the upwards force from the pressure is 2500 MN, 17 times higher.  This ratio will hold for a dome of any size and any internal pressure, if local gravity and construction parameters are held constant.  A real dome naturally will not match this exactly. With stronger or lighter materials (stainless steel, aluminium, carbon fibers, etc.) this number will increase.  With glass panes or a nonhemispherical structure it will decrease (for example, the suboptimal, nonhemispherical curve of the domes shown above will require a greater material thickness).  In neither case does the pressure force change, but instead the changing weight of materials changes the denominator of the fraction.

This is why I suggested that all Martian structures be topped with an adequate level of regolith to serve as a pressure counterweight, limiting pressure containment needs to the horizontal direction where they can be held in with tensile cables, rigged in a manner similar to a suspension bridge.  The design I laid out in that thread is safe, durable, rad-hardened, easy to build, modular, and all-around sensible; I stand by it.  But in recognition of the romanticism of domed craters, the physiological and psychological benefits of open space and sunlight, and the eventual need for large habitation volumes I would like to explore the idea in more depth rather than dismissing it outright.  I will do so in the next post.


-Josh

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#2 2019-05-20 23:54:14

JoshNH4H
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From: Pullman, WA
Registered: 2007-07-15
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Re: Covered Craters, Tented Towns

Two key design principles that I want to mention are failure resistance and failure tolerance.  Failure resistance means reducing the probability of failure.  Above, I used a safety factor of 5.0 in calculating the thickness of the dome.  This means that my thickness was five times higher than the theoretical physical limit needed to contain the pressure.  This ratio is standard for pressure vessels.  Increasing the amount of material means that stress concentration, fatigue, mechanical shock, etc. are much less likely to exceed the dome strength and cause a failure.  Failure tolerance, on the other hand, means that when failure comes (and no matter how well-maintained, how often replaced, how carefully inspected, all systems fail eventually) it is not catastrophic.  If failure means a sudden explosive decompression, that would really catastrophic.  If failure means that a single strut yields and nearby struts bear the load while it's replaced, that's not so bad.  Likewise, if failure means that there's a gas leak which can be located and sealed, that's not so bad.

One open question common to every approach is the need for strong, clear materials to roof in these enclosures.  The strength of this material determines how far the spacing can be between beams of structural material/cable.  Glass is easy to make but has extremely poor mechanical properties, plus its brittle behavior gives it both poor failure resistance and poor failure tolerance (if anything happens it will crack and fracture).  Plastics are better but still weak and much harder to make than glass on a planet with no known oil reserves: HDPE is roughly 20 times weaker than plain carbon steel.  I don't have any novel ideas on this front and I'd be interested in hearing what you all think.

For the purposes of this post I have ignored the question of radiation shielding.  There is a need for it, but with clear-dome designs it's more or less impossible to build in a thick shield.  It's a question that's worth circling back to, sooner or later.

Conceptually, the simplest habitat style that conforms visually to the "domed crater" idea is the partly-buried sphere.  The idea is pretty simple, really: Dig a big hole in the ground and build a spherical pressure vessel within.  Put a clear roof on the aboveground portion. You can understand it as being similar to a large, half-buried ISS module with a glass roof.  The upwards pressure on the domed part is cancelled out by the downwards pressure on the belowground portion, transmitted through the structural members of the habitat.  There's no problem with this approach per se, but there's also no reason to prefer a crater to the flat plain.  Indeed, crater floors seem likely to be substantially harder than the surrounding regolith, and therefore worse locations to dig on; although on the other hand perhaps that makes it a good foundation, and you can pile regolith up around the habitat instead of building down into the ground.

You could build it as an inflatable.  In that case, you'd probably start with a compressive structure (much like a tent, in fact) and wrap your elastic "tent fabric" around it, finally connecting up all the cables with a bit of tension so that the tent fabric expands into it once the structure is pressurized.  I don't know what sort of thickness you'd need for the tent fabric.  A good design might be a multi-layered, clear, elastic plastic.

The second kind is a dome with a counterweight.  In this design the upward pressure force is counterbalanced by weight from below, probably with tethers  coming straight down from bracings on the roof.  In this design, the regolith below needs to be sealed against gas exchange, but the dome can have a foundation like a normal (extremely heavy) building.  To give an idea, a pressure of 50 kPa (0.5 atm) corresponds to about 13,500 kg/m^2 under Mars gravity.  At a bulk density of 1500 kg/m^3, that's regolith to a depth of 9 meters.

You might build it by pouring a concrete foundation on the floor of the crater, flooring it over and building the dome structure, loading the floor up with regolith, putting up the panes of glass or tenting, and pressurizing the dome.  This one is sort of like the structure I described in the other thread for an enclosed building, but upside-down.  I guess this isn't too far from the popular conception of a domed crater, and even has many of the benefits (the crater apron for some craters may be smooth enough to use as a ramp and many large craters are deeper than 9 m), but it still requires substantial modification of the crater floor.

The third method is not something that I have fleshed out very well, but what I'm thinking is that much in the way skyscrapers dig their foundations to rest down on the bedrock, you might do something similar, but instead of gravitational forces pushing down you would anchor your dome to the bedrock pulling upwards.  It might work, and if so could be the easiest way to anchor large domes (there being more than enough weight pushing down on the bedrock to make it work, and the dome pressure adding even more), but it depends on the strength of the bedrock as an anchoring point.


-Josh

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#3 2019-05-21 02:18:12

louis
Member
From: UK
Registered: 2008-03-24
Posts: 4,900

Re: Covered Craters, Tented Towns

Some thoughts:

1. Is gas exchange a problem as such?  A balloon won't burst if you provide an escape point for the gas...Is there any possibility should could reduce the strength of the dome by allowing an "escape route" for the internal atmosphere?  This would of course require a huge energy expenditure as you would have to keep replacing the "air" but would there be any reduction in the pressure on the external structure?  I've no idea - is "pressure" always "pressure" or does it vary with internal convection currents ie is a "still" gas exerting more pressure than one that has (for the same amount of gas) convection currents create by the gas travelling to an escape valve? Could you even have the air on a close loop with air being reintroduced all the time?

2. On a similar theme,is there any possibility you could have your breathable atmosphere holding up a much thicker layer of gas that would normally fall to the surface? I'm thinking of the WW1 experience with poison gases that would sink to the surface.

3. Building on 2 - we know, I believe, that lasers can deflect gas molecules. Could we use that to help trap gases in a crater.

Personally I think the radiation issue rather undermines the "dome" conception, except possibly for natural light farming.

What people are hankering after is a sense of spatial freedom. Well we are not for the foreseeable future going to be able to breathe fresh air on an uncovered open plain...but millions of people live in forested areas hardly ever seeing much of the sky. So maybe that is a better model.

That's why I think covered gorges (either natural or artificial) can provide us with the earth-like environment (ELE) we crave. The span  needed to cover the equivalent areas is much less, and much more doable with ISRU resources. A 50 m. deep gorge might have a span of only 15 to 20 metres.

The risk factor from micrometeorites can be reduced to close to zero because the covering material can be much thicker, there is less exposed area and you can have air lock tunnels interconnecting discrete gorges.

Light levels in the gorges can be enhanced through use of reflectors, light pipes and supplementary natural spectrum artificial light powered by surface PV panels.

The gorges can be planted with natural vegetation - trees, shrubs, grass, flowers and so on - and be landscaped with numerous paths, rope walkways, waterfalls, ponds, play areas, sports facilities and so on.  Along the gorges retail and other units can be carved out of the rock. 

These gorges I conceive of as natural leisure and recreational areas while the working day would be spent in surface habs and you would also have home habs where you slept . The home habs would have a combination of small real windows and also large artificial windows - live TV screens showing the external view at that point.

Then of course you would have the option of getting in a pressurised rover for journeys on the surface - maybe quite extensive joy rides to other bases and various "tourist" locations. 

Diving in heated water pools  stiock with aquatic plants and tropical fish would be another way of enjoying the sense of ELEs.

I think if you combine all that, then people will not feel like they are being confined. But clearly humanity is going to have to adapt pyschologically and culturally to this new environment.

JoshNH4H wrote:

Two key design principles that I want to mention are failure resistance and failure tolerance.  Failure resistance means reducing the probability of failure.  Above, I used a safety factor of 5.0 in calculating the thickness of the dome.  This means that my thickness was five times higher than the theoretical physical limit needed to contain the pressure.  This ratio is standard for pressure vessels.  Increasing the amount of material means that stress concentration, fatigue, mechanical shock, etc. are much less likely to exceed the dome strength and cause a failure.  Failure tolerance, on the other hand, means that when failure comes (and no matter how well-maintained, how often replaced, how carefully inspected, all systems fail eventually) it is not catastrophic.  If failure means a sudden explosive decompression, that would really catastrophic.  If failure means that a single strut yields and nearby struts bear the load while it's replaced, that's not so bad.  Likewise, if failure means that there's a gas leak which can be located and sealed, that's not so bad.

One open question common to every approach is the need for strong, clear materials to roof in these enclosures.  The strength of this material determines how far the spacing can be between beams of structural material/cable.  Glass is easy to make but has extremely poor mechanical properties, plus its brittle behavior gives it both poor failure resistance and poor failure tolerance (if anything happens it will crack and fracture).  Plastics are better but still weak and much harder to make than glass on a planet with no known oil reserves: HDPE is roughly 20 times weaker than plain carbon steel.  I don't have any novel ideas on this front and I'd be interested in hearing what you all think.

For the purposes of this post I have ignored the question of radiation shielding.  There is a need for it, but with clear-dome designs it's more or less impossible to build in a thick shield.  It's a question that's worth circling back to, sooner or later.

Conceptually, the simplest habitat style that conforms visually to the "domed crater" idea is the partly-buried sphere.  The idea is pretty simple, really: Dig a big hole in the ground and build a spherical pressure vessel within.  Put a clear roof on the aboveground portion. You can understand it as being similar to a large, half-buried ISS module with a glass roof.  The upwards pressure on the domed part is cancelled out by the downwards pressure on the belowground portion, transmitted through the structural members of the habitat.  There's no problem with this approach per se, but there's also no reason to prefer a crater to the flat plain.  Indeed, crater floors seem likely to be substantially harder than the surrounding regolith, and therefore worse locations to dig on; although on the other hand perhaps that makes it a good foundation, and you can pile regolith up around the habitat instead of building down into the ground.

You could build it as an inflatable.  In that case, you'd probably start with a compressive structure (much like a tent, in fact) and wrap your elastic "tent fabric" around it, finally connecting up all the cables with a bit of tension so that the tent fabric expands into it once the structure is pressurized.  I don't know what sort of thickness you'd need for the tent fabric.  A good design might be a multi-layered, clear, elastic plastic.

The second kind is a dome with a counterweight.  In this design the upward pressure force is counterbalanced by weight from below, probably with tethers  coming straight down from bracings on the roof.  In this design, the regolith below needs to be sealed against gas exchange, but the dome can have a foundation like a normal (extremely heavy) building.  To give an idea, a pressure of 50 kPa (0.5 atm) corresponds to about 13,500 kg/m^2 under Mars gravity.  At a bulk density of 1500 kg/m^3, that's regolith to a depth of 9 meters.

You might build it by pouring a concrete foundation on the floor of the crater, flooring it over and building the dome structure, loading the floor up with regolith, putting up the panes of glass or tenting, and pressurizing the dome.  This one is sort of like the structure I described in the other thread for an enclosed building, but upside-down.  I guess this isn't too far from the popular conception of a domed crater, and even has many of the benefits (the crater apron for some craters may be smooth enough to use as a ramp and many large craters are deeper than 9 m), but it still requires substantial modification of the crater floor.

The third method is not something that I have fleshed out very well, but what I'm thinking is that much in the way skyscrapers dig their foundations to rest down on the bedrock, you might do something similar, but instead of gravitational forces pushing down you would anchor your dome to the bedrock pulling upwards.  It might work, and if so could be the easiest way to anchor large domes (there being more than enough weight pushing down on the bedrock to make it work, and the dome pressure adding even more), but it depends on the strength of the bedrock as an anchoring point.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#4 2019-05-21 16:27:16

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 16,150

Re: Covered Craters, Tented Towns

Nice combination image of a city in a crater covered with a dome and habitats in the hillside like the frontier design that Robert Dyck was a part of..

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#5 2019-05-21 17:33:53

JoshNH4H
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From: Pullman, WA
Registered: 2007-07-15
Posts: 2,513
Website

Re: Covered Craters, Tented Towns

Hey louis,

With regards to your points 1, 2, and 3, the short answer is no, those techniques could not realistically be used to contain pressure in the open air on the surface of Mars.

In the case of 1, the reason a balloon does not pop while the air is coming out is that balloons are small in comparison to the size of the opening (not to mention their internal pressure is barely higher than ambient) so that by the time you might pop it the pressure has already declined towards zero.  While gas speed can affect pressure (this is called the Bernoulli effect) the effect is so small as to be negligible at speeds corresponding to comfortable wind speeds.

In the case of 2, while you could use a counterweight made from air, the requirement for 13,500 kg/m^2 does not go away.  You would in effect need to pile an entire atmosphere on top of your dome somehow.  I'm no opponent of terraforming, but building domes probably will happen before that.

In the case of 3, it might theoretically be possible to do that by creating a sort of plasma window with the lasers, but the power required would be truly fantastic, on the order of GW per square meter, literally billions of times what you would otherwise need.  You might fry the inhabitants in the process.  I should also add that the plasma windows we've worked with are on the order of 1 cm, so a 100 m across enclosure would have an area 100,000,000 times larger than existing technologies.

As far as your "gorges" go: I'm not opposed to the idea per se but I don't think you've fully thought it through.  You can correct me if you disagree, but the effective difference between a "domed crater" and a "gorge" is that, per square meter of roof, the gorge has more internal volume because you've dug down inside the enclosure.

I have no problem with this (in fact I think it's a good idea) but many of the design constraints for a roofed trench and a domed crater are the same:

  • The fundamental law of space construction still applies: The key design criteria is how you contain the pressure

  • Assuming the pressure is the same (and why would it be different?) there's just as much upwards force on the roof of the gorge per unit area as the roof of a dome

  • No matter how you're containing the pressure, the exterior walls of the gorge need to be sealed against leakage.  Rock and regolith are not airtight and shouldn't be used for this purpose, not just to prevent air loss but also because pressure can, over time, cleave rocks apart and cause a catastrophic blowout.

  • Rock and regolith are poor structural members and probably can't be used for wall or floor support without modification (read: a construction project)

Again, the key consideration for a roofed trench or a domed crater is how you're going to keep the roof and walls on against the internal pressure.  If you'd like the roof to be transparent, the answer is probably one of the ways I described, or a different method that I didn't think of that deals with that force in a different way.  If not, piling regolith on top is probably the best way to get vertical pressure containment and radiation protection at the same time.

As far as radiation protection in the domes, there's some work being done on magnetic, electrostatic, etc. systems which has promise for blocking cosmic rays.  It won't work against X-rays from flares, but it need not do so.  You can call people down to shelters when that happens, because it's pretty infrequent, and people will presumably live "indoors" in a place with an opaque roof--not "outdoors" under the dome.

I definitely agree that this sort of thing is not necessary for people to survive on Mars, but it would be nice to have and I think it's worth thinking about to see if we can come up with something good.  In the medium term I expect settlers to pursue clear-walled buildings for agriculture with lower power usage (whether this tradeoff makes sense depends on your electrical costs, naturally), for thermal management (A properly-designed, two-walled greenhouse can be used either to cool or warm a settlement by varying the air pressure between the two panes in a range between Martian ambient and settlement interior), and as a recreational space.


-Josh

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#6 2019-05-21 20:20:47

kbd512
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Registered: 2015-01-02
Posts: 2,959

Re: Covered Craters, Tented Towns

Josh,

I think Aluminum OxyNitride (AlON) would be a suitable material for the clear sections of the dome.  CNT composites would be suitable for the structural supports, greatly exceeding the mechanical strength of all known metals, and likely far easier to come by than metals on the moon or Mars.  The feedstock on Mars would be the atmosphere or dry ice mined at the poles.  We already know that adding small quantities of CNT to concrete substantially increases its resistance to cracking and abrasion, to the point that foundations in rail yards that barely lasted six months have already gone several years without need of replacement.  The equipment required to convert CO2 to CNT is already used in some manufacturing processes here on Earth.

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#7 2019-05-22 12:57:25

JoshNH4H
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From: Pullman, WA
Registered: 2007-07-15
Posts: 2,513
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Re: Covered Craters, Tented Towns

Hey kbd512,

I've seen on the forums that you're a strong proponent of carbon fiber, CNT, and graphene-based materials (including composites) for their strength and high temperature tolerance.  I will readily admit that I do not know a lot about these materials and haven't kept up with new developments in the last few years.  If you could recommend good sources (orNnewmars threads?) for me to learn about these materials, including current best-practices for their production, molding/forming/use in manufacturing, and bulk properties I would be grateful.

Having said that, I'd like to mount a broad defense of the continued importance of traditional materials, in most applications chiefly steel and to a lesser degree aluminium.  It is of course the case that each particular application has different constraints that call for different materials choices, and while we are very often talking about aerospace on these forums it represents a small minority of our actual material use.  This will remain the case for the foreseeable future, even if we mount a full-scale settlement effort of the solar system.

One important distinction to make is the difference in properties between individual carbon fibers, nanotubes, and graphene macromolecules and the bulk properties of materials formed from these components.  To the best of my knowledge in most practical applications these components are formed into useful engineering materials by forming them in an epoxy adhesive matrix.  This has two major offshoots:

  1. Your ability to produce the epoxy resin (or whatever you're using for your matrix) needs to be taken into account when talking about the bulk cost of the material and synthesis methods

  2. The material properties of the composite are strongly affected by the properties of the matrix and the interaction between the matrix and the fiber

The first is obvious but important, while the second has all sorts of effects.  For starters, while Carbon itself is a very high temperature material, most polymers are not.  Because both materials need to remain within their operating temperature range for a component to function as intended, the actual operating temperature of a carbon composite will in generally be lower than Steel or even aluminium.

The interaction between the matrix material and the fiber has other effects, too.  Composites have failure modes that simply don't exist in metals, things like delamination (layers of fiber reinforcement come apart from each other and from the matrix), fiber pull-out (individual fibers lose their connection to the matrix and pull out from the material), and debonding (the fiber de-bonds from the matrix).  One potentially severe failure mode is thermal strain caused by a difference in thermal expansion between fiber and matrix leading to debonding.  Composite materials also tend to fail in a brittle manner, cracking and losing strength entirely, rather than in a ductile manner, stretching and bending in a visible, gradual way.  Composite materials are also often stronger in one direction than the others.

Finally, composite materials in general don't have a fatigue limit.  Here's what that means: All materials get weaker as they age and go through repeated thermal/mechanical stress cycles.  Some materials, chiefly steel, have a limit to how much weaker they get (Steel will lost half its strength after enough stress cycles but will lose no more after that; other materials get weaker and weaker and weaker until they fail).

As far as the difficulty of manufacture goes, metals are typically easier and therefore cheaper than fibers or composites.  Before tariffs, Steel goes for around $0.25/kg and Aluminium for around $1.80/kg*.  Because composites don't exist as abstract materials in the same way metal alloys do (indeed, the composite doesn't exist at all until its components are molded into a part), it's not really possible to establish a bulk price.  Because polymers and fibers are generally not traded as commodities, it's difficult to pin down a price for them from my computer, but it seems that in general epoxy can't be had for less than a few dollars per kilogram (and presumably you want good stuff for your composite, not the bottom-of-the-barrel cheap stuff) and high-strength carbon fibers a good deal more than that.

For native martian, lunar, etc. materials like we talk about in this thread relative prices will almost certainly be different due to the different structure of the Martian economy.  Just as one example, Mars is not gifted with petroleum oils and fossil fuels like Earth is.  Oil currently trades around $70/barrel ($0.50/kg) and is an important feedstock (it or fossil fuels of other kinds) for most polymers.  One of the main reasons Iron is cheaper than Aluminium is that Iron is produced in a smelting process using coal as an energy source while Aluminium is produced electrolytically.  Carbon Nanotubes specifically are usually created in small amounts electric arcs and then harvested based on their molecular weight, if I understand correctly.

As a final note, I want to point out that traditional forming and machining methods like cutting, CNC, etc. are generally not available for composite materials.  Instead my understanding is that they are molded as a component (in a process I would compare to casting) and sanded down at the end.

By comparison, metals and alloys are strong, common, fairly easy to produce, have a wide variety of forming methods, broadly useful across applications, often fail gently, and can achieve fairly high strength-to-weight ratios.

Having said all that, I want to make clear that I definitely think carbon fiber composites have their uses.  Here's some design paradigms under which it definitely makes sense to look at carbon fibers:

  • High-cost applications where performance is critical

  • Applications requiring not just high tensile strength but high tensile strength per weight

  • Applications with a lower number of cycles ("Low" meaning less than 10,000 or so full loading-unloading cycles)

  • Applications requiring a high-strength, nonconductive material

While the use of carbon fiber composites is growing as costs fall and experience with them increases, for reasons of cost, simplicity of use, temperature tolerance, failure tolerance, and failure resistance, we should look to metal alloys first. 

*Note that because of the difference in densities, the cost per cubic meter is closer, $1950/m^3 for Steel and $4860/m^3 per Aluminium; Tensile strengths for plain carbon steel and aluminium are similar.


-Josh

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#8 2019-05-23 06:10:01

louis
Member
From: UK
Registered: 2008-03-24
Posts: 4,900

Re: Covered Craters, Tented Towns

Thanks for those helpful replies Josh re the possibility of gas sealing of a pressurised atmosphere.

Regarding gorges, whilst it's true the pressure requirement will be the same as for domes, the fact that the span to be covered is so much narrower means you can use much heavier material. That in turn means you can get muich greater radiation protection. I would think in terms of glass blocks maybe placed in a steel lattice that is then placed between the two sides of the gorge. The steel lattice would be weighed down rock on either side. The risk of catasrophic failure would be minimal.

The effect I would imagine would be similar to the Chinese glass bridges:

https://www.youtube.com/watch?v=4v9BS6xOEds

...except of course you'd be underneath the glass blocks, not on top of them.



JoshNH4H wrote:

As far as your "gorges" go: I'm not opposed to the idea per se but I don't think you've fully thought it through.  You can correct me if you disagree, but the effective difference between a "domed crater" and a "gorge" is that, per square meter of roof, the gorge has more internal volume because you've dug down inside the enclosure.

I have no problem with this (in fact I think it's a good idea) but many of the design constraints for a roofed trench and a domed crater are the same:

  • The fundamental law of space construction still applies: The key design criteria is how you contain the pressure

  • Assuming the pressure is the same (and why would it be different?) there's just as much upwards force on the roof of the gorge per unit area as the roof of a dome

  • No matter how you're containing the pressure, the exterior walls of the gorge need to be sealed against leakage.  Rock and regolith are not airtight and shouldn't be used for this purpose, not just to prevent air loss but also because pressure can, over time, cleave rocks apart and cause a catastrophic blowout.

  • Rock and regolith are poor structural members and probably can't be used for wall or floor support without modification (read: a construction project)

Again, the key consideration for a roofed trench or a domed crater is how you're going to keep the roof and walls on against the internal pressure.  If you'd like the roof to be transparent, the answer is probably one of the ways I described, or a different method that I didn't think of that deals with that force in a different way.  If not, piling regolith on top is probably the best way to get vertical pressure containment and radiation protection at the same time.

As far as radiation protection in the domes, there's some work being done on magnetic, electrostatic, etc. systems which has promise for blocking cosmic rays.  It won't work against X-rays from flares, but it need not do so.  You can call people down to shelters when that happens, because it's pretty infrequent, and people will presumably live "indoors" in a place with an opaque roof--not "outdoors" under the dome.

I definitely agree that this sort of thing is not necessary for people to survive on Mars, but it would be nice to have and I think it's worth thinking about to see if we can come up with something good.  In the medium term I expect settlers to pursue clear-walled buildings for agriculture with lower power usage (whether this tradeoff makes sense depends on your electrical costs, naturally), for thermal management (A properly-designed, two-walled greenhouse can be used either to cool or warm a settlement by varying the air pressure between the two panes in a range between Martian ambient and settlement interior), and as a recreational space.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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