Author Topic: Envisioning Amazing Martian Habitats  (Read 869717 times)

Offline Paul451

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Re: Envisioning Amazing Martian Habitats
« Reply #460 on: 11/24/2016 01:54 am »
F(bot), and only F(bot), is the upward buoyancy force.
Thistle Dome lacks that lower surface contact, so there's no liquid displacement,

A dome (inverted bowl) cupped on the bottom of a sink/bath/pool/etc also lacks the "lower surface contact", yet experiences buoyancy.

Your dome is no different than building a dome on the bottom of any lake/ocean on Earth. You cannot just arbitrarily exclude buoyancy.

Offline LMT

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Re: Envisioning Amazing Martian Habitats
« Reply #461 on: 11/24/2016 03:31 am »
F(bot), and only F(bot), is the upward buoyancy force.
Thistle Dome lacks that lower surface contact, so there's no liquid displacement,

A dome (inverted bowl) cupped on the bottom of a sink/bath/pool/etc also lacks the "lower surface contact", yet experiences buoyancy.

Your dome is no different than building a dome on the bottom of any lake/ocean on Earth. You cannot just arbitrarily exclude buoyancy.

No, you just confused force from air pressure with buoyancy force. 
« Last Edit: 12/14/2016 07:50 pm by LMT »

Offline Paul451

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Re: Envisioning Amazing Martian Habitats
« Reply #462 on: 11/24/2016 07:48 am »
F(bot), and only F(bot), is the upward buoyancy force.
Thistle Dome lacks that lower surface contact, so there's no liquid displacement,
A dome (inverted bowl) cupped on the bottom of a sink/bath/pool/etc also lacks the "lower surface contact", yet experiences buoyancy.
Your dome is no different than building a dome on the bottom of any lake/ocean on Earth. You cannot just arbitrarily exclude buoyancy.
No, you just confused force from air pressure with buoyancy force.

Buoyancy is in addition to the uplift from air-pressure.

Offline LMT

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Re: Envisioning Amazing Martian Habitats
« Reply #463 on: 11/24/2016 03:29 pm »
F(bot), and only F(bot), is the upward buoyancy force.
Thistle Dome lacks that lower surface contact, so there's no liquid displacement,
A dome (inverted bowl) cupped on the bottom of a sink/bath/pool/etc also lacks the "lower surface contact", yet experiences buoyancy.
Your dome is no different than building a dome on the bottom of any lake/ocean on Earth. You cannot just arbitrarily exclude buoyancy.
No, you just confused force from air pressure with buoyancy force.

Buoyancy is in addition to the uplift from air-pressure.

It doesn't work that way.

« Last Edit: 12/14/2016 07:49 pm by LMT »

Offline Paul451

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Re: Envisioning Amazing Martian Habitats
« Reply #464 on: 11/25/2016 06:16 am »
F(bot), and only F(bot), is the upward buoyancy force.
Thistle Dome lacks that lower surface contact, so there's no liquid displacement,
A dome (inverted bowl) cupped on the bottom of a sink/bath/pool/etc also lacks the "lower surface contact", yet experiences buoyancy.
Your dome is no different than building a dome on the bottom of any lake/ocean on Earth. You cannot just arbitrarily exclude buoyancy.
No, you just confused force from air pressure with buoyancy force.
Buoyancy is in addition to the uplift from air-pressure.
It doesn't work that way.

So try it.

An air-filled dome fully-submerged on the bottom of a tub of water should (according to your reasoning) experience no buoyancy, provided there's no water underneath the dome. And without the 600kpa excess air-pressure, it should experience no lift at all. In fact the dome should be held down by the weight of the column of water above it.

Go try it.


Offline KelvinZero

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Re: Envisioning Amazing Martian Habitats
« Reply #465 on: 11/25/2016 07:56 am »
How about this? what I have done is place a glass bowl in a sink over the plug, right way up, and taped some plastic sandwich wrap over it, sealing the edges firmly. Then I filled the sink with water.

I argue that all the plastic-wrap that is clearly curved downwards is feeling a downwards force. No part of it attempts to form an upwards bubble.



Interestingly, the flat plastic-wrap covering the bowl itself remained pretty much flat as far as I could tell. It felt strange to the touch, not under tension that I could tell. I think it formed a proper seal and thus no air could be pushed out and down the plughole. Im sure that if I had used a bowl with a way for air to escape down the plughole, it would have formed a downwards curving dome.

Note: when I pulled a corner of the black tape up allowing water to enter, the plastic wrap immediately converted to an upward pulling dome shape, even as the water began pouring down the plughole.
« Last Edit: 11/25/2016 08:00 am by KelvinZero »

Offline Paul451

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Re: Envisioning Amazing Martian Habitats
« Reply #466 on: 11/25/2016 11:29 am »
How about this?

Nice! At least someone is willing to play along. Props for getting a good seal with the tape.

I argue that all the plastic-wrap that is clearly curved downwards is feeling a downwards force.

Mainly an inward force from the sides, I suspect.

I did something similar, but using a plate as a base and a tall narrow glass as a central support. The sides bowed inwards, exactly as yours, but the air gathered around the top and pushed upwards. Essentially it tried to form a buoyant sphere.

[My tape wouldn't stick to the sink surface enough to stop the glass'n'bubble from lifting off (too damp, I expect, even after I dried it). So I had to resort to sticking it to the plate, which had to be held underwater by hand, so no pics. Sorry. I might try with the bathtub tomorrow. It needs cleaning anyway.]

Hence...

No part of it attempts to form an upwards bubble.

...I suspect in your set-up, some air was able to leak through out the drain.

Try repeating it with a plug in place, or on a solid part of the sink.

Interestingly, the flat plastic-wrap covering the bowl itself remained pretty much flat as far as I could tell. It felt strange to the touch, not under tension that I could tell.

You get the same effect when you place a flat plate over a bowl. The bowl remains buoyant, but the plate sits loosely on top of the bowl. Neither lifted by the air, nor pressed down by unsupported the column of water. Its buoyancy is exactly the same as if the air-filled bowl wasn't there.

Did you try getting an air-filled bowl (inverted) to sit on the bottom of the sink, like wstewart's dome?

Interestingly, you can get a solid-but-buoyant object to "stick". With difficulty. If the bottom of the object and the tub are both very flat and you add a hydrophobic layer between them (lard!). In order to lift, it must create a vacuum. A small change in height creates a large increase in "volume" below the object, hence a large reduction in pressure, enough to resist buoyancy.

But no matter how much grease I slather around the base of a "dome", it will not stay down. A small lift creates almost no change in air pressure inside the dome, hence no resistance to buoyancy. The same will be true of wstewart's dome (or worse because of the over-pressure.)

I suspect the only way to "stick" a dome would be to pump the air out.

Offline LMT

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Re: Envisioning Amazing Martian Habitats
« Reply #467 on: 11/25/2016 04:11 pm »
Moon Pool

Interestingly, the flat plastic-wrap covering the bowl itself remained pretty much flat as far as I could tell. It felt strange to the touch, not under tension that I could tell.

Because...?

There's no tension if no net force.  Here weight of water above the film is a force of same magnitude as force from air pressure below. 

Note: when I pulled a corner of the black tape up allowing water to enter, the plastic wrap immediately converted to an upward pulling dome shape, even as the water began pouring down the plughole.

Because...?

Drainage reduces water depth above the film, and weight.  Result:  net upward force, putting film into tension as a dome.

(And of course, no F(bot), no buoyancy force.)



Seal the drain and water enters the dome until base water pressure equalizes with dome air pressure. 

Result:  moon pool

Would be funny to see model astronauts exiting your dome through a moon pool.  Elon would totally twig on that.   :)
« Last Edit: 12/14/2016 06:34 pm by LMT »

Offline KelvinZero

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Re: Envisioning Amazing Martian Habitats
« Reply #468 on: 11/25/2016 10:14 pm »
No part of it attempts to form an upwards bubble.

...I suspect in your set-up, some air was able to leak through out the drain.

Try repeating it with a plug in place, or on a solid part of the sink.
That was by design though. There was always a bit of water trickling in but it was slow enough that it didn't equalise the pressure. When I pulled up the edge, it immediately ballooned upward into a dome.

Ah. I just realised something. I haven't really been following the exact subject that the moot is about. Something about buoyancy vs being weighed down.

I don't dispute that the buoyancy would lift up a sufficient portion of whatever ground the dome is attached to. If the ground is not one monolithic solid object then water pressure can get beneath and start pushing.

I think the actual interesting question is this: Is the force of buoyancy transmitted through tension on the plastic film pulling, or from pushing from beneath, ie does the dome material have to be built to withstand those huge lifting forces?

In my experiment the film over the top of the bowl was flat, not under tension. I believe any lifting pressure was on the underside. IMO your base has to be strong, watertight, and of sufficient mass, but your roof material can be weak.

Im not sure if this is exactly what you guys were discussing though. The above can be tested more simply by just taping a plastic cover under over a bowl and holding the bowl underwater by force. My guess is that the plastic film roof would remain fairly flat and not strain upward, probably dip down sightly due to pressure, though the bowl itself strains upward. So something has to be strong, but not your transparent domed roof. Ideally the strong part is a monolithic lump of rock that is large enough that dome+rock mass/volume exceeds density of water.

(I just tested this and of course it is true. The top of the bowl does not bulge upwards. It had a slight downwards curve but was hard to judge. I expect if the water was deep enough it would bulge downward because the air inside is being compressed by the water sitting on top of it.)



« Last Edit: 11/26/2016 03:41 am by KelvinZero »

Offline Paul451

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Re: Envisioning Amazing Martian Habitats
« Reply #469 on: 11/26/2016 03:57 am »
though the bowl itself strains upward. So something has to be strong, but not your transparent domed roof.

Remember in that case it's the bowl that contains the air, and the bowl that experiences the buoyancy. The cling-film is flat, thus experiences no buoyancy.[*] To replicate wstewart's submerged dome, you'd need to hold an air bubble with the cling-film alone. For example, a flat plate with an air-filled cling-film "dome", or taping an air-filled "dome" of cling-film to the bottom of the sink before filling it.

The buoyancy is present no matter what you do.

Wstewart believes that he can treat it as a ballast force and ignore the buoyancy, that submerging a dome actually makes things easier.

[*Other than from it's own internally generated displacement volume, which is trivial.]
« Last Edit: 11/26/2016 07:18 am by Paul451 »

Offline KelvinZero

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Re: Envisioning Amazing Martian Habitats
« Reply #470 on: 11/26/2016 04:47 am »
though the bowl itself strains upward. So something has to be strong, but not your transparent domed roof.
Remember in that case it's the bowl that contains the air, and the bowl that experiences the buoyancy. The cling-film is flat, thus experiences no buoyancy.[*] To replicate wstewart's submerged dome, you'd need to hold an air bubble with the cling-film alone. For example, a flat plate with an air-filled cling-film "dome", or taping an air-filled "dome" of cling-film to the bottom of the sink before filling it.

The buoyancy is present no matter what you do.
Im not fussed about that part though. The dome has to be connected to a massive monolithic piece of stone. The stone has to perform the roll of this bowl. If the density of the dome+stone is less than that of water, it would rise.

The only interesting point to me is that the material of the dome itself does not feel huge forces, or any particular force at all if the air pressure inside balances the water pressure outside. Even at the edges where it connects to the stone it does not feel any pressure like you would experience trying to hold down a diving bell.

There is huge pressure trying to pull this thing up but it is not transmitted by the roof material tugging upwards. Rather the roof has lessened the weight of the column of water pressing down from above, and it is this imbalance that tries to lift the floor up if any ambient water pressure can get beneath it.
« Last Edit: 11/26/2016 04:50 am by KelvinZero »

Offline LMT

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Re: Envisioning Amazing Martian Habitats
« Reply #471 on: 11/26/2016 05:48 am »
Erebus Montes Prospect
34° N 177° W

Maybe an improvement on the previous prospects for accessible tunnel-ready rock with nearby ice deposits.

Pros:

- Early Hesperian rock, probably without aeolian contaminants
- > 600 TI, exceeding minimum UCS for tunneling
- 1 km sunlit exposure at 34 N, for good PV
- widespread evidence of geologically recent glacial and periglacial effects, for possible remnant subsurface ice

Cons:

- -3800 m entrance elevation, marginal atmospheric pressure for liquid water
- rock type(s) and UCS uncertain
- remnant subsurface ice concentration unknown
- no crater at tunnel entrance locations; wall/plain structure required



Image:  ErebusMontesHypothetical1.png

High TI units outlined, esp. in Arcadia Planitia, from Bandfield et al. 2013.

Image:  ErebusMontesHypothetical2.png

Glacier-like forms, esp. in Arcadia Planitia, from Souness et al. 2012.

Image:  ErebusMontesHypothetical3.png

Block mesas near Erebus Montes, possibly with remnant glacial fill between blocks.  1 km sunlit southern exposures.  Note filled crater 2 km S of mesas, possibly with remnant subsurface ice.

Image:  ErebusMontesHypothetical4.png

Night IR of mesas, showing higher TI of exposures (brighter).

Image:  ErebusMontesHypothetical5.png

Debris apron around mesa, possibly with remnant glacial fill.

Image:  ErebusMontesHypothetical6.png

Concentric crater fill example, 50 km SW of mesas, possibly with remnant subsurface ice.
« Last Edit: 12/14/2016 06:34 pm by LMT »

Offline Paul451

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Re: Envisioning Amazing Martian Habitats
« Reply #472 on: 11/26/2016 07:28 am »
Im not fussed about that part though. The dome has to be connected to a massive monolithic piece of stone. The stone has to perform the roll of this bowl. If the density of the dome+stone is less than that of water, it would rise.

That was what started this. Wstewart believes that submerging the dome reduces the upward force from the air-pressure. That the water acts as ballast and doesn't add an entire extra buoyancy effect to deal with.

though the bowl itself strains upward. So something has to be strong, but not your transparent domed roof.
Remember in that case it's the bowl that contains the air, and the bowl that experiences the buoyancy. The cling-film is flat, thus experiences no buoyancy. To replicate wstewart's submerged dome, you'd need to hold an air bubble with the cling-film alone. For example, a flat plate with an air-filled cling-film "dome", or taping an air-filled "dome" of cling-film to the bottom of the sink before filling it.
The only interesting point to me is that the material of the dome itself does not feel huge forces, or any particular force at all if the air pressure inside balances the water pressure outside.

That's not the case. You are confusing the flat cover (which doesn't experience buoyancy) with the walls of a dome.

The dome will be crushed in from the side, and push up through the top. (Essentially it's trying to form a sphere at the same time as it's trying to rise.)

and it is this imbalance that tries to lift the floor up if any ambient water pressure can get beneath it.

It doesn't require water underneath, it only requires the absence of a vacuum wide enough to overcome the net buoyancy.
« Last Edit: 11/26/2016 07:30 am by Paul451 »

Offline KelvinZero

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Re: Envisioning Amazing Martian Habitats
« Reply #473 on: 11/26/2016 08:01 am »
That's not the case. You are confusing the flat cover (which doesn't experience buoyancy) with the walls of a dome.

The dome will be crushed in from the side, and push up through the top. (Essentially it's trying to form a sphere at the same time as it's trying to rise.)
That might be true. I did actually wonder about that before reading this, and if I should repeat the experiment with something other than a flat surface.

It is still not a big issue to me. For something underwater I don't care about the sides. The roof could be a flat surface and not a dome if it turns out that is the only shape to which this principle applies. Simply by making the area sufficiently large, the cost of the walls becomes a small fraction of the cost of the roof, which does not need any particular strength if the air is at the same pressure as the water. At 30 meters on Mars that would match earth sealevel. I imagine pillars dotted around because even if the roof is at equilibrium, it is not particularly stable and could even exhibit waves.

The transparent roof could be quite cheap. The underside can be as cheap, rigid and as heavy as you like, ideally a massive piece of ground but it could also be a freefloating thing build of concrete or whatever.

Offline LMT

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Re: Envisioning Amazing Martian Habitats
« Reply #474 on: 11/26/2016 01:02 pm »
Cheap Digs

The roof could be a flat surface and not a dome...

Yes.  Thistle Dome has slight curvature for tensioning and aesthetics.

...the roof, which does not need any particular strength if the air is at the same pressure as the water. At 30 meters on Mars that would match earth sealevel.

Yes.  Thistle Dome moon pool (stemwall base) is at ~15 m because dome is at 60 kPa.  Crown exterior is notionally at 3 m (sunlight through shallow water).  Surface in tension, perimeter ring in compression.

I imagine pillars dotted around because even if the roof is at equilibrium, it is not particularly stable and could even exhibit waves.

A single-piece film would be a wavy mess all right, and hard to fabricate at scale.  In contrast, Thistle Dome is conceptually modular:  hexagons fabricated in shirtsleeves, lifted through the fab roof airlock one-by-one for dome assembly.   Titanium ribs for rigidity.  No waves.

The transparent roof could be quite cheap. The underside can be as cheap, rigid and as heavy as you like, ideally a massive piece of ground but it could also be a freefloating thing build of concrete or whatever.

Bedrock is cheap, yep.  ETFE is not as cheap, but you'd expect bulk discount.   :)
« Last Edit: 12/14/2016 06:33 pm by LMT »

Offline Paul451

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Re: Envisioning Amazing Martian Habitats
« Reply #475 on: 11/26/2016 01:31 pm »
For something underwater I don't care about the sides.

Work out the crushing force per square metre.

And you've still got to deal with the buoyancy.

Offline LMT

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Re: Envisioning Amazing Martian Habitats
« Reply #476 on: 11/26/2016 03:37 pm »
Cooler Insulation

If room-and-pillar tunneling should prove feasible, how might the space be thermally regulated?

Near-surface rock averages ~ -60 C at useful latitudes, and living quarters would be in direct conductive contact with that rock.  Insulation would be necessary, and lots of it.

For comparison, the South Pole station is elevated off the ice, minimizing conductive heat loss.  Even so, insulating panels are pretty thick, at 124 mm:

Quote
Sandwich Composite Panels
These panels, made of plywood on both sides with core insulation between them, were used in the construction of modular structures (living accommodation, kitchen, etc.)....  [T]imber frame was used and the panel thickness was 124 mm (12 mm thick marine plywood on each side and 100 mm core polyurethane foam insulation)...   [T]he floor needed additional insulation, because temperatures varied from -4ēC at the floor to 15ēC at the ceiling.

Just as baseline:

Polyurethane foam density is maybe 100 kg/m3.  Marine plywood, maybe 800 kg/m3.   If you double the South Pole foam thickness to handle rock conduction (enough?), the composite panel density becomes 176 kg/m3.

Result:  A cubic room 4 m on a side, with one open wall, would need 3 tons of insulation.

Not that you'd use those materials, obviously.  Other insulating materials are needed, via ISRU, and in bulk. 

Questions:  What can be made via ISRU, and how much is needed, to insulate that 4 m room?

« Last Edit: 12/14/2016 06:33 pm by LMT »

Offline lamontagne

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Re: Envisioning Amazing Martian Habitats
« Reply #477 on: 11/26/2016 07:04 pm »
The best insulation would be a vacuum layer, then you would have only radiation, and could reduce that with low e and reflective materials.

What could be better than nothing at all as far as ISRU goes  :-)  Mars atmosphere is practically vacuum, after all.

We could start with some kind of alumimised  paint that manages to stick to the tunnel wall (already a first challenge). then strips of poorly conducting but pressure resistant materials, molten basalt bricks with lots of hollows might serve.  Arrange these so they occupy no more than 10-15% of the area, and no more that 10cm wide, so 10cm lattices, 1 cm thick (20x2 might be easier to build).  Then a strong wall, metal or FRP, holding in the air pressure, but beeing supported by the lattice, so no need to be very thick.  The outside of the wall could be painted with low-e coatings as well.  The vacuum gap works at all thicknesses, but 2cm min. would be easier to build.
« Last Edit: 11/26/2016 07:26 pm by lamontagne »

Offline Paul451

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Re: Envisioning Amazing Martian Habitats
« Reply #478 on: 11/26/2016 07:16 pm »
Re: Insulation in a tunnel.

You guys do realise that an inhabited space will need to get rid of heat, not retain it?

Offline lamontagne

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Re: Envisioning Amazing Martian Habitats
« Reply #479 on: 11/26/2016 07:44 pm »
Re: Insulation in a tunnel.

You guys do realise that an inhabited space will need to get rid of heat, not retain it?

Yes, but perhaps not necessarily through our load bearing walls, specially if they are full of ice, and that ice is part of the structural strength.
That being said, i'll admit I was just answering the problem posed by wstewart.  Hadn't really thought if there was a problem there in the first place  ;-)

If the thermal flow through the walls is too low, a very likely possibility in plant growing areas, then we might need active cooling.  I've recently done some grow rooms that had loads up to 1000 W/m2, so those needed refrigeration to be livable.  If we practice high intensity underground agriculture, and we probably will, then we may need to provide for some means of circulating a lot of heat and removing it to the surface to radiate away.  Or create sufficiently large heat sinks to have it absorbed by the soil.
I've heard enough stories about frozen geothermal wells though to know these might not be a very effective solution.  Melting ice and then letting it evaporate into the Martian air would probably be the simplest system.

Another possibility might be an equilibrium between underground heat gains and glass domes heat losses.  You can insulate the tunnels, to ensure a simple energy balance, in particular if you design simple domes with high heat losses, and then just use hot air to transport the heat around the base.  You would need to circulate the air anyway, for health, air treatment and plant growth reasons.





« Last Edit: 11/26/2016 07:49 pm by lamontagne »

 

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