An Island getaway on Mars

Tom Kalbfus · 2015-10-26 09:17:25

Lets do Bermuda!

Suppose we wanted to put Bermuda under a dome in a crater filled with water. We'd want to make the radius of the dome far enough so that the wall structure is below the horizon. So how big a dome do we need?
There is an article by Popular Science which discusses how to find the distance to the Horizon.
I supplied the additional Mars Horizon Table myself.

How far away is the horizon?

By Phil Plait | January 15, 2009 7:00 am
I fly a lot. Talks, meetings, whatever. I usually prefer an aisle seat, because then the rude guy who smells funny and spreads over 1.8 seats only irritates me on one side, and I’m not wedged up against the window.

However, sometimes I do like to grab a window seat, especially if I’m flying near sunset, or over a particularly interesting landscape (flying over southern Utah near sunset will change your life). But even then, the landscape blows past, and eventually you wind up flying over eastern Colorado, and there’s nothing to see but flat, flat land, extending all the way to the horizon.

And as I gaze over the amber waves of grain to the line that divides land and sky, I sometimes wonder how far away that line is. The horizon is a semi-mythical distance, used in poetry as a metaphor for a philosophical division of some kind. But in fact it’s a real thing, and the distance to it can be determined. All it takes is a little knowledge of geometry, and a diagram to show you the way.

Follow along with me here. We’re going to find the lost horizon.

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So you’re standing on the Earth. Let’s assume the Earth is a perfect sphere, because that makes things a lot easier. What does our situation look like? Well, it looks something like this:

Diagram showing the distance to the horizon

In this diagram, the circle is the surface of the Earth, which has a radius of R. The Earth’s radius varies with latitude, but I’ll just use 6365 kilometers as a decent average. The dude standing on the Earth is a human of height h (not to scale, huge duh there). The line-of-sight to the horizon is the red line, labeled d. Finding the value of d is the goal here. Note that the radius of the Earth is a constant, but that d will vary as h goes up or down.

The key thing here is that at the visible horizon, the angle between your line-of-sight and the radius line of the Earth is a right angle (marked in the diagram). That means we have a right triangle, and — reach back into the dim, dusty memory of high school — that means we can use the Pythagorean Theorem to get d. The square of the hypotenuse is equal to the sum of the squares of the other two sides. One side is d, the other is R, and the hypotenuse is the Earth’s radius plus your height above the surface, R+h. This gives us the following algebraic formula:

d² + R² = (R+h)²

OK. Now what? Well, let’s expand that last term using FOIL:

(R+h)² = R² + h² + 2Rh

Substitute that back into the first equation to get

d² + R² = R² + 2Rh + h²

Hey, we have a factor of R² on both sides, so they cancel! That leaves us with:

d² = h² + 2Rh

Now, take the square root of both sides, and voila! You get d.

So now we have an equation that tells us how far away the horizon is depending on where we are above the surface. We can use this to put in different values for h, our height, and see how far away the edge of the Earth is. I put this into an Excel spreadsheet, and the numbers are below.

In the table, the first column is your height in meters above the Earth’s surface (really the height of your eyes) and the second column is the horizon distance in kilometers. Columns three and four are the same, but in feet and miles for you Amurcans.

horizon_tables_mars_and_earth_by_tomkalbfus-d9eg3qg.jpg
So lets look at the map again:

We'll assume the highest viewpoint is 50 meters. We build some wave generators at the perimeter of the dome to create some nice surf. Do any particular locations on Mars come to mind?

It appears we'll need a crater that is at least 75 km wide to create a suitable island retreat under a dome on Mars.

Here's a suitable crater on Mars, its right on the equator, and we'll need all the sunshine we can get for this island paradise!
Nicholson (Martian crater)

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This article is about the crater on Mars. For other craters named Nicholson, see Nicholson (crater).

Nicholson

Martian crater Nicholson based on day THEMIS.png
Crater Nicholson based on THEMIS day-time image

Planet
Mars

Coordinates
0.2°N 164.6°WCoordinates: 0.2°N 164.6°W

Diameter
102.5 km

Eponym
Seth Barnes Nicholson

Nicholson is a crater on Mars centered at 0.1° N and 164.5° W. It is 62 miles wide (100 km), and located in the Memnonia quadrangle. Nicholson is a good marker for the equator as it sits almost directly on the martian equator. It is named after Seth Barnes Nicholson, an American astronomer.

Nicholson is notable for its central peak, which rises in a high mound 3.5 km above the crater floor. This rounded peak is riddled with channels, which may have been eroded by wind or even water.

Last edited by Tom Kalbfus (2015-10-26 09:33:42)

RobertDyck · 2015-10-26 12:25:48

Wikipedia: Tensile structure

For a membrane with curvature in two directions, the basic equation of equilibrium is:
$e4bda98b615499a62820ee74abb7ad9f.png$
where:
R1 and R2 are the principal radii of curvature for soap films or the directions of the warp and weft for fabrics
t1 and t2 are the tensions in the relevant directions
w is the load per square metre

Assume spacesuit pressure = 3.0 psi pure oxygen. Dome O2 partial pressure = 2.7 psi, which permits suits to have 10% pressure loss and same partial pressure as astronaut is used to. Dome N2 partial pressure = suit total pressure * 1.2 = 3.0 * 1.2 = 3.6 psi. Earth has 0.9340% argon, 14.69595 psi at sea level, so 0.13726 psi partial pressure. Too much argon will affect voice timber, make your voice sound deep. Say increase argon to 0.2 psi partial pressure. That totals 6.5 psi. That equals 4,570 kilogram-force/square metre.

The dome for Tom's crater is round, so R1 and R2 are both 100km. So w = 2 * (t / R), or (R * w) / 2 = t. So tension = 100,000m * 4,750kg/m² / 2 = 228,500,000kg/m. That means 228,500 metric tonnes force per metre of the dome meeting the ground.

I started to look at cables: Kevlar, Technora, carbon fibre. But I have to go. Let's say this will require extremely strong cables. I have seen on TV a sports dome that has cables holding down the inflated dome: parallel cables in one direction, and an equal number of cables 90°. I don't think that would be good enough. If you arrange the dome in hexagons, with cables woven together to integrate them at each joint, that would provide some redundancy. Polymer film would only have to carry pressure to the hexagon edge. But that still means the force calculated above is the tensile force for a single cable at the dome edge! We're going to have to look up carbon nanofibre cable strength.

Tom Kalbfus · 2015-10-26 12:58:38

The density of glass is 2.4 to 2.8 tons per cubic meter, on Mars it would be 0.912 to 1.064 tons per cubic meter as it is weight that matters, not mass. Air pressure at sea level is 10 tons per square meter. So lets say we inflated a dome made of plastic, and then piled on glass plates, then we inflated the dome more to counter act the weight of the plates as we placed them on top of the dome. Eventually we can have a glass dome that is 10 meters thick, probably thinner at the top as the air pressure would diminish as you climbed 50 km above the surface of the crater on top. The glass would also be useful for radiation shielding as would the air underneath, 50 km of it above the center. the would tend to get rid of most of the dangerous radiation reaching the island in the center.

SpaceNut · 2015-10-26 17:37:29

If you are going to use plates of glass then go with a geodesic dome construction which is more than a half dome as it greater than a half shell. The adjoining plates would allow for many cables to be attached to eat of these points lessening the load to hold the dome down with.

RobertDyck · 2015-10-26 21:41:20

Glass would provide radiation shielding. However, it's really heavy. And you have to worry about dome integrity. Mars atmosphere is very thin, but does have wind. Wind force isn't much because pressure is so low, but speed is high, and a 100km diameter dome is just big. What happens if a cable breaks? What happens if a transparent section breaks? What happens if a meteorite strikes? I tried to emphasize how strong the cables would have to be for a dome that size. And that's with light weight transparent material. Here's an example:

Those images are from Wikipedia. The project's website is linked to this image:

That project uses geodesic domes with structure that holds itself up. A pressurized dome has to be held down. So additional structure you see here will not be required, just the hexagons. And we could use transparent polymer film, not translucent film you see here.

Note that Mars atmosphere blocks most of heavy ion GCR. And a simple film with spectrally selective coating will block alpha, beta, UV, and X-ray radiation. There isn't much X-rays in space, but the coating is metal so thin that you can see through it. The metal coating won't block much, but blocks enough. What will get through is proton, light ion, and gamma radiation. And that will be less than half that of ISS.

Or you could just dream on. No harm in dreaming. One friend here in Winnipeg wants to put a force field over a crater, to hold air in. He thinks that's the most economic way to pressurize a crater. But the only force field that can hold atmospheric pressure is a plasma window. That requires plasma at least 12,000°K, and most researchers studying it today use 14,000°K to ensure anything contacting the plasma doesn't drop the temperature below the critical point where it's sufficiently viscous to hold air pressure. And the first paper published was a flat round window that required 20,000 watts per inch diameter. You may be able to reduce that power consumption if you can find a way to prevent plasma heat loss, both radiative and convective/conductive with air. But that power consumption doesn't sound practical to me. But he too can dream.

Terraformer · 2015-10-27 05:18:55

Isn't there one which uses water vapour instead of plasma, using it's diamagnetic properties?

Tom Kalbfus · 2015-10-27 05:45:31

RobertDyck wrote:

Glass would provide radiation shielding. However, it's really heavy. And you have to worry about dome integrity. Mars atmosphere is very thin, but does have wind. Wind force isn't much because pressure is so low, but speed is high, and a 100km diameter dome is just big. What happens if a cable breaks? What happens if a transparent section breaks? What happens if a meteorite strikes? I tried to emphasize how strong the cables would have to be for a dome that size. And that's with light weight transparent material. Here's an example:
https://upload.wikimedia.org/wikipedia/commons/thumb/f/f2/Eden_Project_geodesic_domes_panorama.jpg/360px-Eden_Project_geodesic_domes_panorama.jpg
https://upload.wikimedia.org/wikipedia/commons/e/e6/Eden_project_tropical_biome.jpg
Those images are from Wikipedia. The project's website is linked to this image:
http://www.edenproject.com/sites/default/files/styles/image_carousel_full/public/building-construction-core-roof.jpg?itok=6BahtDNZ
That project uses geodesic domes with structure that holds itself up. A pressurized dome has to be held down. So additional structure you see here will not be required, just the hexagons. And we could use transparent polymer film, not translucent film you see here.
Note that Mars atmosphere blocks most of heavy ion GCR. And a simple film with spectrally selective coating will block alpha, beta, UV, and X-ray radiation. There isn't much X-rays in space, but the coating is metal so thin that you can see through it. The metal coating won't block much, but blocks enough. What will get through is proton, light ion, and gamma radiation. And that will be less than half that of ISS.
Or you could just dream on. No harm in dreaming. One friend here in Winnipeg wants to put a force field over a crater, to hold air in. He thinks that's the most economic way to pressurize a crater. But the only force field that can hold atmospheric pressure is a plasma window. That requires plasma at least 12,000°K, and most researchers studying it today use 14,000°K to ensure anything contacting the plasma doesn't drop the temperature below the critical point where it's sufficiently viscous to hold air pressure. And the first paper published was a flat round window that required 20,000 watts per inch diameter. You may be able to reduce that power consumption if you can find a way to prevent plasma heat loss, both radiative and convective/conductive with air. But that power consumption doesn't sound practical to me. But he too can dream.

How does a plasma window work? I assume it works by heating air to a plasma and then controlling it with magnetic fields, you basically have to strip atoms of their electrons so you have positively charged particles that can be contained with magnetic fields, as neutral atoms cannot be. I don't think they could be made into force field domes. Besides, if you put 14,000°K next to air, you are going to heat that air, if you surround air with a dome of force fields, you are going to turn what's inside into an oven I imagine! Never mind the amount of energy used!

Of what practical use are force fields? If you want a hangar bay that is open to space, on one side of the force field you have breathable air, on the other a vacuum, I guess the question is whether solid objects can pass though that force field without getting seriously damaged or vaporized, if it can't do that, then force fields would have no purpose.

One possible use, an electromagnetic launch system into space, you accelerate something to orbital velocity within a tube, and releasing it into an atmosphere. Lets suppose you wanted to set up a mass driver on Mars, and hurl rock into space, at the end of the mass driver is a plasma window, the rock is accelerated to orbital velocity and then passes through the plasma window into the Martian atmosphere, atmospheric friction is taken into account as the rock hurls into space. So lets say we wanted to follow O'Neill's plan except using Mars instead of the Moon. Mars has stuff that the Moon does not have much of, such as water for instance, passage through the atmosphere should be brief enough so all the water inside does not boil away as it hurls into space, a catcher intercepts the rocks and they are processed in space into orbital colonies. Mars has everything an orbital colony needs, unlike the Moon.

Last edited by Tom Kalbfus (2015-10-27 05:59:31)

Terraformer · 2015-10-27 08:34:43

How about something like this - http://www.weather.com/series/human-nat … s-sunny#/!

Places like that would be valuable getaways for colonists. You don't need them to be big enough for the walls to be over the horizon in order to obscure that it's enclosed, either, if you plant trees and build cliff faces right.

Hmmm, you could build a valley like this, with a moving sunlamp on a rail and a roof producing diffuse light of the appropriate hue. Have a central river that includes lakes and islands, and maybe a small sea at the end.

RobertDyck · 2015-10-27 10:29:24

Tom Kalbfus wrote:

How does a plasma window work?

Heat air above 12,000°K. At that temperature it becomes viscous, like honey. Viscous plasma will not let air pass through; it can hold 1.5 atmospheres of pressure against hard vacuum. It was first discovered by a researcher in 1995 at Brookhaven National Laboratory, working on an electron beam welder. For an electron gun to work, it requires vacuum. But how do you get the electron beam out of the vacuum chamber without the beam itself cutting a hole in it? The point of a welder is to melt steel. His solution was to put a small plasma window at one end of the vacuum chamber, the electron beam would fire through that. If the electron beam causes the plasma window to heat, so much the better. His plasma window was a cylinder with open ends, one end welded to the vacuum chamber, the other open to air. The cylinder was two curved electrodes on the inside, with electricity passing between. Those electrodes heat air to the required temperature, and the charge from the electrodes holds the plasma in place.

Since I read about this, I did think of the force field of the shuttle bay on Enterprise D on Star Trek - The Next Generation. The original series had clamshell doors that opened, the shuttle bay was decompressed before the doors opened. But ST-TNG had a smaller shuttle bay, with a flat door that opened by sliding up. Before the door opened, an force field was activated to hold in air. The shuttle craft than flew through the force field. As a special effect, they showed a glow around the edge of the shuttle bay door opening, and as the shuttle craft passed through a glow around the shuttle craft where it supposedly contacted the flat force field. Again, the only real life force field that could hold in air is a plasma window. That means a viscous plasma thick enough to hold air pressure. That viscous plasma will stick to anything that passes through, like a shuttle craft. So you have a dense, viscous plasma at 14,000°K in direct contact with the shuttle craft hull. Black tiles of NASA's Space Shuttle can protect against up to 2,300°F = 1,260°C = 1,533.15°K. Reinforced Carbon-Carbon was used for the grey nose cap, and leading edges of the wings. It could withstand up to 3,000°F = 1,649°C = 1,922°K. If a shuttle craft tried to pass through a plasma window, it would melt. Furthermore, they often showed an individual "sending off" a shuttle craft, standing in the shuttle bay as it left. This demonstrated that the shuttle bay was still pressurized. The door was roughly 10-feet high by 20-feet wide, based on an image from the show. So 20,000 watts per inch diameter by 10-feet high would consume 2.4 megawatts of power. If you calculate by width, double that. That means the person standing in the shuttle bay was standing in front of a multi-megawatt plasma window @ 14,000°K. That person would be broiled. The broil element of your kitchen stove typically goes to 500°F = 260°C = 533.15°K.
latest?cb=20060926140943

Could you do something to contain radiative heat loss? The only way is another plasma. Air lets Infrared go right through, but plasma can contain it. So layer a cooler plasma in front of the plasma window? The cool(er) plasma would not contain air pressure, but would contain radiant heat loss, and the cool(er) plasma would be held in place with magnetic and/or electro-static fields. This would dramatically reduce power consumption, and ensure the operator doesn't get broiled. However, a shuttle craft would still have to pass through the dense viscous plasma @ 14,000°K, so it would still melt/fry.

Plasma is gas that has been heated so much that electrons in the valence shell are no longer held by the molecule. That is a soup of positively charged molecules and negatively charged free electrons. Overall the charge is balanced, but the molecules and electrons are all charged. That creates complicated interactions of magnetic, electro-static, electric current flow, pressure, and gas flow.

Fire is plasma. A well stoked bonfire can exceed 1,100°C (2,012°F). (In the following I converted the words "degrees" to the symbol.)

When a fire heats wood to 100°C (212°F), the water inside the wood boils and escapes as steam. As the wood dries, at 300°C (572°F), it begins to release combustible gases that ignite when they contact an open flame. The gases burn, gradually raising the temperature of the wood to 593°C (1,100°F).
...
The color of the fire indicates its temperature, with deep reds at roughly 600°C (1,112°F) and orange-yellow being about 1,100°C (2,012°F).

NASA fact sheet: Space Shuttle Main Engine Enhancements (Again I converted "degrees" to the symbol.)

...hydrogen propellant and oxygen oxidizer mix and burn at high pressures and at temperatures exceeding 6,000°F (3,315°C) to produce thrust.

So 14,000°K is extremely hot! And it's dense viscous plasma that will stick to anything you put into it.

Tom Kalbfus · 2015-10-27 11:19:57

Seems to me that an ablative coating might help when traveling through a plasma window, that is you accept that part of the craft is destroyed and you shed it to get rid of the heat as you pass through.

RobertDyck · 2015-10-27 16:22:55

Found a reference: here
It says Technora rope with 6-1/8" (156mm) diameter has a breaking strain of 3,000,000 lb (13,300 kN). That's 1,360,777.11 kg; round to 3 significant figures to get 1,360 metric tonnes. For safety, each cable will require double the breaking strain calculated above, so that if one cable breaks, neighbouring cables can take up the strain. That requires 336 such ropes per cable, or a single cable 2,859.65mm diameter. That's 2.85965 metres diameter, with one such cable each metre. That already overlaps.

Technora is slightly stronger than Kevlar, and was used for parachute cord by Spirint & Opportunity. But not good enough for this application.

Tom, would you be satisfied with a smaller dome? And what about water? You should build your island on a crater that already has a glacier. So we need only heat to produce water. Does this crater have ice?

SpaceNut · 2015-10-27 18:08:25

I have seen a glass panel tt is a few inches thick that has channelswithin it that makes it so that you do not have a clear sight through it but it lets light throu it. It seems to be like this product. http://www.tgpamerica.com/structural-gl … t_spec.pdf

Lumira aerogel, formerly Nanogel® aerogel, a translucent surface-treated amorphous silica, is a safe and non-hazardous material. It is encased in 16 mm polycarbonate sheeting, which is centered in the Pilkington Profilit channel glass cavity. Combined with Pilkington Profilit channel glass, all the components of the system are safe, recyclable and environmentally friendly. For projects requiring extra thermal performance, contact TGP for custom Lumira aerogel information.

Frame radius bend is on page 20...

Tom Kalbfus · 2015-10-27 18:57:16

RobertDyck wrote:

Found a reference: here
It says Technora rope with 6-1/8" (156mm) diameter has a breaking strain of 3,000,000 lb (13,300 kN). That's 1,360,777.11 kg; round to 3 significant figures to get 1,360 metric tonnes. For safety, each cable will require double the breaking strain calculated above, so that if one cable breaks, neighbouring cables can take up the strain. That requires 336 such ropes per cable, or a single cable 2,859.65mm diameter. That's 2.85965 metres diameter, with one such cable each metre. That already overlaps.
Technora is slightly stronger than Kevlar, and was used for parachute cord by Spirint & Opportunity. But not good enough for this application.
Tom, would you be satisfied with a smaller dome? And what about water? You should build your island on a crater that already has a glacier. So we need only heat to produce water. Does this crater have ice?

I kind of suspect that by the time we are ready to make huge domes like this, we'll already have space elevators on Earth, thus the material will be available for manufacturing these domes, or we could make a more flat roof dome on top of a crater and pile something transparent on top so downward weight would counteract upward air pressure. Perhaps a 10 meter thick glass roof. We start with a plastic membrane and add glass plates on top as we increase the air pressure underneath. The glass should be as clear and as transparent as we can make it!

GW Johnson · 2015-10-28 11:08:30

So, how do you tie your magic cables to the surface of Mars? A bunch of sand, loose rock rubble, etc. There's fractured and intact bedrock down there, but what do you use for cable fittings and stakes? How do you keep the huge cable forces from fracturing the bedrock? It an't but ordinary rock, after all.

GW

RobertDyck · 2015-10-28 11:34:47

Excellent points. The usual design for a large Mars dome is a concrete ring around the base of the dome. Integrated with bedrock for structural strength and to make it air tight. How much force does Hover Dam hold?

Tom Kalbfus · 2015-10-28 11:36:29

GW Johnson wrote:

So, how do you tie your magic cables to the surface of Mars? A bunch of sand, loose rock rubble, etc. There's fractured and intact bedrock down there, but what do you use for cable fittings and stakes? How do you keep the huge cable forces from fracturing the bedrock? It an't but ordinary rock, after all.
GW

There is the walls of this crater for one thing, the air pressure within the dome would push outward against these walls, and its a pretty big crater, about 100 km in diameter, so these crater walls are pretty thick. Then we start with a fairly flat dome roof made of plastic, and we pile glass on top while increasing the air pressure underneath to carry the weight. So most of the containing force will be the weight of the glass on top of the dome roof. Plastic will simply seal the gap between the glass roof and the crater walls and prevent the air from escaping in between.

We would have to make sure the glass is as clear as possible, fill the crater with water, and we'll have an island in the center as you see there. From the island the water will stretch to the horizon, I don't know if the crater wall mountains are high enough to rise above the horizon, maybe it will or maybe it won't. Another question is what color would you like the sky to be? Would you like the sky to be pink as it is now, or should we tint the glass to make the sky blue?

Last edited by Tom Kalbfus (2015-10-28 11:40:58)

GW Johnson · 2015-10-28 16:50:18

RobertDyck:

Doesn't matter what loads Hoover dam holds. That dam just doesn't hold loads of these magnitudes, and the loads it does hold are horizontal, not vertical. That's largely shear (horizontal) vs tension (vertical), and rock is just plain lousy in tension (all the vertical load cases for pressure domes).

Any "concrete ring" must be supremely massive to have the weight necessary to hold back the air pressure loads (even at 0.38 gee) yet near 1 atm pressure. I think y'all are talking about materials technologies very far from what we have available now or in the near (next several decades) future.

Tom:

What happens if you start losing air pressure inside your weight-stabilized dome? Ever thought about the weight compression loads it must carry without collapsing (!!!) if it does depressurize? Rock and glass are good in compression; plastic, not so much.

Further, what makes you think that a crater rim has rock with enough structural integrity to carry the radial component of your dome's restraining forces, even at steady state conditions? It's an impact crater. Its rocks are all fractured all to hell-and-gone, just because it's a crater! There's no rock strength there.

GW

RobertDyck · 2015-10-28 18:01:48

GW Johnson: Yes, one reason for calculating this was to show that a dome this big has problems. Notice the conclusion is Technora cables are just not strong enough, and that's an aramid slightly stronger than Kevlar. Any proposed dome has typically been the size of a small town, not an island nation. The concrete base ring will be modern concrete, with steel reinforcing rods. And the base would have rock bolts drilled into bedrock, and steel rebar attached to the rock bolts before pouring concrete on top. The dome would be low, so most of the force would be lateral, compressive to the ring.

But you do raise valid points. Rock in the rim of a crater is shocked to an extreme. Just making the ground beneath the ring air tight could be an issue. And I calculated pressure based on 6.5 psi instead of making the argon:nitrogen ratio equal Mars ambient. Based on Viking 2 data, that would be 8.433 psi. Of course that much argon would affect voice timber.

Others have suggested a "world house", composed of multiple smaller domes. I envision something like Project Eden, in England. Images for that are above. And I've said a Mars settlement may be a pressurized building, similar to an interconnected shopping mall, rather than a dome. The large downtown shopping mall in my city is called Portage Place, located on Portage avenue. A multi-level mall with enclosed/heated skywalk over Portage avenue to the remaining large downtown department store, another to what is now the arena, and another in the back to a major apartment building. The back is directly connected (without a skywalk bridge) to the downtown YMCA. The arena is has a skywalk bridge connecting to a major hotel, and another skywalk to another mall called City Place. A series of downtown buildings are connected, from City Place, to the downtown library, to office buildings, to an underground mall called Winnipeg Square, beneath a large office building. That's connected underground to a smaller underground mall, beneath another large office building, and a major hotel. If you've ever lived through a winter in Winnipeg, you would understand why. Here's the front of Portage Place:
portage-place-40.jpg?resize=450%2C300
The skywalk to "The Bay" department store. This skywalk has stores on it.

Inside the main mall looking west:

The main mall looking north:
interior-of-portage-place-shopping-mall-on-portage-avenue-winnipeg-c6e2f3.jpg
I could go on, but you get the idea.

Tom Kalbfus · 2015-10-28 22:20:05

GW Johnson wrote:

RobertDyck:
Doesn't matter what loads Hoover dam holds. That dam just doesn't hold loads of these magnitudes, and the loads it does hold are horizontal, not vertical. That's largely shear (horizontal) vs tension (vertical), and rock is just plain lousy in tension (all the vertical load cases for pressure domes).
Any "concrete ring" must be supremely massive to have the weight necessary to hold back the air pressure loads (even at 0.38 gee) yet near 1 atm pressure. I think y'all are talking about materials technologies very far from what we have available now or in the near (next several decades) future.
Tom:
What happens if you start losing air pressure inside your weight-stabilized dome? Ever thought about the weight compression loads it must carry without collapsing (!!!) if it does depressurize? Rock and glass are good in compression; plastic, not so much.
Further, what makes you think that a crater rim has rock with enough structural integrity to carry the radial component of your dome's restraining forces, even at steady state conditions? It's an impact crater. Its rocks are all fractured all to hell-and-gone, just because it's a crater! There's no rock strength there.
GW

Its the same principle as an impact crater holding water. The water in the crater pushes outward on the walls of the crater just as compressed air would, the only difference is that the compressed air also pushes up but water does not!
karakul.jpg?zoom=2&resize=564%2C372
9. Kara-Kul Crater: high altitude crater. This crater was formed about 10 million years ago, and is located in Tajikistan, near the Afghan border. In total, the crater is about 45 km in diameter and is partially filled with a 25 km-wide lake. This might be the “highest” impact crater, almost 6,000 m above sea-level in the Pamir Mountain Range. It was found only recently from satellite images.

bosumtwi.jpg?zoom=2&resize=496%2C395
10. Bosumtwi Crater: built of bedrock. The last crater on our tour of impressive impact craters is this located in Ghana, Africa. It is about 10.5 km in diameter and about 1.3 million years old. The crater is filled almost entirely by water, creating Lake Bosumtwi. The lakebed is made of crystalline bedrocks.
These impact crater seems to hold water just fine. There seems to be enough rock strength to hold in these crater lakes.

Last edited by Tom Kalbfus (2015-10-28 22:21:24)

Antius · 2015-10-29 11:19:43

As Mars has substantial gravity, I would suggest a semi-transparent masonry shell for very large enclosed structures. That way you avoid the need for enormously thick tensile structures. The shell would essentially be a layer of bonded regolith and rock, penetrated by sunlight tubes, each of which is capped by a semi-circular polymer cap on the inside and a dust cover of the outside.

The weight of the shell per unit area must be sufficient to balance internal pressure. If the shell is thick enough then the sunlight tubes could account for most of its cross-sectional area and little sunlight would be lost. For example, if the density of regolith and rock is 3t/m3, internal pressure is 3t/m2 and sunlight tubes account for 2/3rds of the shell area, then the shell would need to be ~10m thick on Mars.

You could build the shell as relatively thin reinforced concrete sections with the sunlight tubes sticking out the top of each section like a porcupine. Next, use earth moving equipment to heap rock and regolith over the top. It would be wise to divide the structure into cells, such that damaged sections could be isolated for repair. Maybe a set of stabilised soil dams could achieve this whilst providing additional support to the shell, such that it could support its own weight following depressurisation. It would also allow incremental construction and habitation.

Tom Kalbfus · 2015-10-29 12:53:28

What's wrong with glass? It is transparent and fairly heavy, it works out to about 1 ton per cubic meter under Mars Gravity. I am sure there is plenty of material for making glass on Mars, and with modern production methods, you could make that glass crystal clear so that most of the light that you want gets through.

Antius · 2015-10-29 14:54:30

Tom Kalbfus wrote:

What's wrong with glass? It is transparent and fairly heavy, it works out to about 1 ton per cubic meter under Mars Gravity. I am sure there is plenty of material for making glass on Mars, and with modern production methods, you could make that glass crystal clear so that most of the light that you want gets through.

Nothing wrong with glass. I'm not sure about the transmittance of a sheet of glass 3m thick. Producing glass sheets of that thickness would also be energy intensive and therefore expensive. Dirt is dirt cheap.

GW Johnson · 2015-10-29 15:07:59

Tom:

"Its the same principle as an impact crater holding water." No, it's not. You need to learn your science better.

Strength of the material has very much to do with whether the material is already broken, and anywhere around an impact crater, it most certainly is already broken.

Here on Earth, holding water in a depression depends upon the difference between inflow and outflow. That's the classic "leaky bucket" problem. Same as your wallet or your bank account. The rate of change of what you have inside = what enters - what leaves.

In the absence of surface streams, inflow depends upon averaged precipitation. With surface streams entering the depression too, it depends upon averages of both precipitation and incoming stream flow (a highly-variable quantity). Outflow depends upon flow seeping down into the earth through voids and cracks in the material (the very thing you have all around the vicinity of an impact crater), plus any streams that leave the depression (again a highly-variable quantity).

Don't lecture a professional aerodynamicist with 3 degrees how to balance massflow into, and out of, a control volume. You'll make a fool of yourself every single time.

GW

RobertDyck · 2015-10-29 15:15:08

Antius wrote:

I would suggest a semi-transparent masonry

Antius wrote:

Nothing wrong with glass. I'm not sure about the transmittance of a sheet of glass 3m thick. Producing glass sheets of that thickness would also be energy intensive and therefore expensive. Dirt is dirt cheap.

What do you think glass is? Glass is just sand, melted.

You start with pure white silica sand, add caustic soda (sodium hydroxide) and lime (calcium oxide). Soda and lime add strength. Lime is formed by baking limestone, which is a mineral of calcite and dolomite. Calcite is calcium carbonate, dolomite is calcium magnesium carbonate. You can use calcite instead of limestone, although you will also need oxygen to combine with carbonate to form CO2. Sodium hydroxide is formed by dissolving salt in water, then electrolysis across a membrane. Hydrogen bubbles off the negative electrode, chlorine gas from the positive. Add salt water to the positive side, sodium hydroxide builds up on the negative side. Theoretically water becomes hydrogen gas and hydroxide, salt becomes chlorine gas and sodium, so nothing left. In practice there's always something else in the water or salt, so spent brine has to be removed or recycled.

If you don't find any white silica sand on Mars, you can use silica gel. That's a byproduct of making aluminum.

If you want to make a transparent block from dirt, isn't that just a less pure grade of glass? Most dirt won't form anything translucent (or semi-transparent). The only "dirt" that will is sand, or highly pure minerals such as plagioclase feldspar. Anorthite and bytownite are types of plagioclase feldspar that will dissolve in acid, the first step to extracting aluminum. Another mineral that will form something translucent is quartz, a crystal of silicon dioxide. That's the same thing as glass, just a crystal structure instead of amorphous. Sounds like you're just talking about grades of glass.

GW Johnson · 2015-10-29 15:20:02

As for the ongoing dome discussion, would it not make more sense just to build simple small cylindrical pressurized buildings in which to live, plus ice-covered ponds in which to farm-by-aquaculture? The ice covering gets buried under regolith cover to prevent its sublimation. The building roofs (cylinder ends) get covered in a thick layer of regolith for radiation protection.

All you need to build such buildings on Mars is a worthy substitute for earthly concrete on cold, dry Mars. That and some steel rebar, and some clear panels for the transparencies (which could be glass or plastic). The form of the building could initially be low and squat. Later, you build tall ones. Steel and glass-or-plastic are initially brought from Earth, later they get produced on Mars.

Sealing gas flow through fractured bedrock is fairly-easily done with composite materials based upon ice as the matrix.

All that really proves is that the most important resource at any potential base or colony site is massive deposits of freshwater ice that can be easily mined. Not with shovels, but with steam down a drilled well.

GW

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