Shell Worlds

Calliban · 2023-10-05 06:58:41

Kenneth Roy discusses shell world stability.
https://www.researchgate.net/profile/Ke … bility.pdf

For bodies some 40% more massive than Ceres, the effects of scale height are enough to provide a counteractive force that will restore a displaced shell. On these worlds, pressure containing shells could be built without the need for any physical connection to the central body. For smaller worlds, cables or towers would be needed to physically anchor the shell in place and prevent it from displacing and impacting the surface. Roy doesn't seem to have accounted for the compression that would result from a displaced shell. Given that speed of sound is only 343m/s under standard conditions, any movement of a planet sized shell will compress the gas underneath faster than it can escape. This has nothing to do with scale height and would provide an additional restoring force. So I think his shell concept could be applied to smaller bodies than he anticipates.

I think a more probable concept to shell worlds is the tunnel world. A large asteroid like Vesta could be riddled with tunnels and cavities as a result of mining activities. These could be hundreds or even thousands of metres in diameter. If these are more than about 200m beneath the surface, the overbearing weight of the rock would contain the upward force resulting from pressurisation.

Tunnel worlds could be built up incrementally as a byproduct of the mining needed to build space habitats. They aren't things that necessarily require a dedicated effort to build and can start small, growing bigger as excavation removes more material. This makes them a more probably outcome in my opinion.

The conventional shell might eventually be used to enclose outer solar system objects like KBOs. These bodies are 50% water ice by mass. A sufficienctly powerful thermonuclear heat source could be used to melt this ice. An inflatable skin would then be placed over the surface and inflated with air to low pressure. The liquid water will then be pumped from beneath the skin onto its surface, where it would freeze into solid ice. As the ice layer thickens and its weight increases, air pressure will grow to balance it, allowing net zero shell stress. Eventually, we have a mostly rocky world, with a gap of say 10km between its surface and a thick icy shell above, with the weight of the shell largely balanced by air pressure, leaving only a weak compressive stress within it. Waste heat removal would favour keeping most of the above the shell liquid. So the roof itself will need enough integrity to prevent water from draining out of tge shell onto the ground.

Last edited by Calliban (2023-10-05 07:19:21)

Void · 2023-10-05 09:27:49

I liked your post. If you can tolerate it maybe we can talk about a radiator that has a two-part cycle.

If you have a radiator of metal perhaps, and you discharge evaporation into it, then the air pressure inside can be kept low, by freezing.

If the shape of the radiator were basically a cylinder, when the level of frost was high then you could pull a sock over it as thermal insulation.

Then you could allow the air pressure to build up inside, and so the cooling output from the heat source you want to sink, could melt the ices. So, maybe a phase change cooling in this part of the cycle?

Of course, then you need a method to draw off the fluids that result from melting or evaporating the ices.

Rinse and repeat.

I just got up so maybe there is a flaw, but please let me know what you think.

Done

Last edited by Void (2023-10-05 09:30:34)

Void · 2024-02-08 13:08:07

I guess this could be a candidate for a shell world, as the sea is so young, we may not anticipate life: https://www.msn.com/en-us/news/technolo … r-BB1hYRUB Quote:

Indy 100
Hidden ocean discovered beneath the surface of moon
Story by Harry Fletcher •
4h

I would not go so far as to make a high pressure atmosphere under such a shell, just a method to reach the ocean.

For instance, if you harvested the ice shell to fill up aqua habitats, you would eventually reach the ocean, and then with robots you could mone the core.

If Calliban wishes he might do some calculations of conditions of such a terraformed world.

Done

Last edited by Void (2024-02-08 13:11:36)

Calliban · 2024-02-08 16:38:58

Void, that is a fascinating discovery. Well done for digging it up. Here is the space.com article that goes into more detail.
https://www.space.com/saturn-death-star … face-ocean

At -100°C, the thermal conductivity of ice is 3.5W/m.K. Mimas has a radius of 200km and the ice shell is expected to be 30km thick. By my estimates, the temperature difference between the sea and the surface, must be at least 150K. To keep the sea liquid, about 8GW of heat must be generated by core-ocean interactions. Maybe tidal flexing induced by the gravity of Saturn and its other moons provides this heat.

But 8GW is not a huge amount of heat on a planetary scale. It is the heat generated by two 1400MWe nuclear reactors. In the future, a human colony of a million people could generate this sort of waste heat. And a small icy world gives that colony a place to dump waste heat without huge radiators. So aquaforming is something we may be seeing a lot of.

It occurs to me that an ice shell 100km in diameter and several km thick in a gravity field of ~0.001g, has both a short radius of curvature and low compressive shell stress. It should be possible to inflate an air bubble between the ice layer and the ocean that humans can live in. Because the ice layer is curved, it should function as an arch.

Last edited by Calliban (2024-02-08 17:04:03)

Void · 2024-02-08 19:44:42

Thanks, Calliban. I have some new notions about shells. I will stay ambiguous as I want others to consider if they have input.

To get to the ocean a tunnel method is needed, I suppose that would be a first base.
Getting to the ocean, you might do the things you have suggested. Now I wonder about a simple spherical shell to spin around the moon.

A simple version is just that a sphere, that is outside of the moon and spins and has artificial gravity on its equator.

Doing this then something needs to be done about the stability of ice. and the sea.

Frankly I want the shell to be quilted sort of multi-layer, so that rupture is less dangerous. I originally thought that the interior of the shell would not be pressurized. But now I wonder if it can tolerate a certain level of O2. Maybe as high as 1/3 g???

The notion of a mega satellite for Ceres invokes magnetic bearings. So here I am supposing the moon might have magnets that point magnetic north poles outward, and the shell would point magnetic north poles at those magnets.

You have a sort of an air bearing if the distance between the shell and the moon is enough. Don't know if we can get away with it though.

If you could have a 1/3 bar or maybe less O2 atmosphere you could use air breathing aircraft to pass from the moon to the shell.

Granted, you could probably have towers at the spin poles that would allow trams between the moon and the shell.

So, the shell would have enough gravity at its "Equator" whatever sufficient gravity were.

The outer shell could have a rectenna on it, and power plants may send power to the apparatus. I know solar is not popular for Saturn, but concentrating mirrors are simple and can be rather thin, so solar is not ruled out for Saturn and company.

And in the Saturn system we have Nitrogen for making a greater atmosphere simulation.

Done

So, notions of modifications and concerns are welcome.

Done

My profile for the moon is probably not correct, but it is hard to get the correct information.

https://phys.org/news/2024-02-mimas-tin … ocean.html

OK, hard to get the information I want but just now I think that perhaps 90% of the moon is melted or rock. Not that much rock, but still if it were even 5%, then that is something.

The anti-humans don't want our germs anywhere in space. Basically they wish we were dead, I think. The want their servants to serve them and then be dead, I think.

Done

Last edited by Void (2024-02-08 20:41:15)

Void · 2024-02-09 09:08:45

I am not sure that Mimas is the best case for a spin shell. It could be a good case, but what about other worlds? The further away from the world, then the less gravitation on the shell. Spin would help hold the shell up at least near the equator. But can you have a very large spin shell around a small world? For instance Vesta.

Then if you wanted could you cook up a partial pressure atmosphere for the spin shell? Using magnetics to keep the small world centered in the spin shell, and can the air be like an air bearing?

Could you fly aircraft from the little world to the interior of the shell and land at the equator? Of course a crash might rupture the shell, so I anticipate that the shell would have many layers and some method to patch a hole.

As far as Nitrogen goes I guess you might have spaces with a higher pressure and Nitrogen.

If you have a sphere for a shell, then you have various spin gravity gradients which could accommodate various industrial processes.

You could actually have microgravity at the poles, if you had subshells that could be counter spun.

But the interior of the shell does not have to be pressurized, in some cases I simply imagine multiple shells with pressurized compartments where needed. This would be less vulnerable to accident and military actions.

Done

Last edited by Void (2024-02-09 09:15:46)

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