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Gravity Balloons
#11
Another thought: compression.

The idea of a gravity balloon is that the weight of the shell is balanced against gas pressure. Something I ran into with Hawkings Not Here is shells placed in compressional stress are very vulnerable to buckling, especially large shells. I would've loved to make HNH 50 or 100km in diameter, but the shell thickness became ridiculous (tens of kilometers with diamondoid). HNH, of course, has a vacuum center and is entirely dependent on the structural shell, unlike a gravity balloon.

But in larger volumes, you can't depend on constant air pressure. Differential heating will create differential pressure across many square kilometers.

I suppose you could over-pressurize a bit to keep the shell prestressed in tension against any reasonable pressure drop, but the structure is vulnerable to catastrophic collapse if there's a lot of air loss.
Mike Miller, Materials Engineer
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"Everbody's always in favor of saving Hitler's brain, but when you put it in the body of a great white shark, oh, suddenly you've gone too far." -- Professor Farnsworth, Futurama
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#12
Most artificial worlds have dynamically supported suprashells, so they should resist buckling (so long as the orbital loops continue to work). Only very small ones wouldn't need this. Birch compared this to a balloon, by the way.
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#13
(03-07-2014, 02:47 AM)stevebowers Wrote: Most artificial worlds have dynamically supported suprashells, so they should resist buckling (so long as the orbital loops continue to work). Only very small ones wouldn't need this. Birch compared this to a balloon, by the way.

Yep, I was just looking through OA's supermundane world and artificial planet articles. I was sketching out a campaign setting for Dungeons and Dragons (of all places) set on a supermundane world with Jupiter as a center.

One thing that struck me was that the diameter of the shell wasn't much larger than the diameter of the planet, and if I crunched the numbers correctly then a low density, low gravity planet like Saturn would see the shell nearly touching the planet. Drop the planetary gravity below 1G and the shell should come in contact with the planet.

And then you're back to gravity balloons, which offer the nifty ability to build really large shells (e.g., supermundane worlds) without dynamic compression beams. You'd need some active stabilization against the shifting atmosphere, like ballast weights under the shell, but buckling is a lesser concern because you have that universal gas pressure support.
Mike Miller, Materials Engineer
----------------------

"Everbody's always in favor of saving Hitler's brain, but when you put it in the body of a great white shark, oh, suddenly you've gone too far." -- Professor Farnsworth, Futurama
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#14
(03-06-2014, 11:18 AM)Cray Wrote: I like the self-gravitating approach to resisting 1-bar of pressure.

Instead of the many layers of flow dividers, you could just use a single-layered, closed aluminum (or plexiglas, or whatever) shell. The total air frictional losses between the solid shell and the internal spinning habitat should be about the same as the many-layered flow divider, and a single shell simplifies a lot of design issues (dissipating frictional heat, maintenance access, etc.) Poke a few holes in both shells and you've got free ventilation, delivered by the same electric motors that maintain the internal shell's spin against drag.

As for walking out of the spinning habitats, I'm not sure how practical that approach would be. For habitats that keep their angular rotation down to stomach-friendly rotation rates (1rpm or less for most people; 1-3rpm with adaptation), you're looking at ~2km-diameter habitats for 1G. You'd need mechanical transport - elevators or some parallel "subway car" system" - to debark. It wouldn't be hard to move between adjacent habitats, but it'd be a move of more than a few kilometers.

This is good. Thanks for making technical points on this, because more eyes are needed.

The problem with the single-layered solution is that it would have a much larger frictional torque. Increase the diameter to 2 km and it's hopelessly huge. To start with, how many times larger would the outer layer be? 50% larger? 2 times as large? Make it infinitely large - we have equations for that. Seriously, I've actually done this calculation. For a 2.3 km diameter, energy loss is around 700 W/m^2. Maaaybe you could tolerate this, and you'll just be feeding several kW per every inhabitant. If you want rural density, it will be megawatts.

Now, let's say that the single-layer is stationary. I can't easily tell you how much that will change the power consumption, but I do know it will make it greater. If it is co-rotating to some degree, it can decrease the friction... and you're on your way to full circle back to the multi-layer option.

It's not clear from what you wrote, but what you could do is to fill the area between the tube and the outer shell with a lighter gas (like Helium), so that it will have lower friction. Even better, because Helium might be much easier to procure in deep space. Obviously, this would conflict with your other ideas about cooling, because the gases must remain separated. Actually, that's why I never went into that. Even with no pressure difference, maintaining the seal between gases over a moving seal is hard. Recycling the gases via separation is even more energy intensive.

The ventilation doesn't work like you hope either. The problem is that you've under estimated the fluid pressure head from 1 km (actually 500 m equivalent) of elevation change. The simple act of letting the air through holes in the floor raises the temperature of the air. It raises it more than what you would be cooling in the first place! The rotating tubes are a giant centrifugal pump, and one that is surprisingly powerful. Now your idea would still work, but only by using several times as much energy as what the inhabitants are using to begin with. Then all the work you put into the torque to keep it rotating is just going to heating up the air that you let out of the vents.

Otherwise, the mechanical transport system is pretty close to what I had in mind. Elevators, in particular. Actually, this works very well with my ventilation solution. If you allow the air to go in one end and out the other (done using flow dividers), natural circulation of the air (due to energy use on the inside) will drive the flow, and this is decently well-matched to the available flow area, estimated energy consumption, and comfortable range of air temperatures. That means you could have a nice 1 m/s flow of air traveling through the center of the tube. So the elevator takes them up to the center, and they literally ride the air flow out.

How do they get in? I don't know. Slides?
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#15
(03-07-2014, 12:13 AM)Cray Wrote: Another thought: compression.

The idea of a gravity balloon is that the weight of the shell is balanced against gas pressure. Something I ran into with Hawkings Not Here is shells placed in compressional stress are very vulnerable to buckling, especially large shells. I would've loved to make HNH 50 or 100km in diameter, but the shell thickness became ridiculous (tens of kilometers with diamondoid). HNH, of course, has a vacuum center and is entirely dependent on the structural shell, unlike a gravity balloon.

But in larger volumes, you can't depend on constant air pressure. Differential heating will create differential pressure across many square kilometers.

I suppose you could over-pressurize a bit to keep the shell prestressed in tension against any reasonable pressure drop, but the structure is vulnerable to catastrophic collapse if there's a lot of air loss.

There are some important points in here too, so I'll add another reply.

Compressional stress will lead to problems, and this is why a perfect gravity balloon would have no compressive stress. Only isotropic pressure. But that is for a perfect gravity balloon. In reality, you'll have various stresses due to its imperfections. For extremely large sizes (like Virga), I think there are some major global stability issues. But porosity data suggests that asteroids have at least an order of magnitude more strength than what's needed to keep the intermediate sizes (up to maybe 70 km or more diameter) nice and structurally happy.

Oddly, I went in the completely opposite direction regarding the over/under stressing. My take is that you'll choose to have additional compressional strength. That is, the pressure is lower than what you could sustain via the rock's self-gravity. The reason is because this amounts to asteroid caves as they already exist. Some rearranging might be possible once people have lived there for a long time and have gained lots of understanding of these bodies.
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#16
Finally got round to making an image of one design of Gravity balloon, with a rotating radiator ring to cool the interior. This one has a number of quite large rocks on the outside to counterbalance the pressure from the interior.


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#17
Very niceSmile Although, would it be possible to add some additional color to the rock and surface textures? It is very uniform at present and it's not obvious that these are rocks on the surface.

Todd
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#18
Yes; it currently looks a bit like a sesame seed bun. I'll see what I can do.
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#19
A version which looks less like a sesame bun, and more like a balloon covered in rocky ballast


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#20
Gravity balloon page here
http://www.orionsarm.com/eg-article/54a180bebfacc
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