Steel is the most obvious material but I’ve seen basalt fibre suggested. The purpose is to rotate it to produce 1g of gravity, which would induce similar stresses to the surface being a suspension bridge of equal length (I think the circumference is around 24km so pretty long). How could steel be manufactured on such a scale? Forging huge pieces with hydraulic presses in space and welding hundreds of thousands of them together? Having some sort of extruder that pushes out plate like a really big steel sheet and then welding the long strips together to form a cylinder? Advice appreciated so I can buy the materials and get to work
It’s like someone in the 1800’s saying “How would you make enough cement and asphalt to create a road from one coast to the other? How would you deliver it? There aren’t enough horses…”
The scale of what you ask about is immense beyond what we do today. Even moving enough atmosphere to fill it up is a substantial task. The infrastructure to begin to build something like that boggles the mind. If I had to make a wild guess, it would be made as a single piece by a “Printer” that melts the steel and connects it to the existing structure, much as a 3D printer works today.
3D printing ‘robots’ come to mind
Well, the premise of the question is a little shaky:
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No, the stresses wouldn’t resemble those in a suspension bridge. The mass of the structure would be accelerated in a direction normal to its surface at 9.81 m/s[sup]2[/sup], yes, but because it’s a cylinder, the structure itself would be loaded in essentially pure tension (in the “hoop” or circumferential/tangential direction). If this were not a true cylinder but more like a ferris wheel with radial members like the spokes in a wheel, then the stresses would be closer to those in a suspension bridge. But you don’t gain any substantial structural efficiency with radial members, especially if you’re using anisotropic materials (see below).
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This structure would never be made of steel. In a situation like this, low mass pays off twice: once in reducing the energy required to put the material into orbit, and once in reducing the stress in the structure due to the structure’s own mass. Because the stresses are highly anisotropic (highly directional) you’d want to make it out of an anisotropic material. If this were being built today, that would mean something like carbon fiber, Kevlar or Dyneema. In the future, one would use carbon nanotubes.
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Even if steel were somehow required, you’d forge everything on earth and launch it into orbit. It takes millions of times less energy to put the finished product into orbit than it does to put a whole foundry up there. Plus, steelmaking and steel fabrication are really energy-intensive processes…you’d have to boost up a bunch of fuel to run those processes. There’s no way you’d fabricate anywhere but on Earth (or on an iron-rich nearby planet, if this is a sci-fi premise).
Molten iron will vaporize quickly in space especially with no atmosphere / pressure.
I remember an SF story where they took a roughly cylindrical asteroid, bored a longitudinal shaft that they filled with bladders of water. They then spun it on its axis while heating with solar mirrors. By the time the surface heated to a malleable consistency, the water boiled and they let it blow up like a balloon.
Clearly, many issues but the ‘glass blowing in space’ idea was clever.
You could never haul the material up from Earth’s gravity well. Or rather, if you could you got enough magic to do whatever you like. Space habitats on this scale will have to be made from in situ materials.
Bubbleworld. (The concept, not the story title.)
“Substantial” is a bit of an understatement. Assuming 14.7 psi throughout, we’re talking about 2 billion tons of air. I don’t think you’d be moving that up from earth; you’d be using solar power to extract atmospheric gases from an asteroid.
As for the structure itself…that’s a lot of steel. The limiting factor will be hoop stress. Considering only atmospheric pressure - never mind the weight of the steel shell itself, or whatever topsoil/water/buildings/living meat are being contained by it - if you allow a safety factor of 3 using A-36 mild steel, we’re talking about a shell of steel 16 feet thick. That thick of a shell weighs a lot more (per square inch of internal real estate) than the atmosphere it’s impounding, which means steel won’t work for a structure this large: if you make your shell thicker to account for its own weight, you’ve increased its weight, and the shell has to be designed even thicker, ad infinitum. If you’re going to do this, you’ll need a material with a much better strength-to-weight ratio, like one of those suggested by EdelweissPirate.
Not a lot of nitrogen in your average asteroid. Might be better off mining comets.
The best way to do this is to just extend your cylinder all the way around the sun. It orbits the sun normally. You get your one G gravity by rotation of the cylinder. Sure, that means you get tension on the cylinder but at 1 AU the curve is so negligible you can just neglect it. Now you don’t even need scrith for your bucatiniworld megastructure.
Now, why exactly do we want to create a 1 G megastructure in space? If you want 1 G you don’t need a megastructure. Large accelerations and megastructures don’t play well together. You can incorporate little 1 G environments into your megastructure, but if you’re insisting that the whole thing be a 1 G shirtsleeve environment why not just stay on Earth?
You’re probably thinking of Higher Education by Sheffield and Pournelle.
How good are these materials at resisting degradation from radiation levels in space ?
Maybe you could repurpose the junk that’s orbiting the Earth?
The essential idea is okay but it would actually be better and more flexible to start with a water ice-silicate matrix reinforced with an outer long fiber overwrap to give the habitat strenght to resist hoop stresses. I’ve previously described the design and construction of such a habitat ([POST=14053719]here[/POST]) which would largely use simple materials available in space with minimal processing and be capable of being expanded up to the limits of the strength of the overwrap and availability of materials. The advantages of using ice versus a more standard terrestrial construction material such as steel, aluminum, or concrete are not only that it is readily available and only requires heating to a modest temperature to mold into shape, but also the latent heat of fusion gives it thermodynamic stability, the thickness of the shell plus the water layer provides protection from solar charged particle and cosmic radiation comparable to Earth’s surface, and water ice is “self-healing” under pressure; it will flow and freeze to fill any defect provided the overwrap prevents it from gross fracture or catastrophic rupture. This is all plausible with materials that exist today and the only part that requires complex fabrication is the reinforcing fiber and the initial “balloon” skin.
Constructing a space habitat the way we do a building or a automobile on Earth is a non-starter; not only would that require either transporting all of the individual components from the surface of the Earth into space or first setting out some kind of complete manufacturing infrastructure to make not only the major structural elements but also all of the other manufactured components such as fasteners, jigs, et cetera. Even if this were not cost prohibitive the logstics would be fantastically difficult, representing by far the largest construction and transportation effort ever attempted, dwarfing even the most audacious wartime logistics efforts in volume, and needing to be sustained for decades on end.
Beyond the fairly primitive water ice habitat structure described above, future habitats will likely be “grown” as carbon nanostructure fiber structures secreted by some kind of dedicated microscale assemblers somewhat akin to how coral reefs are formed by Anthozoa polyps. This is well beyond our capability to engineer today but makes far more sense than trying to mine and smelt relatively rare structural metals. Titan, a moon rich in complex hydrocarbons, may become the motherlode of the solar system as the source of basic structural material as well as a useful cold sink for industrial processes; provided, of course, that it isn’t already inhabited by some non-terrestrial form of life.
Stranger
And the other advantage of using volatiles as your structural material is that you need those volatiles to run the biosphere of your habitat. Having lots of extra megatons of CHON hanging around means you’ve got a cushion.
Could you build a massive ring attached to a furnace and just extrude a massive tube by keeping on adding metal in the back? Like how they make plastic pipes.
Hey, I’m back!
So yeah, in situ resource utilisation is the order of the day - I don’t intend to be driving the materials all the way from my hardware local to the site (Ceres), I’ll just pick them up from the bulk seller next door.
With regards to atmospheric pressure: at least half sea level atmospheric pressure is fine, like you’re living in Kathmandu but a bit higher and less steep. Nitrogen is a good inert gas if you want to have a working ecosystem, but as someone pointed out it can be quite hard to get unless you go all the way to Titan and back or manage to snag a sale at a comet. As long as the partial pressure of oxygen is high enough for unrestricted human life, and there’s enough extra pressure from an inert gas that we don’t start sweating blood, we’ll be fine. Does the calculus change with that assumption? I seem to remember Gerard K. O’Neill in The High Frontier supposing a significantly lower thickness than 4.877m.
@Delicious, that sounds like an excellent idea. You would probably need some very light spin while it’s extruding to keep stability high, and you would necessarily have to go very slow.
One idea I thought of was to have a ring of extruders all extruding a slightly bent thickness of steel, like a measuring tape. It only leaves the extruder when it’s solid enough to stay a little bit stiff (so very slowly). All the output steel tapes go to a further ring that lines them up against each other and holds them firm there, while friction stir welders weld the joints in a continuous line in 100m segments (whereupon the whole assembly stops and the FSW probe is replaced due to wear, before restarting). How does that sound?
How fast would molten steel evaporate in vacuum? Could you surround the liquid area in a bag to catch it? If the losses aren’t huge you could potentially just swallow them like you do shavings. There is a lot of almost everything we’re using in Ceres, more than could ever feasibly be used for this structure or another fifty like it.
That can’t be right. Here on Earth, we have cylindrical steel tanks holding compressed gases at more than 2 atm (that is, more than 1 atm difference in pressure between the inside and the outside), and the walls are much less than 16 feet thick.
Not advocating for Machine Elf’s specific numbers, but don’t forget LaPlace’s Law. Cylinder wall tension scales directly with both pressure and radius. We don’t have a lot of cylindrical steel tanks on Earth that are 8 km in diameter. For the same pressure, such a tank would have 100,000 times the wall tension of a fire extinguisher with a radius of 4 cm, and would need correspondingly thicker walls.