I would be worried about tunnelling subtly changing the rotational inertia of a rubble pile, and it suddenly reshaping itself in a series of avalanches that change the rotation again and trigger the next avalanche.
Quote from: mikelepage on 11/21/2018 07:34 amBecause you'll gradually lose kinetic energy to friction/heat, you'll need to keep accelerating the train just to stay at the same speed/gravity-level. This will, in turn, gradually de-spin the entire asteroid.This is not quite correct, because of conservation of angular momentum. If the train is losing kinetic energy due to friction, it is not just being lost to heat, some of it is being converted back into kinetic energy of the asteroid, spinning it up again. So you are not going to gradually de-spin the asteroid.
Because you'll gradually lose kinetic energy to friction/heat, you'll need to keep accelerating the train just to stay at the same speed/gravity-level. This will, in turn, gradually de-spin the entire asteroid.
There was a similar error in a short story you wrote, where you had people running to spin up a space station. Actually, as soon as they stop running again, their efforts will amount to nothing. Angular momentum is conserved. The real way for them to manually increase the spin of the station would be to lift mass towards the center.
Quote from: KristianAndresen on 11/23/2018 11:18 amQuote from: mikelepage on 11/21/2018 07:34 amBecause you'll gradually lose kinetic energy to friction/heat, you'll need to keep accelerating the train just to stay at the same speed/gravity-level. This will, in turn, gradually de-spin the entire asteroid.This is not quite correct, because of conservation of angular momentum. If the train is losing kinetic energy due to friction, it is not just being lost to heat, some of it is being converted back into kinetic energy of the asteroid, spinning it up again. So you are not going to gradually de-spin the asteroid.So... you agree with me that some of the energy is being lost to heat? Yes, there is a portion of the energy due to friction that will be exerting a "spin up" effect on the asteroid, but the portion of the energy lost to heat will not. Therefore the total angular momentum of the asteroid decreases, but conservation of energy is maintained.QuoteThere was a similar error in a short story you wrote, where you had people running to spin up a space station. Actually, as soon as they stop running again, their efforts will amount to nothing. Angular momentum is conserved. The real way for them to manually increase the spin of the station would be to lift mass towards the center.Yeah I rewrote that section when I realised my error In that case, the heat energy of the friction is being retained by the system (so it wouldn't work), while in the asteroid case above, I assumed it would be lost to space. Not sure how big the effect would be though.
Quote from: KristianAndresen on 11/23/2018 11:18 amQuote from: mikelepage on 11/21/2018 07:34 amBecause you'll gradually lose kinetic energy to friction/heat, you'll need to keep accelerating the train just to stay at the same speed/gravity-level. This will, in turn, gradually de-spin the entire asteroid.This is not quite correct, because of conservation of angular momentum. If the train is losing kinetic energy due to friction, it is not just being lost to heat, some of it is being converted back into kinetic energy of the asteroid, spinning it up again. So you are not going to gradually de-spin the asteroid.So... you agree with me that some of the energy is being lost to heat? Yes, there is a portion of the energy due to friction that will be exerting a "spin up" effect on the asteroid, but the portion of the energy lost to heat will not. Therefore the total angular momentum of the asteroid decreases, but conservation of energy is maintained.
Quote from: TrevorMonty on 11/01/2018 09:23 amThe shielding can also be in an outer shell that doesn't rotate. Need a lot of shielding but support structure required to hold it in place is very light as its 0g.IMO, no large rotating settlement, such as an O'Neill, will ever be flown "naked". It'll either be built inside an asteroid, or surrounded by a non-rotating shell serving the same role.Edit/Lar: Topic split... near term and inflating asteroids may or may not be useful in the same sentence...Post-split edit:I asked Lar to split this off the other thread. Thanks Lar.For a rough topic description: How to use asteroids to build large (up to O'Neill scale) habitats and settlements in space. And how to use large habits and settlements to exploit asteroids.
The shielding can also be in an outer shell that doesn't rotate. Need a lot of shielding but support structure required to hold it in place is very light as its 0g.
I like the way trains theoretically allow for practically any radius.One concern I have with trains and even shielding independent of the spinning portion is how to maintain it without stopping everything. How do you confidently avoid a sort of cascade effect if anything goes wrong, with shrapnel continually gaining more destructive kinetic energy as it bounces back and forth between the two surfaces?
Being one piece (and outside the asteroid) often seems reasonable to me. Beyond a certain scale, a 10 meter water reservoir beneath your feet does not seem a big deal, and the lakes will have other uses too.
I think this will happen too, especially once in-space fabrication gets going. Almost like two classes of spin gravity habits: additively manufactured (free spinning - with water-filled expandable modules for shielding), and subtractively manufactured (inside asteroids - using regolith for shielding). The distinction will probably get fuzzy once space industry gets big enough.
My third "ceres-class" option is centrifugal habitats in oceans melted by waste heat, deep enough under the ice for earth pressure. For example a parabola-shaped spinning cup/diving bell where the direction of gravity is always aligned to the surface. You could swim into the diving bell from a minimal hole at the bottom, where there is no spin and only the 3% gravity of ceres, then swim up to the beach which is in low gravity with no doubt very interesting waves, and continue walking up to the beach-front properties and cafes in full gravity.Unfortunately you cannot just have this diving bell spinning in water. I did a bit of investigation and apparently even with theoretical infinitely low friction you still have drag. I think the theory assumes at least an atom-width of water is carried along or something like that which still interacts with the rest of the water.You could of course have something moving around in an evacuated chamber, like a hyperloop in a circle but I hope there might be a much better trick with a paint-thin counterrotating layer that gives you a stationary outer surface rather than a theoretical-frictionless one.. and ideally is self healing eg a magnetically suspended liquid layer. So the idea is practical but hard work, or easy but hand-wavy science fiction
Pretty sure the friction issues make it a non-starter. Assuming there is a liquid subsurface ocean, you'd basically be attempting to set up an ocean-wide whirlpool, with corresponding energy requirements. Seems like you're introducing an insane amount of complexity so you can surf some cool waves
QuoteI'm familiar with the concept from Niven, The concept precedes Niven. At least back to Dandrige Cole/Roy Scarfo in the very early 1960's.One other major problem with this design, beyond the difficulty in actually making it: it's dynamically unstable. Unless either constantly actively controlled, or connected to another identical rotating habitat in the same way the O'Neill habs were meant to be, a cylinder like this won't rotate on its long axis. It might start off that way, but it'll soon end up tumbling end-over-end.
I'm familiar with the concept from Niven,
So that's why I think it will be a long time before the internal diameter of space habitats ever gets so large that the ceiling can't be easily reached with a cherry picker (or similar) to perform such repairs. I guess that limits us to something as tall/wide as a 4-6 storey building...?
It's not like there isn't an uncountable number of micrometeorites/debris flying about at speeds sufficient to create such a hole. These events are a matter of when, not if.
When your habitat is measured in kilometers, the wall thickness - assuming materials such as steel - is measured in *meters.* An impactor dangerous enough to poke a hole through *that* would be detectable and interceptable. Even Bernal almost a century ago assumed electron beam directed energy weapons to zap impactors.
But the risk of puncture/breach gets us back to shielding. Having a non-rotating bulk shield that can deal with any impact that isn't on a structure-destroying scale. (Some other system would be required for larger asteroids.) I would expect the shielding to outmass everything else. Trying to put that shielding on the rotating structure itself drastically increases the structural loads.
My question to you is, if you're capable of creating kilometer-scale habitats out of meters-thick steel, you're presumably also capable of creating dozens/hundreds of smaller habitats for the same price, so - aside from romantic notions of huge open spaces - what is the reasoning for making the single huge habitat?
I'm curious how you see the asteroid mine working, such it's worth it (from an economic perspective) to have an "underground" mine going deep into the more compacted material, versus an "open cut" mine, skimming and processing loose material on the surface. .
A primary *psychological* reason for "bigger is better" is that habitats such as these will likely be where the vast majority of humanity will *eventually* live, and so you want them to be *good* places to live. Turning them into a vast armada of orbital Cabrini Greens and Pruitt Igoes? No thanks. Less "romanticism" than "how do we keep the humans from turning feral?"