Author Topic: Asteroids as habitats  (Read 26359 times)

Offline Paul451

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Re: Asteroids as habitats
« Reply #60 on: 01/15/2015 05:15 PM »
No idea if this thread is still being watched by the original participants. But...

Re: Dust/difficulty.

It's interesting that when people criticised the idea of building a base inside the asteroid, their alternative suggestions always contradicted the very reasoning they use to criticise the asteroid-base.

For example, paraphrasing, "tunnelling will be difficult/expensive, and tunnelling will produce dust", therefore... "you should build a stand-alone rotating space-station with shielding made from the asteroid material..."

... which you have to dig off the asteroid, separate out the preferred ore (high hydrogen, low metal), process (shape/sinter) the chosen ore into suitable blocks/panels or bag and seal, and transporting it to the ring-station construction site, presumably using fuel also made from asteroid material (which you have to dig off the asteroid, separate out...) And somehow all that is not only going to be easier and cheaper, but create less dust around the asteroid, than digging a hole - once - and then just living in the hole?

Pretty much by definition, if you can make enough shielding for a rotating station by mining an asteroid, then you've figured out how to dig material off the surface of an asteroid without ruining your own equipment with dust. If so then wouldn't it be even easier to dig one smallish hole, line it, add a dock, and move all further mining/etc, underground. No more dust problem except at the "coalface". Solves your radiation problem instantly, long before you've mined enough material to shield a large space station, long before you've even built the large space station. Plus fewer problems attaching equipment to the "surface" (inside), and fewer problems even attaching equipment to the actual surface (outside) since you have a rapidly deepening "anchor" to attach to.

because a tunnel-boring machine for a 1-km wide hole would probably have a mass of 10^6 tons by order of magnitude.

I think you misread the idea. The tunnels for the ring-hab were not 1km wide. Someone mentioned inflating a 1km bubble, but that's a different concept and doesn't require a TBM (except perhaps to deliver the inflatable.)

(However, since you brought it up, a TBM able to bore a 1km wide tunnel doesn't have to be 1km wide. You can use a regular TBM running in a helical pattern with a 500m radius, separating out an "apple core" 1km wide, even though the actual TBM, and hence the width of the cut, might only be a few tens of metres wide.)

Offline Paul451

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Re: Asteroids as habitats
« Reply #61 on: 01/15/2015 05:30 PM »
(Last one, promise. But it goes on for awhile...)

I wrote about one of the simplest ways to generate artificial gravity - a train track inside an asteroid

Is there a reason you went with a train on tracks? Why not a torus-station spinning in free-fall inside the ring-tunnel? Then your wall-track is merely to keep it centred, which should create much less pressure and vibration grinding at the walls of the tunnel compared to having a multi-tonne train, off-centre, unbalanced.

Your track has to be strong enough to support the entire mass of your train anyway. So why not separate it from the walls and have it spinning with the modules attached. Then the modules don't need heavy rail-bogies able to support the whole weight of the module (in 1-artificial-g) on the track. You'll just need a much lighter guide system to keep the ring centred.

You also don't need the habs to immediately fill the entire ring, you just need any modules balanced by a counterweight. (Nods towards the trillion tonne asteroid to hint where you can find free counterweight mass even at the earliest stages of development.) That way, the big stuff in your very first construction (ring-spine and guide-track) is dumb bulk material, while the complicated stuff (habs) don't have to be the equivalent of heavy rail cars. (The first ring-spine might be a clever/expensive composite truss, to make it as light and low volume for transport from Earth. But it's still easier to engineer than the habs, so off-loading the loading to that bulk-structure makes more sense than adding that complexity to the habs.) You still get all the benefits of radiation and solar shielding, thermal uniformity, etc. And without the bogies, the modules can initially be simple inflatables attached to the spinning ring, until your IRSU is capable of building new habs.

Speaking of thermal issues. Have you considered how you are going to cool the damn thing? I mean it benefits from not being in direct sunlight, and the asteroid has a huge thermal mass so the tunnel walls will radiate a uniform (IR) temperature. But eventually the habs will warm the tunnel walls and will be reabsorbing their own reflected heat. You'll need to connect it to radiators somehow.

(Obvious solution, pressurise the tunnel not the modules, circulate the gas in the tunnels through external radiators. But, even though you're only moving at 60kmh relative velocity, that leads to issues with drag, hence energy costs, plus a more rapid altering of the asteroid spin. And the solution to the latter (more rings, counter-rotating), increases the energy cost further. Still, unpressurised modules is a nice bonus.)

Speaking of energy. Another useful thing about asteroid habs, you can shove a nuclear reactor a few tens of metres underground, but well away from the base, and not only is the hab/work-area shielded by the asteroid bulk, but even the reactor's external radiators are shielded simply from having the reactor itself 20-30m beneath the ground under them. Not only reduces degradation of the radiators, but allows astronauts/vehicles to work around them freely without worrying about their own position relative to a reactor-shield shadow.

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Additional observation: QuantumG's proposal is a 894m radius, that's simply the minimum needed to produce 1g at 1RPM. But since radius is proportional to the square of the rotational period, increasing the RPM gives an exponential reduction in radius. Experiments suggest people can probably handle 2-3 RPM without difficulty. And we can expect the earliest workers to be trained astronauts, selected against motion sickness and experienced in the weirdness of micro-g; many of them should be able to handle 4-6 RPM.

At 3 RPM, your radius is reduced to just 100 metres for 1g. At 6 RPM, it's down to just 25 metres. At 4RPM and 0.2g, your radius is just 11m. At 6RPM and 0.1g, radius is just 2.5m  If we ever do this, it's worth exploring higher RPMs, the potential savings are enormous.

(At 6PRM and 0.1g, your 5m diameter fits entirely within a BA-330 inflatable module. At 4g, 0.2g, rotational velocity is just 16kmh and the 22m diameter is small enough to carve out the entire disk. Line the cavern and pressurise it, and have an unpressurised module and counterweight on a pair of simple rotating arms, with the hub simply bolted to the walls. BTW, I'm not suggesting these are optimal sizes, just using them to illustrate the size of the effect.)