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[thobal]

I read an interesting idea where in you mine out a stoney asteroid, pack it full of ice, then focus big solar mirrors on it until the rock turns to putty. The ice melts/vaporizes and the entire thing inflates like a balloon. At the appropriate moment, you've got these big pressure seals that pop open and 90% of the water/water vapor blows out into space(they didnt say at which point you "turn the heat off" but I imagine it would be a bit before this). So you're left with a big hollow asteroid filled with a fair amount of water. Then you install a big docking bay on one end, pump it full of air and set the thing spinning.

Boom, giant kickass space hab.

[mainiac]

Friction doesn't work that way.  The station as a whole keeps it's angular velocity so it returns to its configuration if you reset the position of the dock.  The only change possible is when a craft docks with the station.  That would be a tiny change and could be offset when the craft launches.

No, he's on about the central part remaining stationary i.e. no rotation whilst the rest of the station rotates. There would have to be at least one part of the non-rotating part of the station touching the rotating part, leading to friction between the non-rotating part and the rotating part. This would lead to either:
A) The stationary part rotating with the station or
B) the rotating part of the station becoming stationary

So you'd have to keep expending energy preventing the friction from counteracting what the station is doing. You could have the central part rotate with the station until a ship gets close, but that only minimises the friction, it doesn't remove it.
[/quote]

Any friction would exert an equal and opposite force on the docking ring and the rest of the station.  So the angular velocity and momentum of the station as a whole would remain unchanged.  Likewise when you accelerate the docking ring to get it to slow down to stationary (i.e. accelerate it in the direction opposite station rotation) an opposite force is applied to the rest of the station so the station as a whole maintains its kinetic energy.

There is an even simpler way to understand this though.  Just apply Newtons first law, a body in motion stays in motion without an outside force.  The station as a whole is a body.  So the station as a whole is not going to have it's motion changed unless an outside force acts on it.  Internal forces can act on the station until the cows come home, that wont change the overall energy of the station.  So if you slow down the station slightly by walking in the direction of the rotation that only lasts as long as you are walking.  As soon as you stop walking, the station returns to it's original speed. 

I guess that means there would be tiny fluctuations in the speed of rotation throughout the day but even together they would be too small to measure because there is no net change over time.  The only noticeable changes would be if a ship docks with the station, giving an outside force.  But that outside force could be very carefully measured before it even happens so by planning launches and landings you could maintain the station's spin.

[thobal]

And dont forget, as long as we're talking about Newton,any attempt by the stationary part to keep the spin of the spinning part going will set the stationary part spinning in the opposite direction.

[andrea]

the station as a whole maintains its kinetic energy.

That is wonderful. But I see a problem. Friction isn't conservative, which means that part of the energy involved is dissipated into heat. While overall energy doesn't change, kinetic energy does. the difference being the energy released as heat.
Unless you can convert that heat back into rotation, friction will slow down the station.
Unless I am missing something.

[mainiac]

That is wonderful. But I see a problem. Friction isn't conservative, which means that part of the energy involved is dissipated into heat. While overall energy doesn't change, kinetic energy does. the difference being the energy released as heat.
Unless you can convert that heat back into rotation, friction will slow down the station.
Unless I am missing something.

Ok, you are right, I wasn't thinking the energy thing through correctly.  The kinetic energy is lost because the angular velocities of the docking ring and the station are brought more in line with each other.  In this process momentum is conserved but kinetic energy is not.  So an electric motor could reintroduce the lost kinetic energy by restoring the docking ring and the station to their original speeds relative to each other.  Momentum is conserved in this process again.

So kinetic energy would be lost but it's energy that could be replaced without launch propellant.  Still, that's a good catch. 

[andrea]

I half hoped to be wrong. conservative friction would be awesome.

to bring back a topic from a page or 2 ago, I would be very interested to see what are the effects of 0g and vacuum on properties of metals ( such as better or worse mechanical qualities due to different solidification). Has anyone ever done experiments in this field? or at least proposed theories?
I wonder how small could an experiment be, while still being meaningful. small enough, and it would be feasible to launch it to ISS or something. ( although I guess waiting list is huge).
if fast enough, even freefall could be a good substitute for a 0g environment.

edit: although alloy making would be most interesting too, but testing it would make the experiment more complex, and different alloys may behave differently.
I should ask my professor if he knows anything about this stuff, when the break ends.

[mainiac]

I must admit I don't know the specifics of this.  Poking around the internet a little it seems that it's important to keep oxygen out of the process (because you can't make slag float to the top without introducing artificial gravity) and that you can produce interesting crystal structures from the expansion or through the introduction of inert gas for bubbling.

It seems that the lunar fan community operates on the assumption that you want pure carbon since this site lists methods to extract it from the common forms it's found on the moon.

[andrea]

well, keeping oxygen out is always one of the most important things when smelting, perhaps second only to actually picking the right ore.
no gravity takes away slag, that is true, but 0g environments would usually be found in vacuum, which elegantly solves the oxydation problem by removing any oxygen.
Lack of oxygen alone would be a quite big advantage to have.

slower cooling time however may pose a problem, but nothing that can be solved with a cooling system. still a downside, however.

[mainiac]

I thought smelting was done with CO2 typically.  I was trying to say that a substitute for CO2 would need to be produced.  Is CO2 not used in smelting?

[Aseaheru]

Carbon dioxide?
its a byproduct...


Im in favor of getting humans off earth, as long as we dont use up earth to do so.
and the biggest problem is food, water and O2.
O2 and food are dealt with via hypodermics or other systems and water can be recycled in a closed environment.

[MonkeyHead]

IIRC crystalline solids form with larger crystal dimensions under zero-g for some reason. So, applying this to the polycrystalline nature of metals might give harder, stronger but less plastic metals that underwent less ductile flow and more brittle fracture. Maybe.

[andrea]

right, right. I said smelting, but I was thinking about casting. vacuum should be great for casting, as far as I know. in a few weeks I should be able to give a better answer on that, but I don't recall anything about gases used for anything that isn't protection.

Smelting... depends. I only studied smelting of steel, and there carbon is required. Usual way is to fill the blast furnace with both iron and coke, and let chemical reactions do their magic, both deoxidizing iron ( since iron ores are oxides) and heating it to melting point ( melting posint of the resulting high carbon alloy, mind you.). This process involves mainly CO and also CO2 generated by oxidation of coke.
Furthermore, to actually make steel rather than pig iron, you need to blow air or pure oxygen into it to release carbon as gas. ( and heat the stuff to the melting point of the new, lower carbon, alloy).

you would need quite a bit of oxygen for both steps, if done this way. but nno gravity would mess with convection, so we would probably need an entirely new way.

[Aseaheru]

there wont be all that much materials on mars...
due to lack of tectonics...

[MonkeyHead]

there wont be all that much materials on mars...
due to lack of tectonics...

The soil has magnesium, sodium, potassium and chlorine. Lots of Olivine too, wihch is Fe and Mg based. Dont forget small amounts of Magnetite, in various forms. A lack of tectonics now doesnt mean a lack of tectonics before, and geologic time is very long.

http://en.wikipedia.org/wiki/Composition_of_Mars is useful reading.

[Aseaheru]

all right. i meant that the deposits are fairly small.
where they exist...