Hot or cold?

[atpollard]

Originally posted by Malenfant:
I'm a bit rusty on this side of physics, but vacuum isn't an insulator - it's actually fantastic at removing heat by radiation but just sucky at conduction and convection (because there's nothing there to conduct into or material to to convect around).

You don't really need to 'work' to remove heat at all - you just have a radiator that has the hot stuff going through it (either heated directly or having hot water going through pipes) and a lot of surface area to radiate the heat away into space and that cools things down. Though for best results the radiator itself has to be in the shade so it's not itself being heated by the sun.

A thermos bottle, in it's simplest form, is a vacuum space between two thermally conductive spaces and keeps hot foods hot and cold foods cold precisely because it allows heat loss only through radiation and not through conduction and convection. Radiation is much less efficient at removing heat than the other methods.

Radiators are precisely how a contemporary space craft “works” at removing heat. If the radiators on the ISS failed, the occupants would die of heat and not from cold. The people and electronics generate too much heat for human comfort. Survival depends on adding equipment to radiate the excess heat. ... And THAT is ironic given how cold space is (in the shade).

 

[Malenfant]

Originally posted by atpollard:
A thermos bottle, in it's simplest form, is a vacuum space between two thermally conductive spaces and keeps hot foods hot and cold foods cold precisely because it allows heat loss only through radiation and not through conduction and convection. Radiation is much less efficient at removing heat than the other methods.

True, except that thermos flasks are also designed specifically to reduce radiaton as well (by having silvered flasks) - they're good at keeping things hot or cold because efforts are made to prevent all three modes of heat transfer (not sure how well they'd work if the flask wasn't silvered - they probably still would work quite well, but more heat would leak out by radiation).

But then, radiation is how heat is transferred from stars to their planets - the light (and IR) travels through vacuum. So it can't work that badly in a vacuum or we'd be frozen on Earth

.

 

[avi.hecht]

Of the three heat transfer methods (conduction, convection, radiation) radiation is the least efficient by far. The Sun heat on earth 'works' because its outer surface burns in the 5780K range.
'Stefan-Boltzmann law' describes heat transfer in radiating bodies. Power output (W) is in direct relationship to - Area * (Temperature)^4.
So to relay on regular unassisted heat radiation is problematic.
Things just get worse, Traveller technology, without some 'hand waving' Thermodynamics does not really work...

 

[Malenfant]

Of course, Traveller's never really bothered with dealing with excess heat and radiators and so on in their vehicles and spacecraft, and AFAIK no attempt has ever been made in any of the canon to explain where the heat goes. It's just another aspect of the realism debate really.

 

[Anthony]

Originally posted by AviH:
Of the three heat transfer methods (conduction, convection, radiation) radiation is the least efficient by far.

Not precisely. It's the slowest option at low temperatures, but at high temperatures it is quite fast.

 

[atpollard]

If we assume that the sun qualifies as "high temperature", the transfer of heat from the sun to the earth across a vacuum (radiation) would still be slower to bring the bodies into equilibrium, than if we brought the sun and earth into direct contact (conduction).

 

[Chaos]

If you (or the inhabitants of that world) are ready to think in *really* large-scale engineering terms, you (or they) could set up a circulation system all around the planet, continuously pumping some fluid (or gas?) in a circle around it. The city would be part of that circle.

While the city is on the day side, pumping the (cooler) liquid from the day side through it would cool the city; the liquid will then cool down again while flowing across the night side.
While the city is on the night side, pumping the warm liquid coming from the day side through the city will heat the city; the liquid will then be heated again while flowing across the day side.

Depending on the materials and technologies used, this is probably monstrously inefficient, though.

 

[far-trader]

Another Mega-Engineering approach...

I forget where I read the idea originally, but how about a moving city? It keeps pace with the terminus so it's always in a nice comfy twilight as it trundles around the planet. Though a week long "day" may be too fast for a surface mobile city so maybe you'd need a grav city.

 

[Valarian]

Originally posted by far-trader:
I forget where I read the idea originally, but how about a moving city? It keeps pace with the terminus so it's always in a nice comfy twilight as it trundles around the planet.

This is in one of the Star Wars books - Lando Calrissian has a city on the back of AT-AT walker frames on the planet Nkllon. Ships have to come behind shieldships to protect them against the glare of the primary.

I think it's one of the Timothy Zahn books (Heir to the Empire?).

 

[Pickles]

Originally posted by far-trader:
I forget where I read the idea originally, but how about a moving city? It keeps pace with the terminus so it's always in a nice comfy twilight as it trundles around the planet.

There's also one on Mercury in Kim Stanley Robinson's Blue Mars book, kind of a big railway carriage. 'Big' as in city-sized.

 

[far-trader]

Originally posted by Valarian:
</font><blockquote>quote:</font><hr />Originally posted by far-trader:
I forget where I read the idea originally, but how about a moving city? It keeps pace with the terminus so it's always in a nice comfy twilight as it trundles around the planet.

This is in one of the Star Wars books - Lando Calrissian has a city on the back of AT-AT walker frames on the planet Nkllon.</font>[/QUOTE]

Originally posted by the Bromgrev:

There's also one on Mercury in Kim Stanley Robinson's Blue Mars book, kind of a big railway carriage. 'Big' as in city-sized.


It might be the one on Mercury in Robinson's work I was remembering, though I seem to recall it as more a featured setting. Maybe it was a different (golden age I'm thinking) sf story. In any case thanks Valarian and Mike

 

[atpollard]

A NASA article on placing a floating city over Venus mentioned that by orbiting closer to the poles, you could adjust the lenth of the day/night cycle in the city. The same concept (adjusting the latitude to determine the length of day for a moving city) should work in other settings.

 

[Icosahedron]

Originally posted by flykiller:

2) something that precipitates the energy into matter. every week you open up the AC and shovel out the new dust, sort of like cleaning out a coal-fired furnace.

At last - a mechanism for hiding ships in space!
No matter how big your drives, you're only going to create a few invisible milligrams of dust. You wouldn't need much of a shovel using E=mc^2.
TL17-20, possibly?