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( 4 comments — Leave a comment )
kengr
Apr. 15th, 2018 08:59 pm (UTC)
Trouble is, there are only three ways to get rid of heat.

Conduction (ie it moves along/thru a substance). Fastest, but impossible in space because vacuum.

Convection (ie hot air rises, cold air sinks. Applies to many fluids) This is how the fridge gets rid of heat. It pumps the working fluid thru the coils on the back of the fridge, where conduction heats the fins, and convection carries away the heat. Won't work in space because vacuum.

Radiation. Hot surface emits energy (microwave, IR, Light, etc). Microwave for really *cold* temps. IR for cool to very hot, Red at "red-hot" etc.

Works in space, but two problems. First, any portion of the radiated energy that hits another part of the radiator gets reabsorbed. So the radiating surface has to be a flat plate, any other radiators can't be visible from that plate.

That's why the panels on the ISS are all in the same plane, and *only* in one plane.

Second, the amount of energy radiated (in terms of watts per square meter) depends *solely* on the temperature of the radiating surface. and it's a 4th power law. so at twice the (absolute, ie measured from absolute zero) temp, it radiates 16 times the energy.

This is great if you are getting rid of heat from something like a rocket motor or a reactor. At those temps a lot of heat radiates from a fairly small area.

But keep temps low enough for people requires *huge* amounts of radiator area because the watts per sq meter is so low at those temps.

You could use a "heat pump" (that's essentially what a refrigerator is) to raise the temp of the radiators. But that uses energy, which winds up as heat. Which means you need more radiator area to get rid of *that*.

Basically a losing proposition except in special cases.

Oh yeah, you can get rid of heat by evaporation/sublimation. That's how spacesuits do it. But you are using up the water (one of the best substances for evaporative cooling) in the process.

Not suitable for cooling a station or a spaceship.

ps. the cooling lasers in Brin's "Sundiver" are pure handwavium. Heat energy is high entropy. Laser beams are low entropy.

fayanora
Apr. 16th, 2018 01:24 pm (UTC)
"any portion of the radiated energy that hits another part of the radiator gets reabsorbed."

So make the ship a d20 or higher number sided polygon solid. Put a radiator on every side, you've got at least twenty of them, none aimed at each other. They wouldn't be able to aim at each other even if one wanted them to be able to do so. Oh and yeah, minus the docking side and the rocket, or course. But that's till at least 18 sides. More, if you use a larger number of sides for your ship.
kengr
Apr. 16th, 2018 11:39 pm (UTC)
The problem is that heat generated is pretty much volume based, and heat radiation goes by area.

So the bigger the ship/station/whatever, the less surface area (relative to volume) you have.

Volume goes up by the cube of the dimensions, area only goes up by the square. So if it's twice as big, it has 4 times the surface area, but 8 times the volume.

So you have to add extra surface area just for the radiators.

This is why the radiators on things in space are "wings" or "fins".

Look at the ISS. The radiator panels are arranged in a single plane with the station on the other side.
kengr
Apr. 16th, 2018 11:43 pm (UTC)
Oh yeah, the higher the number of sides, the closer you get to being a sphere. And a sphere has the *least* surface area for a given volume of any shape. In effect a sphere is an infinite-sided polyhedron.

Fewer sides are better. Best is a big flat, thin, plate.
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