Cooling for Satellites.

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dic_penderyn
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Cooling for Satellites.

Postby dic_penderyn » Wed Nov 19, 2008 9:09 am UTC

How do Satellites manage to cool the instruments and equipment on board?
Given that the satellite is in a vacuum what is the method of heat transfer to the outside?
I am sure there is a simple solution that I haven't thought of but I am obviously missing something.
If the temperature was high enough then a simple heat dump radiator would work because the heat could be carried off via radiation.
What about temperatures that are not high enough for this process?
Do they use Peltier pumps to increase the temperature of a radiator panel?
Apologies if this has already been answered, I did a quick search without finding any previous posts.

Thanks

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Zamfir
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Re: Cooling for Satellites.

Postby Zamfir » Wed Nov 19, 2008 2:33 pm UTC

There is nothing simple about it, heat management, or thermal control, is a huge issue in satellite design.

Basically, every surface of the satellite absorbs some portion of the external radiation falling on it, and emits radiation, roughly like a black body. Surface materials have different absorbtance of the infalling radiation (high for black, low for white and metals), and different emmittance compared to a pure black-body radiator ( low for metal, high for plastic).
So if you know how your satellite is oriented to the sun or other radiating bodies, you can play around with those properties to get surfaces that are good at attracting energy from the sun, or are good at radiating energy away.

Usually, heat-generating components are connected by heat conducting elements to a radiator. In larger systems, there will be an active system that pumps a fluid around. For very temperature sensitive instruments, like a thermal sensor, their might be a reservoir of liquid nitrogen as coolant.

dic_penderyn
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Re: Cooling for Satellites.

Postby dic_penderyn » Wed Nov 19, 2008 11:15 pm UTC

Thanks for the reply zamfir.
The main problem I have in understanding this, is how one would get the heat from the satellite to be lost to space (vacuum).
What is the mechanism for radiating the heat from the satellite?
A standard radiator is pretty useless because the vacuum would not allow heat transfer. The only way I can think of getting the heat out is to use peltier's or groups of peltier's in order to get the radiator plate sufficiently hot so that the radiator actually gives off radiant heat.

Am I way off in thinking this?

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Mr. Beck
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Re: Cooling for Satellites.

Postby Mr. Beck » Wed Nov 19, 2008 11:52 pm UTC

Yes. All object that are above absolute zero lose energy via blackbody radiation. Even in a perfect vacuum, heat is still radiated- for "normal" temperatures, this radiation in the infrared range. This is how Thermal Cameras work- they measure the frequnecy of emitted EM and use it to determine the radiating object's temperature.

In the case of a satellite, Peltiers or Heat Pipes carry the heat away from hot places and dump it onto a flat, metal radiator hanging off the side of the craft. Then, the heat becomes infared EM and flies off into space.

asad137
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Re: Cooling for Satellites.

Postby asad137 » Thu Nov 20, 2008 2:18 am UTC

Yep, all these guys are correct. Photons have no problem traveling through vacuum, and a radiator for a spacecraft is just some area with high emissivity used to emit hot photons into the universe.

Mr. Beck wrote:This is how Thermal Cameras work- they measure the frequnecy of emitted EM and use it to determine the radiating object's temperature.


Well, not exactly -- they are just cameras that are sensitive to a different band of frequencies than normal cameras, but they don't actually measure the frequency that enters camera, it just cares whether or not it's in that band (yes, pedantic, but an important distinction, as something that measures frequency/wavelength would likely be called a spectrometer).

Asad

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Mr. Beck
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Re: Cooling for Satellites.

Postby Mr. Beck » Thu Nov 20, 2008 3:20 am UTC

asad137 wrote:Yep, all these guys are correct. Photons have no problem traveling through vacuum, and a radiator for a spacecraft is just some area with high emissivity used to emit hot photons into the universe.

Mr. Beck wrote:This is how Thermal Cameras work- they measure the frequnecy of emitted EM and use it to determine the radiating object's temperature.


Well, not exactly -- they are just cameras that are sensitive to a different band of frequencies than normal cameras, but they don't actually measure the frequency that enters camera, it just cares whether or not it's in that band (yes, pedantic, but an important distinction, as something that measures frequency/wavelength would likely be called a spectrometer).

Asad

Ah, my bad. You're right of courses- somehow when I posted I thought for a minute that coloring was actually related to frequency, not intensity.

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evilbeanfiend
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Re: Cooling for Satellites.

Postby evilbeanfiend » Thu Nov 20, 2008 7:22 am UTC

just to add a few things. due to the restrictive heat management of satellites then the design of instruments can often be power limited.
as said heat pipes are used to transfer heat around the craft (at least in non air tight craft, i imagine craft with human payloads can at least partly use convection in the air) and ultimately all the heat is radiated from the spacecraft. luckily this is relatively efficient in space compared to on earth as you can radiate towards 4K space which means your radiator isn't absorbing much. lastly reflection is also important given that most of the surface may be bathed in sunlight at some point most of the craft is highly reflective in order to be a poor absorber (unfortunately this means its a poor emitter too).
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