[amsat-bb] Re: Satellite Thermal Lesson
bill at hsmicrowave.com
Wed Nov 18 08:52:42 PST 2009
Very interesting work. I appreciate you sharing the results. This issue
of thermal stabilization has been a very interesting "art" although some
will say it's a science. Your aluminum cleanliness find goes a long way
to helping me understand it as science.
Regards...Bill - N6GHz
Robert Bruninga wrote:
> Lesson learned on Satellite Thermal.
> For years, we have been trying to demonstrate to students the
> extreme differences in Temperature of a satellite based simply
> on its color. In space, far from earth, here is what you should
> get for three identical satellites:
> Black will be about +55 deg F
> White will be about -60 deg F
> Aluminum will be about +225 deg F
> (from memory anyway)... But we have never been able to see the
> extreme temperature of the aluminum in our vacuum chamber. Of
> course, we are not using a SUN, but an incandescent lamp which
> has 95% of its radiation as heat and only 5% as light, so we
> attribtuted our wishy-washy results as due to the lack of real
> solar spectrum.
> This year, we finally have a Tvac chamber that has a liguid
> nitrogen cold plate so we can actually better simulate the
> blackness (and cold) of space.. Again, same result. Aluminum
> was about the same as black. This was a frustrating result from
> this new chamber.
> Then I noticed the fingerprints on the aluminum. I cleaned the
> aluminum with a swab of alcohol to eliminate all the surface
> contaminants, and fingerprints and re-did the test...
> BINGO. Now the temperature of the aluminum goes up and off
> scale HOT, way way different from the black or White. And now
> the WHITE also goes colder..
> So just the thinneest innvisible layer of surface contamination
> completely changed the thermal emissivity of the Aluminum. And
> we all know this anyway, since "low-E" coatings of glass and
> other materials is now so common. It is only the outer surface
> of molecules that set the absorbtivity and emissivity... And
> the differnce between Black and Aluminum is 30-to-1 (if the
> aluminum is clean)... That's why we wrap baked potatos in
> Aluminum foil! (don't use greasy hands)...
> Of course we always clean our actual spacecraft to clean-room
> conditions before flight, so this does not impact our on-orbit
> results, but it sure does make a difference in the lab when we
> are demonstrating absorbtivity and emissivity in the chamber
> with samples handled by the students!
> When I get a chance, Ill post the results...
> Bob, WB4APR
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