[amsat-bb] Re: Satellite Thermal Lesson

[Bill Ress]

Hi Bob,

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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