[amsat-bb] Satellite Thermal Lesson
bruninga at usna.edu
Wed Nov 18 06:34:18 PST 2009
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
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...
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