[amsat-bb] Re: Satellite Thermal Lesson
g.shirville at btinternet.com
Wed Nov 18 11:46:42 PST 2009
As it happens we have included a material science experiment/demonstration
into the requirements for the educational payload of FUNcube. It will
actually measure the temperature of two pieces of aluminium facing the same
direction in space.
We had already concluded to have a black anodised finish on one and a silver
finish on the other but we will now ensure that a final careful cleaning
process is undertaken just before the satellite is placed in the deployer.
A very timely "heads up" for us.
----- Original Message -----
From: "Robert Bruninga" <bruninga at usna.edu>
To: <amsat-bb at amsat.org>
Sent: Wednesday, November 18, 2009 2:34 PM
Subject: [amsat-bb] Satellite Thermal Lesson
> 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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