[amsat-bb] Re: Satellite Thermal Lesson
bruninga at usna.edu
Wed Nov 18 10:25:14 PST 2009
The huge difference in Aluminum, White and Black will only be
seen in space where there is only radiation heat transfer and no
Conduction and no convection. On Earth, in air, convection
cooling dominates. These numbers only apply to Radiation heat
Black absorbs 90% of sunlight and emits 90% heat - nice
White absorbs 30% of sunlight and emits 90% heat - cool
Al... Absorbs 30% of sunlight and emits 3% heat - gets hot!
BUT, if you have air, and especially moving air, then anything
can get rid of most of its heat to the air which is a larger
Second Lesson: BUT "natural convection" which we all know cools
things... depends on gravity! Remove gravity, and there is no
natural convection... That is why you have to wear earplugs on
the Space Station... Because every little thing has to have a
FAN to move the air. Without it, things and the air around them
just get hotter and hotter...
> -----Original Message-----
> From: Idle-Tyme [mailto:nss at mwt.net]
> Sent: Wednesday, November 18, 2009 11:33 AM
> To: kk5do at amsat.org
> Cc: bruninga at usna.edu; amsat-bb at amsat.org
> Subject: Re: [amsat-bb] Re: Satellite Thermal Lesson
> Morning Bruce and everyone.
> This really amazes me. Now is these spec's below for in a
> vacuum of space? Or on the earth's surface with Air.
> Reason asking is, if this is indeed true, then why does
> every solar heat panel be painted black?
> Not air panels by the numbers given below an Air panel being
> made black is near perfect it re radiates almost everything
> it absorbs.
> But a closed loop liquid system where tubing is in the panel
> with fins attached to gather the energy. absorb it, and let
> the liquid take the heat away.
> Every one of these are also painted black.
> in this case wouldn't it be better to leave it bare aluminum?
> for it absorbs it but doesn't re radiate it away so it has
> more efficiency of getting the heat into the liquid.
> Joe WB9SBD
> The Original Rolling Ball Clock
> Bruce wrote:
> On 11/18/2009 8:34 AM, 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
> This was sent by Bob on April 26, 1996. I found it
> interesting and kept it.
> KEEPING ELECTRONICS COOL IN THE SUN.
> WHile building a GPS unit for mounting on my dashboard
> and noting the
> comming summer months, I looked up the difference in
> absorption and
> emissivity for Aluminum, Black paint, and white paint.
> Satellite builders
> are well aware of these facts, but many of us
> landlubbers are not.
> ALUMINUM will get 30 TIMES hotter than WHITE paint!
> (in a vacuum)
> The following table is for a vacuum and accounts for
> RADIATIVE effects. It
> does not account for convective or conductive cooling
> Absorbtion Emissivity Ratio Temp C
> ALUMINUM .4 .03 11:1 400
> STEEL .6 .4 3:2 150
> BLACK PAINT .9 .9 1:1 110
> WHITE PAINT .25 .85 1:3 72
> Most people are aware that Black gets hotter than
> white, but the fact that
> bright, reflective, shinny Aluminum gets 10 times
> hotter than BLACK is a
> surprise to most people...
> So, if it sits in the sun, paint it white! If you dont
> believe this, put
> an aluminum baking sheet in the sun. I baked my first
> roof mount GPS
> stand alone tracker thinking that the upside down
> baking pan would reflect
> the sun... WRONG! Painted it white and it is now as
> cool as a cucumber.
> The difference in Aluminum is the POOR EMISSIVITY at
> infrared. It can't
> radiate the heat away...
> Sent via AMSAT-BB at amsat.org. Opinions expressed are
> those of the author.
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