[amsat-bb] Re: Satellite Thermal Lesson

Idle-Tyme nss at mwt.net
Wed Nov 18 08:33:06 PST 2009

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.



*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
> snip...
> This was sent by Bob on April 26, 1996. I found it interesting and kept it.
> 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 (air)..
>              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...
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