[amsat-bb] Re: Satellite Thermal Lesson

Edward Cole kl7uw at acsalaska.net
Wed Nov 18 08:55:20 PST 2009

At 05:46 AM 11/18/2009, 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.
>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...
>Sent via AMSAT-BB at amsat.org. Opinions expressed are those of the author.
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Yep, a hard lesson for the uninitiated working in the desert in 
summer: lay those shiny metal tools in the sun for ten minutes and 
you better wear gloves to pick em up.  They get extremely hot and 
will burn skin.  We always took care to lay the tools in shade or 
cover them with a cloth.

Regarding painting dishes white, all the dishes at Goldstone were 
painted white.  What you may find interesting was that receive 
waveguide was painted white while transmit waveguide was 
flat-black.  The heat buildup in the transmit waveguide required 
water-cooling by silver soldering cooling tubes to the surface of the 
waveguide.  I suppose the black color aided black-body radiation.

73, Ed - KL7UW, WD2XSH/45
  BP40IQ   500 KHz - 10-GHz   www.kl7uw.com
500-KHz/CW, 144-MHz EME, 1296-MHz EME
DUBUS Magazine USA Rep dubususa at hotmail.com

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