[amsat-bb] Re: Satellite Thermal Lesson

[Bruce]

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.

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 (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...