[amsat-bb] ANS-199 AMSAT News Service Special Bulletin - AMSAT Fox-1C Launch Opportunity Announced
g0mrf at aol.com
g0mrf at aol.com
Sun Jul 20 14:41:58 UTC 2014
I must quickly point out some real data:
Which shows an equilibrium of around +20 degrees after 64 minutes of sunlight.
Black solar cells on a black surface but some polished Aluminium in the structure.
During eclipse, The Earth facing side begins to increase in temperature at around -16 degrees, but then cools down rapidly as the cube rotates. The temperature is still heading down rapidly as it exits eclipse after 34 minutes and at around -24C on the outside surfaces.
From: Phil Karn <karn at ka9q.net>
To: amsat-bb <amsat-bb at amsat.org>
Sent: Sun, 20 Jul 2014 11:59
Subject: Re: [amsat-bb] ANS-199 AMSAT News Service Special Bulletin - AMSAT Fox-1C Launch Opportunity Announced
On 07/19/2014 09:23 PM, Robert Bruninga wrote:
> I cannot believe that. The equilibrium of a nominally black (solar panels
> on all sides) spacecraft is something like about 0 to 30 C (32F to 90F) a
> very benign operational range. The only time you DO have thermal issues is
> when you DO have attitude control and have things that are not equally over
> time seeing the sun and dark sky.
See Dick's paper for the details; I'm just quoting his results. I know
the basic physics of heat transfer in space but I would never call
myself an expert. He is.
But I can do a back-of-the-envelope calculation that tells me he's right.
The solar cells they're using have an absorptivity and emissivity that
is both 0.98, as I recall, so a cubesat covered with them is essentially
a perfect blackbody.
A blackbody cube with one face normal to the sun at 1 AU will reach an
equilibrium temperature of -21.35 C. The problem is that the ratio of
radiating area to absorbing area for a cube is 6:1 (with the sun normal
to one surface). A sphere would be warmer because its ratio of radiating
to absorbing area is only 4:1. A thin flat plate normal to the sun (like
a solar wing) would be even warmer -- 2:1.
And that -21.35 C figure is for continuous sunlight. Throw in eclipses
and things get much worse. Yes, it would be a little better when the sun
shines on a corner rather than normal to a face, and Earth albedo and IR
radiation will warm things a little, but not enough to matter.
PS: Temperature of 10 cm blackbody cube at 1 AU:
Area facing sun: .01 m^2
Solar constant: 1367.5 W/m^2
Absorbed power = 13.675 W
Total radiating area: .06 m^2
Emissivity = 1.0 (perfect blackbody)
Stefan-Boltzmann constant = 5.6703e-8 W/(m^2K^4)
T = (13.675 W / (5.6703e-8 * 1.0 * .06)) ** (1/4)
= 251.8K == -21.35 C
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