Hi Phil,
The reality is, even with no battery heater on FUNcube-1 we seem to have an
acceptable battery temperature of between 0 and +5C. The temp sensor is, of
course, actually external to the battery itself.
Our orbit is sun synchronous so we "suffer" eclipses for approx 33% of the
orbit ..but then we are relatively close to the earth!
I would also comment that any active attitude control system will consume
power...which we don't have much of..
Probably, if you need continuous operation of the radio system, then a 2U
with deployable solar panels is the minimum configuration for a CubeSat
operating on microwave bands with an active attitude control system.
best 73
Graham
G3VZV
-----Original Message-----
From: g0...@aol.com
Sent: Sunday, July 20, 2014 3:41 PM
To: k...@ka9q.net ; amsat-bb@amsat.org ; bruni...@usna.edu
Subject: Re: [amsat-bb] ANS-199 AMSAT News Service Special Bulletin - AMSAT
Fox-1C Launch Opportunity Announced
I must quickly point out some real data:
www.warehouse.funcube.org.uk
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.
Thanks
David
-----Original Message-----
From: Phil Karn <k...@ka9q.net>
To: amsat-bb <amsat-bb@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.
--Phil
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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