[amsat-bb] ANS-199 AMSAT News Service Special Bulletin - AMSAT Fox-1C Launch Opportunity Announced
karn at ka9q.net
Mon Jul 21 03:59:14 UTC 2014
On 07/20/2014 08:09 AM, Graham Shirville wrote:
> 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.
You must be using different solar cells, or perhaps you don't fully
cover the exterior with them. I did wonder why Dick didn't consider the
same thing for Fox-1; if you're going to dissipate some of the
electricity they generate in resistance heaters, you might consider
covering less than 100% of the surface with them and cover the remainder
with thermal blankets. I haven't worked out this alternative, and as I
said I'm not a thermal design expert.
At the time I believe Dick said he was considering gold plating the
inside surfaces of the cells to isolate them from the interior, since
there was little or no room for any other kind of insulation. In that
case you would no longer have thermal equilibrium and my back of the
envelope calculation would not apply.
There's one additional factor to make things worse, though I haven't
quantified it. Space-rated solar cells are getting good enough that the
spacecraft as a whole actually converts a non-insignificant amount of
the sunlight hitting it into transmitted RF, and that power is no longer
available to keep the spacecraft warm. So it gets even colder.
That's right, turn on the transmitter and the spacecraft cools down --
at least if the solar cells are not thermally isolated from the interior.
> Our orbit is sun synchronous so we "suffer" eclipses for approx 33% of
> the orbit ..but then we are relatively close to the earth!
Yes, and you do pick up longwave IR from the earth even on the night
side. According to my copy of "Spacecraft Thermal Control Handbook Vol
1", this is only about 150 W/m^2 in LEO, so it doesn't seem to help that
much. Albedo is greater (about 250 W/m^2) but only over the subsolar
point so again it doesn't help much on average.
It *would* help a lot if you could insulate those sides facing dark sky,
and that's where attitude control comes in.
During his after-dinner talk at the AMSAT Symposium a couple of years
ago, astronaut Sam Durrance described just how cold that dark sky is. He
flew with Ron Parise on the Astro-1 and -2 shuttle missions. Because
these were astronomy missions, the shuttle payload bay spent long
periods pointed at dark sky, as opposed to its usual practice (when not
docked to the ISS) of keeping the payload bay toward earth. He said it
got so cold near the orbiter's overhead windows that they had to don
But the real issue that Dick drove home to me in his talk was just how
variable the thermal situation was when you can't control your attitude
and when eclipse durations and beta angles vary so drastically over a
year. Even if you could design for acceptable equilibrium temperatures
under one set of conditions you can't maintain them as they vary so much.
> I would also comment that any active attitude control system will
> consume power...which we don't have much of..
Yes, but active attitude control lets you keep those panels pointed at
the sun to produce much more power on average. Only two of the six
surfaces of a 1U cubesat even need solar cells with my stabilized
design. You can do much better with deployable, steerable panels --
which pretty much demands active attitude control.
The fact that virtually every military, commercial and scientific
spacecraft launched today is three-axis stabilized should settle the
question: active attitude control is the way to go, if you can do it.
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