Yes, that is exactly the temperatures of PCSAT in similar eclipse periods.
IE, anything more or less uniformly BLACK (solar panels) no matter the shape and size will assume that average temperature with 35% eclipses. This is because the absorbtivity and emissivity of "black" are both 0.9. (assuming there is some thermal communication within the spacecraft to move the heat evenly (such as an aluminum frame)...
If it ever gets into a 0% eclipse period (full sun) those average temps will rise to about 30-40C. Still safe for most electronics.
Bob, WB4APR
On Sun, Jul 20, 2014 at 11:09 AM, Graham Shirville < g.shirville@btinternet.com> 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.
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: g0mrf@aol.com Sent: Sunday, July 20, 2014 3:41 PM To: karn@ka9q.net ; amsat-bb@amsat.org ; bruninga@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 karn@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 _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb