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