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 _______________________________________________ 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
