[amsat-bb] Re: AO-51 magnet reversal tonight
Alan P. Biddle
APBIDDLE at UNITED.NET
Thu Dec 3 15:17:00 PST 2009
According to my prelaunch Echo manual, the S/C has black and silver kapton
tape along the edges to impart spin. The documentation mentions that the
earlier microsats used painted antenna elements to achieve the same result.
Given enough time, the equilibrium rotation rate will be the result of a
balance of the torque from the tapes, and "drag" caused by the internal eddy
currents. I am not aware of any data for AO-51, but normally spacecraft
passively stabilize in a matter of days, weeks, or rarely months. Since the
spin rate is falling slowly, it suggests, but does not prove, that the cause
is a decrease in the applied torque from the kapton tapes.
Kepton is widely used in S/C, but is subject to deterioration due to UV and
particularly atomic oxygen. Going back to the Long Duration Exposure
Facility and other tests in the 1980s, it was found that it and similar
"soft" materials had significant changes in their optical and mechanical
properties. Atomic oxygen levels have the usual exponential dependence on
altitude, plus a factor of 10-1000 variation due to solar and magnetosphere
conditions. Effects on kapton by atomic oxygen were of considerable
interests to shuttle, ISS, and HST designers, though the reason was
primarily optical and contamination. They of course have active pointing.
It would be interesting to know from the Olde Tymers whether similar changes
were found in much earlier LEO S/C, and how other LEO S/C using the same
system as AO-51 have faired recently.
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