Drew posted
On the 2242UTC pass of AO-51 tonight, we will try for the first time to reverse the magnet and "flip" the orientation of the satellite. While we do this, the 435.300 transmitter will be shut off for about 10 minutes. If the reversal is successful, we should see some changes in signal levels and patterns. This particular maneuver has not attempted since launch.
I'd appreciate any signal reports or observations from this evening and the next few days be emailed to ao51-modes@amsat.org or ko4ma@amsat.org.
I'm really glad we are finally testing this AO-51 experiment! Thanks for listening to my request. I'm most interested in seeing the results of this test, but I don't see the ao51-modes reports, so please keep me posted offline.
Since angular momentum must be conserved when you "flip" the s/c. The s/c body should precess (probably looking like tumbling) until the momentum dampers absorb the change. Keep an eye on the solar panel currents and I'll bet the spin rate will change.
Concerning your other note:
In related news, as the spacecraft spin inexplicably continues to slow, we are seeing increasingly larger dips in the spacecraft voltage when the panels are poorly aligned to the sun.
My guess is that the momentum loss is due to eddy currents induced in the s/c body by the earth's magnetic field. Sometime drop a piece of (non-magnetic) aluminum into the poles of a big magnet -- you'll see the aluminum change speed noticeably. Back in the antique days of mechanical speedometers in cars, the speedo cable turned a permanent magnet above an aluminum or brass disk connected to the indicator. As the magnet spun faster, eddy currents would drag the disk/pointer to higher values.
There are some good treatments of eddy currents& angular moment on YouTube at
http://www.youtube.com/watch?v=39r8moW_p-w , http://www.youtube.com/watch?v=WZ-3pxKdT8Q , http://www.youtube.com/watch?v=jmfKmUq2EI8&feature=related , http://www.youtube.com/watch?v=H3xqH1arESA&NR=1 and http://www.youtube.com/watch?v=brNBVDCeECg&feature=related .
In these clips, note that moving brass or aluminum items (acting just like the satellite body) "feel" the static magnetic (like the earth's magnetic field). For some (boring) physics theory, follow the links at http://en.wikipedia.org/wiki/Eddy_current.
73 de Tom, K3IO
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.
Alan WA4SCA
participants (2)
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Alan P. Biddle
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Tom Clark, K3IO