[amsat-bb] Re: AO-51 magnet reversal tonight

Tom Clark, K3IO tom.k3io at gmail.com
Thu Dec 3 00:48:08 PST 2009

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 at amsat.org  or ko4ma at 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

73 de Tom, K3IO

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