[amsat-bb] What is the Mystery Effect?

["Stacey E. Mills" <w4sm@xxxxxxxxxx>]

It's nice to see folks thinking about this again.  AO-40 is currently spin 
stabilized; it's a gyroscope.  When you spin a gyroscope and gently apply 
upward force on one end of the axis,  the axis moves 90 degs to your 
efforts, either left or right depending on the direction of spin.  The spin 
of AO-40 is such that to move the spin (Z) axis "left", decreasing ALON as 
we observe, there must be a force trying to rotate the + end of the  Z-axis 
upward (northward).  If this force originates off-center to the Z-axis,  it 
must be "despun".  That is, as AO-40 rotates, the sum of the force must not 
equal zero.  Any mechanical effect  due to something hanging off the 
satellite is not despun, and the sum of the effect goes to zero during each 
rotation.  Forces can be despun electrically, and this is what is done to 
magnetorque.  Despinning for magnetorquing depends on the sun as a fixed 
reference.  Theories about currents from the solar panels causing the 
mystery effect are intriguing because they are, in effect, despun by the 
sun.  However, the wiring is such that we believe insufficient magnetic 
force can be generated by these currents to cause the effect.  More 
importantly, we currently have perigee eclipses which would completely 
eliminate this effect, and yet the observed mystery effect is completely 
unchanged.

Any net force/field moving through the center of the Z-axis is by 
definition despun.  The momentum wheels contain permanent magnets and the 
fields of these magnets are not completely closed.  We believe that the 
magnets in all three orthogonal wheels are oriented in the same way.  This 
creates a net magnetic field along the Z-axis.  Imagine that the south pole 
of this magnetic field emerges from the +Z axis.  The earth's magnetic 
field will interact with this field, trying to pull it upward to align with 
the earth's north pole.  By gyroscopic effect, this will move the Z-axis 
"leftward" decreasing ALON. ... precisely what we see.

The mystery effect  appears to have changed (decreased) somewhat as we have 
increased perigee, and the amount of change is consistent with an 
interaction with the earth's magnetic field. Althought drag was initially 
considered when the perigee was rather low, our current 1000+ km perigee 
completely excludes that possibility.  If the solar panels were opened we 
would be able to tell by changes in the solar panel current patterns, 
changes in magnetorquing efficiency, changes in center of gravity as seen 
in the photos, changes in temperatures of the panels and objects shaded by 
them, etc.  There is no evidence for any of these things so it is clear 
that the panels remain closed.   (Even if one were open, any effect from 
this would not be "despun" as discussed above.)

If the mystery effect is due to AO-40 having a net Z-axis magnetic field, 
then aligning that field with the earth's magnetic field, should cause the 
magnetic force and resultant mystery effect to disappear.  This is one 
approach we are considering for counteracting the ME during perigee under 
3-axis control.




-- 
  _______________________________________________________________________
  Stacey E. Mills, W4SM    WWW:    http://www.cstone.net/~w4sm/ham1.html
   Charlottesville, VA     PGP key: http://www.cstone.net/~w4sm/key
  _______________________________________________________________________

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