


On Sun, 24 Nov 2002, fr5cy wrote:

> The sat's spin is stop or lower by a magetic system . It's call
> "magetotorqeur " , how is this mecanic  system called ?
> Where I could find any papers , URL or any bibliography about it ?....

Yes, a magnetorquer is an "active" system where you pulse a magnetic coil
to generate forces on the satellite to change its attitude.  But if you
only want to "lower" its spin, then any satellite made out of a
conductive material (metal) will have a force to slow it down.  The
spinning conductor in the earths magnetic field will generate an opposite
torque (from eddy currents) to tend to slow it down.  But it can not go to
zero because if there is no spin, then there is no torque.

On PCsat, we can see this effect.  PCsat has black/white painted antennas
about its "equator".  This causes a spin of about 0.5 RPM during average
eclipse periods of 25%.  But when PCsat precesses into a full-sun period
for 2 weeks, this spin goes up to 0.8 RPM.  What is interesting to see is
this 0.8 RPM decay back down to 0.5 RPM in about a week when it goes
back into eclipses.  This is the result of the magnetic eddy currents
noted above.

Also, since the radiometric torque to spin-up PCsat could spin-it up
forever to very high speeds, there must be something to limit the final
speed.  That limit is the eddy current torque mentioned above.  The
spacecraft spins up until the Solar Torque equals the eddy-current
counter-torque and that becomes its steady-state rotation speed.
In our case, about 0.8 RPM in full sun...

This does not answer your question about magnetorquers.  (Id like to learn
more about them too), but it does say something about your original
question about "slowing" it down.

 Hope that helps..

de WB4APR@amsat.org, Bob

CUBESAT Designs     http://www.ew.usna.edu/~bruninga/cubesat.html
APRS SATELLITES     http://www.ew.usna.edu/~bruninga/astars.html
