Satgen505  Inertial Stabilization  by GM4IHJ  (BID SGEN505) 1998-11-28

The property of matter we call INERTIA, is that which causes it to
continue in its existing state of rest , or, uniform motion in a
straight line , unless that state is changed by an external force.
Sounds perfect for heavy wheel rotating at high speed keeping
satellites pointing in the right direction. But there are snags.

In practice satellite stabilization takes 3 possible forms:-
a. Spin stabilization . Where by the satellite is spun at between 10
   to 30 rpm.
b. Gravity Gradient stabilization . Using a large weight attached to 
   the satellite by a length of line.
c. Inertial Stabilization . Using heavy wheels rotating at high speed 
   Often with 3 wheels, one each axis, providing 3 axis stabilization.

Spin stabilization has been used very successfully on Amsats Oscars 10
and 13, while gravity gradient weights have stabilized several Uosats.
Both systems working well for years because they were simple , and
allowed fixed magneto torquing coils operating against the earths
magnetic field , to be used to point the satellite as required.

Inertial stabization is much more precise than the two simpler systems
but it is far less reliable in the long term. The problems are that
the very heavy high speed wheels need strong electric drive motors
fitted in the wheels. These motors have a great many turns of thin
wire which does not react well to the large mechanical stresses which
are sometimes present when the system is trying to stabilize against
large disturbing forces. These mechanical stresses also affect the
wheel bearings after a time . A situation usually first revealed, as
wheel current levels start to rise.

Inertial wheel problems were well known to NASA/JPL engineers before
the Hubble Space Telescope was launched. Noting that the Solar Maximum
Mission satellite had lost all but one of its wheels before NASA could
refurbish it. Unfortunately however limited time prevented the
manufacture amnd fitting of a solid state design of stabilizer before
Hubble was launched.

What has followed has been exciting or worrying depending on your
outlook. Long before Hubbles first in space refit was due , the
duplicate set of wheels was defective, and with one more failure of
its six wheel Hubble would have had to be shut down. Fortunately
replacements were fitted in time. But here we are 2 years on and
Hubble is losing stabilizing wheels fast, and there is concern that it
may have to be shut down for a time until its next in space
maintainance period, which is some time away. 

Like Hubble , Mir has also been breaking stabilization wheels too
often for comfort . But at least there, there was uplift of
replacements and a permanent crew to fit them. Several Amsats use
Inertial wheels (P3D, Techsat, Sunsat). Although Sunsat has also got
gravity gradient stabilization and will only use the inertial wheel
when imaging.