Satgen190   Satellite Stabilization Pt4   by GM4IHJ             14 Nov 92
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Some satellites require extremely accurate pointing in order to line up
telescopes, lasers, antennas or special sensors. In some cases the
slightest movement of say a tape recorder or a relay. or, an astronaut
moving about, can upset a delicately pointed experiment. Stabilization of
this sort cannot be achieved by magnetic loops, gravity gradient weights,
or thrusters. So builders have adopted the well known gyroscope principal,
whereby a large spinning wheel acts to restrain any motion in one axis.
For full 3 axis stabilization you need 3 wheels one for each axis. These
wheels are spun up to high speed by electric motors, and every time they
dampen any motion they need more power from their motors. The power is
supplied by large solar panels. A point which makes this system rather
awkward if the satellite goes into earth shadow, temporarily cutting off
the solar electricity supply.
The Russian space station Mir, has used 3 axis inertia wheel stabilization
for about 5 years. At first the Mir unit alone had only thruster
stabilization. Then the Kvant 1 science complex was added . Kvant needed
accurate pointing so that its fixed telescopes could be aligned by turning
the whole Mir complex so they pointed as suitable X ray targets such as
Super Nova 1987. This steering of Mir in space was achieved by onboard or
ground control adjustment of tiny precession weights located on motor
driven screw threads near the perimeter of each wheel. Unfortunately there
were problems. The inertia wheel stabilization and steering of the big
space station took a lot of electric power. It was found that Mir+Kvant
did not have enough solar panels for the job, so more were added. Then
secondly the inertia wheels were not at the Mir+Kvant centre of gravity,
so that took more power and last but not least the high speed wheels and
associated motors proved to have a relatively short life expectancy.
Recently more and bigger wheels have been added in a different position
outside Mir. But Mir now has two other science modules attached and looks
like a letter "T", a shape which is impossible to balance. So Russian
engineers have been thinking of adding 2 more modules to form a letter "H"
shape which could balance. But this mod may be overtaken by a completely
new Mir2 .
Meanwhile several other satellites , notably the American Solar Maximum
Mission  (SMA) were demonstrating exactly the same problems as Mir. SMA
lost nearly all its inertial wheels and could not be pointed at the Sun.
It was rescued by Space Shuttle astronauts and fitted with new wheels but
it soon lost orbit height and was dragged down into the thick atmosphere,
burning up during the last Solar Sunspot Maximum. So perhaps it is
understandable that Radio Amateur satellite builders have not yet opted
for 3 axis inertia wheel stabilization on any Amsat bird. Although at
least one forth coming Amateur sat is being described as 3 axis
stabilized. This is Arsene the French satellite due for launch in 1993.
But in this case I understand the stabilization is by thrusters not
inertia wheels. Thrusters can position a satellite in 3 axes , but they
cannot give the fine control instant and continuous stabization available
with inertia wheels. Given Arsene's orbit and usage however the luxury of
extra fine control ( and the risk of early demise of any inertia wheels )
would appear not to be necessary. 73 de GM4IHJ @ GB7SAN