Satgen 188  Satellite Stabilization Pt3   by GM4IHJ          31st Oct 1992
The BID of this bull is sgen188  Please retain this Bid if you retransmit

As mentioned in sgen187. When a satellite goes more than 20000 kms from
the earth , its stabilization is very important if it is to keep its high
gain antennas pointing at the earth. Active magnetic, or, thruster,
induced spin, will keep it pointed at earth for some time , but if the
satellite is to repay its huge building and launch costs , it must be
stable for many years.
It is this long term requirement which causes the problems, because the
Sun and the Moon both exert gravitational force on the satellite, pulling
its orbit so that either or both, its orbit eccentricity, and, or, its
orbit inclination with respect to the equator , change with time
In commercial satellites going beyond 20000 kms, it is common practise to
use very large amounts of thruster fuel over a 6 year life. Just to keep
the satellite earth pointing and on its designated orbit station. This is
very expensive and commercial companies are now asking for 10 to 12 years
of satellite on station life expectancy, in order to recoup the enormous
cost. In the Oscar 13 Phase 3 case we cannot presently counter these solar
and lunar effect, once the satellite is up, because we have no correction
thrusters.
So where do amateur and commercial satellite builders go from here ? In
the Amsat Phase 3 case, we can probably improve things by carefully
arranging to launch to an orbit which is least likely to be upset by
solar/lunar perturbation, BECAUSE THE PHASE 3 ORBIT IS NOT RIGIDLY FIXED
Orbits where the Sun does not lie along the extension of the orbits major
axis offer a rough approximation to what we need.
In the commercial case however, most of them are rigidly fixed geosats .
Required to stay well within a degree of their designated equatorial
station. Despite the Sun pulling them up and down in latitude with its
seasonal change of position , or, the irregularities in the gravity field
over the equator pulling them east or west . In this commercial situation
the answer seems to be a more economic type of thruster.
Today, most sat thrusters use UDMH Unsymetrical DiMethylHydrazine. This
can be stored relatively safely without much leakage ( unlike most rocket
fuels ). But when this complex liquid is fired through a hot grid made of
a suitable metal catalyst, the UDMH breaks up into two components which
interact explosively producing thrust. But this method use a fuel of low
atomic weight , so you use a lot of fuel to get the required thrust . What
is needed is a high atomic weight fuel such as Xenon or Ammonia. These are
not explosive , but if they are ionised they can be propelled at very high
speed in a controlled manner by a suitable electric field. To get the
power for the field we need large solar panels. Fuel usage is low because
the Sun provides most of what we need. It makes sense to equip not only
commercial geosats but also Amsat Phase 3 birds with these ion thrusters.
So Phase 3D designers are considering using them aboard this next launch.

Comet Swift-Tuttle (reported satgen172 in respect of Perseids meteor
shower) has finally reappeared and is presently visible in binoculars in
the constellation of Hercules in UK NW evening sky . 73 de GM4IHJ @ GB7SAN