(word processor parameters LM=8, RM=78, TM=2, BM=2)
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                                  August 3, 1990

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                                   APPLICATIONS

           The most   important  application  of  auxiliary-propulsion  ion
       thrusters is north-south stationkeeping.

           Gravitational forces of the Sun  and  Moon  tend to increase the
       inclination of the geosynchronous orbit.  Through the  use of proper
       thrusting, centered about  the  nodal  crossings, the geosynchronous
       orbit will not incline and will remain in the equatorial plane.

           The duration of thrust each day  depends  on the spacecraft mass
       and thrust level and on how closely the thrusters can be alined with
       respect to a  north-south  line  and  still have a thrust  direction
       through the center of mass of the spacecraft.

           Three-axis stable-platform  spacecraft allow the thrusters to be
       alined directly north-south.

           Effects of solar pressure and the triaxiality (gravitational
       nonuniformity) of the Earth will cause the east-west position of a
       geosynchronous spacecraft to  vary  unless  thrust  corrections  are
       properly applied.

           The triaxiality of the Earth is a much weaker disturbing force
       than that caused by the Sun and Moon and requires only  1/26  of the
       total impulse for proper correction.

           The thrusters   can   easily  perform  east-west  stationkeeping
       through the use of the gimbal system;  their  thrust is deflected in
       the correct direction   and  they  are  started  simultaneously   at
       specific times throughout the orbit.

           Station walking,  or  changing  the  east-west  location  of the
       geosynchronous spacecraft, can also be accomplished.

           Because the center of solar pressure  and  the center of mass of
       the spacecraft are rarely the same point, the speed of flywheels, or
       momentum wheels, must be gradually increased in order  to  hold  the
       spacecraft in the power orientation.

           Electric thrusters   can  provide  a  counter-acting  disturbing
       torque, which can be used to dump or reduce the velocity of the
       momentum wheels.  This reduction can  help lower excessive momentum-
       wheel spin rates.







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           Jerry W. Decker.........Ron Barker...........Chuck Henderson
                             Vangard Sciences/KeelyNet

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