Attitude Control and Station Keeping
Like all permanent bodies in low Earth orbit, the Orbital Servicing
Facillity will require reboost engines to make up for atmospheric drag.
Attitude control is also a concern for the servicing facility.
Ion thrusters are a neat idea, but unfortunately they don't exist. They
were going to be developed for the International Space Station, but the
prohibitive. The station will probably be using the traditional
hypergolics, with nitrogen tetroxide and monomethyl hydrazine, which is
available off the shelf.
For attitude control, we'd probably use gryoscopes, a cluster of four
of them. Since we launch the station thing only once and accelerate it only to
make up for atmospheric drag, gyros are better then thrusters. Even with
gyros, however, thrusters will be needed to desaturate the gyroscopes once
in a while, as well as to reboost the facility twice a year.
The thrusters will probably be shipped up with their tanks, as a
single unit. We'd operate that way until we can refuel the facility from
the moon. Maybe someone will develop some good ion thrusters by then, and
we can use lunar material for the reaction mass.
The propulsion unit would be a box standing on legs with openings for
thruster nozzles. The ends of the thruster nozzle can be flush with the
faces of the box; that will make for easier packaging and shipping. The
box itself would be about one meter on a side. (A scaled version of the
International Space Station's propulsion modules, in other words.) It
would be rectangular and have mitred corners. The box's legs would be
attached to the main truss somewhere so that the thrusters don't plume
anything important. A couple of redundant boxes like that on opposite sides
of the facility would be appropriate. Legs could be splayed a bit for
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