Mir and Iridium

Philip Chien (kc4yer@amsat.org)
Tue, 1 Sep 1998 21:41:52 -0400

I saw Mir on Tuesday morning, about 5 am local time (EDT).  No other object
was visible (limiting magnitude about 4 in light polluted skies) so I'm
assuming that the Progress has redocked with Mir.

The Delta Iridium launch is still on hold due to the Delta 3 investigation.
While they're moving towards a September 4 launch it's by no means certain.
When Boeing puts out its formal launch notice I'll forward it.

Jeff Hunt <jhunt@Radix.Net> said:

>What's involved in moving a launched sat designated for a given plane to
>be moved to another plane of the same inclination. Is it more than
>raising or lowering the orbit to change the period?

That's basically it, but it's also a function of how quickly you need the
satelltie in its operational plane.  A satellite's precession is a function
of its inclination, perigee, and apogee.

To place satellites in to separate planes you launch them in to elliptical
orbits with the apogee at or close to the planned operating altitude and a
fairly low perigee.  As the orbits precess you raise them in to their
operational planes.  This requires more propellant on the satellite, which
can be put in to the satellite's design (e.g. for normal missions the
satellites aren't completely loaded and the launcher places them close to
their operational orbits, for cases where you need different planes you put
more propellant on the satellites).

The key question is how quickly your orbit precesses and therefore how long
it takes for you to put your satellites in to their proper slots.
Obviously you launch the rocket in to the plane where the satellites are
required most rapidly and to minimize the "travel time" for the other
satellites to their planned destination orbits.

The Globalstar Zenit-2 launches are dependent on this approach with 12
satellites launched on each rocket.  As each group of four satellites
reaches their plane they're raised up to their operational orbits.  It
takes about 2 months from the first to the third plane to complete the
Zenit deployment.  Globalstar's in a 52 degree inclination where the
precession rate is faster.

I'd have to look up the specific formula for precession rate, but as I
recall it's a function of the inclination, semi-major axis, and
eccentricity.

Changing inclinations is not practical since it's a function of the orbital
velocity times the cosine of the amount of inclination change, and rather
expensive in terms of propellant.  It's a requirement for geostationary
launches (at least for any launch site not on the equator) and a very
significant portion of the apogee burn.


Philip Chien, KC4YER
Earth News
world (in)famous writer, science fiction fan, ham radio operator,
all-around nice guy, etc.