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Date: Thu, 7 Mar 85 23:49:35 pst
To: space-network-source@mit-mc.ARPA
From: fisher@dvinci.DEC
Subject: Launch Windows

> ...why are there launch windows for geosync satellites?

>> ...they have to be at the right place at the right time to end up
>> in the right place over the earth

Well, actually there is more to it.  If you end up in the wrong place in
geosync orbit, it is pretty cheap to move a satellite, as long as you are
not in a real big hurry.  You just give it a push and let it drift.

In general the OPENING of the launch window is usually defined by when
the AOA (Abort Once Around) landing site (usually Edwards AFB) will have
enough light.  Thus try not to launch earlier than one orbit-time before
sunrise at EAFB.

Another important consideration is that satellites are typically designed
for the conditions they will encounter during their operational life, and
not a whole lot more.  Ex:  In operation, many geosync sats spin, and are
in sunlight MOST of the time.  Thus they are not designed to tolerate
darkness very often, and they are not designed to tolerate direct sunlight
when they are not rotating.   The launch timing is critical (I think typically
the window closeing is based on) the sun angles on the satellite while it
is climbing to geosync altitude.

Aside:  I have always been amazed at how fussy and specialized space vehicles
are.  And you wonder why it costs $millions to keep a space vehicle like
Voyager going once it is up there?  Someone has to keep monitering the critter
to make sure it does not exceed some weird design limitation!


Date: Wed, 13 Mar 85 04:10:54 pst
To: space-network-source@mit-mc.ARPA
From: fisher@dvinci.DEC
Subject: Launch Windows

> ...the comsat uses batteries until it can reach geo, stop spinning, and
> unfold its solar panels

This may be true for some of them (the stop spinning and unfold part), but
the ones I know about are either 3-axis stabilized and don't spin at all
(like TDRSS) or are spin stabilized and spin during their entire life
(like Palapa and Westar).

I don't think this invalidates any of the other stuff about sun sensors,
etc, since a spinning satellite would still have lots of constraints.  For
example, stuff that can't spin must either be mounted on the satellite's 
spin axis, or on a "despun" section, which itself must be attached to the
satellite on the spin axis.  Not to mention mass being distributed evenly
around the cicumfrance, fuel having to drain evenly. must be pretty
hard to change the attitude of a spinning satellite, so you want to do that as
little as possible.  What a pain!  With all that, I wonder why the launch 
window is so wide!


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