Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!rutgers!bellcore!faline!thumper!karn
From: karn@thumper.bellcore.com (Phil R. Karn)
Newsgroups: sci.space.shuttle
Subject: Re: Launch Window question
Summary: geostationary launch windows
Message-ID: <1329@thumper.bellcore.com>
Date: 19 Sep 88 23:48:17 GMT
References: <15019@ames.arc.nasa.gov> <2234@ssc-vax.UUCP>
Organization: Bell Communications Research, Inc
Lines: 53

> If the purpose of the mission is to deploy a geosynchronous satellite,
> why is there a launch window?  What difference does it make when one
> launches into earth orbit?

There are two main classes of geostationary communications satellite: spin
stabilized and 3-axis stabilized.

At separation, both types of satellite are generally oriented by the
launcher (either expendable or STS) with the proper attitude for kick
motor firing, and spun up. The satellite spins, maintains this inertial
attitude during the ~5 hour coast up to geostationary altitude, trimming
it as necessary with attitude control fuel (usually hydrazine or cold
gas). The firing attitude puts the kick motor thrust vector nearly in
the plane of the equator and perpendicular to the major axis of the
transfer orbit. (I.e., at apogee the satellite's spin is parallel to the
earth's surface directly beneath it). For a launch in the northern
hemisphere the nozzle is actually pointed slightly north of west so the
transfer orbit inclination can be reduced to zero.

The operational attitude for a spin-stabilized spacecraft always has the
spin axis parallel with the earth's axis. Since the kick motor is also
aligned with the spacecraft spin axis, the firing attitude is therefore
nearly perpendicular to the normal operational attitude.  In order not
to starve the solar arrays during this period, the sun must be aligned
roughly with the major axis of the orbit. In other words, the apogee and
perigee points of the transfer orbit must lie above spots on the earth
that are near local noon and midnight (or local midnight and noon). So
you generally have two launch windows per day -- one centered around
local noon at the orbit injection point and another around local
midnight.  Since the launch always takes place from a site to the west
of the injection point, the local time at the launch site will be a
couple of hours before noon or midnight.

A second consideration is eclipses -- you want to avoid them during the
transfer orbit, if you can help it. Particularly if you have deployable
solar arrays, you are working from a limited budget during the transfer
orbit phase and you need all the power you can get. So the daytime
launch window often closes during the spring and fall seasons, when the
transfer orbit apogee would be on the night side of the earth and in the
earth's shadow.  That's why you see so many nighttime launches -- they put
the apogee on the day side of the earth.

Three-axis spacecraft are generally operated in a spin-stabilized mode
during the orbit transfer and kick motor firing phases, and their
deployable arrays are usually folded up against the side of the
spacecraft. So for the purposes of setting launch windows, these
spacecraft are the same as spinning spacecraft.

There are some exceptions, notably payloads riding on IUS (which doesn't
spin), and their launch windows are probably based on eclipse
considerations more than sun attitude.

Phil