Newsgroups: sci.space.shuttle
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Why are there launch windows?
Message-ID: <1989Mar24.230920.17804@utzoo.uucp>
Organization: U of Toronto Zoology
References: <36700005@hcx2> <256@cbnewsl.ATT.COM> <7230@thorin.cs.unc.edu> <1665@ubu.warwick.UUCP>
Date: Fri, 24 Mar 89 23:09:20 GMT

>>>In article <36700005@hcx2> tom@hcx2.SSD.HARRIS.COM writes:
>>>>Why do earth orbit missions have launch windows?

Since nobody seems to have really answered the original question...
(The following ignores some details and fine points for simplicity.)

The main constraint on comsat launches is power requirements during the
climb to Clarke orbit.  To keep the satellite rigid and strong for the
final motor firing, solar arrays generally aren't fully deployed during
this period.  This puts a premium on getting as much sunlight on the
arrays as possible.  Most satellites (even three-axis-stabilized types)
are spin-stabilized during this climb, with the motor pointing along the
spin axis and the spin axis lined up for the final motor burn.  This
fixes the spin axis with respect to the Earth, making it impossible for
the satellite to turn to point the solar arrays at the Sun.  So the
trajectory has to be chosen to put the Sun in the right position.  This
fixes the trajectory in space; to get the satellite into that trajectory,
launching from a rotating Earth, it has to be launched at the right time.

A weaker but significant factor in the choice of trajectory is avoiding
eclipses by the Earth.  This affects even non-spinning satellites.

Thermal considerations may be a factor, too, although I'm not sure of
this.  I think even IUS payloads, which aren't spin-stabilized during
climb, in fact are spun very slowly ("rotisserie mode") for temperature
control.
-- 
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