Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: $Revision: 1.6.2.14 $; site umn-cs.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!ihnp4!stolaf!umn-cs!atchison
From: atchison@umn-cs.UUCP
Newsgroups: net.columbia
Subject: Re: Launch Windows
Message-ID: <17100001@umn-cs.UUCP>
Date: Fri, 8-Mar-85 20:22:00 EST
Article-I.D.: umn-cs.17100001
Posted: Fri Mar  8 20:22:00 1985
Date-Received: Wed, 13-Mar-85 01:13:51 EST
References: <225@tellab2.UUCP>
Lines: 28
Nf-ID: #R:tellab2:-22500:umn-cs:17100001:000:1694
Nf-From: umn-cs!atchison    Mar  8 19:22:00 1985


One of the big factors which effects the launch window is the gyroscopes.
At some point in the launch countdown (I don't know exactly when it is),
they set the on board gyroscopes.  Once these are set, the launch must occur
at a specific time within a couple hours (?) or so.  The reason for this is
because the earth is rotating which continuously changes the gyroscope
settings. This is the basis of the launch window.  If they cannot launch 
within this time, they must cycle the countdown back to some point before 
the time the gyroscopes are set and start over again.

This is just one of the many things that, once started, must finish at a
predescribed time period.  Other factors include fueling.  Once they
send in the liquid hydrogen/oxygen, they must launch within a particular
time period.  There are many examples of these types of things.

The closer the time to launch, the smaller the launch window becomes as
more and more critical (time dependant) things are accomplished.  If these
time dependancies are not met, the count must be stopped and cycled back
to some previous point in time and continued from their.

As an interesting example, when the count reaches about T-3seconds, the
onboard computer takes over.  At some point past this (I don't know the
exact time), the launch window is only a few milliseconds long.  If they
(the computer) must delay the launch longer than a few milliseconds, the
launch must be aborted until much later.  This happened with the first
shuttle launch, the computer stopped the launch at T-3seconds, and it was
stopped longer than possible and they had to recycle the launch.  In that
example, they had to cycle back, I believe, 3 days.