Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!purdue!haven!aplcen!wb3ffv!idsssd!bruce
From: bruce@idsssd.UUCP (Bruce T. Harvey)
Newsgroups: sci.space.shuttle
Subject: Re: holds in countdown
Summary: Dangers of going straight through
Message-ID: <538@idsssd.UUCP>
Date: 25 Mar 89 15:20:46 GMT
References: <8.UUL1.3#5131@mvac.UUCP>
Organization: In*sight Distribution Systems, Hunt Valley, MD
Lines: 30

In article <8.UUL1.3#5131@mvac.UUCP>, space@mvac.UUCP (Sci.Space netmail) 
writes:
> ...  Countdowns for launches begin much earlier than the time alloted in
> the countdown timer (ie a launch is set for 72 hours away, but clock
> says T-50 hours, or whatever).  I know that 'built in holds' are the
> reason why the two clocks do not agree, but my question is why are they
> there?

I believe that in the 1940s and 1950s, when expectations were high,
rockets were small, and men were very impatient, the scientists and
engineers discovered that their "straight-through" countdown (i.e., with
no planned holds) forced them to do _everything_ perfectly the first time.
If there were any problems that took longer than a few minutes to fix, the
launch attempt was thrown off track -- systems would start before the
previous task was complete, and so on.

As I understand it, the planned holds are both "breathing space" (where
you can check in with everyone to make sure their part is finished) and
"end-of-task-markers."  The launch is divided into a series of tasks, each
complete in itself, which is why you can hold at T-9 minutes for either 10
minutes or 3 hours, depending on what needs to be done.  Once each subtask
is complete, and is verified to be complete, and the weather is still
good, then the count resumes.

Without the holds, you would see many more aborted countdowns, I'm sure.
-- 
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Bruce T. Harvey  (B-}> |             ... cp1!sarin!wb3ffv!idsssd!idssup!bruce
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