Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uunet!pilchuck!seahcx!phred!petej
From: petej@phred.UUCP (Pete Jarvis)
Newsgroups: sci.space.shuttle
Subject: Re: How do they get it pointing up?
Message-ID: <2871@phred.UUCP>
Date: 4 Dec 89 15:11:15 GMT
References: <1989Nov29.094841.19907@jarvis.csri.toronto.edu> <648@sirius.ua.oz.au>
Reply-To: petej@phred.UUCP (Pete Jarvis)
Organization: <648@sirius.ua.oz.au>o
Lines: 18

>From article <1989Nov29.094841.19907@jarvis.csri.toronto.edu>, by hogg@db.toronto.edu (John Hogg):
>>
>When the SSMEs fire the whole stack is noticeably pushed over (ie nose down)
>then about 2 seconds later starts to swing back again, like a big pendulum.
>-- I guess the SRBs are flexing, or maybe the actual launch mounts. At the
>
>Department of Computer Science                  Francis Vaughan
>University of Adelaide                          francis@cs.ua.oz.au
>South Australia

When the SSME ignite, causing the stack to sway forward and back again,
this is called "twang". The entire stack pivots. The SRB's do not flex.
Lift-off is designed for the moment when it returns to vertical again.
By this time, the SSME's are up to operating temperatures and pressures.  
These are "fast start" engines which require only a few seconds to get
going, as opposed to the old Saturn V - type design where it took about
9 seconds.
 ----------- Peter Jarvis --------- Physio-Control, Redmond, WA.