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From: francis@chook.ua.oz (Francis Vaughan)
Newsgroups: sci.space.shuttle
Subject: Re: How do they get it pointing up?
Message-ID: <648@sirius.ua.oz.au>
Date: 29 Nov 89 23:38:41 GMT
References: <1989Nov29.094841.19907@jarvis.csri.toronto.edu>
Sender: news@sirius.ua.oz.au
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From article <1989Nov29.094841.19907@jarvis.csri.toronto.edu>, by hogg@db.toronto.edu (John Hogg):
> In article <117.UUL1.3#5131@mvac23.UUCP> mvac23!thomas@udel.edu writes:
>>I assume that one of the reasons that shuttle's engines start before the
>>solid boosters is to help counteract the tendency of the whole thing to
>>fall on the orbiter's back?
>
> Were that a problem, it could be solved by starting the boosters and
> main engines simultaneously.  But the stack is resting on the SRBs
> before they're ignited, so the centres of thrust and gravity should be
> fairly well matched anyway.  Remember that the SSMEs don't thrust
> through the centre of mass of the orbiter, but rather through that of
> the whole stack.  The sideways ``walk'' resulting from this angled
> thrust is very noticeable at liftoff.  This means that the mains would
> supply little or no righting torque initially.
>
> --
> John Hogg                     hogg@csri.utoronto.ca
> Department of Computer Science, University of Toronto


One thing I noticed when watching the movie "The dream is alive" (what a buzz,
the launch shots really are quite something) is that the exact timing of the
ignition sequence may be dependant upon some mundane mechanical factors.

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
instant the stack is pointing upright again the SRBs are ignited and the thing
jumps into the air.

I remember someone on the net earlier mentioning that the external tank was
not capable of supporting its own weight unless internally pressurised. Of
course this probably does not apply to those parts where the orbiter and SRBs
are attached. However the nose of the external tank seems to take the brunt of
the aerodynamic load.


Department of Computer Science                  Francis Vaughan
University of Adelaide                          francis@cs.ua.oz.au
South Australia