Path: utzoo!utgpu!jarvis.csri.toronto.edu!db.toronto.edu!hogg
Newsgroups: sci.space.shuttle
From: hogg@db.toronto.edu (John Hogg)
Subject: Re: How do they get it pointing up?
Message-ID: <1989Nov29.094841.19907@jarvis.csri.toronto.edu>
Organization: University of Toronto, CSRI
References: <1989Nov23.165606.27671@utzoo.uucp> <117.UUL1.3#5131@mvac23.UUCP>
Date: 29 Nov 89 14:48:41 GMT

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.

The overwhelmingly persuasive argument for starting the SSMEs first is
that they can be shut down, but the SRBs can't.  Once the boosters are
lit, the launch is on, regardless of what may follow.  The sequenced
starting therefore allows the main engines to be checked out while
running before the shuttle is irreversibly committed to launch.

-- 
John Hogg			hogg@csri.utoronto.ca
Department of Computer Science, University of Toronto