Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!uunet!convex!mozart!dodson
From: dodson@mozart.uucp (Dave Dodson)
Newsgroups: sci.space.shuttle
Subject: Re: How do they get it pointing up?
Message-ID: <3516@convex.UUCP>
Date: 29 Nov 89 23:22:07 GMT
References: <1989Nov23.165606.27671@utzoo.uucp> <117.UUL1.3#5131@mvac23.UUCP> <1989Nov29.192322.6761@utzoo.uucp>
Sender: news@convex.UUCP
Reply-To: dodson@convex.COM (Dave Dodson)
Organization: Convex Computer Corporation, Richardson, Tx.
Lines: 25

In article <1989Nov29.192322.6761@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes:
>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?
>
>Yup.  The other reason is that you can abort a launch if problems occur
>in the orbiter's engines, but once the SRBs fire you are committed, so
>it makes sense to light the orbiter engines slightly early and see if
>they work.  (There have been one or two aborts at that point.)

Don't forget that it takes several seconds for the turbopumps to ignite and
come up to speed to start pumping the fuel and oxidizer into the combustion
chamber.  Then it takes a while before the thrust reaches acceptable levels.

Remember that the Saturn V and many other launchers (maybe all of them?)
were held to the pad for several seconds until the thrust built up.  They
didn't want the thrust to get to where it slightly exceeded the weight,
have the vehicle start to move, but not have enough excess thrust to
control it and still keep it moving upward.

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Dave Dodson		                             dodson@convex.COM
Convex Computer Corporation      Richardson, Texas      (214) 497-4234