Path: utzoo!utgpu!water!watmath!clyde!att!ihlpl!knudsen
From: knudsen@ihlpl.ATT.COM (Knudsen)
Newsgroups: sci.space.shuttle
Subject: Re: "Walking"
Summary: Walk deliberate
Message-ID: <7418@ihlpl.ATT.COM>
Date: 28 Oct 88 17:59:25 GMT
References: <665@sas.UUCP> <4964@hplabsb.HP.COM> <29792@think.UUCP>
Organization: AT&T Bell Laboratories - Naperville, Illinois
Lines: 22

Don't forget that a certain amount of "walk" is deliberate.
Because the SSMEs are off-center, and angled, it is necessary
for the whole stack to clear the tower a little off-vertical,
so the SSMEs are more nearly under the center of gravity of the
whole thing.  Otherwise the SSMEs would pitch the stack right over.

So on liftoff the shuttle has to walk under the tank a little bit.
I predict that if you check the tapes carefully you will see that
the top of the stack (ET) moves sideways less than the bottom,
so the whole thing goes up a little tipsy.  

I've heard that the SRBs are ignited just as the "twang" effect is
swinging back to the neutral point.  This rotational momentum
is in the right direction to help the shuttle walk its way under
the ET.

Thanks to Phil Karn for explaining the above to me years ago;
apologies to him if I screwed it up here.
-- 
Mike Knudsen  Bell Labs(AT&T)   att!ihlpl!knudsen
"Lawyers are like nuclear bombs and PClones.  Nobody likes them,
but the other guy's got one, so I better get one too."