Path: utzoo!attcan!uunet!tektronix!percival!bucket!leonard
From: leonard@bucket.UUCP (Leonard Erickson)
Newsgroups: sci.space.shuttle
Subject: Re: Rockets Pulling (was Re: Challenger tragedy)
Message-ID: <1027@bucket.UUCP>
Date: 27 Aug 88 07:34:36 GMT
References: <1001@scicom.alphacdc.com> <44700006@pyr1.cs.ucl.ac.uk> <6204@ihlpl.ATT.COM> <1988Aug15.162352.22902@utzoo.uucp> <3058@ttidca.TTI.COM>
Reply-To: leonard@bucket.UUCP (Leonard Erickson)
Organization: Rick's Home Grown Unix; Portland, OR
Lines: 39

In article <3058@ttidca.TTI.COM> jackson@ttidcc.tti.com (Dick Jackson) writes:
<Henry Spencer wrote, re. rocket motors pulling instead of pushing:-
<
<=>With rockets, the force is *along the axis of the rocket*, regardless of
<=>where gravity happens to be relative to the axis.  With a force along the
<=>axis, it doesn't matter where the nozzle is, and stability must be attained
<=>by other means (spin, fins, active control).  Gravity does not pull on the
<=>tail any harder than it pulls on the nose; there is no tendency for gravity
<=>to pull a rocket vertical in flight, regardless of where the nozzles are.
<=>--
<=>Intel CPUs are not defective,  |     Henry Spencer at U of Toronto Zoology
<
<I agree with this BUT -- how about firework rockets, which gain their
<stability from the long stick hanging down? Is the function of the stick
<just to increase the moment of intertia so that any pitching motion takes
<longer? Come to think of it, at school we used to take the sticks off the
<rockets and glue fins on (a la V2) and they went quite well. But intuitively
<it seems that a stickless rocket would just tumble.

As a former model rocketeer, I'm all to familiar with "Goddard stability"
and similar ideas. All you need for a stable flight is to have the center
of pressure sufficiently behind the center of gravity.

Calculating the CP can get a bit hairy, but there are a lot of methods
that are siimple and tend to err in such a manner that the calculated
CP is *ahead* of the real CP. So the rocket will be overdesigned.

The stick, by virtue of being in the airstream acts like a fin! It is also
possible to design a stickless, finless rocket that is stable. You just have
to weight the nose enough to bring the CG far enough forward. (This may sound
self-defeating, but there is a model rocket event where you try to attain 
the highest altitude with a given impulse and a standard payload. Since
I had to have the weight *anyway* and the fins were just adding drag... :-)

-- 
Leonard Erickson		...!tektronix!reed!percival!bucket!leonard
CIS: [70465,203]
"I used to be a hacker. Now I'm a 'microcomputer specialist'.
You know... I'd rather be a hacker."