leonard@bucket.UUCP (Leonard Erickson) (08/27/88)
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."johnson@c10sd1.StPaul.NCR.COM (Wayne D. T. Johnson) (08/29/88)
In article <3058@ttidca.TTI.COM> jackson@ttidcc.tti.com (Dick Jackson) writes: >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. > >Dick Jackson If I remember right (Its been a few years since I designed any model rockets), The stability of the rocket depends on the center of gravity being ahead of the center or pressure. The center of pressure can be moved tward the rear of the rocket by adding (or making larger) fins. The center of gravity can be moved forward by adding weight to the nose. In the case of a bottle rocket, the majority of the weight is at the top and the stick acts to move the center of pressure back. -- Wayne Johnson (Voice) 612-638-7665 NCR Comten, Inc. (E-MAIL) W.Johnson@StPaul.NCR.COM or Roseville MN 55113 johnson@c10sd1.StPaul.NCR.COM These opinions (or spelling) do not necessarily reflect those of NCR Comten.