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[DPRG] Rockey Guy

Subject: [DPRG] Rockey Guy
From: Mike McCarty jmccarty at ssd.usa.alcatel.com
Date: Wed Jul 5 11:14:09 CDT 2000

On Wed, 5 Jul 2000, Ed Koffeman wrote:

> > Why would a physical princple (that of stability of
> > an object which is vertical being unconditional when the center of mass
> > is *below* the point of suspension, and being critical when *above* the
> > point of suspension) not apply to a rocket?
> 
> Because the rocket isn't "suspended" from the point of thrust.
> 
> Since the thrust will change its angle as the rocket tilts, you have no
> corrective force.  That's why it doesn't matter which end it is on.

This makes sense only if the rockets are not mounted on some sort of
gimbal, i.e. are rigidly mounted to the body of the rocket. What makes
you think that the guy who built the rocket has mounted the rockets
rigidly to the body of the rocket?

> However, if you have the thrust coming from the same point as the center of
> gravity, then imperfections in the thrust that cause a side force will not
> tilt the rocket.

True.

> You either need thrust vectoring (Goddard used paddles in the exhaust,
> modern rockets tilt the motor, but you could have side thrusters), spin
> stabilization, or tail fins.  Tail fins are reliable if you can get moving
> in the right direction initially.  Model rockets use a wire guide to keep
> them straight until the speed is high enough to depend on the fins.

Or gimbals (or socket joint...).

> Ed Koffeman
> 
> ----- Original Message -----
> From: Mike McCarty <jmccarty at ssd.usa.alcatel.com>
> To: John Alway <jalway at icsi.net>
> Cc: <dprglist at dprg.org>
> Sent: Wednesday, July 05, 2000 9:48 AM
> Subject: Re: [DPRG] Rockey Guy
> 
> 
> > On Sat, 1 Jul 2000, John Alway wrote:
> >
> > >    This isn't a robotics website, but I thought it would interest most
> here.
> > > This is *amazing*.   This guy is going to build a rocket and launch
> himself
> > > 30 miles up, and parachute down in the capsule.   Now, there may be a
> flaw
> > > in his theory.   I haven't checked into this, but he believes that by
> > > providing the thrust from the nose of the rocket, it will be more
> stable,
> > > because of the "pendulum effect".   According to someone I know on
> another
> > > board, this is a widely held misconception.  The pendulum effect won't
> work
> > > on a rocket.
> >
> > That's news to me. Why would a physical princple (that of stability of
> > an object which is vertical being unconditional when the center of mass
> > is *below* the point of suspension, and being critical when *above* the
> > point of suspension) not apply to a rocket?
> >
> > >
> > >    Anyway, this worth looking at:
> > >
> > > http://www.rocketguy.com/rocket.html
> > >
> > >    Check out the stills of the computer animation, which shows one
> beautiful
> > > design.
> > >
> > > http://www.rocketguy.com/movies/anim.html
> > >
> > >
> > >     ...John
> > >
> > >
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> > >
> >
> >
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> 
> 
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