On Mon, Aug 24, 2009 at 01:41:51PM -0700, Jamey Sharp wrote:
> Thanks for this feedback! If anybody takes on implementing better
> physics models, please consult with Dan for advice. :-)
> 
> On Mon, Aug 24, 2009 at 1:15 PM, I<kirk...@pdx.edu> wrote:
> > Quoting Jamey Sharp <ja...@psas.pdx.edu>:
> >> Physics modelling: I don't care what piece of rocket physics you want to
> >> model as long as it involves torque.
> >
> > I'm curious why you want to involve torque? Our rocket actually needs to
> > model fins as a force applied some distance from the center of mass and the
> > center of pressure. Torque (or if you will, a "moment") is generated when a
> > force vector is crossed with a position vector, assuming the force and the
> > vector sum of forces are NOT co-located. It's literally as simple as f x r
> > (where x is the cross product and f and r are vectors).
> >
> > The reason this is done this way in industry is because lateral fin forces
> > contribute to rotations *AND* translations, and this coupling is what makes
> > the rocket a non-minimum phase system. It's crucial that this relationship
> > is present in the simulation dynamics, because it creates the need for a
> > different control strategy than would be needed if you neglect it.
> 
> First disclaimer: I'm not that good at physics. That's why I'm asking
> for help. :-)
> 
> I don't understand the distinction you're making. I understand that
> torque comes from applying force off-center (for some value of
> "center" that I understand only in abstract terms). I also understand
> that a force like wind will tend to have both a linear and a
> rotational effect on the rocket body. But right now Josh and I only
> understand how to implement linear components, so I'm looking for
> someone to add the rotational components to the model. Are you saying
> I don't want what I thought I wanted, or that I'm using the wrong
> terms?

If I understood the distinction correctly, it sounds like the complaint
related to the use of "torque" rather than "force", where the latter
creates the former as well as other things.  If so: yes, we want to work
in terms of forces, and the problems we have arise because we don't yet
understand all the details of applying forces to the rotational
components of our rocket model.

Also, I'd like to know exactly what you meant by "non-minimum phase
system".  I've read http://en.wikipedia.org/wiki/Minimum_phase and I
still don't fully understand it.

> >> Our current simulator never causes
> >> the rocket to rotate at all. Two possibilities are to model crooked fins
> >> (which leads to spin) or to model wind (which applies more force at the
> >> fins than at the nose, causing the rocket to lean into the wind). If
> >> several people are interested you can work together or model different
> >> kinds of forces. And you can start with a very simple model and make
> >> incremental improvements.
> >
> > The role only model is given on the roll control page, though I owe an
> > explanation to make it useful. It will be expanded to include a damping
> > force caused by the AOA of the main fins during rotation. I've done a planar
> > 3DOF model (vertical and horizontal translation, plus one rotation) that I
> > plan to show on that page after I finish my fin testing. Finally, I will
> > expand the models to the general 6DOF model with unlimited force/position
> > inputs, but maintaining the rigid body assumption. I think higher order body
> > mechanics are unnecessary here.
> 
> Awesome!

Definitely awesome!  I look forward to seeing that.

- Josh Triplett

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