# Re: [psas-avionics] looking for a project?

```Thanks for this feedback! If anybody takes on implementing better
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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?

>> 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.

http://psas.pdx.edu/rollcontrol/

>> We also have a to-do item to implement a decent numerical differential
>> equation solver, such as Runge-Kutta.
>
> That may be a start, but there are better algos out there, especially if are
> dealing with numerically "stiff" equation sets. Investigating them will help
> with singularities that will certainly when you are considering cm scale
> fins with planetary scale orbits.

Ooh, I look forward to having that class of problems.

Anybody considering taking on the ODE solver task: we'll happily take
a simple algorithm like Runge-Kutta first, if that's where you'd like
to start, and you can improve from there.

Jamey

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