Thanks for this feedback! If anybody takes on implementing better physics models, please consult with Dan for advice. :-)

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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. Awesome! I take it you mean this page? 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 _______________________________________________ psas-avionics mailing list psas-avionics@lists.psas.pdx.edu http://lists.psas.pdx.edu/mailman/listinfo/psas-avionics