Have wondered exactly the same thing. Would love to hear a proper explanation that doesn't rely on the pendulum fallacy.I've always wondered why bottle rockets like this are stable. I know that if you make the stick long enough so the CG is just behind the motor nozzle, they fly straight, but it sure seems to me that CP is likely in front of CG in that configuration.
I don't trust the other values when OR sims an unstable flight in this scenario. If you look at the side view plot, the shows the flight doing funny things that these rockets don't do in real life and that would affect the predicted altitude.
Well the bit of the finstick that's at the very end has an extremely long lever. Might have more luck simming it in OR as a single tubefin having the same surface area.
You are correct. Barrowman (and OR) assumes the body tube contributes nothing to the CP, which is accurate for "normal" designs.I'd bet it has got to be this. I think that the Barrowman equations assume that the body tube itself doesn't provide any stabilizing force, which is probably mostly correct for a "normal" rocket. But that long stick has probably a fair bit of drag, and at the end of a long arm, it can provide an effective restoring force. That'd be my answer if this was a test question.
That is exactly what I would do as a starting point... possibly, though, there could a bit more nuance to the UI to make it clearer.My best idea is to have it move as you rotate the rocket with the slider on the left side of the rocket design window... What OR actually does is calculate the CP at all angles, and present the worst case -- the one that's farthest forward.
No -- it sims it as behaving like an unstable rocket. Run a sim and plot the angle of attack. It shows it as looping (like an unstable rocket would do), which kills all the results.As NateB stated, I too, have used a single freeform fin shaped like the stick I use that I know has worked in real life scenario but OR does not show it as stable at all. Just wondering if I can still rely on the other stats like apogee, velocity, etc...
Hmm.... this is actually a case where the cardboard cutout method might be reasonably close.... the only case I think I've ever seen.Are we saying that the Barrowman equations simply weren’t made for this scenario? If that’s the case, would it be helpful to measure CP via the old-fashioned cardboard cut-out method?
Spent a little time playing with your bottle rocket this morning --
What I expect is the biggest problem is that Barrowman (and OR) assume thin planar fins. To the best of my recollection, OR's drag calculations don't consider the effects of off-center drag -- and my best guess is the main contribution the stick makes to stability is off-center drag, so OR doesn't really deal with it at all.
Something else worth mentioning is that CP really doesn't behave like CG -- it isn't a single point that can be calculated or measured. If you think of a design like a Bomarc that has wings, the restoring force in pitch is very different from in yaw -- in effect, you've got a different CP depending on which one you're looking at. I'd like to think of a good way to present that in OR, but any idea I come up with seems like it would have more capacity for confusion than clarity. My best idea is to have it move as you rotate the rocket with the slider on the left side of the rocket design window... What OR actually does is calculate the CP at all angles, and present the worst case -- the one that's farthest forward.
Your bottle rocket design, having only one fin, only has a restoring force in yaw, nothing in pitch. So OR is showing you the CP of the nose cone and the transition. If I remove your fin completely, the CP doesn't move. I can add a second fin identical to the first but at 90 degrees around the body tube, and I see the CP gets a little less unbelievable -- it's still in front of the nose, but closer to it.
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