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Old 06-09-2009, 11:48 AM
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CPMcGraw CPMcGraw is offline
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Join Date: Mar 2005
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Quote:
Originally Posted by Pyro Pro
One of the little things that's been bugging me (I've been working on a Saturn 1B model, where this noticeably occurs, but it has come up in other cases too) is how RockSim handles fin placement over transitions. The fact that it can calculate the correct curvature for fin roots in these situations is nice for making patterns, but there are a few issues:

1) There isn't any way to turn off the feature that I can see, such as in the case of AMRAAM, where the rear fins overhang, but do not conform to, the boattail. I haven't tried using freeform fins for this yet, but from what I remember, it didn't help.

2) The conforming process seems to take one more iterative "step" after any other process. Ie, if a fin's position is set to a point there the conforming could be applied, it will remain unconformed until some other value (I usually add a bit to the span, then change it back) is adjusted, at which point it figures itself out. I've attached an example of what it looks like when this happens.

3) When the fins, even when owned by a body tube, are moved forward of that body tube onto a transition in front of that tube, they will conform to it, but instead of simply dropping an extended portion of the fin to do this, the entire front of the fin drops along the transition, while the rest of the fin is "cut" to fit what lies behind. This can be worked around with a bit of trig to compensate for how far the span is reduced, but my main issue is with the fact that RockSim will continue to print out the original fin span of the rocket, even when that span is noticeably reduced by the conforming process. As shown in the diagram, the rocket, with fins straight on the tube, has a span of 4". When the fins are moved forward, their effective span decreases, and yet the span is still, reportedly, 4". You can see that RockSim is still using the span assumption in the way it resizes the window (pictures 2 and 3 are shown at the same scale, even though picture 3 ought to be able to be zoomed in more due to the decreased span), and I worry that it would not be handling stability calculations correctly in these cases, if it is assuming a large fin span than is actually present.


David,

Items two and three are close to being bugs in the program, both in the calculation routine and in the rendering routine. I know they're using the Qt library for the GUI, but I don't know about the renderer. You probably should send Tim a note asking about what's happening.

Item one might be the addition of a simple button for toggling the contour feature in or out.
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