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I'm making little tweaks to my planned 18mm rocket in OpenRocket. Right now, it's sitting with a stability factor of 1.25 cal. The fins have a length of 2.2 cm. Pulling the tip points in the shape dialogue in to 2.1 cm length drops the stability factor to 1.16, and 2.0 cm drops the stability factor to 1.05. That's a 7.2% loss of stability factor from a 4.5 % reduction in fin length, and a 16% loss of stability from a 9.1% reduction in fin length. The way I reduced the fin size in the dialogue also took some off the leading and trailing edges, but not that much.
Looking at the exported sim data, with the 1.25 stability factor in the design panel, it actually clears the rod at 1.08 calibers and gets to 1.25 only ~0.05 seconds later and ~1.7m past the end of the rod. It grows to 2.14 calibers at burnout.
Checking export with the 1.05 stability factor in the design panel, it actually clears the rod at .896 calibers and gets to 1.05 only at about the same ~0.05 seconds later and ~1.7m past the end of the rod.
Things are unlikely to get crazy in that short duration and distance, but I'm more comfortable coming off the rod above 1.0 calibers.
I started this post pointed in the direction of the first paragraph. The predicted stability factor of the model seems really sensitive to pretty small changes in the design. The question this leads me to ask is how accurate is it typically? Obviously, accuracy of weights and dimensions affects the c.g. calculation, that's a given. But since it's just calculating CP, how big are the error bars that should be applied?
Would people push a design to 1.0x in the design panel, or is it generally recommended to keep a cushion of something like 0.2 or more? When it's really sensitive to certain factors, do you give yourself more cushion on those?
Looking at the exported sim data, with the 1.25 stability factor in the design panel, it actually clears the rod at 1.08 calibers and gets to 1.25 only ~0.05 seconds later and ~1.7m past the end of the rod. It grows to 2.14 calibers at burnout.
Checking export with the 1.05 stability factor in the design panel, it actually clears the rod at .896 calibers and gets to 1.05 only at about the same ~0.05 seconds later and ~1.7m past the end of the rod.
Things are unlikely to get crazy in that short duration and distance, but I'm more comfortable coming off the rod above 1.0 calibers.
I started this post pointed in the direction of the first paragraph. The predicted stability factor of the model seems really sensitive to pretty small changes in the design. The question this leads me to ask is how accurate is it typically? Obviously, accuracy of weights and dimensions affects the c.g. calculation, that's a given. But since it's just calculating CP, how big are the error bars that should be applied?
Would people push a design to 1.0x in the design panel, or is it generally recommended to keep a cushion of something like 0.2 or more? When it's really sensitive to certain factors, do you give yourself more cushion on those?
