Fin area

[Doug GerrardParticipant]

Well there’s no point in discussing this anymore with someone that is 100% convinced. But for the rest of you, remember those assumptions that are used to calculate the Center of Pressure? I looked them over last night and I won’t list them here for brevity, but I recommend you review those from time to time because if you violate the assumptions that the equations were based upon, you cannot rely on the answer for the location of the Center of Pressure.

And if you are designing a high performance rocket and have ~1 caliper stability margin (based on the Barrowman method), you shouldn’t be surprised if it does go unstable. It might fly straight but if it doesn’t, it won’t be because you had 1 gram off center by 1 mm but rather it is much more likely that you did violate one (or more) of the assumptions that those calculations were based upon. I can’t imagine trying to mount the altimeter and battery or the recovery method to within 1 mil position in order to precisely get it’s center of mass in the center of the rocket. All rockets likely have their center of mass off from the centerline of the rocket even if they are not carrying camera payloads.

Doug

[Anonymous]

Regarding medusas and off-axis mass… to me, there is one pretty compelling piece of evidence that suggests the multi-throat nozzle is to blame. Loki K350s have been flown *very* straight by several of us. That motor has an off-centered core. While the K350 core doesn’t extend to the exterior of the grain like a traditional c-slot, the core is considerably larger than a c-slot, and it is quite a long ways from being placed in the center. That motor uses a single-throat nozzle.

My Loki K350 shot in July was, if I say so myself, incredibly straight. That was in “benchmarK”, which has done some skywriting on L330s and K250s. The only common denominator to the less-than-straight flights was the medusa.

Given the cost of motors like K250s and L330s, I’ve seen enough to not want another one of either 😯 I will probably fly another K350 next year in a minimum diameter rocket. Great motor!

[Author]

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