0FNC: Some further thoughts

Chris Michielssen pointed out his build of the Centuri (American Rocketeer) Finless Rocket. Somehow I missed that one! Interestingly, he says it’s stable on a B6-4 but unstable on a C6-5.

I tweaked my Finless sim to get it close to what Chris built.

Screen Shot 2015-09-03 at 8.03.03 AM

With a B6-4 the CG is 3.284″ ahead of the (OR-computed) CP. With a C6-5, the distance is 2.907″. Those are 1.58 and 1.4 calibers, respectively, if you use the diameter of the cone base to define a caliber.

I thought the Rxiiiab was pretty similar in shape to the Finless when I saw the picture of the latter, but in fact while it’s nearly the same (body) diameter it’s quite a bit shorter, with a cone that’s smaller in absolute size but larger relative to the rocket’s length. So it’s not entirely obvious how to apply lessons learned from the Finless to the Rxiiiab. But my sim says if I want 1.6 cone base calibers from the CG to the CP with an A10-3T, I need 0.28 oz nose weight… just about the same as what I flew it with last month! Or should I keep the absolute distance the same, since the body tube diameter is the same? In that case I’d need about 0.4 oz. But I did have two stable flights with 0.3 oz.

All this assumes the CG is correct for both sims, and that’s open to question. (I checked for the Rxiiiab, and it was close.) The difference between stable and unstable for the Finless is not very large compared to my confidence in its calculated CG position and in the relevance of OR’s computed CP. But what all this does strongly suggest to me is that the Rxiiiab should not get its nose weight reduced by much if at all.

Guess I should go stick that clay back in.



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