Great shell! One option, mentioned by Shimizu, is to use a finer mesh (more energetic) burst for the inner petal. For instance, if you use BP on rice hulls for the outer petal, you might try granulated BP in the inner petal. You could also just add more booster to the center of the shell. I made a 3” double petal recently… it turned out pretty well, but not perfect. Cut stars, so symmetry was imperfect. I used independence red in the inner petal too (it’s a good choice, nice and bright). 1.5” newspaper hemis for the inner petal, BP:hulls 5:1 throughout, and 3g slow flash in the middle. The petal size ratio was good, as the outer petal was blonde streamer (fast burning, smaller radius). With double petals, relative star burn time (and brightness) of inner and outer petals is a big factor.
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