At best this is a marginally stable design, a quick RockSim simulation (attached) based on the photos you provided gives a stability margin of only 0.75 calibers with a D12-5 motor installed. It will take at least 1/2 ounce of nose weight to bring the stability margin up to 1.05 calibers for a stable flight. 290 feet above ground level (AGL) on a C11-3 and 748 feet AGL on a D12-5 with the added nose weight. Due to the large fin surface area, I would not fly this design in winds above 7mph. For a better simulation you need to provide me with measurements and other design details. As already mentioned lengthening the main body tube by about 3 inches will also bring the stability margin to the same levels as adding nose weight.

Aerodynamically, if you did a wind tunnel simulation of this design, I would guess that the lower set of fins are in the turbulent zone generated by the upper set of fins, thus they would have no effect on the the stability of the rocket! Taking this into account gives an unstable situation of -0.92 calibers with the D12-5 motor and requires about 6 ounces of added nose weight for stability at 1.01 calibers or lengthening the main body tube by 17 inches for the same margin. The performance is greatly reduced to 81 feet AGL for the C11-3 and 263 feet AGL for the D12-5 with the added nose weight, lengthening the main body tube more than doubles these attained altitudes.

Attached Thumbnails

   

Attached Files

Cubix.rkt (83.9 KB, 20 views)

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Bruce S. Levison, NAR #69055