Sport Rocketry

28 MAY/JUNE 2017 SPORT ROCKETRY Fins I know what you're thinking: "These fins are way too small!" You're not the only one. The fins are considerably smaller than one would expect to see and seem to buck traditional "best practices." They made for a lot of discussion, debate, and unrest (up to and including pre-flight safety review on the day of launch!). Typical model rocket mass distribution has the bulk of the mass top and bottom with the recovery/nosec- one mass top and the fin can/motor as- semblies bottom; leaving marginal mass in between. CP calculations require (among other things) adequate fin surface and drag to achieve optimum stability. But Mestler has the bulk of her mass just forward of dead center (red area). This alternate mass distribution pushed her CG forward, re- quiring less fin surface area to achieve a CG/CP margin of 1.98 (see Figure 14). For comparison, if I remove the lander payload mass and associated hardware I would have to increase my fin surface area to achieve a CG/CP margin of 2.08 (see Figure 15). This produces the expected traditional visual sizing (albeit still smaller than one typically expects). Regardless, even though I had done the math and run hundreds of sims re- sulting in stable flight, I needed to verify the fin design further. I built a scale model (weighted accordingly) to prove the flight dynamics were sound. At approximately 1/30th scale the model proved the fin de- sign provided stable flight and the design question was settled. Recovery Systems There would be four recovery events: two main events (the launch vehicle and the lander) and two minor events (two payload bay doors). I targeted the launch vehicle descent rate at a modest 10-12 Figure 14. RockSim showed that the Mestler had a stability margin of 1.98 even with the small fins. Figure 15. Another RockSim plot showing that without the Mestler payload, the rocket would indeed require a more typical fin size for a two-caliber stability margin.

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