Sport Rocketry

42 JANUARY/FEBRUARY 2020 SPORT ROCKETRY Stability From a stability standpoint, the SS-25 is both simpler than the Minuteman, and considerably more challenging. Unlike the finless American missile, the SS-25 sports a set of eight fins that eliminate the need for a base flare and very large nose weight. However, the SS-25 fins are very differ- ent from those commonly used in hobby rocketry. The SS-25 uses two sets of four grid fins (i.e., two different types) that are folded up along the body of the rocket while inside the launch tube and then piv- ot down to flight position after the missile exits the tube. SpaceX has recently brought grid fins back into vogue by utilizing them on its Falcon 9 rocket to control the first stage during its spectacular descent and landing. RockSim offers no direct way to deter- mine stability with grid fins. As a best and hopefully sufficient approximation, the SS-25's eight grid fins were represented in Rocksim with 16 sets of eight-fin groups with the same net length of the grid fin segments, for a total of 128 rectangular fins. Each of these 16 sets has a length, height, and thickness that corresponds to one of the model fin's various struts or cross pieces. Because the Rocksim stability method is a generalization of the Barrow- man method that is supposed to work for an arbitrary number of fins, modeling grid fins in this way might provide a reason- able result. Aerodynamic stability calculations are further complicated by a set of four pods near the base of the nosecone. Rocksim claims the ability to model such things, but I do not know how well that capabil- ity is validated. The best I could do was to verify that modeling the pods as fins with the same cross-sectional shape did not change the predicted stability in any meaningful way. To guard against the possibility that the modeling approach described above overestimates the stabilizing effect of grid fins (though my hunch is that, if anything, it is likely to underestimate the stability because of collective effects that closely- spaced adjacent fin elements may have on disrupting air flow and increasing drag) or underestimates the destabilizing effect of the forward pods, the nose was weighted to provide a large predicted margin of sta- bility. The calculated difference between the center of pressure and the pre-launch center of mass was equal to 3.0 times the maximum airframe diameter. This places the rocket in the overstable regime, but there appears to be a consensus among many who have flown in this regime that it is not problematic for rockets that leave the launch rod/rail at greater than 50 fps. (See, for example, http://www.rocketry forum.com/showthread.php?125718- What-to-do-if-rocket-is-overstable.) With the planned motor, the SS-25 was predict- ed to exit the launch tube at 56 feet per second. This relatively high speed should be adequate even if the stability margin is as high as predicted by RockSim. Construction The method used to connect the SS- 25's three main airframe sections was very similar as that described above for the Min- uteman, but with a few notable differences. While all of the Minuteman airframe tubes FOR ALL YOUR MOTOR NEEDS www.BalsaMachining.com We carry the complete Estes, Quest & Aerotech motor lines. Go to our website and see how you can save 25% off high power propellant and 20% off high power hardware using our pre-order program. Figure 7. SS-25 model flies on an AeroTech M1297W motor. Figure 8. Modeling SS-25 grid fins in RockSim.

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