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NARCON 2021: Design, Evolution, & Prod. of Long burning Rocket Motors
It's also a nice overview of the history of composite motor evolution in hobby rocketry.
At around 35:45 he says, "I learned about the use of three vs four fins to reduce or eliminate coning" and then doesn't specify WHAT he learned. I found the answer in the paper linked below and suspected the correct answer before finding it since sounding rockets with few exceptions have four. NARCON 2021: Design, Evolution, & Prod. of Long burning Rocket Motors by Gary Rosenfield https://www.youtube.com/watch?v=7iCoICrGXUA Sounding Rocket Fin Design to Mitigate Roll Lock-In http://rsandt.com/media/Sounding%20...l%20Lock-In.pdf Abstract - Roll lock-in is a persistent high angle of attack, nonlinear coning motion sometimes observed in the flight of reentry vehicles and sounding rockets. For example, it has occurred during Aerobee sounding rocket and Sidewinder missile flights prior to the introduction of rollerons. This paper focuses on how fin design effects the probability of lock-in. The high angle of attack response is driven by misalignment and/or offset thrust and drag forces, amplified by during pitch-roll resonance. High angles of attack engender nonlinear roll moments which cause the roll rate to follow the pitch natural frequency. It is well known that such roll moments can arise when the center of mass is offset from the vehicle symmetry axis. However, this paper explores another source of nonlinear high angle of attack roll moments, interaction between vorticity shed from a fore body and tail fins. Both kinds of roll moment have similar magnitudes. However, lock-in due to center of mass offset is, apart from static margin, not affected by fin design. VII. IMPLICATIONS FOR FIN DESIGN [big snip] By far the most important fin parameter for mitigating lockin is the number of fin panels, N. Most sounding rockets have flown with 4 fins with a small minority using only 3 fins, and almost none with 5 or more. Our model shows that there is very little difference between 3 and 4 fins when attempting implementation of the first part of our strategy. Small tweaks in taper ratio or exposed fin span can bring about very similar performance in these two cases. But, increasing the number of fins to 5 or more leads to dramatic improvements. Our model shows that with 6 fins major improvement is relatively easy, and it becomes trivially easy with 8. Note that mortar bombs are commonly flown with 8 fin panels.
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