Sport Rocketry

upon deployment or later in flight the fins would over-rotate into a downward-orient- ed vertical position. To alleviate this con- cern, the deployed fins were tested to veri- fy that calculated aerodynamic drag forces should not cause over-rotation. The calcu- lated drag forces for the two fin types were 1.4 lbs and 1.2 lbs at 560 ft/sec, assuming a drag coefficient of 0.17. When tested with a static load of 3 pounds (a safety factor of more than two) none showed any indica- rocket body without breaking. If, on the other hand, a grid fin contacts the ground upon initial impact, or if the body rotates about its symmetry axis as it lays down on the ground, then there is a high probabil- ity that one or two grid fins will be dam- aged or destroyed. This risk likely cannot be fully mitigated unless the grid fins are constructed of steel or similar-strength material. However, these grid fins are de- signed to be quickly and easily removed and replaced, even in the field. Therefore, damage to one or two would not prevent rapid relaunch if spares are available. Fur- thermore, the rocket does not require all eight fins for stable flight. Relaunching with four fins rather than eight would re- duce the calculated stability margin from 3.0 to 1.8-2.0, depending on what four-fin configuration is used. 44 JANUARY/FEBRUARY 2020 SPORT ROCKETRY Figure 11a. Model SS-25 missile Transporter Erector Launcher. tion that it was close to over-rotating. Finally, it was clear that there was a real risk that one or more of the plastic grid fins would snap or break on landing. If the rocket body lands in a near verti- cal position and then lays down on its side without significant rotation, then there is a reasonable chance that the fin(s) on the under side will fold back up along the Figure 11b. Model SS-25 missile TEL with launch tube elevated to firing position. Figure 10. SS-25 missile Transporter Erector Launcher. Source: Wikimedia Commons.

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