Issue link: https://www.e-digitaleditions.com/i/1197609
SPORT ROCKETRY JANUARY/FEBRUARY 2020 41 of nosecone!) before the main parachute deployed and arrested the ascent at ap- proximately two thirds of the nominal height. This was a surprise, despite a rela- tively loose fitting nosecone and lack of pressure relief hole, because the predicted peak force that might drive premature separation was only about 17 pounds (11 pounds due to drag and 6 pounds due to atmospheric pressure differential), and the three shear pins reportedly should hold up to about 60 pounds. My best guess is that the thin copper inserts built into the nose section at the shear points were more effective than the configuration in which the pins were tested. Ground testing could of course confirm or refute this hypothesize, but remains to be done. Fortunately, the Minuteman's premature nosecone separation did not compromise the recovery process in any way, and the certifier exercised his discretion to de- clare the flight a successful Level II attempt. In any sub- sequent flight, a tape wrap will be added to increase the nosecone's friction fit, and a pressure relief will be added for good measure. The SS-25 Model In addition to everything it was in its own right, the Minuteman II was instrumental in proving features that would also be incorporated into my Level III SS-25. The basic approach to constructing a pseudo-three-stage rock- et with multiple transition sections was the same, and the two would similarly use a breakaway sabot in a launch tube. Beyond those commonalities, though, the SS-25 model is more complex, higher technical risk, and of course larger. The model SS-25 is 8'1" long, with a maximum airframe diameter of 8". At that size, it is approximately 1:9 scale (according to the Federation of Ameri- can Scientists, the SS-25 has a first stage diameter of 1.8 meters and a length of 20.5-21.5 meters). The empty weight of the model is about 43 pounds, and its launch weight is about 53 pounds when loaded with an AeroTech M1297W mo- tor. See Figure 7. Figure 6. Minuteman II first flight launch sequence. Photo credit: David Wilkins