Sport Rocketry

were phenolic, the mock first stage on the SS-25 is a beefy 8" diameter fiberglass tube from Performance Hobbies. This provided a better scaling match to the actual missile than would available phenolic tubes, but more importantly facilitated more solid mounting of the grid fins. Carbon fiber and fiberglass sleeve overlayment was then extended from near the tip of the nosec- one down to just past the upper edge of the mock first stage, rather than continu- ing down to the aft end of the rocket. Another departure from the Minute- man construction was the experimental use of epoxy clay in several components of the SS-25 model (others may have tried this, but for me it was a first). First, clay was used in place of expanding foam to shape the lower two transitions. These have higher height-to-thickness aspect ra- tios than the corresponding components on the Minuteman model, making the prospect of obtaining uniform fill with ex- panding foam a difficult one. Second, the four nosecone pods were constructed from plastic Easter eggs that were filled, supple- mented, and reinforced with the same ep- oxy clay. Finally, epoxy clay was use for the tip of the custom-built nosecone (see Fig- ure 9). Unlike the Minuteman, the SS-25 in- cludes an avionics bay constructed be- tween the concentric airframe tubes just below the lower transition section. The SS-25 also includes a number of vent holes that are sized to equilibrate its various in- ner compartments on a uniform timescale that is about one tenth of the expected as- cent time. The folding grid fins were constructed number of risks that required mitigation during design, construction, and testing. One possibility was that the plastic grid was too brittle to sustain the requisite aero- dynamic forces. This risk was mitigated by calculating a maximum drag force of ap- proximately 0.8 lbs/in 2 at 500 mph (sig- nificantly higher than the predicted peak speed of 385 mph), corresponding to a maximum lateral force of only 0.4 lbs per grid element. The plastic grids were then tested to a static pressure of 7 lbs/in 2 (more than 10 times the calculated maximum) and a static lateral force of 80 lbs per grid segment (more than 100 times the calcu- lated maximum). Another risk was that the fins would not deploy properly, or would deploy too asymmetrically or too slowly to provide stability during early flight. Because the fins were tested to deploy under gravity while at rest, it seemed that gravity assisted by aerodynamic forces should drive rapid deployment. There was also a potential risk that SPORT ROCKETRY JANUARY/FEBRUARY 2020 43 ANYTHING NEW IN ROCKETRY? ANYTHING NEW IN ROCKETRY? Do you have news relating to model, mid, or high power rocketry? The NAR will post a link to your news item on the NAR.org website. Please send topics of interest to: latestnews@nar.org. Figure 9. Plastic Easter eggs in the service of nosecone pods. from sections of plastic ceiling panel that is sold at hardware stores for suspended light panels. These rectangular grid sections are the fixed between G10 fiberglass struts that are attached to angle brackets bolted onto the lower body tube. This unorthodox fin design raised a TO FIND FELLOW TO FIND FELLOW MODEL ROCKETEERS MODEL ROCKETEERS NEAR YOU, VISIT: NEAR YOU, VISIT: WWW.NAR.ORG WWW.NAR.ORG CLICK ON: CLICK ON: "FIND A LOCAL CLUB FIND A LOCAL CLUB"

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