Sport Rocketry

Stability Aerodynamic stability is a particular challenge for this finless rocket. The Min- uteman II includes a conical transition sec- tion at the top of each of its three stages, and each of these moves the center of pressure a little further aft. Unfortunately, the combined effect is still not enough to provide stability with a reasonable weight distribution. Addressing this problem re- quired borrowing from the design of the earliest Minuteman variant, which had a flared skirt at the base of its first stage. SPORT ROCKETRY JANUARY/FEBRUARY 2020 39 WRITE IT WRITE IT Sport Rocketry is looking for full length feature articles with high quality photos/illustrations. Articles such as building projects, technical topics, and "how to" subjects are needed Sport Rocketry is also looking for additional, shorter, perhaps less technical, but still substantive articles. Examples of shorter articles that have been published include: outreach events, local or regional launch- es, and simpler builds. Feature article authors will receive $350 after publication of their article. Authors of shorter articles will receive a free year's NAR mem- bership. Decision on payment is based on the quality and subject mat- ter of the article and is at the discretion of the Periodicals Committee Chair, Mark Bundick. Do you know someone in your club or a NAR member attending your launch who has an interesting project or model you think would make a good article? Please send those suggestions, along with any contact information on the project owner, to Periodicals Committee Chair, Mark Bundick for follow up. and get paid! ed to examine how far forward the center of pressure might move if the rocket de- viates from straight flight. Those calcula- tions predicted that even at large angles of attack (approaching 90 degrees) the center of pressure would move forward less than four inches. This should leave a comfort- able stability margin. In addition, a small- scale prototype powered by an Estes D motor was used to demonstrate the stabil- ity margin needed for stable flight. Results from the small-scale model were expected to be conservative because it had only one transition section (in addition to the base skirt) instead of three. Construction The basic approach to design and con- struction is nested airframe tubes, with transitions filled out using expanding poly- urethane foam. A 54mm diameter motor mount is fixed within the base tube, from which it protrudes by a length of about 7". Though the mock third stage section is only about 7" long, the corresponding airframe extends down through the entire second stage section and to the upper end of the first stage. This third stage section is mounted via a pair of centering rings to the section of motor mount that protrudes from the first stage. Another set of center- ing rings connects the mock second stage section (which is therefore non-structur- al and basically along for the ride) to the mock third stage within. See Figure 4. The scratch-built nosecone includes a conical tip, a short cylindrical section, and a conical transition. Three pounds of lead were cast into a solid piece that includes the conical nose tip and a cylindrical sec- Figure 4. Airframe construction diagrams (not to scale). Adding this to the Minuteman II model in place of that actual missile's cylindrical skirt, and enlarging it somewhat relative to the real thing, was sufficient to move the center of pressure back to a few inches be- hind the top of the first stage section. Still, the design required nearly seven pounds of lead in the nose section to place the center a mass 1.5 body diameters (defined based on the first-stage diameter) ahead of the center of pressure. Prior to construction, both calculations and experimental testing were used to build confidence in the design. Center of pressure calculations were performed with RockSim and verified by hand. Results pre- sented in Robert Galejs's "Wind Instability: What Barrowman Left Out" were conduct-

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