Modular rockeTs Building Baffles - National Association of Rocketry

Photo 1. The basic parts for themiddle and outer chambers ofthe baffle, along with the bulkheadparts. From left to the right: thehuge 5.38" x 11" standard coupler;the heavy and rigid 5.25" x 10-7/16"x 1/8" thick coupler stiffener; andthe 10" BT-3.00 tube used as themiddle chamber of the labyrinth.Down in front is the 1/4" bulkheadwith the standard 1/4" eye bolt andthe 5/16" eye bolt upgrade.ring. That centering ring would be 1" insidethe rear body tube. That puts the endof the motor tube inside the body tube by0.5" (instead of flush with the aft end of thebody tube). Therefore, the overall lengthof the motor tube required would be 28".The forward (third) centering ring wouldbe epoxied in place 10" back from the forwardend of the motor tube. This centeringring would be 5.5" inside the fin can withthe motor tube exposed 4.5" above the fincan (or 1" shorter than the baffle length).See Photo 2.I had the motor tube with the twoworking upper centering rings epoxied inplace (the aft centering ring is not epoxiedin place yet). The part of the motor tubethat extended beyond the upper centeringring would form the inner chamber of thebaffle. I again dry-fitted the middle andouter tubes of the baffle in place to be surePhoto 2. This clearly shows howthe motor tube is designed andplaced in the 24" Minie-Maggbody tube. Note that the 28"38mm motor tube has the uppercentering ring epoxied 10" fromthe end. The upper centering ringis placed 5.5" into the body tube(half the length of the 11" coupler).The two lower centering ringswill form the fin tab cage.The aft centering ring is NOTepoxied in place at this time.The motor tube is recessed 0.5"inside the aft end of the body tube.all lengths were correct (see photo 3).The next step was to epoxy the motortube into the fin can body tube. I used30-minute epoxy for this assembly. FirstI placed the motor tube in its exact position.I marked the locations of the centeringrings from the both the aft and forwardbody tube openings. I wanted to epoxy thearea between the correct positions of thecentering rings. I pulled the motor tubepartly out to do the upper centering ringarea first. Then with the tube still partiallyout the back I swabbed the area just abovethe fin tab slot. I pushed the motor mountback in and rocked it in and out to smearthe epoxy in place. I finally placed the motormount in its correct position and leftthe assembly to harden. The reason for allthis jockeying was that I did not want afillet above the upper centering ring thatmight prevent the baffle from seating correctly,yet I wanted a full body/centeringring bond. Photo 4 shows the motormount expoxied in place (the aft centeringring is still not in place at this time).Photo 3. Dry-fit test of the baffleparts against the upper centering ring.Looks good at this point.With the motor tube secure in thebody tube I returned to the assembly ofthe baffle itself. I mixed up a large batch of30-minute epoxy and quickly swabbed theinside of the 5.38 coupler. I finished just asthe mixing dish was getting warm due tothe epoxy begging to set. I set the couplerover the 1/4" bulkhead (as a spacer) andproceeded to push the 8-ounce stiffenertube into its place. It went in stiffly due toPhoto 4. Bottom view of the assemblies.At the left is the inner chamber/centering ring assembly. On the rightis the middle and outer chamberstest-fitted together. Note the epoxylayer on inside of middle tubechamber to protect the tube.30 MAY/JUNE 2010 Sport Rocketry