I wanted to make a very strong bird that could handle some motors with a punch and not shred. I also wanted them to take some hard landings just in case. The idea is use 6 point through the wall fin mounts three centering rings, two of which rest on and are epoxied to the fin tabs on the top and bottom with the tube coupler and bulkhead. This gives the lower rocket section a lot of strength. Keep in mind that I had this design done in RockSim first. This gave me the direction I needed to go. This section is missing some picture because I didn't have my camera with me when I started. You can see what I did with the design from the RockSim files. Please email if you have any questions. mbennett at mbrocketry dot com
I started off with deciding what size of motor I wanted to use. I wanted to put it up on an Aerotech J350W reload. The 38/720 case from Dr.Rocket is 13.573 inches with both fore and aft closures installed. This is the casting that fits a J350W. So I made the motor tube 14.5 inches long and cut this with my tube cutting jig (see tube cutting jig). After I had cut the motor tube and knowing that I was going to do an anti zipper design, I then measured the lower body tube to house the motor tube at about 12.5 inches. With the coupler tube being added to the top, this allowed clearance with the motor tube about an inch just under the tube coupler bulkhead (Picture 1). I cut the 12.5 inches off one 3 inch PML body tube using the tube cutting jig again. It takes a little more time to cut using this jig due to the thickness of the PML tubes. Some times I have used my table saw to cut body tubes as well. After I cut the fin can tube I lightly sanded the end and made sure it was flat and level. Then I wicked in some CA super glue on both ends of the fin can tube.
Then I cut the fins using a scroll saw and a template from RockSim for the fins. I sanded the edges and the surface lightly to clean up the edges. I don't have a cool jig to put edges on the fins yet so I did it by hand. The Estes tube alignment kit in handy for marking the fins. Using the big block for marking the BT-80 tubes, I extended the lines over the sides (Picture). I then set it on the block to mark the lines. It does not fit into the PML 3 inch tube so you have to make sure it is lined up. Next I used my routing table with a 1/8 inch straight bit. I cut the fin slots for the fins.
I had done a lot of research on different designs and how they are put together. I voted to use 3 centering rings plus the coupler with a 1/4 bulkhead. The centering rings were a pain to put in along with the fins because I wanted the centering ring to sit on the fin tabs to have another mounting point. I started by installing the middle centering ring just above the fin slot grooves and the motor tube and the upper center ring. This held the motor mount tube straight. Then I installed all the fins and held them in with masking tape. This is just temporary. At this point I checked the fin tab depth and sanded the fin one at a time to make a perfect fit for the fin tab depth. Once the tab depth is set for all the fins, I took out the top centering ring and put it in the bottom. This gave me access to the middle centering ring from the top. I pushed the centering ring down onto the fin tab. I then used 30 min epoxy to glue the middle center ring from the top only and to the body tube only not the motor mount yet. This allows the middle centering ring to be at the right place and position for fin and motor alignment. Make sure the motor mount is still in and on a level surface so the epoxy stays on the outer edge inside the body tube only. Once the epoxy died for and hour, I then took out the lower center ring and placed it in the top again. I then aligned the fins and tacked them in with CA super glue. Then I epoxied the fins in one at a time using 30 minute epoxy again to fill the small gaps between the BT and fins. I used two hour epoxy to make fillets with tape dams to keep it from running off. I epoxied all six points, both side on the outside of the BT, both sides on the inside of the BT and both sides of the MM tube. I used the lower center ring to help keep the MM tube from being off center. Just be careful not to glue it in yet. I just set it up on a small incline to keep the glue from running out. This was the most time consuming portion next to the sanding at the end (Picture 2).
I took the bottom centering ring and drilled the two holes for the T-nuts for the motor retention. I then cut the spike points off the T-nuts and epoxied them in the holes. I finished installing the lower and upper centering rings (Picture 3).
I glassed the tube using three long strips between the fins, and then I overlapped the small strips and the fins with more fiberglass. I let this dry (Picture 6) and it worked out very well (Picture 8).
I then install the coupler and bulkhead assembly and epoxied it into place. I drilled four 1/4 inch holes for the motor ejection if I choose to use it (Picture 1).
