Building up the fincan this weekend. The goal is to use axial reinforcement so all the thrust will be transferred up through the structure to the bulk plate of the payload bay. Should make it very sturdy.
I've done quite a few scratch builds of varying power levels. To keep my build clear in my head, I usually fully diagram out what I will do. Here's what's planned for this build. I'll try to remember to take pics at each step as I'm doing them this weekend. I tend to get into the build and forget, though.
1. Fiberglassed centering rings are done. MMT tube is sanded to cut the glassine and then wiped down with rubbing alcohol to remove the dust. Reinforcing plywood splines are all cut to length.
2. JB Weld applied to the bottom of the MMT, and the bottom ring is slid on flush to the end of the MMT. After that cures, the short reinforcing splines are tacked on with JB Weld, and the 2nd centering ring is epoxied to the MMT and to the top of the reinforcements. Once cured, small filets are added to the reinforcements.
3. The middle centering ring is epoxied on using the fins (not shown) to measure the proper height. The CR should bump flush against the leading fin tab edge. This is held with tape until cured.
4. Like before, the top reinforcing splines are now epoxied to the MMT and fileted once they're set.
5. A 1" x 36" kevlar strap is epoxied to two sides of the MMT to act as an attachment bridle for the shock cord. 12" on the MMT both sides.
6. To insert the fincan, I leave the top CR off for now. This will let me go in and filet the splines to the airframe for maximum support. For now, add JB Weld up the sides of the frame where the splines attach and where the middle CR will rest just above the fin slots. Slide the slotted, fiberglassed booster frame down the fin can to about where the next CR is.
7. Add JB Weld to the airframe just below the fin slots for the next CR. Continue inserting to just before the next CR.
8. Add the last JB Weld for the bottom CR and finish sliding in. I like to add strap ties to the bottom to make sure I have good contact while curing.
9. Next day, come in and add filets to the top splines. When cured, drill small holes near the rear of the tube to inject some Rocketpoxy for adding a filet to the rear splines. When all finished. Epoxy on the top CR to the MMT, splines, and airframe.
10. Then it will be on to fin attachment. But I'm a ways off from that.
Not shown are the blocks of wood I added to the bottom CR as an attachment point for the bottom 1515 rail guide.