Started by finishing up step 18 now that I have my motor retainer…
Step 18 – Mark the motor tube 6 1/8” and 1/2” from one end of the motor tube. Epoxy two of the three centering rings on each of the marks. IMPORTANT: make sure the motor tube is clean of epoxy from the centering ring that is 6 1/8” from the end for the remaining 15 or so inches of the motor tube because the fin roots will need a clean surface to attach to. At this point it is a good idea to test fit the retainer because it is much easier to sand the motor tube now rather than later.
Step 19 – Sand the slotted main body tube for at least 1/2” around the fin slots inside and outside to rough the surface for the fin fillets. Also sand where the centering rings will attach in front of the fin slots on the inside. You can wrap sand paper around a large dowel to help sand the inside of the body tube.
No photos of this sanding process.
Step 20 – Test fit each of the fins in the fin slots and file the slots to make sure you have a snug but smooth fit. Also sand the sides of the fin tangs so the entire tang plus at least 1/2” of the exposed fin above the body tube is roughed up so epoxy will stick to the fin. You can clean any residue from the machining process using acetone. It is a good idea to number your fin pairs and slots to remember which fin fits best in what slot.
Lots and lots of sanding. The slots in the main body tube required a little bit of work with a dremel.
Step 21 – Use a door jamb to mark half way between two fins on the slotted main body tube. This line will be used later to align the rail buttons.
Step 22 – Test fit the motor tube assembly inside the slotted main body tube to ensure a smooth fit. The two centering rings should go in first. When you are satisfied with the fit, spread epoxy on the inside of the body tube above the fin slots and slide the motor tube in until the centering rings clear the forward fin slot. Make sure any fillet on the forward centering ring is clear of the slot also so the fin root can seat against the motor tube. The aft end of the motor tube should be recessed about 1/2” inside the aft end of the body tube. IMPORTANT: do not use epoxy on the 3rd centering ring yet. Insert the 3rd centering ring temporarily inside the aft end of the rocket to make sure the aft end of the motor tube remains centered while the epoxy sets.
Testing, sanding, testing, sanding, and finally epoxy them into place.
Step 23 – With the 3rd centering ring temporarily still in place, tack each of the fins in place using CA on the root edge of the fin. Start with the aft fins first because the forward fins overlap the aft fins. Make sure each fin seats properly to the motor tube before applying CA. Readjust your slot if needed. IMPORTANT: make sure you do not accidentally glue the 3rd centering ring in place. Clamp a scrap of wood to the fin pair to make sure they stay in alignment with each other. Remove the aft 3rd centering ring after the glue sets
I printed a fin alignment guide from PayloadBay.com and cut it out of some thin plywood.
I used JB Weld to tack the aft fins in place and let it sit undisturbed overnight.
Next, I tacked the forward fins in place, using angle irons and clamps to ensure perfect alignment.