The BOTC84 motor mount and fin can will be of a standard construction with a total of three centering rings. The fins will be attached with the through-the-wall method. Fillets will be made at the motor mount, inside the body tube, and outside the body tube.
BOTC84 Motor Mount and Fin Can
20 July 2018
- The top centering ring of the motor mount has eye bolts that will attach to the lower recovery shock cord. I drilled two holes in this ring and attached eye bolts, washers, and nuts.
- Before epoxying these nuts, I fit the centering into the body tube and over the motor mount tube to ensure that the eye bolts will not hit either one.
- Tightened the nuts with Loctite in the threads. Once the Loctite dried, epoxied the nuts with 20-minute epoxy.
- Sanded the top 1 inch of the motor mount tube. Attached the top centering ring with 30 minute epoxy. Once this dried, applied a slow-hardening epoxy fillet on the top of centering ring around the motor mount tube.
21 July 2018
- Flip the motor mount over and set it on the nosecone to keep it horizontal. Applied another colloidal silicon slow-curing epoxy fillet to the upper centering ring/motor mount joint.
- Next, I inserted the motor mount into the lower body tube. Inserted the bottom centering ring and thrust plate into the lower body tube. I fitted the fins into the slots on the lower body tube. Numbered each slot and fin to ensure that I use fins that were fitted to a specific slot.
- Next, I measured the distance from the bottom of the thrust plate to the top of each fin. The distance is 15-13/16 inches.
- I marked the motor mount tube to align the bottom of the middle centering ring at 15-13/16 inches from the bottom of the mount. Sanded the motor mount in preparation of the attaching the ring with epoxy.
- Set the position of the middle centering ring with 5-minute epoxy. Next, I added a slow-hardening epoxy fillet at the top the the ring to the motor mount.
23 July 2018
- Fitted bottom centering ring into the lower body tube with the thrust plate. Marked the spots on the centering ring where the thrust plate will be bolted to it.
- The instructions that came with the thrust plate suggested that the plate be connected to the centering ring with wood screws. To make this connection stronger, I am drilling holes in the ring and gluing T-nuts into them.
- Drilled the three holes in the bottom centering ring to accommodate the T-nuts that will hold the attachment screws.
- Attached each T-Nut to the bottom centering ring with JB Weld epoxy. I greased and inserted screws into each nut to ensure they stay in alignment during this operation.
24 July 2018
- Sanded the motor mount tube where the fin roots and fillets will be placed. Placed a cover made of paper for the top of the motor mount tube. This paper will remain in place as I epoxy the upper centering ring to the lower body tube. It will protect the inside of the motor mount tube from spurious epoxy.
25 July 2018
- Today, I fitted the motor mount tube assembly into the lower body tube to the correct depth. I placed the bottom centering ring in place but will not epoxy it at this time. It is there to insure that the motor mount tube is aligned properly.
- I tightened the body tube around the bottom centering ring with a hose clamp so that the mount will not move during epoxying.
- Mixed slow-hardening epoxy for the top centering ring to body tube attachment on the top of the ring. As this joint is deep in the body tube, I taped a syringe to a dowel to reach the joint.
- With the help of an assistant (wife), I lowered the syringe into the lower body tube and using another dowel to push the plunger, applied the epoxy.
26 July 2018
- Measured 17 1/2 inches from the bottom of the rocket and drilled a 1/4-inch hole in the body tube. This hole will be used to apply an epoxy fillet to the top of the middle centering ring using a syringe.
- Also drilled a hole 29 inches from the bottom of the rocket. This hole will be used to apply an epoxy fillet to the bottom of the top centering ring.
- Injected slow-hardening epoxy mixed with colloidal silica into the lower hole over the middle centering ring. Slowly rotated the lower body tube so that the fillet will make contact with the full circumference of the body and centering ring.
27 July 2018
- Did the same procedure as yesterday to fillet the connection between the lower body tube and the bottom of the top centering ring. I injected the epoxy into the drilled port and worked the epoxy around the tube and rotating and spinning the lower body tube. I set it upright to let it cure.
- Washed the fins with dish soap and warm water to remove mold release and other substances that could interfere with epoxy and paint adhesion.
- Sanded the parts of the fins that will be attachment surfaces to the rocket body and motor mount.
29 July 2018
- Using a weighted string as a plumb bob, aligned the first slot to the vertical.
- Using 5-minute hardening epoxy, attached the fin root of the first fin (number 2) in this case.
- Waited for the epoxy to cure on the first fin and then taped that fin into place to help support it during the attachment of the other fins.
- Repeated the steps above for each fin.
- Next comes the fillets to create the actual load bearing connections. I am starting with the internal fin-to-body tube fillet. To start I sealed the slot where the fin passes through the body tube. This will keep the epoxy inside the body tube when the fillet is created. I also taped the motor mount tube and body tube where the bottom centering ring will be attached later.
- Mixed slow-hardening epoxy with colloidal silica. When it was mixed well, I poured the epoxy into the body tube on the joint in question. I tilted the body tube to insure the entire joint was covered. I set the rocket in its cradle and made sure it was level to insure the epoxy will settle evenly.
- Was able to do two joints today.
30 July 2018 and 31 July 2018
I was able to finish the fin-to-inside body tube fillets in two more days. I could place one fillet in the morning and then place another in the evening after the first one had set up.
1 Aug 2018
- Started on the motor mount-to-fin fillets today. The geometry of the fins will allow me to do two of these at a time.
- Created a dam out of painters tape at the end of each fine. Then mixed two small cups of slow-hardening epoxy with colloidal silica and poured one on each joint. I tilted the body tube to get the epoxy to both ends of the fin.
- Was able to do the fillets on two fins today.
2 Aug 2018
- Finished up the last fin today.
3 Aug 2018
- Started on the external fin fillets. Mixed slow-hardening epoxy with colloidal silica. Dammed each end of the fillet and poured the epoxy onto the joint.
4 Aug 2018 & 5 Aug 2018
Spent these days applying the rest of the fillets to the external fin-to-body tube joints.
8 Aug 2018
- Prepared the bottom of the rocket for the installation of the bottom centering ring. Sanded back the fillets and excess epoxy from the fin attachment work.
- Taped off the three screw attachment points on the centering ring. Placed wax paper between centering ring and the thrust plate and screwed the thrust plate to the centering ring.
- Mixed two ounces of slow-hardening epoxy with colloidal silica. Placed one-third of the epoxy in each cavity of the fin can.
- Placed the centering ring into the body tube and used a rubber mallet to seat the centering ring at the proper depth for the thrust plate.
- Set the tube upright and let the epoxy flow to the base. Then spun the tube to push the epoxy to the outer joint.
The rest of the BOTC84 rocket construction pages can be accessed from this link: The BOTC84 Rocket Build