Super Big Bertha -- Part 3: Another motor mount!
The Thrifty Rocketeer continues...
The first step in this construction is supposed to be the motor mount. And often I will plunge in and do that right away, so that it can be drying while I work on another part.
But this time, I waited until after I had done the fin assembly.
I carefully sliced out the three centering rings and assembled the various parts for this motor mount. Much later, a screw on motor retention device will be added, but after the rocket is pretty much assembled.
Now, the instructions indicate that you are to measure 1/2" from the tail end of the motor mount tube for the last centering ring. The middle ring is 3 inches from this end, and the top ring is 7 1/2" from that same end. (This also can be seen as 1/2" from the top end, but who's counting here.)
You can assemble and glue all three in position without much trouble at all, and I did this. Then I realized that I had rushed into it without thinking it all the way through: When assembling through-the-wall fins, it is often helpful to leave the last centering ring unglued, until after the fins have been inserted.. This is so that you can add fillets and glue to the tab edge of the fins as they impact the motor tube. It makes for a much tighter, more secure, stronger assembly. But it takes so fore-thought and planning...which I missed.
Also, if you're thinking of replacing that rubber Estes shock cord with a Kevlar cord or string, you might also want to measure out your string and secure it under that top centering ring. You might also want to cut a small notch to pass the cord through between the motor mount tube and the centering ring. None of these things I did... (partially because I don't have Kevlar string, and partially because I plunged ahead.)
Once the centering rings are dried and/or filleted again for strength, you'll want to dry fit the motor mount into the body tube. I try not to do this too many times, as it can wear or bend the edge of the centering rings a bit. So I wait until I have everything in place...fins, motor mount, rings, slots, etc.
In my case, while the centering rings were drying and too late to alter, I noticed the center slotted ring wasn't quite square all the way around the tube. But it wasn't enough to stop or re-do. I would live with it.
I took my photo-copy of the tube markings and cut out the guide, attaching it to the bottom of the body tube and tacked it in place with more tape. When I was sure it would not slip, I began to trace the slots with my Exacto knife and removed the slots paper. Then I repeated this slicing into the body tube through that template slot. A brand new, sharp Exacto knife blade is best for this. Smooth steady cuts are best.
After completing the slots, I inserted the motor mount tube assembly and checked the assignment. I dry fit the fins into slots and immediately reminded myself why the fins weren't going all the way in. Remember, the middle centering ring is slotted, to receive those slotted fin tabs. If the motor mount tube assembly isn't aligned with the slots facing those new slots you just cut, it's not going to fit together.
I removed the slot marking guide, and prepared to insert some wood glue inside the body tube, to a point just beyond the slots. This ring of glue will be impacted by the forward centering ring, and the ring will squeegee the glue forward with it into position. In the best of all worlds, the middle ring of glue that you have spread between the slots, will have enough lingering on the body tube all to clue the middle slotted ring in place. But this is why I should have left the bottom ring off and free until after this step. (Once inserted, I can't access the middle slotted ring to fillet or supplement the glue... the same for the fin tabs.)
As it is, I applied two rings of glue, and then in one smooth motion, after aligning the slots, slid the motor mount in, stopping just as the "tail pipe" of the motor mount matched the outer bottom edge of the body tube. I double checked the slots, and spun the motor mount slightly until the fins would insert. I again re-checked the tail pipe was even with the end of the body tube.
I debated letting the motor mount dry completely before starting to glue my fins through-the-wall. I decided if the assignment wasn't right, I'd rather adjust while the glue was soft and the mount could turn, rather than be trapped with a slight mismatch.
So I proceeded to apply wood glue onto the edge of the fin tab, and along every surface that was going to contact the body tube or motor mount. The fin went in smoothly, and I was pleased that it would dry squarely if not disturbed. I checked the perpendicular angle of the fin before leaving it to dry. I let it dry for about 20 minutes before I rotated the rocket and glued another in place. Again, I let it dry perpendicular, and after another 20 minutes, rotated around to do the third fin...and after another 20 minutes, the last fin.
I discovered something not often noted. Although each of my fins contacted the motor mount, there's a slight gap between the fin and the body tube. Something is keeping each fin from seating tightly. (Now, I can bridge this gap with wood glue and fillets, but it's there.) After racking my brain, I remembered that I had applied a fillet to both sides of the middle slotted centering ring without regard of how the fin tab was going to impact this dried fillet. I am convinced now that I should not have filleted this, but waited with the third and final centering ring off, to be done after the fins were inserted. Again, another lesson that experience should have taught me, but I didn't wait.
As I wait for the fins to all dry in place, and add additional fillets to the fins, I'll break off here until next time.
Don't forget to cap your glue and save your spare balsa... you never know when you'll need it!
The first step in this construction is supposed to be the motor mount. And often I will plunge in and do that right away, so that it can be drying while I work on another part.
But this time, I waited until after I had done the fin assembly.
I carefully sliced out the three centering rings and assembled the various parts for this motor mount. Much later, a screw on motor retention device will be added, but after the rocket is pretty much assembled.
Now, the instructions indicate that you are to measure 1/2" from the tail end of the motor mount tube for the last centering ring. The middle ring is 3 inches from this end, and the top ring is 7 1/2" from that same end. (This also can be seen as 1/2" from the top end, but who's counting here.)
You can assemble and glue all three in position without much trouble at all, and I did this. Then I realized that I had rushed into it without thinking it all the way through: When assembling through-the-wall fins, it is often helpful to leave the last centering ring unglued, until after the fins have been inserted.. This is so that you can add fillets and glue to the tab edge of the fins as they impact the motor tube. It makes for a much tighter, more secure, stronger assembly. But it takes so fore-thought and planning...which I missed.
Also, if you're thinking of replacing that rubber Estes shock cord with a Kevlar cord or string, you might also want to measure out your string and secure it under that top centering ring. You might also want to cut a small notch to pass the cord through between the motor mount tube and the centering ring. None of these things I did... (partially because I don't have Kevlar string, and partially because I plunged ahead.)
Once the centering rings are dried and/or filleted again for strength, you'll want to dry fit the motor mount into the body tube. I try not to do this too many times, as it can wear or bend the edge of the centering rings a bit. So I wait until I have everything in place...fins, motor mount, rings, slots, etc.
In my case, while the centering rings were drying and too late to alter, I noticed the center slotted ring wasn't quite square all the way around the tube. But it wasn't enough to stop or re-do. I would live with it.
I took my photo-copy of the tube markings and cut out the guide, attaching it to the bottom of the body tube and tacked it in place with more tape. When I was sure it would not slip, I began to trace the slots with my Exacto knife and removed the slots paper. Then I repeated this slicing into the body tube through that template slot. A brand new, sharp Exacto knife blade is best for this. Smooth steady cuts are best.
After completing the slots, I inserted the motor mount tube assembly and checked the assignment. I dry fit the fins into slots and immediately reminded myself why the fins weren't going all the way in. Remember, the middle centering ring is slotted, to receive those slotted fin tabs. If the motor mount tube assembly isn't aligned with the slots facing those new slots you just cut, it's not going to fit together.
I removed the slot marking guide, and prepared to insert some wood glue inside the body tube, to a point just beyond the slots. This ring of glue will be impacted by the forward centering ring, and the ring will squeegee the glue forward with it into position. In the best of all worlds, the middle ring of glue that you have spread between the slots, will have enough lingering on the body tube all to clue the middle slotted ring in place. But this is why I should have left the bottom ring off and free until after this step. (Once inserted, I can't access the middle slotted ring to fillet or supplement the glue... the same for the fin tabs.)
As it is, I applied two rings of glue, and then in one smooth motion, after aligning the slots, slid the motor mount in, stopping just as the "tail pipe" of the motor mount matched the outer bottom edge of the body tube. I double checked the slots, and spun the motor mount slightly until the fins would insert. I again re-checked the tail pipe was even with the end of the body tube.
I debated letting the motor mount dry completely before starting to glue my fins through-the-wall. I decided if the assignment wasn't right, I'd rather adjust while the glue was soft and the mount could turn, rather than be trapped with a slight mismatch.
So I proceeded to apply wood glue onto the edge of the fin tab, and along every surface that was going to contact the body tube or motor mount. The fin went in smoothly, and I was pleased that it would dry squarely if not disturbed. I checked the perpendicular angle of the fin before leaving it to dry. I let it dry for about 20 minutes before I rotated the rocket and glued another in place. Again, I let it dry perpendicular, and after another 20 minutes, rotated around to do the third fin...and after another 20 minutes, the last fin.
I discovered something not often noted. Although each of my fins contacted the motor mount, there's a slight gap between the fin and the body tube. Something is keeping each fin from seating tightly. (Now, I can bridge this gap with wood glue and fillets, but it's there.) After racking my brain, I remembered that I had applied a fillet to both sides of the middle slotted centering ring without regard of how the fin tab was going to impact this dried fillet. I am convinced now that I should not have filleted this, but waited with the third and final centering ring off, to be done after the fins were inserted. Again, another lesson that experience should have taught me, but I didn't wait.
As I wait for the fins to all dry in place, and add additional fillets to the fins, I'll break off here until next time.
Don't forget to cap your glue and save your spare balsa... you never know when you'll need it!
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