I started building the motor mount, and had a scary moment when I realized it wasn’t going to work the way I was about to do it. Fortunately there’s an easy fix.
Here’s the problem. The motor tube is 4″ long. The fin tabs are 3½” long. So the forward face of the forward centering ring goes ⅛” from the front of the tube and the rear face of the rear centering ring goes 3/16″ from the rear of the tube. Except that won’t work in my case, because the retainer shoulder is longer than 3/16″. To put the retainer on, the rear centering ring has to not be where the fin tabs want it to be.
Luckily I figured that out before I glued the rear ring on. In fact, I wasn’t going to glue the rear ring on right away. I’ll be gluing it later. But further forward, because I’ll be cutting ⅛” off the rear of each fin tab.
For now, though, the rear ring is just dry fit. I glued the front ring (and the thrust ring). Since I’m not using the motor hook, I’m repurposing the hook slot as a Kevlar slot — meaning I’m using the intended rear ring in the front and vice versa. I tied Kevlar around the motor tube and pushed it into the glue fillet. Then I cut the laser-cut fins out of their sheets. Four fins, two on one sheet and two on another, and when I picked up the second sheet I thought, “Am I imagining things? This feels heavier.” So I got out my scale and weighed all four fins.
Yikes. The heaviest fin is just very slightly less than twice the weight of the lightest one! (Weights shown are in grams.) Maybe you can see the difference in the grain between the fins from the light piece of wood and the ones from the heavy piece. They’re quite distinct.
On the other hand, the estimated weight of the whole rocket is 150 grams, so a difference of 6 grams is rather small in comparison to that. I’m not worried enough about it to go to the trouble of cutting new fins… but I do intend to make sure the heavy fins are opposite one another, not both on the same side of the rocket.
The wood’s ⅛” thick and feels pretty tough. I think I’ll go with CWF, not paper.