Beautiful rocket! Now for the silly question part. How did you manage to drill the holes through the tube fins, and into the air frame ensuring that everything lined up?
With a 4" diameter, I can't fit a hand drill into the tube---so I'm assuming I'd need to measure and mark hole locations on both airframe and tube fins, then bolt them in?
Sounds like something I'm also certain to mess up? Any tips? A jig perhaps?
Explain it to me like I'm five years old. My wife says I often am! (Seriously, thanks for the advice)
Great question. As others have noted, You can just epoxy the tube fins on and you should be fine. This is also your only real option if the main airframe is going to be minimum diameter.
The way I approached the PEM nuts and bolts worked out well for me:
- I marked a line up the fin tubes and carefully measured out the six evenly spaced lines around the body tube (note, for this build, the fins were couplers because I also wanted to have room for straight fins in between the tube fins.)
- I measured the exact position of the bolt holes on each tube fin and the airframe
- Using a drill press, I drilled the holes in the body tube the correct size for the PEM nuts and, this is key, the holes in the tube fins a little bit larger than was needed for the bolts. This gave me a bit of room for error.
- Most importantly, when you epoxy the PEM nuts into the airframe, use a slow or medium cure epoxy to give yourself some working time. Epoxy the PEMs in position for 1 tube fin and, while the epoxy is still wet, bolt the tube fin on (you need to hold the PEM nut on the inside with your finger, being careful not to get epoxy on the threads), position the tube fins while the bolts are still a little loose, then tighten down the bolts and let the epoxy on the PEM fully cure.
- Move onto the next tube fin
I chose to do this installation at the point in my build where I had installed the motor mount assembly, but had not yet epoxied in the rear CR. This allowed me to access inside the body tube and install the PEMS. Once they were in place, I removed the fin tubes (Numbering them so I knew which fin location they went back to). Then I placed a small piece of tape over the inside of the threads on the PEMs and installed the straight fins. While I was laying in the interior fillets, I reinforced the epoxy around the PEM nuts (the previously placed tape protecting the threads). Once the straight fins were completely in and filleted, I placed epoxy on each of the three contact edges of the tube fines and on the bolt threads and bolted the tube fins in.
One tool that helped me out greatly on this build (and many others) was this Dewalt angle head for my electric screwdriver. It would have been difficult to screw the bolts in securely without it.
Once again, I want to emphasize that there are many techniques in rocketry and other posters are completely correct that epoxied on tube fins work fine. I chose to do it this way and it has worked for me. This rocket has gotten a lot of abuse and some hard landings, but no damage. I am confident I could stand on these fins and they would hold me up.
This would allow reaching into a tube fin for tightening a screw. Or you could use the method I used for the 4". Note that my tube fin rockets are not minimum diameter. Finally, there are fly away button carriers available but consider them single use.
