Broken Fin

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Bob in Phoenix

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Now I've done it. While trying to repair a loose fin, I've managed to break it off completely. Sadness...
And no, it's not surface mounted. The rocket is an old THOY Falcon. Four inch diameter, 54 mm motor mount. The fin root is 9.375 inches, and the fin tab WAS 4.125 inches.
So I removed the remains of the fin tab from the fin. But the tab - almost all of it - is still in the body tube clinging to the motor mount.
I am considering two methods: 1. Find a way to remove the broken fin tab from inside the body tube. Then I just make a new fin and reinstall it. 2. Mount the now tabless fin in some kind of flange or bracket, and install that back onto the body tube. Strength restored!
Your suggestions will be appreciated..
Bob in Phoenix
 
I don't fly very aggressively, so my opinion may not be valid. I often have invalid opinions, so listen to smarter people than me.

In the past, when I've had that situation, I would leave the broken tab in the rocket if it seemed to still be rigidly attached. I would clean any loose stuff, but leave the messed up parts if it looked like epoxy would help attach it back. I would remove some/all fillet, depending on how it broke and reattach the fin to the inner part/remaining fillet groove. I would fix the fillets and then use lightweight fiberglass on the repaired fin from its tip to the root of the next fin at least on both sides. Depending on how the rocket was finished (or not), I might tip to tip it all. I wouldn't use heavy fiberglass, just some thin smooth glass.

If the part in the rocket was loose, I would get it all out and put a complete new fin and patch all the yuk that happens when you Dremel (or other) out a fin from an existing rocket. A 4" with 54mm mount should have a fairly shallow tab, maybe you wouldn't do too much damage, especially if you can figure out a guide to help reduce unintended drift of whatever tool you use to get rid of the old fin.

I imagine pictures of the current state could help those with more experience to lead you to the prefect repair. My suggestions are just what I've done and might not at all apply.

Sandy.
 
Use a Dremel router and a jig to make a flat surface for it to run on. The fin I repaired was broken above the root so I was able to make a new piece and cut a tongue and groove to attach it (its nearly as strong as it originally was). In your case it might be possible to use a narrow straight or upcut router bit (1/8" or so) and making light cuts to keep the router controllable cut it all the way back to the motor tube, it would be a time consuming process but worth the effort as you would be able to make an original form fin and attach it normally.
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I took your advice. Using a variety of Implements of Destruction - small Dremel drills, small Dremel router bits, and the ubiquitous #11 blade, I managed to trim the broken tab down to half its, what - depth? Then reached into the slot with a 6 inch steel ruler, I manged to break loose the remaining broken fin tab. It came out cleanly! It took a while to fish it out through the slot. A couple more days of shaking will remove all the splinters and sawdust.
Now I am building a replacement tab for the existing fin. I will attempt to attach a piece of basswood to the fin using epoxy and four steel pins into the root edge. I hope it works.
Bob in Phoenix

Use a Dremel router and a jig to make a flat surface for it to run on. The fin I repaired was broken above the root so I was able to make a new piece and cut a tongue and groove to attach it (its nearly as strong as it originally was). In your case it might be possible to use a narrow straight or upcut router bit (1/8" or so) and making light cuts to keep the router controllable cut it all the way back to the motor tube, it would be a time consuming process but worth the effort as you would be able to make an original form fin and attach it normally.
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