Question: Fin Repair

I recently flew my unique 12" diameter sonotube scratch build level 3 certification rocket and unfortunately the main parachute didnt open. This lead to one of the fins breaking in half and cracking and another fin fillet to crack. The fins are made of 1/2" plywood.

My plan was to cleanly cut the fin on the rocket, then attach a new fin section to match the rest of the fins. I would attach it using metal strapping and bolts, then fiberglass over the top with two layers of 8oz e-glass and epoxy resin. Would this be a strong enough solution to be reliable and not be a weak spot? or would it be more worth it to not risk failing the L3 flight again and building a new rocket. See images attached

If there are different ways you would repair these fins how would you do it?

This is out of my league rocket wise...
But as a wood worker, there is not enough of the fin left to section another piece of plywood to it to make it very strong.
It's your call on this one...it is your L3 attempt.

Woody's Workshop said:

This is out of my league rocket wise...
But as a wood worker, there is not enough of the fin left to section another piece of plywood to it to make it very strong.
It's your call on this one...it is your L3 attempt.

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what are your opinions on fitting the two pieces back together like puzzle pieces then fasten it together with epoxy fiberglass and mechanical fasteners?

In my opinion, fin is shot and band aid work is questionable.

Looks like a Polecat kit. Where you go from here depends on what you did for internal fillets--what technique did you use internally?

Clean up the pieces so they fit together. Mix a batch of West with Colloidal Silica filler (or your preferred epoxy with microballoons) to a cold-honey consisancy. Slather it between on both faces of the break and fit them together. Slather more on both sides of the fin. Lay a sheet of wax paper over each side of the fin. Sandwich the fin/epoxy/waxpaper between 2 pieces of 1x4 or 1x6 and clamp the whole thing until the epoxy cures. Pull it all apart and sand until you're back to shape. Glass if you want, but you won't really have to.

I would scrap the fin, and see if you can get the internal piece out so you could make a new fin that's through the wall down to the motor mount. I only do surface level replacements or jigsaw puzzle fin repairs on LPR rockets.

tollyman said:

I recently flew my unique 12" diameter sonotube scratch build level 3 certification rocket and unfortunately the main parachute didnt open. This lead to one of the fins breaking in half and cracking and another fin fillet to crack. The fins are made of 1/2" plywood.

My plan was to cleanly cut the fin on the rocket, then attach a new fin section to match the rest of the fins. I would attach it using metal strapping and bolts, then fiberglass over the top with two layers of 8oz e-glass and epoxy resin. Would this be a strong enough solution to be reliable and not be a weak spot? or would it be more worth it to not risk failing the L3 flight again and building a new rocket. See images attached

If there are different ways you would repair these fins how would you do it?

Click to expand...

Every time I attempted to repair broken plywood fins (on smaller scale), the new attachment point / glue joint became the week point, and the fin would (eventually) break there again.

If feasible, I would remove/cut/dremel-out all of the remnants of the old fin, and epoxy in a brand new fin.
If fully removing the broken fin is not a viable option, then proceed with your stated plan, assuming the airframe is thick and strong enough to anchor the bolts.

Good luck,
a

afadeev said:

Every time I attempted to repair broken plywood fins (on smaller scale), the new attachment point / glue joint became the week point, and the fin would (eventually) break there again.

If feasible, I would remove/cut/dremel-out all of the remnants of the old fin, and epoxy in a brand new fin.

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Yes, I agree, but again, big question is how he attached the root edge inside the frame. Given that it is probably a 98mm mount inside a 12" airframe, if there is an "extreme" attachment on the root edge, there unlikely to be able to excavate without damage. If you have a basic epoxy filet internally, then breaking free and installing a new fin is a snap (after dremelling at airframe joint of course).

tollyman said:

what are your opinions on fitting the two pieces back together like puzzle pieces then fasten it together with epoxy fiberglass and mechanical fasteners?

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I wouldn't recommend it.
The plywood is to badly damaged.
Full fin replacement (as others suggested) is probably your best bet.
Since this is out of my league rocket wise, I can not offer advice outside the plywood condition itself.

I understand that this is scratch built using sonotube. Did you use Baltic birch plywood or construction grade? The strength of the core is different. I’m just curious.
I had a similar break on a plywood fin. I used a Fein tool to cut the fillets. Snapping the fin/motor tube off was tricky. I hit it with a hammer and was then able to pull the broken fin out.
I recommend not having the grain of any plies of the plywood fins parallel to the axis of the rocket. Line up the outer grain like you would a balsa fin.

Steve Shannon said:

I understand that this is scratch built using sonotube. Did you use Baltic birch plywood or construction grade? The strength of the core is different. I’m just curious.
I had a similar break on a plywood fin. I used a Fein tool to cut the fillets. Snapping the fin/motor tube off was tricky. I hit it with a hammer and was then able to pull the broken fin out.
I recommend not having the grain of any plies of the plywood fins parallel to the axis of the rocket. Line up the outer grain like you would a balsa fin.

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That looks to be construction grade ply not Baltic Birch, several reasons why, the cores plys are way to coarse grained, and wrong colors, skin plys have too pronounced of grain, they look to be pine/fir. Best thing would be to totally remove fin and replace if possible.

So to answer some of your questions. Attached is an image of how i attached the fins internally. Using quite a bit of rocket poxy with fiberglass over top of the 75mm mmt and the fin tabs. I feel that if i try to remove the fin that it will damage the sonotube so much so that it would not be flyable and that the root cord connection would be significantly weaker than original. With that being said, i dont know the condition of the internal fillets. I think what i plan on doing is trying to fix the fin with some of the suggestions above, and then seeing how strong it is. If its too weak well then i can build a new rocket and i only spent a little time and money on the fix.

Hmmm . . . Assuming that you have internal fillets, getting that fin out would be very difficult. I think I would glue the broken fin back on with lots of wood glue. Cover with wax paper and clamp boards onto it real tight. Then after it dries, make it as nice as possible with Bondo putty and a palm sander. That should be pretty strong but if it breaks again you're no worse off than you are now.

Nathan said:

Hmmm . . . Assuming that you have internal fillets, getting that fin out would be very difficult. I think I would glue the broken fin back on with lots of wood glue. Cover with wax paper and clamp boards onto it real tight. Then after it dries, make it as nice as possible with Bondo putty and a palm sander. That should be pretty strong but if it breaks again you're no worse off than you are now.

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You dont think that epoxy resign with fiberglass and mechanical fasteners would be stronger than wood glue?

tollyman said:

You dont think that epoxy resign with fiberglass and mechanical fasteners would be stronger than wood glue?

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As you have probably heard before, wood glue joints are stronger than wood. The question is how strong is the plywood in your broken fin parts. You're right, it is pretty shredded. If you are going to repair it by gluing those parts together it may need some additional reinforcement as you described. Like fiberglass.

tollyman said:

So to answer some of your questions. Attached is an image of how i attached the fins internally. Using quite a bit of rocket poxy with fiberglass over top of the 75mm mmt and the fin tabs. I feel that if i try to remove the fin that it will damage the sonotube so much so that it would not be flyable and that the root cord connection would be significantly weaker than original. With that being said, i dont know the condition of the internal fillets. I think what i plan on doing is trying to fix the fin with some of the suggestions above, and then seeing how strong it is. If its too weak well then i can build a new rocket and i only spent a little time and money on the fix.

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Based on your photographs, you’re right; you wouldn’t be able to remove and replace the fin all the way to the motor mount. You used a very strong construction technique which is much stronger than the fin material.
Aligning the fin with the outside grain parallel to the central axis of the rocket left you with a fin that could be more easily broken than if the grain were diagonal to the central axis and parallel to the leading edge of the fin.
If the parts fit together tightly, I would glue them back together. Either a good wood glue or epoxy would be sufficient. The wood’s not going to be any stronger because you used one instead of the other. Then I would do as you suggest and laminate the fin with two or three layers of fiberglass and epoxy resin. You may as well laminate all of the fins while you’re at it.
If the parts don’t fit tightly together you may not have any other option but to cut the original fin flush with the body tube and surface mount a fin in its place using epoxy and fiberglass.