Reattaching a Broken Fin

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MurphysLaw42

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If you were at the NSL later in the day Monday you might have seen my DX3 XL go up. The up part went great, but the down part not so much so I'm looking for some tips on how to do a little rocket surgery.

Booster detached from my Kevlar and took a bit of a tumble. Landed right on one of the fins and actually turned it sideways and jammed the root into the body tube. I don't have a good picture of the initial condition, only after I'd tried to pull the fin free a little bit. I've attached the photo I do have but it doesn't really show just how sideways it went.

One of the other fins looks like the fillet has a hairline crack as well and because the fin that broke was jammed in so hard (it was pretty much lying tangent to the body tube) it might have deformed the airframe enough to break my internal joints between the fins and body tube or motor mount.

I'm looking for ideas of the best way to inspect for damage and repair the break, so any and all help would be appreciated.

 
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EeebeeE

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Try jiggling the fin that was most separated back and forth until it and the root come out. Inspect the fin and airframe for damage. You may need to cut a new fin, and you can use the old for a template.
Assuming the airframe is paper... If the airframe isn't too damaged, the fillets for the new fin should strengthen up that part of the airframe. If it is damaged to the point of repair, straighten it out as much as you can. Sand it down through the white paper. Mix 15 min. or 30 min. epoxy together and thin it down with alcohol. Apply to the damaged area of the airframe, then soak a piece of bond paper the size of the damaged area in the same mixture and apply across the damaged area. You've essentially made the damaged area phenolic. The thinned epoxy will soak into the damaged section of airframe and set, re-bonding the broken fibers of the paper tube. If you want to save time, you can use thin (not the gel) slow-drying CA into the damaged area. In both cases you will need to sand when finished to smooth out the surface.

If the other fin is cracked at the fillet, sand the fillet off and make sure the crack was in the fin. It may have been just the fillet. If so, replace the fillet and you're done. if the fin was slightly cracked but not too loose and you are fairly certain the bond is strong at the root/mmt, sand the paint off the entire fin. Use the same epoxy mix above on the crack, then cut paper in the shape of the fin and paper both sides.

If the fin is loose, and sanding off the fillet makes it even more loose, try wiggling it back and forth some more and removing it, then do as you did with the first fin.

After repairs are done your rocket should be about 85-90% as strong as original which should be OK as long as you do not try to push it to the limits.
 
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MurphysLaw42

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It's a Madcow DX3 XL all-fiberglass kit (4" dia., 92" long, 75mm mount). Fins themselves are fine, it was the epoxy that broke but I can't get into the airframe to see if the epoxy inside broke as well.
 

djs

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I had something similar happen to one of my rockets. I ground down the fillets and was able to pull out the fin completely (so much for internal epoxy!). Then I ground and sanded down everything and then started over completely. I think it was just a bad epoxy job in the beginning, as eventually the other two fins popped loose.

Anyway, see if you can pull the fin out intact, and go from there with a Dremel.
 
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