Fin at body crack repair

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Mr G

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A perfect flight on an AT H180 sent my 3" LOC Shadowhawk to 1,350 feet and then back down gently to what appeared to be a soft landing.
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On inspection, a crack was discovered along the seam where the fillet attached to the booster tube. This was not totally unexpected as this was the first high power construction project and materials and techniques were in the early stages. Now, with Rocket Poxy in hand, I am ready to go back down to bare cardboard and birch if needed to build the fillet up again like it should have been the first time.
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Anyone have suggestions on how to do this? I don't want to take any cardboard or wood material away if it can be helped. Are there any particular tools that are good for this job? Should the void just be filled or does the whole filet have to be removed? I know I'll have to redo the paint job too.
 
If you think the fin is still bonded well to the motor tube, I would fill the crack with thin CA. Then use Bondo Glazing Putty or equivalent to smooth the area up for paint.
 
If you think the fin is still bonded well to the motor tube, I would fill the crack with thin CA. Then use Bondo Glazing Putty or equivalent to smooth the area up for paint.

+1 if you feel the fin is still bonded securely, then fill the crack either with either the CA or epoxy then prep for repainting. You can use various Dremel type bits/burrs/wheels to grind out the fillet and just redo it, its a pain but very doable.
 
A Dremel tool will cut through the crack in a few seconds. I usually take my repairs down to the cardboard. If you weren't using H motors then I'd just squirt some CA in the crack.
 
This is a dual deploy version that is relatively heavy so it'll go up on mostly H and I motors and a possible J now and then. The fins are really big compared to most rocket designs so there is a bunch of torque on the body/fin joint at touchdown.

Are folks expecting issues with flutter with larger motors and speeds or just the weight on landing stressing the fins at the body tube?
 
A Dremel tool will cut through the crack in a few seconds. I usually take my repairs down to the cardboard. If you weren't using H motors then I'd just squirt some CA in the crack.

Dittos on the Dremel. I've had good luck with using a tapered sanding cone in the Dremel to remove the original epoxy fillet, and this minimizes digging too much into fin and body tube. With this clean, unpainted surface exposed, you can assess the damage, and re-epoxy the joint, then re-fillet the joint and re-paint. For minimal cracks, I've also dribbled CA into the joint. That may be enough, but epoxy can still be applied before re-painting.
 
This is a dual deploy version that is relatively heavy so it'll go up on mostly H and I motors and a possible J now and then. The fins are really big compared to most rocket designs so there is a bunch of torque on the body/fin joint at touchdown.

Are folks expecting issues with flutter with larger motors and speeds or just the weight on landing stressing the fins at the body tube?

Looking forward to hearing about a J-shred, I'm not sure the Estes tubes could handle a J without shredding, even the low thrusting long burn varieties.
 
"Are folks expecting issues with flutter with larger motors and speeds or just the weight on landing stressing the fins at the body tube?"

The problem with that sort of damage is that you can never quite tell how many layers of cardboard became delaminated. So if you squirt some CA in it, you might bond some of the layers together, but there still might be a major loss of strength to the repair. It won't matter as much with the lower impulse motors.

On the other hand, by removing all of the cracked areas, you can be assured that you repair will be a structural repair.
 
Sounds like the prudent thing to do is take it all the way down and build it up again. As mentioned, that provides the opportunity to evaluate the extent of the problem and take appropriate steps to assure the fin is properly attached to the body tube and motor mount.

The LOC cardboard tubing is quite robust so there are no qualms with a slow burn J motor (other than finding the rocket after it goes out of sight).

Good suggestion on using a Dremel tool. I have one so I'll put it to work.
 
I dont think the fillet failed, the fillet itself is not cracked- .just what it was bonded to lifted off the tube .

If it doesnt rock it maybe zap the edge and addrss the paint.

Looks l Iike it hit first when landing..not during boost to me

Kenny
 
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Using a Dremel tool and a wood chisel, the filet was cleared away revealing that the epoxy had adhered to the birch fin quite well but not as well on the cardboard.

The culprit for the cracking was separation of the fin from the motor mount tube. The fin must have bonked the ground at just the right angle and velocity to put too much shearing pressure on the fin/motor mount joint. Not unsurprisingly, the cardboard is what gave. The fin has a 4 3/4 inch cord and a 3/4 inch tab inside the body tube so there is roughly a 6 to 1 mechanical advantage from the tip to the other end.

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This appears to be a classic situation where internal filets are warranted for this design. With the fin completely detached, we can now clean everything up and make room to add strength to the base of the fins. The plan is to have enough space to get inside and reach the adjoining fins. This way (hopefully) an internal filet can be applied to at least one side of the other fins.

Can anyone provide advise on a good method for internal filets in this situation?
 
Using a Dremel tool and a wood chisel, the filet was cleared away revealing that the epoxy had adhered to the birch fin quite well but not as well on the cardboard.

The culprit for the cracking was separation of the fin from the motor mount tube. The fin must have bonked the ground at just the right angle and velocity to put too much shearing pressure on the fin/motor mount joint. Not unsurprisingly, the cardboard is what gave. The fin has a 4 3/4 inch cord and a 3/4 inch tab inside the body tube so there is roughly a 6 to 1 mechanical advantage from the tip to the other end.

View attachment 268993 View attachment 268991 View attachment 268992

This appears to be a classic situation where internal filets are warranted for this design. With the fin completely detached, we can now clean everything up and make room to add strength to the base of the fins. The plan is to have enough space to get inside and reach the adjoining fins. This way (hopefully) an internal filet can be applied to at least one side of the other fins.

Can anyone provide advise on a good method for internal filets in this situation?

inject the fillets, drill one or two small holes that can be covered by the external fillets, and inject your adhesive into the holes, epoxy will flow out flat with the rocket laid flat on a cradle, you can do two sets of internal fillets at a time.
 
IMG_4785.jpgIt was a bit of a hack job, but by threading a coffee stir stick carefully through the fin slot, one side of the base of the fins on either side got a good dose of Rocketpoxy. Next, the motor mount got a thick coat, then the fin was inserted with filets laid in as the Rocketpoxy was thickening up. Hopefully this will protect three out of four fins from loosening up.
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Thanks for starting this post. It was very helpful in working through a similar situation which recently happened to a rocket (Blue Tube "Executioner XL") I built a couple of years ago and finally found time to fly this summer.

I actually built two at the time, the XL and an Estes Executioner kit.
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I love the Executioner and have built a lot of them of the years. It is a relatively low cost kit (internet sales and Hobby Lobby coupons) that is easy build, looks great, is fun to upscale, fly on HP or use for clusters. I am beginning to wonder however, if the kits are cursed or I am, as mine always seem to meet an untimely demise. A nasty CATO from a previous stretched Executioner:
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The Estes Executioner kit in the first photo met an equally gruesome fate on its maiden flight when the ejection charge failed to ignite and the rocket lawn darted spectacularly at the last Southern Thunder:
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So I waited a couple of years to undertake the maiden flight of the Blue Tube Executioner XL, to allow ample time for a cosmic realignment and the bad juju to pass. The XL's inaugural flight on a Cesaroni I345 was a thing of beauty, at least what I saw of it before it disappeared into the low ceiling.
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When if finally reappeared below the clouds, with the chute deployed, the XL managed to find its way back to within the 50 feet of the launch site.
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It had been a sound decision to wait out the stars. Unfortunately, the wait did nothing to improve my faculties, as I somehow deduced that using an undersized X Form chute, for a heavy blue tube rocket with 1/8" non-reinforced plywood fins, made perfect sense. Brilliant! My mental acuity rewarded me with a cracked fin along the body:
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Rather than file 13 this baby, with all my other mishaps attributable to operator error, I decided I would attempt to repair the fin using some of the many great techniques shared on the Forum. So I commenced to strip down the rocket and assess the damage; an especially painful exercise as Mark at StickerShock23 had done such a great job on these vinyl decals!:
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Not only had the fin cracked at the body tube, but it had creased in the middle of the fin to the right of the crack. The further I got into the repair, it was evident the damage was worse than I initially thought.
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I concluded it was going to take more than filling the crack with epoxy to stabilize this fin and prevent future cracks of these thin fins from future rough landings. This repair was going to require my first foray into fiberglass and carbon fiber.
 
The first step was to straighten/realign the fin and fill the crack with epoxy.
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Straight enough. Now the epoxy:
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After shutting off my brain and camera, i followed tfish's video on tip to tip glassing of fins (a great resource!). I did think a layer of carbon would be a nice touch for the Executioner, so I added a layer of carbon fiber on top the layer of glass that went on first. I also read through, several times, Jim Jarvis' Guide on Carbon Fiber which was extremely helpful, given I had no clue on how to work with CF. Again - apologies for the absence of pics on these steps. I agonized over this and overworked it a lot but, the final results were acceptable for my first attempt with these techniques:
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Fortunately, the sunlight and high gloss shine of the epoxy mask the many flaws on this fin can. I used too much epoxy on the glass and CF, and there were a lot of scratches, low spots, etc which I could not get out after countless attempts at sanding. Eventually, I had to say "uncle" as I was "done" with sanding! It was a good learning experience nonetheless and the results will only get better from here, hopefully. The last step was to order new decals from Mark at StickerShock23, which he turned around in no time as usual. The finished repair of the cracked fin:
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it was a lot easier to do than I built it up to be in my head!

Where are you guys flying HPR now that the field in Manchester, TN was lost?
 
We have a couple fields near Woodville AL owned by a farmer that we can use Oct - Mar/Apr weather permitting!

Flights to 10,800 standard
 
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