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viciouspeanut

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Hey everybody,

I'm currently working on my 2.6" Big Bertha upscale and am debating on the type of wood for the fins. I had a sheet of 3/16 basswood but it has warped so I figured I could rethink this now. It has a 3x24mm cluster mount, but I imagine it will only fly on D12s and E9s so it shouldn't have too much stress put on it. My only concerns are with how big the upscaled fins are I don't want them to split on a hard landing. I do not plan on doing TTW fins, but I can alter that if it would be prudent - currently planning on drill a few small holes for glue to seep through and roughing up the coating of the body tube for adhesion. What do you all think? Basswood good enough or should I do 1/8" ply and cut some slits for TTW?
 

dr wogz

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I would go with Ply, good birch ply. I would also strongly recommend TTW, as the fin are large, and do hang well below the tail end of the rocket. They will meet the ground first in just about every landing..
 

viciouspeanut

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TTW would be nice, but as a three motor cluster and four fins it would prove complicated I think. Hmm..
 

EXPjawa

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You can do 4-fin TTW on the 3-motor cluster mount, but it'll require a little planning and making sure you align the mount correctly. I did the same thing when I upscaled an Astron Cobra. I had 2 fins with deep tabs and 2 fins with stubby tabs. I also built essentially what you're doing now, except that I had one 29mm mount and I used 1/8" balsa for fins (per the original Super Big Bertha). I've broken fins cross-grain twice now, so you're definitely stepping in the right direction with 3/16 bass. But I think I'd avoid single-grained material. I used 3/16" ply on the Cobra, which has a similar fin condition.
 

viciouspeanut

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I'm weighing stepping to a LOC body tube now instead of BT80, or just beveling each fin to fit in the slot of the motor mount gap. Not sure I'm accurate enough for that lol
 

Peter Olivola

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Using LOC tubing isn't necessary if you're not going to fly it on anything bigger than black powder motors. It will add weight, which will have a negative effect on landing impact stresses. One of the ways to minimize fin damage is to build light overall. Bass would be a good choice. Light ply might be better. Five ply aircraft birch would be overkill and add significant weight. Another consideration would be using delayed chute deployment permitting the use of a larger chute without the risk of excess drift.

I'm weighing stepping to a LOC body tube now instead of BT80, or just beveling each fin to fit in the slot of the motor mount gap. Not sure I'm accurate enough for that lol
 

jlabrasca

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I did a BT70/BT80 upscale of an Estes Scrambler (the K-37, 3 engine cluster). I also had to deal with big fins and badly warped stock (3/16 balsa). I papered the wood before I cut the fins. I sandwiched the balsa between two pieces of printer paper, adhered with the thinest coats of Tite-bond that I could manage. I clamped it tightly between two pieces of 3/4" plywood and left it for the weekend. Came out dead flat and very rigid. As a bonus, you can print the fin templates on the paper before gluing it on. I screwed up a little trying to get CA onto the exposed wood on the edges of the cut fins. The drips showed through the paint. Otherwise it worked pretty well.
 

Woody's Workshop

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Your are basically making a Super Big Bertha, or Broadsword except you are using engine clustering.
I would use balsa for the fins. In the kits, they provide extra balsa pieces to glue next to the root edge and the BT for extra support.
Papering is also a good idea. Use the Elmers wood putty and then paper, it comes out really nice that way.
Be sure to posts some pics, Berth's are always welcome!
 

jlabrasca

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TTW would be nice, but as a three motor cluster and four fins it would prove complicated I think. Hmm..
Not sure how complicated is "complicated". I had an idea for TTW fins, not anchored to the MM tube, for an upscale Gyroc. Unfortunately, I didn't have the idea until AFTER the Gyroc disintegrated shortly after take-off.

Basically, extensions on the fin tabs that fit around rings of coupler stock at the top and bottom. Gives you some material for internal fillets and the extensions provide some tensile support. I am using a modification of this design on the two rockets currently on my bench.

berthatfinA.png
 

Peter Olivola

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Elegant solution.

Not sure how complicated is "complicated". I had an idea for TTW fins, not anchored to the MM tube, for an upscale Gyroc. Unfortunately, I didn't have the idea until AFTER the Gyroc disintegrated shortly after take-off.

Basically, extensions on the fin tabs that fit around rings of coupler stock at the top and bottom. Gives you some material for internal fillets and the extensions provide some tensile support. I am using a modification of this design on the two rockets currently on my bench.

View attachment 321931
 

CORZERO

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Estes or similar paper BT80 tube, TTW, 1/8" Basswood, gtg. My 29mm Estes Phoenix with TTW and 1/8 basswood weighing in at 32oz loaded (subtract spent fuel) lands with no issues at 19fps on grass/hard dirt. Not even a stress crack in the ppg clearcoat.
 

TangoJuliet

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Why not do it like the MDRM? Cut a 1/8" Plywood core (with lightening holes) then sheet both sides with your choice of balsa sheet thickness. And I would go through-the-wall.
 
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