Question on fin fillet size for large sonotube and plywood rocket

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Crooksz_Rocketz

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Good afternoon fellow rocketeers - I have a design question regarding fin fillet size. My current build uses 8.5" outer diameter sonotube wrapped twice with 10 oz fiberglass as the airframe. Fins are TTW to a 4" MMT. The fins are 33.5" root edge, 14" height and about the same tip chord. I am wondering what size fin fillet I should use. I've heard on forums that the fillet radius should be approximately 4 to 8% of the root chord but wanted to check if that's appropriate. 8% yields a 2.68" radius fillet, 4% yields a 1.34" radius fillet.
The fins are 0.5" baltic birch 9-ply and I will glass over the fillet and fins with 1-2 more layers of 10oz fiberglass after the fillets have cured. Using RocketPoxy for the fillets.
Also, the rocket will end up weighing approximately 100 lbs. Hopefully I didn't miss any details for your recommended fillet size!

Thanks!



T.J. Crooks
 
Do you know Tom Cohen? All of his rockets are Sonotube and ply, and weigh over 100#. You should come out to MDRA this weekend and talk to Tom.

Meanwhile, knowing very little myself, I can only speculate that if you are glassing over the fillets, then they don't need to be as robust. But there is a formula out there for fillet radius to breaking strength somewhere. I don't know it though.
 
Do you know Tom Cohen? All of his rockets are Sonotube and ply, and weigh over 100#. You should come out to MDRA this weekend and talk to Tom.

Meanwhile, knowing very little myself, I can only speculate that if you are glassing over the fillets, then they don't need to be as robust. But there is a formula out there for fillet radius to breaking strength somewhere. I don't know it though.

Yes, I do know Tom. As a matter of fact some of the sonotube that Tom gave me over a year ago has been used to build this rocket. My brother Rich and I used most of that sonotube in building our upscale King Kraken called "Cthulhu". I will be at MDRA tomorrow. I'm also bringing the rocket in question along with me.

T.J.
 
Excellent. Look me up! I'll probably be with Tom when he launches, but I'll also have two little blond kids with me.
 
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