all carbon fins question

[r1dermon]

im toying with the idea of scratching my laid up fiberglass over ply fins for some same size, but considerably lighter and thinner carbon fins...

several years ago i was asking an accomplished L3 about an all carbon design for a mach buster (actually mach shredder), and he told me that it's always a wise choice to go with fiberglass underneath the carbon fiber because it's more elastic and less prone to catastrophically shatter. given the propensity for the fins to flutter in, lets face it, my less than "perfect" designs, i'd like a real world, or pretty educated opinion on the issue.

just as an FYI. i realize i could make the carbon fins thicker, and that would in turn reduce the risk, however, this is about shaving both weight, and drag, while increasing strength, so i'd like to compromise as little as possible in those areas in the name of reaching maximum velocity.

 

[rockets2000]

If you're going for maximum velocity, your fins shouldn't flutter....

 

[troj]

If you get fin flutter at Mach, kiss your rocket goodbye.

Sounds to me like a mess of nonsense.

-Kevin

 

[ewallace]

You could go with something thin like .047 G-10 FIBERGLASS SHEET then lay up carbon to the thickness you desire. Then tip to tip carbon to hold it on your carbon BT:confused2:

What size rocket are you working on?

 

[r1dermon]

It's a hair over 23" overall

I have ply fins already laminated and beveled with 6oz cloth. My plan was, and still is, to lay 3k carbon over them, and then tip to tip that with 3k fiber, and since it's almost minimum diameter, the TTW design extends not even 1cm into the BT.

I was browsing a carbon fiber suppliers website when I came across pre-made "plates" of already laid up carbon fiber, and was wondering if there would be a significant advantage in using something like that, over the components I already have.

I'm not looking for specific component suggestions, I am looking for experienced opinions as they pertain to my curiosity about a particular application of this particular component (carbon fiber).

thanks again.

 

[ewallace]

what diameter rocket are you doing ?

 

[r1dermon]

It's a LOC aura kit that's being converted into a 38mm 3FNC.

 

[ewallace]

I would think tip to tip 5.7 oz carbon would be plenty I had a loc weasel I flew on 38mm motors with out tip to tip.

 

[troj]

The pre-made plate is really nice stuff, but is also pricey. Basically the same thing was what the PR Mongoose fins are, if you've ever seen them.

-Kevin

 

[cjl]

Honestly, for fins, the stiffer the better. They should not be flexing or fluttering, so you shouldn't have to worry about any dampening or resistance to large bends. If your fins are fluttering, the solution is to make them stiffer rather than to worry about some theory about fiberglass cores.

One good reason to use fiberglass cores though is that the center of a plate does very little to resist bending stresses, and because of that, you barely lose any strength by using a fiberglass core (but you do lose a decent amount of the cost of the fin).

 

[bobkrech]

It's a hair over 23" overall

I have ply fins already laminated and beveled with 6oz cloth. My plan was, and still is, to lay 3k carbon over them, and then tip to tip that with 3k fiber, and since it's almost minimum diameter, the TTW design extends not even 1cm into the BT.

I was browsing a carbon fiber suppliers website when I came across pre-made "plates" of already laid up carbon fiber, and was wondering if there would be a significant advantage in using something like that, over the components I already have.

I'm not looking for specific component suggestions, I am looking for experienced opinions as they pertain to my curiosity about a particular application of this particular component (carbon fiber).

thanks again.

Your rocket is already overbuilt. Adding additional mass will only slow it down. For a given material, stiffness is proportional to thickness squared, and the thickness of the skin is the dominante factor. For a 38 mm rocket, you can use a foam core with thicker stiff skins for the lightest fins, but a plywood core a good alternative and much lighter than FG or CF sheet.

TTW fins are unnecessary in a rocket with a stiff composite airframe provided that the fins are also stiff and well bonded to the airframe and the fillet is reinforced by a layer of fabric in the resin fillet. If the fins are stiff, tip to tip covering only adds additional weight.

Bob

 

[r1dermon]

thanks for the answers...