L3 Fins

I am trying to get my L3 rocket "Big Boy" back on the front burner. I am going to fly it with the Largest AT 75mm L motor before going to the M. It is made from 7.5" Giant Leap Flex Phenolic with 2 layers of 6 OZ Glass Laminated with West System Epoxy. The Fins are .125 G10 to the 75mm motor tube with Dowels layed in the full internal Fillets. The Fin can section is also filled with 2 Part foam. My first NAR L3 guy is no longer doing that (I started this thing before anyone was NAR L3 certified). My next guy did not like the .125" G10 for fins because of vibration (I tend to agree with him). My first idea was to epoxy 1/16" aircraft plywood to each side of the fin and then use 2 layers of 6oz. glass layed tip to tip over that. The plywood was to damp the vibrations and the fiberglass was for strength and to hold the plywood on. Now Im am thinking of doing away with the ply wood and just going with the glass. I have heard that I don't want to be able to pluck the fin and have it act like a tuning fork. This is not a really high performance rocket, but a pretty conservative design (7.5"x12'single recovery) The booster has a coupler doubling it leaving just enough room for the payload compartment shoulder It really is pretty heavily built. I tried to design everything so that it does not depend on epoxy alone.
I am a ways from flying the thing, but I would like to start slinging epoxy on the fins again. All ideas are welcome!

Yours,

Dwight
(Leica)

Go to U.S. Composites and get yourself some carbon fiber cloth. You can either cover the fin area in carbon fiber or do what I'm doing for my level 3. Take a 6" strip of carbon fiber and go from tip to tip over the fins. Then cover the whole fin area with fiberglass. It makes the fins super stiff. No flex at all.

Here's a pic with the fiberglass over it. There are some bubbles but they will be filled tonight.

Leica,

Have you tried using FinSim? That is the only program I know of that will tell you if a rockets fins are strong enough. Visit the Apogee Roskets web site at:

https://www.apogeerockets.com/finsim.asp

I haven't used the software, but if its anything like RockSim it should be of the highest quality!

Bruce S. Levison, NAR #69055