Article: 1920
By Robert Krausert, January 2010

Julian Picard asked:
My name’s Julian Picard, and I’m new to the list. I’m working on a building a 54mm minimum diameter, and I’m looking for suggestions on how to better attach the fins. What I did before is to make shallow fin slots and do some initial gluing, and then solidly glassing (or with carbon fiber) the fin can. I’m using a pre-glassed phenolic tube, with G10 fins.

Thanks so much for you help!

Scott Barfield commented:
I just got the fins on my 75mm min diameter. I cut slots and stuck in a plastic wrapped motor case. Tacked the fins into the slots with epoxy, butted up against the case. Pulled the case. Added fillets to the fins with epoxy mixed with milled fiberglass. Then applied two layers of glass, one halfway and one tip to tip. Seem rock solid.

Julian Picard asked:
You said you used epoxy mixed with milled fiberglass: two questions, how large did you make your fillets? Any larger than normal? Also, I’ve heard of people using fiberglass boat-body sealent as fillet material, have you or anyone else heard is that’s a successful method?

Greg Clark commented:
I used chopped Kevlar in my fillets, but just about any ‘chopped’ material will add some needed strength. For my 3-4″ rockets, I lay out the epoxy, cover it with a plastic sheet, and “form it” with a piece of 3/4″ PVC pipe. Result? Perfect fillets!

Important, since sanding kevlar is no fun…

Mike Fisher commented:
JB Weld with milled fiber. You’ll need the Industrial sized tubes, not the little ones.

Lay it in heavy on both sides of the same fin, then press 1″ pvc pipe into both fillets until it squeezes out the ends. Then rubber band the pipes together and then rubber band them to the airframe on both ends. When the JB Weld is totally cured, tap the PVC pipes off with a hammer and smooth out the squeeze out with a drum or flap sander on your Dremel tool. You can do all the fillets at the same time if you have enough PVC pipes cut and some coordination. Perfect molded fillets with just a little work to fair them in at the ends.

You can tip-to-tip glass over the top of that or not. I’ve tested it to mach 2 without any overlay.

I’ve done the same thing using Bondo, but it is non structural and polyester does not stick to epoxy composites so well.

The JB Weld sands easily enough anyway. Wear a mask.

Jeff Moore commented:
I like Mikes method too! I’ll have to try it myself.

The last minimum diameter rocket I built, I wanted the fins to stay on. So I drilled a series of small wire diameter holes along the root edge of the fins. I left enough material around each hole that I was confident it would take some serious torque to break the G10. I tacked the fins on with enough epoxy to ensure a decent joint and to hold alignment. Once the epoxy setup, I took a length of picture hanging wire and proceeded to thread it through the holes running from fin to fin around the airframe until I had run it through every hole. The result was a stiff wire cage holding the root edge of the fins against the body tube. I then used a screwdriver blade to bend the wire down as close to the body tube as possible. Fillets were next using milled fiberglass mixed in with the epoxy. Then some microlight body filler to cover over the wires against the body tube and smoothe things out. Then a layer of unidirectional carbon fiber tip to tip with the tow oriented from tip to tip. This covers the wires quite nicely and smoothes everything out and also inhibits the fins from flexing in flight.

This is probably over-kill for a 54mm rocket, but those fins aren’t coming off. They may break above the fillets but the root edge will still be attached to the body tube.

If I use the technique again, I’ll probably try a thick kevlar thread instead since it will lay tighter against the body tube and will be easier to cover over – probably won’t be quite as strong though!