"PL Premium polyurethane glue (from Lowes) to secure the fins"

You are using this to attach the fins internally? The MMT connection makes sense.

How about the internal fillets?

I foam my fin cans, but have been making rockets that are on the heavy side due to generous epoxy fillets on the inside. I'm starting to think this is overkill. The foam is polyurethane as is the glue you recommend. Seems like a great idea. They should stick like crazy.

Using the PL Premium polyurethane glue only in the fin can and then foaming it seems appealing. I'll still use epoxy fillets on the outside of the main tube. Is this what you do?

I assemble my motor mount and fins outside the rocket, then slide into the airframe. I use the PL Premium on the MMT to fin connection. I don't do internal fillets to the airframe. Never have. I don't know why. I don't foam the fincan either. Never wanted to add weight to that end of the rocket. On the outside I'll go over the gap between the airframe with some very very thick epoxy to form nice fillets. After that I'm done.

I have before run a 1/4" to 3/8" fillet of PL Premium and then put a strip of fiberglass over that onto the fin a bit so that it won't show through the airframe. That really stiffens them up and gives the glass a radius vs. a sharp bend.

I build my rockets as light as possible and for a proper flight. I could build them to come in hot or a parachute malfunction and be okay, but why build to survive failure. I've had much more success building them for flight and concentrating on having all my events happen properly.

Edward

As it differs with my approach. Your experience is invaluable.

I think I'm overbuilding, think 57 Chevy. I'm going to try something radically different along your lines and see how it goes.

This is interesting and a definite improvement in my build philosophy.

Taking 20% of my build weight is significant. I did a quick calculation and feel it's an easy weight percent to drop.

Great info from all... Thanks