Best Fin-to-Body-Tube Glassing Technique for HUGE Fins?

The fin template on the left is 22"wide by 28" long, and has a root of 12". Three fins will be attached to a 98mm Blue Tube. I'll be using 1/4" Baltic birch plywood stock for the fins.

I'll have a 24" dia. "centering ring" made from the Baltic birch sitting on top of the fins as an added 10" horizontal attachment point. I have read the very informative Fiberglassing Techniques thread.

But I thought I should ask specific advice about the fin/body tube fiberglassing process for such large fins as these. I thought of possibly reinforcing the bottom "leg" portion of the fins with layers of fiberglass and/or 1/4" plywood on either side, but I wasn't planning to glass the entire fin.

I'd appreciate any thoughts/directions/techniques/critiques. The finished rocket will weigh about 45 pounds. The football-shaped body will also be glassed over the 1" insulating foam circular layers, but I have experience in glassing an oddly-shaped large body from a previous project (McDonnell Douglas "Delta Clipper").

Thanks!!!

These fins are going to be extremely challenging; they are very likely to break on landing. I'd be tempted to add two outer rings to reinforce the fins and create a more solid fin can around the body tube. (Then your foam sections could fit forward, between and aft of the outer rings.) However, even with that, those aft bits of the fins are going to be fragile, and very likely the part that touches down first on landing.

JohnCoker said:

These fins are going to be extremely challenging; they are very likely to break on landing. I'd be tempted to add two outer rings to reinforce the fins and create a more solid fin can around the body tube. (Then your foam sections could fit forward, between and aft of the outer rings.) However, even with that, those aft bits of the fins are going to be fragile, and very likely the part that touches down first on landing.

Click to expand...

Thanks John. I think I'll make the fins from 1/2" plywood, and make the nosecone "neck" longer to add more forward BB's/epoxy weight. And I appreciate the idea of additional plywood rings to reinforce the fins.

My plan is to have a landing speed of no more than 15fps, and I'm looking at ways to deploy a second main chute at 300 or so feet to additionally slow the final descent (JL Chute Release being the easiest).

And forgot to mention in my original posting that the nosecone section will represent about 2/5th of the rocket's weight, and will descend under it's own parachute.

jmmome said:

Thanks John. I think I'll make the fins from 1/2" plywood

Click to expand...

Going with thicker plywood is going to add a ton of weight, which will affect your motor choices and perhaps require more nose weight.

Do you care about flying this more than once? If not, landing damage isn't a big deal. If so, maybe make the aft ends of the fins break-away in some way and just replace them if needed. I could imagine a butt joint with white glue so that the feet can shear off without putting too much stress on the main fin/body joint. Maybe someone with more knowledge of mechanical engineering would be worth consulting.

You could also consider incorporating some (clear) polycarbonate sheets between the body tube and fins to add some more support, but that adds a lot of weight and I'm not sure how much strength.

This most likely will be a "one and done" rocket. The extra weight will be fine, as I had always planned to use an "L" motor, and it still looks like the rocket will come in at 45 to 50 pounds.

I'm buying the fin stock tomorrow, and will cut one to confirm the weight which I've calculated via approximating the square inches the fin represents. I haven't built the nose section yet, but I have the flexibility to add length to the nose cone tube (inside the outer foam shape) which will allow me to add more BB's/epoxy to increase the nose weight.

Lastly, I've found some gardening "kneelers" that are of a different material than the normal ones with the glossy overcoating. They REALLY absorb a lot of energy, compared to the normal kneelers. I may try to affix a 2" thickness to the bottom of the fin feet to see if that reduces the stress upon impact.

Does the paper mock up in the first post indicate the position of the fins on the rocket? If so, are you tied into that location?

Can you move them well aft? Making fins larger does nothing to help the CP if the fins are too far up! I'm not sure that I'd call the LDRS version stable, and I fear that your version in its current planform will be even less so (unless you can move those fins aft a ways).

As John said, 1/2" plywood is problematic due to it's weight and frankly for the size of the project also overkill.

As Eric said, move the fins farther aft similar to the bottom snapshot in your first post to help stability.

Even with the fins exposed on landing, if you build them light and strong, you can still be OK. If money is plentiful, I would look at honeycomb material with carbon fiber sheathing (https://www.cstsales.com/a-carbon-fiber-sandwich-panels.html). A more budget friendly option is a minimal plywood frame with cardboard honeycomb inserts laminated with CF like in my build here: https://www.rocketryforum.com/threa...cal-engineer-in-the-house.68686/#post-1501721

External shock cord attachment on the tail (will need a divot on the nose close shoulder as a "pass through" for the shock cord)

Pretty easy to hide attachment point. Remove cord for display. At launch, attach the cord, load your laundry and feed extra cord into body of rocket. Run the remaining external section on the rail button side hidden from view. A small segment slips between nose shoulder and body tube.

Launch rocket, deploys as usual but dangles inverted, your delicate fin tips are the last part to touch down.