What exact calculations? Plywood breaking strength? Epoxy holding strength?

Start with shear strength of the epoxy, but all have to be considered. Theyre not typically difficult.

Force / Area = Stress. Rearrange as needed.

Let say that you have three centering rings. The area of each centering ring where is touches the motor mount tube is 1/4 inch (the thickness) times the circumference of the hole the tube passes through, right (2 x pi x radius). Just to make life easier lets say thats a 2 inch radius (4 inch motor mount) and well cheat and use 3 instead of pi. Thats less than pi so were actually being conservative. So, the area of the glue joint for a single centering ring is 1/4 x 2 x 3 x 2 = 3 in^2. As someone pointed out, this doesnt include fillets. I think it really emphasizes the importance of fillets in fact.

Looking at the material properties of Aeropoxy 6209:

https://www.ptm-w.com/technical-lib...oducts Bulletins/AEROPOXY ES6209 Bulletin.pdf
we see that the aluminum to aluminum tensile lap shear strength at 77°F is 2900 psi. Thats the only lap shear strength I found in my 15 seconds of googling. Notice how it goes down to 700 psi at 140°F.

Again, were going to err significantly on the side of caution. Well assume you painted the rocket black and youre flying it at Airfest on one of the hot days, so its entirely possible the inner spaces, sitting in the sun, are hot. We use 700 psi. Psi is actually lbs./in^2.

700 psi x 3 in^2 = 2100 lbs.

In other words, the glue joint at the interface between the centering ring and motor mount tube, for a single centering ring, holds a ton+ and you have multiple centering rings, plus fin tabs, etc.

So epoxy isnt the weak link. Using the same areas look up material properties for your different components. Heres a document discussing the shear strength of birch plywood.

https://ac.els-cdn.com/S18777058173...t=1519295597_5f755dd82c4ed9313804ab72b0ba03ea
It lists the shear strength of plywood as 7.11 mPa, which is 1031 psi.

The force of the motor is simply the peak thrust. I would apply a margin of two or more just to be conservative.

As long as you prepare your joints right, I see no problem.

Steve Shannon