Question about Plywood?

So PGerringer and I are discussing (loudly as we always do) plywood. I'm wondering if I can just go to Lowes or Home Depot to buy large sheets of birch 1/4" plywood to use in my rockets instead of the "craft" plywood that you get from hobby and craft stores. It's a heck of a lot cheaper to buy it this way. I can buy a 4'x4' piece of birch for what I pay for a 12"x12" piece of the craft plywood. The density of the birch looks just as dense as the craft plywood. The birch seems just as strong as the craft.

Is there an advantage to using the craft plywood? Any opinions?

Check the number of layers on the craft plywood compared to that at Lowes. The craft plywood should have more layers, which makes it stronger; that's why it is more expensive.

I know for 1/2", 9 Ply is a popular standard for bulkheads and centering rings while the 1/2" plywood at Lowes is only 5 ply.

Carl

At the 1/4" level it appears the number of plys is five. I will look into this aspect of it. Has anybody done any strength testing on the different types of plywood? I would think that in smaller rockets such as the A2 where 29mm motor mounts are used in conjunction with smaller rocket tubes that the commercial grade plywood would strong enough.

I use 4 mm plywood (20/127" ;-) for fins, centering rings, bulkheads and what not. Strength is absolutely sufficient. If you're building LPR models though, 3 mm (1/8") hobby plywood would be better since it's much lighter, and 3 mm is prolly not available at Home Depot/Lowe's etc. But for LPR models, I'd use balsa wood anyways...

Oliver

Balsa wood? That is interesting. Balsa does have an incredible strength to weight ratio but I never thought that balsa would be strong enough for center rings or fins in HPR. I would think it would be much easy to mill as well.

LPR is low power, _H_PR is high power... Actually, I'd think 3 mm balsa with 1 layer of 1.5 oz glass on each side work for HPR in the level 1 range... I made fins that way, they're of course not as strong as G10, but as strong as plywood.

Oliver

ooops... Looked right over that LPR/HPR thing. Must be getting old, my eyes deceived me.

I have used birch face plywood, luwan, and regular 1/4 inch plywood for fins and centering rings. I do find layed birch plywood to be stronger and will usally increase the thickness about a 1/3 when using a substitute. So 1/4" instead of 3/16" is often how it works. I often use regular 1/4 inch plywood for centering rings, altimeter bays and other internal structures as it is cheap and the slight extra thickness makes little performance difference. However the fins will be made from 3/16 four layer craft birch plywood for better aerodynamics and chip resitance on landing. The edges of layered birch plywood also sand into a smoother profile then Luwan or birch face plywood as the core is soft in these materials.
Happy flying Mark B