I use 2 layers of 1/4" Baltic Birch ply (glued together, to create a stepped bulkhead) and large fender washers to spread the load of the eyebolt or U-bolts and tie rods. The motor size has nothing to do with the thickness, its the expected shock loadings thats important.While you can expect a wide variation in the strength of different types of plywood, when it comes to low and slow M power builds are the bulkhead generally fabricated from ¾” or 1” thick ply? Something else?
Bulkhead, not centering rings.If you are doing through-the-wall fins then 1/4" is more than ample IMHO. The structure formed by the rings and the TTW fins is what provides the stiffness and thus takes the majority of the thrust to the airframe in a typical HPR rocket.
Oops. My bad.Bulkhead, not centering rings.
The motor size has nothing to do with the thickness, its the expected shock loadings thats important.
That may be, but an approximate mass and diameter would be a more accurate gauge, M motor and low and slow does usually mean draggy and heavy. I still go back to figuring shock loads should be the metric that determines what material is used in this case.I think the OP was hinting at the rocket's mass by mentioning the M-motor in combination with low & slow.
That may be, but an approximate mass and diameter would be a more accurate gauge
If you are doing through-the-wall fins then 1/4" is more than ample IMHO. The structure formed by the rings and the TTW fins is what provides the stiffness and thus takes the majority of the thrust to the airframe in a typical HPR rocket.
[edit] Oops. I answered according to centering rings, not bulkheads.
Wasn't really a hint ;-) Build material used is always going to be a function of mass. You wouldn't build rocket that fly's on maybe F motor out 1/2 ply.I think the OP was hinting at the rocket's mass by mentioning the M-motor in combination with low & slow.
when you read the OP wrong...and take a long time to write a reply..and guys are correction it's not centering rings..it's bulkheads....I'm gonna leave the below text as is and....I will be using some of these as bulkheads and not sure how I'm going to put this material to use as bulkheads yet...
Love the pics, thx! Especially the "MDF Like" note. As I mentioned in my OP lots of variation in ply, but if you are not going for an altitude record ;-) one can assume heavier/thicker is better. What I'm gleaning from the posts here is that 1/2" is pretty typical The rocket on the drawing board is a 10" sonotube so I'm right back to a 3/4" birch ply to accommodate the large dia. spans.
In reading all the comments, can you tell us what structural force or condition do you feel you are designing for that would require you to go with a 3/4" thick ply? A well designed motor & fin area will come together as a pretty high-performing "structural assembly". What is your concern, or what do you think is going to be the possible failure mode? Not passing judgement, I'm just curious. Like tfish, I work with alot of student rocketry teams, and it's always interesting to hear what they think is going to happen and why.
Since weight is not a factor beyond a safe rail departure velocity for a BIG L or small M the failure mode would be some "bad" ply in a bulkhead that is not a centering ring. Trading weight for assurance the structure can't fail.
So the issue is the quality of the material you're using. A low-grade plywood vs. a high-grade plywood. I get that.
This is the key- size and amount of force.I use 2 layers of 1/4" Baltic Birch ply (glued together, to create a stepped bulkhead) and large fender washers to spread the load of the eyebolt or U-bolts and tie rods. The motor size has nothing to do with the thickness, its the expected shock loadings thats important.
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