Strong enough bulkhead

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Ryan S.

Well-Known Member
Mar 24, 2003
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Some have you have seen the bulkhead in my Mission IMpossible post, and if you have you will know I broke one of them with my bare hands, part of this was due to the wood, and I am getting better wood, but I was wondering, is there a better place to cut out the 2 circles so the bulkheads will be as strong as possible, and will not break? Thanks alot
If that's plywood, then no matter where you cut the holes, it will always retain the same inherent weak spots. You may just have to get thicker ply.

One consideration would be to remake it, but put in two ring mounts and run a 3-point sling rig so you can put the rings off to the sides of the main holes. This will give the overall structure more to hold itself together.

Of course, the downside to the sling method is that it puts a lot of compressive forces on those U-rings, as oppose to the purely linear force you have now.

I used 1/2" 5 ply plywood and I am going to get some different plywood, this time with more ply. The 2 u bolts doesnt sound like a bad idea, but that will probably add alot of weight.

What if I move both holes to one side, or put the U bolt off to one side of the bulkhead?
Another thing you could try: get the new plywood, 9 ply would be best, then, before you cut the hole, laminate the wood with 6oz. or heavier glass on each side. That should help.
sorry to be vague, I was flexing it and I heard it cracking, I couldnt break it though, then I noticed some of the ply that was sandwiched was cracked, as I further inspected I started pulling and I pulled the ply apart, and the rest of it easily cracked in half. So, the ply pulled apart, and it was cracked on the inside, and there were also gaps....lemme try to get a pic
Ryan. not sure but you may have got a hold of some low grade ply

B grade
which can contain plugs in any of the layers and the layers are
thrown together haphazardly which can mean ALL of your grain is ui-directional, (craft store)

A grade
which can have plugs but only on the inner layers again haphazard grain... (craft store type)

AA grade
which can have plugs on the inner layers only but the sheet are aligned cross grain
(this is what most rocket companies use)

AAA grade
the wood has no plugs and each sheet
is run cross grain,
strong enough to cut it in any direction

Baltic birch is a vague reference
if you order Baltic Birch, depending on the company you ordered it from, you can wind up with
one of a selection of woods from 4 different areas, Finland, Russia (three of thier
ex-satellite countries make Birch Ply)... Finnish Birch is the best since it uses a
waterproof black glue that is much stronger than the other 3.
and doesn't warp near as easy

....or you eat alot of spinach
sorry styme, what are plugs?

from the pics you can see that there are only 3 plys there, that is because it peeled apart and then I cracked it.

btw I am got the plywood from my dad, I am not sure where he is getting it from, I told my uncle about what happened and he said he had a bunch of it laying around the job, he said it was ply and he though 9 ply
My recommendation? Head to your local hobby store and get some aircraft ply. It's typically the top quality, and the 1/4 inch sheets I bought are 5 layers thick. It makes outstanding rings for MPR birds, and would probably be sufficient for small HPR as well.


I am not sure at all that I understand what the two large holes are for (I don't think I have seen your rocket design).
Are these for smaller motor-mount tubes to pass through this bulkhead?
Is there some other reason these holes need to be this large?
Can you use three or four smaller holes, and leave more material in the portions of the bulkhead that 'bridge' between the holes?

If you must have these two large holes, you can improve the overall bulkhead significantly by using the better grade of plywood that wwattles suggested. Also, check out the 9-ply you said your uncle offered. If they are nine plies of about the same thickness, that plywood would also be better.

For a bulkhead with two large holes, instead of installing your U-bolt across the middle, try adding a stiffening web that extends from top to bottom on both sides. Attach a smaller U-bolt up near the top or down near the bottom. Or for an even smaller footprint, get someone to tack-weld an eye-bolt (so the perimeter of its 'eye' is completely closed) and bolt it into a corner.

Your broken bulkhead looks like it was made from 'veneer'-type plywood. These have very thin surface plies and a few, thick middle plies. They are NOT made for structural purposes, but for appearance (the surface ply is usually supposed to be a pretty wood). For model rocketry, look for lots of plies, low rates of gaps or cracks within each ply (peek at the edges of the plywood stock to look for ply quality), and good quality wood used throughout (not some thick layers of cheapo wood to fill up the middle). Home improvement stores in my area sell a style of plywood that is very strong and excellent quality but is quite pricey----I think it is called Scandinavian or something like that. One 4x8 sheet of that stuff would last you a lifetime of bulkheads.
This needs to be thicker than 1/4" the rocket weighs about 15 lbs right now and will probably weigh another 10 in the end. The 2 holes are for the altimeter tubes. I am using the smokin rockets method; 2 54mm tubes are mounted and then the altimeters are slid into them.

Powderburner, I think you are right, this plywood has a very thin layer of what looks likie birch on the top and then thock cruddy wood underneath. As for the changeof location, I think if I did that I would move the 2 holes to the side then put the U bolt on the other side
I got some new plywood, this is the suprlus stuff, it is 5 ply agian but this time all the plys are about the same thickness. I am not sure what kinda wood this is, but it seems strong. I am going to cut out some bulkheads to see. Also, I can see no cracks in the ply.....THink it will work?

also, it is covered with a thin layer of cardboard (really thin) and I guess this is so it will sand easily....I think I am gonna sand that off for a better bond