Bulkhead ply thickness used for low and slow M power builds.

[jahall4]

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?

 

[David Schwantz]

I used 1/4" ply on a 12" V2. Just backed up the area where any attachments were. She flies on an AT M2050 X.

 

[rharshberger]

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?

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.

 

[OverTheTop]

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.

 

[rharshberger]

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.

Bulkhead, not centering rings.

 

[OverTheTop]

Bulkhead, not centering rings.

Oops. My bad.

For smaller rockets I would think 1/4" ok, but once you get past about 6" I would be going for perhaps double thickness of 1/4". The extra surface area (function of square of diameter) when the pressure is applied can cause flexing. Note that if you double the thickness you end up with eight times the stiffness (cubic law).

 

[tsmith1315]

The motor size has nothing to do with the thickness, its the expected shock loadings thats important.

I think the OP was hinting at the rocket's mass by mentioning the M-motor in combination with low & slow.

 

[rharshberger]

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, 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.

 

[tsmith1315]

That may be, but an approximate mass and diameter would be a more accurate gauge

Indeed, what I consider to be a low and slow M-flight could be downright lethargic to others.

 

[tfish]

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.


Funny timing..
I work with a high-school rocket club..and they wanted to build a show piece rocket this year..I have most of the stuff to build an ARLISS.org style of rocket with av75mm mount. I was driving around today scroungingaterials for centering rings and bulkheads.
. I've used some leftover snap together flooring. (#4) that I had on this rocket. It only has 2 centering rings and no fin tabs..5 flights on M1100 research motors..rocket 6" dia 42 pounds on the pad.



Today a local floor store gave me two 4 foot pieces of (1#)

A local used and surplus building goodwill sort of place sold me five 3 foot pieces of (3#) for $5 bucks..
(2#) is from a cabinet maker buddy of mine..





Info only..always ground test before drinking and driving.

I feel that you have to look at it as an engineered system..you're not just pushing on that plywood centering ring..It's attached to and supported by the body tube..the motor mount and maybe even the fin tabs...It would take a lot to move that plywood...as long as it's glued/screwed or bolted together correctly.


Tony

 

[jahall4]

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.

thx, worth noting, IMO a centering ring is a "specialized" bulkhead, just that a non-ring bulkhead will not usually be strengthened from something like the structure of a fin can (tube and/or fins) so it would necc. need to be thicker.

 

[jahall4]

I think the OP was hinting at the rocket's mass by mentioning the M-motor in combination with low & slow.

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.

 

[jahall4]

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.

 

[user 30299]

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.

 

[jahall4]

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.

 

[user 30299]

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.

 

[jahall4]

So the issue is the quality of the material you're using. A low-grade plywood vs. a high-grade plywood. I get that.

Not specifically, but if more weight is a non-issue (or even desired in this case) why not accommodate for lesser grade material.

 

[bjphoenix]

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.

This is the key- size and amount of force.

When 1/4" plywood is mentioned then the next question is what type of plywood and where is it purchased from? Maybe we can get hobby plywood in 1/4" but in Lowes and Home Depot 1/4" plywood is likely to be fairly weak stuff. I agree with using Baltic Birch, it should be much more solid than normal construction plywood, and if the 1/4" doesn't look solid enough then go with 1/2" minimum. And I also agree with using large washers. A 1/4" threaded rod is very strong in tension but that small nut would pull through plywood pretty easily.