Basswood, Craft Ply, Birch Ply

[SharkWhisperer]

Opinions on benefits/limits on comparative characteristics of the above materials for a few purposes. I have a few thicknesses of basswood that I've used for fins before, but not sure I'd trust it in a single ply for nosecone bulkheads or ejection baffles. All three are in AC Supply's inventory, and are Midwest's products but at similar prices than direct from manufacturer. I know it's usually best to get a looksee for warpage/gaps, etc., but that likely won't be happening in this instance. Probably considering multi-packs of 4" x 12" because my largest dimensions currently won't go over 4". Not incidentally, all plywood is out of stock on Midwest's site as of Feb 10, 2021.

I have a Big Daddy whose nosecone needs revision and a bulkhead, and several other 2.6" (BT-80) scratch-made units that variously require ejection baffles and tough motor mounts to handle some hot D-G core burner thrust and ejection (one with stuffer tube, several without), but swappable to typical power/ejection of similarly sized Estes 29-mm motors. I can glue up some balsa or basswood cross-grain 2-ply and sand it down, but am wondering: 1) would plywood be more suitable and 2) what the primary differences are between Midwest's 1/8" poplar craft ply and 1/8" aircraft grade birch ply, besides twice the cost for birch? Worth the price difference for rather crude applications such as bulkheads/baffles vs more precision fins?

 

[Nytrunner]

My personal favorite for lightweight homade ply is 1/32 basswood skins over a balsa core (1/8" or 3/32). Gets stiffness on the outside where the bending stress is higher, and leaves the core nice and lightweight.
Cross grain of course.

2-ply crossed basswood would make a real nice baffle material. If you want to install an eyescrew for recovery, I'd add a couple more plys for thickness and strength.

Without seeing the Midwest stock, I'd be willing to wager the aircraft ply has a greater number of plies (thinner naturally) and is higher quality
1/8" plywood is fine for motor mounts of that size/power, and you shouldn't need to spring for the more expensive high-ply stuff. (Use that for fins if you go fast)

The cheap stuff is fine for rings and baffles, save your money.

 

[jqavins]

Of course we all know that (pretty nearly) the only reason for going plywood rather than single sheet of the same species is cross-grain, i.e. splitting strength. A ring or bulkhead is the place I'd be least worried about that, since it is supported all around. It can't flex in any direction without stretching along the grain, so I should think that 1/8" plywood and 1/8" sheet of the same species would perform almost the same, with one important exception. For rings and "passive" bulkheads I'd go with the basswood. Which is no guarantee I wouldn't live to regret it, but I'd feel quite confident.

The exception is a bulkhead with an attachment point. Under a point load like that I'd be queasy about localized splitting allowing the screw eye (or other hardware) to pull through. So there I'd go plywood.

If you want to install an eyescrew for recovery, I'd add a couple more plys for thickness and strength.

I wouldn't thicken the whole disc that way. I'd build up a boss on the back side tall enough to accommodate the threads (they need enough thickness to bite into) and to spread the load, but I shouldn't think more than a 1/2 inch across is really needed. OK, maybe an inch. Or use an eye bolt with a 1/2 inch or one inch washer under the nut.

All of the above is free advice, worth every penny you payed for it.

 

[NOLA_BAR]

My personal favorite for lightweight homade ply is 1/32 basswood skins over a balsa core (1/8" or 3/32). Gets stiffness on the outside where the bending stress is higher, and leaves the core nice and lightweight.
Cross grain of course.

2-ply crossed basswood would make a real nice baffle material. If you want to install an eyescrew for recovery, I'd add a couple more plys for thickness and strength.

Without seeing the Midwest stock, I'd be willing to wager the aircraft ply has a greater number of plies (thinner naturally) and is higher quality
1/8" plywood is fine for motor mounts of that size/power, and you shouldn't need to spring for the more expensive high-ply stuff. (Use that for fins if you go fast)

The cheap stuff is fine for rings and baffles, save your money.

The 1/32 basswood over balsa core is one of my favorite scratch build methods. Makes a strong smooth fin. If I was building the new 3” red max this is the method I would use for the fins. Either use the Estes supplied balsa or just make my own core.

 

[SharkWhisperer]

The 1/32 basswood over balsa core is one of my favorite scratch build methods. Makes a strong smooth fin. If I was building the new 3” red max this is the method I would use for the fins. Either use the Estes supplied balsa or just make my own core.

Tx for the responses. I'm pretty familiar with the solid woods (balsa and basswood) but haven't much used different hobbyist plywoods (though I've made my own 3-ply). Apart from wanting the highest quality specs for ply fins, I was curious about ply spec requirements (or lack thereof) for structural components like bulkheads and baffles. Aircraft quality birch ply is perhaps 6x the cost of the same cubic inches in generic poplar "craft" ply (I thought birch and poplar were closely related...). Joe, appreciate the bulkhead backside build-up for a screw eye if single ply--thanks--pretty straightforward stuff. NY and Nola, also thanks, and I'll hold that homemade 3-ply for fins in mind (actually ordered some 1/32" basswood yesterday)--some of my "research" motors are coreburners that pump out a lot of early thrust and probably don't qualify as typical LPR/MPR, i.e., 90g of hot willow BP in a "typical" fireworking motor (.75" ID x 7.5" total motor length--6" max propellant with the rest nozzle/delay/ejection...).

I already have 1/16" basswood I use for fins instead of 3/32" balsa on some MPRs. When you use 1/32" basswood over balsa, do you laminate with regular wood glue? I have many adhesives, but Titebond 2 has been my go-to wood glue for awhile (I'll use Gorilla wood glue if I misplace my Titebond--same, same to me). I also use CA to strengthen balsa fins and cardboard BT ends and motor mount rings, but not much else. Never thought to use it to laminate ply for fins--too brittle? What adhesive for lamination?

I'm as much of a low-n-slow guy as a speed demon, and rarely go for max altitude so I tend to build heavy. And my BP "research" motors (this is the only website where they earn that designation) can be tuned anywhere from strict constant-thrust endburners to scalded-ape-off-the rod coreburners, so I know how to lift a load. I've dabbled in several alternative fuels (sorbitol, APCP...) but do most of my work with BP. And I've flown plenty of Estes A-F's but can almost always outperform their larger ones, at a fraction of the cost, with just a little "research". Spindle length, milling time, and charcoal source...

So I'd like a good laminate for MPR fins, though, to date I've never ripped off a single-ply TTW basswood fin on take-off (but have lost a few balsa fins with hot motors and underbuilds). So what adhesive do you use or recommend for bonding 1/32" basswood on either side of 3/32" balsa? Is it still of benefit to harden fin edges with CA in a balsa-core laminate?

For ejection baffles, I usually use two-ply cross-grain balsa bound with wood glue and the motor side painted with sodium silicate solution (waterglass/driveway sealant). I'll be trying out some reasonably priced 1/8" craft poplar 3-ply I have on order for baffles (and MMT centering rings), on BTs above 2" ID, I'm thinking. Overkill?

Opinions?

 

[jqavins]

Last thing first. Overkill? Maybe, but not drastically. It's 1/8" thick light weight wood, so it's not much weight and who cares if it might have been done with less?

As for glue selection for the laminating, there needs to be a sign in every rocketeer's workshop that says

All PVA Glues Are
Stronger Than
Wood and Cardboard
​

The only lamination I've done was two layers of corrugated cardboard, which I did with school glue*, and that worked just fine. I'm certain TBII will get your basswood-balsa-basswood sandwich together fine and dandy.

* People hold up Elmers school glue as the exception to the "All PVA glues..." rule, but I build the whole Office Supplies rocket with nothing but that, and it's flown three times on D12s with no problems at all.

 

[NOLA_BAR]

For fin laminating with wood I have used TBIII and Elmer’s Glue-All. Both have worked fine. Why? They are more fluid and can be dispensed with a syringe. Do both sides, then dry under weights like a pile of books. This is for MPR F-G motors.

I built an upscale Thrustline Arapahoe that I wanted to fly on an CTI H54, so I just changed the wood to 3/32 basswood core and 1/16” balsa skins. Flew great! I probably could have done 1/6 basswood cores for a thinner fin profile. Again I do this for scratch builds because I don’t have the tools to cut plywood easily, and also just to experiment.

I prefer longer burning white motors with only just enough kick to make the flight safe like the H54, H110, G40, G25 etc. For a scratch build I generally already have a motor in mind that I want to fly with that design.

I don’t seal the edges with CA. I use CWF and sand to shape. Good luck!

 

[SharkWhisperer]

For fin laminating with wood I have used TBIII and Elmer’s Glue-All. Both have worked fine. Why? They are more fluid and can be dispensed with a syringe. Do both sides, then dry under weights like a pile of books. This is for MPR F-G motors.

I built an upscale Thrustline Arapahoe that I wanted to fly on an CTI H54, so I just changed the wood to 3/32 basswood core and 1/16” balsa skins. Flew great! I probably could have done 1/6 basswood cores for a thinner fin profile. Again I do this for scratch builds because I don’t have the tools to cut plywood easily, and also just to experiment.

I prefer longer burning white motors with only just enough kick to make the flight safe like the H54, H110, G40, G25 etc. For a scratch build I generally already have a motor in mind that I want to fly with that design.

I don’t seal the edges with CA. I use CWF and sand to shape. Good luck!

Love this little razor saw. Makes short work of 1/4" plywood or 3/32" basswood!!! Highly recommended. https://www.amazon.com/gp/product/B000EROWJ4/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

Thinking I'll keep the basswood on the outer layers for faster filling/finishing than balsa...

 

[SharkWhisperer]

Last thing first. Overkill? Maybe, but not drastically. It's 1/8" thick light weight wood, so it's not much weight and who cares if it might have been done with less?

As for glue selection for the laminating, there needs to be a sign in every rocketeer's workshop that says

All PVA Glues Are
Stronger Than
Wood and Cardboard
​

The only lamination I've done was two layers of corrugated cardboard, which I did with school glue*, and that worked just fine. I'm certain TBII will get your basswood-balsa-basswood sandwich together fine and dandy.

* People hold up Elmers school glue as the exception to the "All PVA glues..." rule, but I build the whole Office Supplies rocket with nothing but that, and it's flown three times on D12s with no problems at all.

I guess I was thinking more along the lines of which glue had better heat resistance for a proximal ejection baffle ply, not overall strength at STP.

Appreciate your inputs.

 

[jqavins]

I'd give more or less the same answer. I don't have actual data, but with glue situated between layers of wood, I'd bet comfortably that if the wood isn't scorched then the glue won't be damaged in a short event like ejection.

 

[58pan]

I usually use wood glue to coat the bottom of my homemade ejection baffles to keep the wood from charring.
Haven't had any issues with it though I'm sure epoxy would be better.