How do you select the right plywood for your rocket?

[rocketlabdelta]

What engineering process or rules of thumb do you use when you select plywood for a scratch-built high power rocket?

For example, I’m designing a 65mm (2.6"/BT-80) rocket with a 38mm motor mount for G–J motors. I will be using through-the-wall fins and will be bonding the recovery harness to the motor mount. This means the aft centering rings will be butressed by the fins when transferring the motor thrust to the main airframe and the fore centering rings don't have to anchor a recovery hardpoint.

My plan was to use 5/32" 8-ply birch plywood from Aircraft Spruce and laminate centering rings with faces exposed to exhaust or ejection gasses with 4oz fiberglass. This design decision is a SWAG that's light on science.

1. Does this seem reasonable?
2. How could I make a more informed decision?

 

[David Schwantz]

That is very good ply. Will work fine. Although some may say overkill, but I like em strong. Coating with just epoxy would do the same sealing over the wood. Be a bit lighter than glassing. But no reason that you can't. I like to pick out ply in person. Make sure it is flat, no twists or warps. Also I do not want any knots or repairs. Grain to be running straight.

 

[JohnCoker]

I've never seen 1/8" (roughly) plywood used for J motors. Even if you use a very gentle motor on the way up, you will need to use a very large parachute on the way down to prevent the fins from breaking on landing. 1/4" or 3/16" plywood is more typical for HPR rockets.

 

[rocketlabdelta]

1/4" or 3/16" plywood is more typical for HPR rockets

That makes sense. I thought I might be able to get away with thinner plywood with a higher ply count—sounds like no.

Even if you use a very gentle motor on the way up, you will need to use a very large parachute on the way down to prevent the fins from breaking on landing.

The fins are going to be made with others materials on this particular rocket. In order to shape the airfoil profile I selected I will be using a thin (.040") G10 core laminated with 2 × 3/32" sheets of balsa and skinned with 4oz fiberglass. The fin cores will also be tip-to-tip laminated to the motor mount with 4oz fiberglass.



I had originally planned on using .06" G10 for the core but AeroFinSim didn’t show flutter with plain balsa stock for the air speeds I’m expecting. That means the composite reinforcement is a “bonus” for flight stresses and can help with recovery stress and overall durability. (Assuming I simulated everything correctly.)

 

[jqavins]

Back to the original question, is there any method for choosing material thickness in between CFD and flutter analysis on one hand and a wet finger it the air on the other?

 

[rocketlabdelta]

Back to the original question, is there any method for choosing material thickness in between CFD and flutter analysis on one hand and a wet finger it the air on the other?

Right. And not just for fins, for other parts of the aerostructure like centering rings, bulkheads, etc.

I feel like there should be a table of material characteristics for aircraft plywood of various grades and thickness but if such a thing exists I can’t find it.

 

[JohnCoker]

That makes sense. I thought I might be able to get away with thinner plywood with a higher ply count—sounds like no.

I didn't say "no", I said it's more typical to use thicker plywood. But that might make it too heavy for a G. I encourage experimentation, so you should definitely try it; the fiberglass reinforcement will definitely help.

The fins are going to be made with others materials on this particular rocket. In order to shape the airfoil profile I selected I will be using a thin (.040") G10 core laminated with 2 × 3/32" sheets of balsa and skinned with 4oz fiberglass. The fin cores will also be tip-to-tip laminated to the motor mount with 4oz fiberglass.

The different layers will definitely give it more strength, creating a truss structure if there is reinforcement in that sandwich. I made a video of similar process for a Nike fin:
https://jcrocket.com/nike-custom-fin.shtml

 

[RocketTree]

This might not answer your original question, but something to consider. When purchasing plywood - the thickness is usually based on the rough cut or unsanded (unfinished) measured thickness. A "sanded-both-sides" finished 1/4" plywood can be closer to 1/8 or 3/16 thickness.

I use 1/4" mahogany veneer underlay plywood, factory sanded-both-sides @ 0.165" thickness. Works well on 29mm/G motors. A 4x8 foot sheet was $19 CDN and will last dozens of rockets. This one has 3 ply core and 2 thin mahogany outer veneers that accept paint nicely.

Best to check with a caliper. Not sure about hobby grade plywood.....

 

[David Schwantz]

Another thing is grain orientation. Make sure the grain runs just like you would do a balsa fin. NOT parallel to the body tube. This will give 2 extra plys in this direction.

 

[user 30299]

What engineering process or rules of thumb do you use when you select plywood for a scratch-built high power rocket?

For example, I’m designing a 65mm (2.6"/BT-80) rocket with a 38mm motor mount for G–J motors. I will be using through-the-wall fins and will be bonding the recovery harness to the motor mount. This means the aft centering rings will be butressed by the fins when transferring the motor thrust to the main airframe and the fore centering rings don't have to anchor a recovery hardpoint.

My plan was to use 5/32" 8-ply birch plywood from Aircraft Spruce and laminate centering rings with faces exposed to exhaust or ejection gasses with 4oz fiberglass. This design decision is a SWAG that's light on science.

1. Does this seem reasonable?
2. How could I make a more informed decision?

Do you have a basic design already laid out for the rocket? Can you post it?

 

[user 30299]

Right. And not just for fins, for other parts of the aerostructure like centering rings, bulkheads, etc.

I feel like there should be a table of material characteristics for aircraft plywood of various grades and thickness but if such a thing exists I can’t find it.

When you get into the specialty plywood, such as aircraft grade, you won't find the table you're looking for; MOE, Tension, Compression, Shear in the Plane, etc. Those manufacturers usually don't publish the numbers. They operate under a different criteria than those set by the manufacturing associations for your everyday regular plywood and OSB used in construction.

 

[rharshberger]

That makes sense. I thought I might be able to get away with thinner plywood with a higher ply count—sounds like no.


The fins are going to be made with others materials on this particular rocket. In order to shape the airfoil profile I selected I will be using a thin (.040") G10 core laminated with 2 × 3/32" sheets of balsa and skinned with 4oz fiberglass. The fin cores will also be tip-to-tip laminated to the motor mount with 4oz fiberglass.

View attachment 445955

I had originally planned on using .06" G10 for the core but AeroFinSim didn’t show flutter with plain balsa stock for the air speeds I’m expecting. That means the composite reinforcement is a “bonus” for flight stresses and can help with recovery stress and overall durability. (Assuming I simulated everything correctly.)

You can get away with the 1/8" ply, but it needs to be laminated with fiberglass or carbon fiber.

 

[jqavins]

How about a table, based on collective experience, of what materials are recommended for body tubes, fins, CRs, etc. based on parameters like maximum speed and thrust? When should one stop using balsa for fins and go to basswood, birch ply or glassed balsa? When should one go to heavy cardboard tube and when go to glass there? Moving up and up in size, thrust, and speed, it'd be good to have a reference list of rules of thumb based on the experience of those who've already gone there. (Of course opinions will vary among experienced flyers, so such a table would surely have to indicate a range of answers for each question, and one would will still have to pick for ones self how cautions one wants to be as compared to building light. At least such a table would provide a starting point.)

 

[boatgeek]

I think it depends more on fin span and rocket weight than motor power.

I've seen 1/8" ply used on a 54mm rocket that flew to M1.6 on a baby J with no problems. I've also used 1/8" for a 3" rocket with pretty big fins, though I don't try to go supersonic on that because I would expect it to flutter.

 

[Charles_McG]

I've used 1/8" ply on a 4" version of the Estes NASA Pegasus. Fins were built up exactly like the Mega Der Red Max, but centering rings and tube tabs were plain. It flew on a J for my L2.

I'll second @boatgeek - a better answer needs to know weight/Gs, max V, and spans internal and external.

 

[jqavins]

Well, y'see, that's just the sort of thing I'm thinking of. I was asking about thrust in terms of the aft centering ring, which is in the load path. Speed for both fin flutter and aerodynamics tending to break or detach fins even if they don't flutter first.

I appreciate - a lot - the advice from you guys, like the above. And wouldn't be great to go to a table, pamphlet, or some such rather than having to come ask over and over?

 

[Nytrunner]

(Of course opinions will vary among experienced flyers, so such a table would surely have to indicate a range of answers for each question,

That range would go from "3/4 inch, anything less is asking for disaster and won't pass my RSO table!" to " 1/8 inch is fine if you know the 1st thing about gluing fillets properly...."

I think it depends more on fin span and rocket weight than motor power.

Yep ^^. Add in duration at max speed too

 

[TonyL]

To answer the original question, it can break into two interpretations:
1. What process do you use to select materials [what many have answered]
2. What engineering process should you use to select materials.
For doing engineering design of fins, the correct process is flutter analysis [AeroFinSim, etc] and dynamic pressure calculations. To overachieve, one should calculate landing loads too. You will find a forest of 'tribal knowledge' here that does neither. That is not a criticism, just an observation.

Flutter analysis at the hobby level likely means using AeroFinSim, but to do that one needs the shear modulus of the material. Metals are generally well documented. For plywood, some data exists in the wood industry, and a 4 point bending test is documented out there for taking a measurement. For composites there is nearly nothing published as it is laminate specific. An FEA model is probably easier to validate for composites. If some motivated person out there with good testing capability wanted to build and publish a database of material properties for flutter analysis, it would enable a tremendous number of people to do 'real rocket science'. BTW using AeroFinSim results for balsa and assuming that a g10 laminate is more conservative is a false assumption. Critical velocity is driven by 'mass to stiffness ratio' distribution across the fin. It is not something that is practical to estimate without a history to draw from.

Dynamic pressure is very straightforward compared to flutter, just pick a velocity, altitude and angle of attack and do the math. One can even proof test the design. My observation from talking to people is that almost no one does this. I only do it for bolted fins, as the loads are generally low [and if the rocket is supersonic and sideways, keeping the fins on is not a high priority].

Getting back to #1, what to do in reality?

Copying fin designs that have flown in the velocity regime intended for the new design is a very reliable approach. I can tell you that if you refit an EZI-65 with an M motor, the fins all flutter and exit the vehicle at exactly the same time [danger sensitive fins], so subsonic and supersonic are not the same. Also getting regular balsa fins to survive mach 1 is a challenge. Odd shaped fins are difficult to predict without FEA or trial and error.

1/8 plywood seems fine for fins, except cheap 1/8 plywood is quite flexible and warps easily, so some high quality flat material is important. Not breaking fins is a direct function of parachute sizing, not that complicated. Trapezoidal fins and clipped delta fins are harder to break on landing than fancy swept back pointy fins.

While it is hard to estimate flutter velocity for composite [includes plywood] fins, one does know that lighter AND stiffer is better [higher natural frequency]. This means that the core should be lightweight and rigid and the face sheets should be as rigid as practical. Carbon cloth on a balsa core is in the upper range of high natural frequency. Carbon cloth on a G10 core is not particularly mass efficient. Not to say it does not work, just not very efficient [i.e. is heavier than it needs to be]. That said it is much easier to do than laminating fancy fins and generally is successful.

hope this helps,

Tony

 

[mbeels]

Yep ^^. Add in duration at max speed too

I've read that when going supersonic, it is better to punch through the transonic zone, in order to minimize the time spent in that transition. But I don't know the details of how or why. I'd be curious if anyone more knowledgeable had an explanation.

 

[boatgeek]

That range would go from "3/4 inch, anything less is asking for disaster and won't pass my RSO table!" to " 1/8 inch is fine if you know the 1st thing about gluing fillets properly...."



Yep ^^. Add in duration at max speed too

And air density at the time of max speed. The table would get really messy really fast. Getting reliable shear modulus numbers for hobby plywood would be helpful, as would having someone go through some design iterations with that in FinSim or the like. (delta, clipped delta, rectangular by aspect ratio, then piled into a few tables/charts (below M0.6, M0.6-M1.1, M1.1-1.5-ish). Any faster and you're on your own. This would be a nice research project for an undergrad if they were interested and could get faculty advising.

Of course, none of that would help with the batwing fins on my 3" rocket mentioned above.

 

[David Schwantz]

I've read that when going supersonic, it is better to punch through the transonic zone, in order to minimize the time spent in that transition. But I don't know the details of how or why. I'd be curious if anyone more knowledgeable had an explanation.

General Chuck Yeager and the F-86. Read about that and you can learn a lot about transonic.

 

[TonyL]

My observation is that when one does not know what one is doing [e.g. the very early days of supersonic flight], minimizing the time spent absorbing energy from the air flow was a strategy to try to get the aircraft to survive long enough to get data. Flutter is a twitchy phenomenon that does not 'have to happen' as much as it 'can happen' [and often does]. I have had a supersonic flight where the balsa fins survived past motor burn out, but on the coast down had enough dwell time to get in enough fin oscillations to snap them all off [again]. "Punching through mach 1" has a nice sound and feel to it, but I don't think of it as a design strategy for a rocket from a flutter perspective. In terms of drag force it makes sense to spend less time at max q if one wants more altitude.

br/

Tony

 

[JohnCoker]

It's easy to focus on flight stresses, but one should not forget transportation and landing damage. I had a Loc kit fly just fine on a big motor, but just happen to land with the fin edge on a rock and break it. My thought with the "J" motor was also the added rocket weight also means more force on landing.

There's sort of an interesting vicious cycle here: overbuilding makes rockets heavy which increases the forces on them. which tempts us to overbuild more...

 

[B_RadB]

There's sort of an interesting vicious cycle here: overbuilding makes rockets heavy which increases the forces on them. which tempts us to overbuild more...

I spent most of my engineering career at GM fighting "mass begets mass" issues...

 

[rocketlabdelta]

Lots of good stuff here! Time for a recap.

A rocket should be designed to withstand:

• In-flight stresses (see above comment from @TonyL)
  • Fin flutter analysis
  • Dynamic pressure
  • Aerodynamic heating (in extreme cases; see commentary in Mach 3.5 Loki L Altitude Record Attempt Build)
• Recovery stresses
  • Deployment events in ideal conditions (at apogee)
  • Deployment events under non-ideal conditions (something went wrong and the recovery system deployed under ballistic conditions)
  • Impact with the ground (depends on the landing site and the decent rate)
• Wear-and-tear
  • Transportation & handling
  • Being dropped by accident
  • Being stored in a hot car in the desert sun
  • etc.

Guidelines:

• An ideal fin is both stiff and light
• For many rockets, the fins encounter the greatest forces on recovery not during flight
• Internal structures need to be strong enough to survive the largest anticipated load
• Making something stronger (usually) requires adding mass which will amplify the forces encountered (I’m sure @B_RadB has lots stories about this)

Constraints:

• Most hobbyists don’t have access to FEA software (FreeFEM notwithstanding) and/or don’t know how to use it properly
  • This will help you understand the forces the rocket will encounter but won’t tell you how strong your aerostructure is without good data on the characteristics of the materials
• Our ability to accurately use software we do have— e.g. AeroFinSim— is constrained by not having accurate data on the modulus of the materials we use
  • Specialty plywood manufactures may have this information but the dealers that sell it to the hobby market aren’t making this available if it exists
  • This is a opportunity to do some materials science research for the good of the community; rocketmaterails.com (RIP) had some information about this but it’s far from a full dataset
• It is difficult to accurately guess the characteristics of a complicated material (e.g. aircraft plywood laminated with carbon fiber and fiberglass) without testing it

Options:

• You can vary the materials you use, how they are bonded, and how they are assembled to get different mechanical characteristics
• You can build copies of the same parts using different methods and do destructive tests (even qualitative ones are better than nothing)
• You can try to make everything as light as possible and risk a mechanical failure
• You can try to make everything as strong as possible and then cram a big motor and bigger parachute in it
• You can judiciously apply reinforcement and hope for the best

---

But it seems like the simplest answer to my original question is buy some different thickness of plywood (ideally in person), build some things, and see how it goes. You can get a feel for about how thick it is “normally” by looking at kits and seeing what other people do. To be more precise than that requires materials testing and FEA modeling.

@jqavins call for an easily digestible list of materials and their applications is laudable. Personally, I’ll leave that out of scope until I’m retired or something.

---

What did I miss?

 

[dhbarr]

Lots of good stuff here! Time for a recap.

A rocket should be designed to withstand:

• In-flight stresses (see above comment from @TonyL)
  • Fin flutter analysis
  • Dynamic pressure
  • Aerodynamic heating (in extreme cases; see commentary in Mach 3.5 Loki L Altitude Record Attempt Build)
• Recovery stresses
  • Deployment events in ideal conditions (at apogee)
  • Deployment events under non-ideal conditions (something went wrong and the recovery system deployed under ballistic conditions)
  • Impact with the ground (depends on the landing site and the decent rate)
• Wear-and-tear
  • Transportation & handling
  • Being dropped by accident
  • Being stored in a hot car in the desert sun
  • etc.

Guidelines:

• An ideal fin is both stiff and light
• For many rockets, the fins encounter the greatest forces on recovery not during flight
• Internal structures need to be strong enough to survive the largest anticipated load
• Making something stronger (usually) requires adding mass which will amplify the forces encountered (I'm sure @B_RadB has lots stories about this)

Constraints:

• Most hobbyists don't have access to FEA software (FreeFEMnotwithstanding) and/or don't know how to use it properly
  • This will help tell you understand the forces you will encounter but won't tell you how strong your aerostructure is without good data on the characteristics of the materials
• Our ability to accurately use software we do have— e.g. AeroFinSim— is constrained by not having accurate data on the modulus of the materials we use
  • Specialty plywood manufactures may have this information but the dealers that sell it to the hobby market aren't making this available if it exists
  • This is a opportunity to do some materials science research for the good of the community; rocketmaterails.com (RIP) had some information about this but it's far from a full dataset
• It is difficult to accurately guess the characteristics of a complicated material (e.g. aircraft plywood laminated with carbon fiber and fiberglass) without testing it

Options:

• You can vary the materials you use, how they are bonded, and how they are assembled to get different mechanical characteristics
• You can build copies of the same parts using different methods and do destructive tests (even qualitative ones are better than nothing)
• You can try to make everything as light as possible and risk a mechanical failure
• You can try to make everything as strong as possible and then cram a big motor and bigger parachute in it
• You can judiciously apply reinforcement and hope for the best

---

But it seems like the simplest answer to my original question is buy some different thickness of plywood (ideally in person), build some things, and see how it goes. You can get a feel for about how thick it is "normally" by looking at kits and seeing what other people do. To be more precise than that requires materials testing and FEA modeling.

@jqavins call for an easily digestible list of materials and their applications is laudable. Personally, I'll leave that out of scope until I'm retired or something.

---

What did I miss?

Not a thing, your summary is a pleasure to read. I just look at kits about the size and weight of the bird I'm building and pick a similar thickness of high quality plywood.

 

[Arsenal78]

I've never seen 1/8" (roughly) plywood used for J motors. Even if you use a very gentle motor on the way up, you will need to use a very large parachute on the way down to prevent the fins from breaking on landing. 1/4" or 3/16" plywood is more typical for HPR rockets.

SBR Rocketry uses 1/8” on his high power kits and I frankly do not trust it. The Thor used to have 3/16” fins(I have the original fins) and they were plenty strong but the new one is 1/8” fins.Truly anything that’ll fly on more than an H deserves more than 1/8” fins. I even get nervous flying my PML Mini BBX with the thin G10 fins. I use a larger than normal parachute on it.

 

[rharshberger]

SBR Rocketry uses 1/8” on his high power kits and I frankly do not trust it. The Thor used to have 3/16” fins(I have the original fins) and they were plenty strong but the new one is 1/8” fins.Truly anything that’ll fly on more than an H deserves more than 1/8” fins. I even get nervous flying my PML Mini BBX with the thin G10 fins. I use a larger than normal parachute on it.

The newer SBR Thors have been upgraded to 1/4" iirc, only the early re-releases had the 1/8" ply hence the K550 warning.

 

[Joshua Smith]

That makes sense. I thought I might be able to get away with thinner plywood with a higher ply count—sounds like no.


The fins are going to be made with others materials on this particular rocket. In order to shape the airfoil profile I selected I will be using a thin (.040") G10 core laminated with 2 × 3/32" sheets of balsa and skinned with 4oz fiberglass. The fin cores will also be tip-to-tip laminated to the motor mount with 4oz fiberglass.

View attachment 445955

I had originally planned on using .06" G10 for the core but AeroFinSim didn’t show flutter with plain balsa stock for the air speeds I’m expecting. That means the composite reinforcement is a “bonus” for flight stresses and can help with recovery stress and overall durability. (Assuming I simulated everything correctly.)

I worry a bit about your fins' trailing edges. Right about here I should point out tho I have a BS in aeronautical engineering, I haven't used it since about 1998 and I definitely don't have much HPR experience to lean on. I just remember in college that trailing edges (like most of the aircraft) were always a balance between lift, drag, stiffness, and manufacturability. If the trailing edge comes down to too fine a point, it may be easily cracked on a hard landing? Glassing it will strengthen it, of course, but also reduce the minimum tip angle, so that might be enough to keep them from breaking. The blue beards on here will likely know from experience if this a real concern or not

 

[rocketlabdelta]

trailing edges (like most of the aircraft) were always a balance between lift, drag, stiffness, and manufacturability. If the trailing edge comes down to too fine a point, it may be easily cracked on a hard landing?

I’m worried about this too. Reinforcing the trailing edge was the impetus for including a G10 plate in the core of the fin. That being said, I have also broken off knife-edge G10 fin tips during normal wear-and-tear.

I have some ideas about different things to try but I’ll leave them for the forthcoming design/build thread for the launch vehicle.