Fiberglass versus Baltic Birch Playwood for Level 3 fins

[knarfster]

I like the ease of working with fiberglass fins, but when you get into big rockets they price jumps.

I am making a 10 1/2 foot tall 7.5 inch rocket for my level 3 attempt and can't decide whether to use 1/2 in Baltic birch or .125 or .187 inch fiberglass for the fins.

I saw Vern K used .125 fiberglass for one of his big rockets.

Whats the proces and cons Other than .125 fiberglass sheet is about $80 (3/16 or .187 is about $130) versus $20-$30 for the plywood.

[RandyM]

I'm in the process of building my L3 rocket and I'm using 1/2" Baltic Birch and will then tip to tip glass it. My opinion is it give the best compromise between price and performance and maybe leans a little more towards price.

[quickburst]

Get the best of both worlds. Use two 1/8" layers of birch and laminate them with glass cloth.

That's what I did on my L-3.

I put four layers of 8 oz cloth between the 1/8" plywood sandwich. Afterward I glassed three more layers on each side tip to tip. This made some very strong fins.

I corrected my mistake, they were 1/4" thick fins made of two 1/8" layers.

Sorry for the goof.

[blackjack2564]

If you use G-10...cut fins, bevel edges, install in airframe, inject fillets....done

Ply wood, can warp...needs to be glassed, after install, glass again.


Depends on how much labor you want to do. G-10 is cheaper if you consider the amount of time needed to do wood and glass.

That said if it is a labor of love or a journey down the road of knowledge, then the time factor is mute.

If you want to build a rocket and fly it, glass is the way to go.

After all.... it's just a motor holder.

[kramer714]

I agree with the fiberglass over wood approach, the glass in the center of the fin isn't really buying you that much it is the outer layers that do all the work. For the same weight as a solid glass fin you can go to a thicker wood / fiberglass fin.

Then again, I used solid graphite fins for my Level 3.

[Brent]

I am in the process of trying some 1/2" honeycomb fiberglass from Giant Leap. I will have to put hard wood strips on the leading, trailing and fin tips and then bevel them though. I saw some made up at at one of the LDRS events in Amarillo and they looked great.

[dlb]

My L3 used (2) 1/4 luan board from Home depot, with Fiberglass sandwiched inside and added it on the leading edge too, Handled a N4000 with no effect seen and these Fins are HUGE.

[sandmantoy]

I laminated my Army Hawk fins with carbon fiber and then installed them like they where G10. The fins are over 3ft. long 1/4" plywood. Recovered it from a river once and 3 years later they are still nice and straight. I like the G10 but carbon fiber would be my second choice

[troj]

Get the best of both worlds. Use two 1/4" layers of birch and laminate them with glass cloth.

That's what I did on my L-3.

I put four layers of 8 oz cloth between the 1/4" plywood. Afterward I glassed three more layers on each side tip to tip. This made some very strong fins.

Holy cow!

Two layers of 1/4" floor underlayment with a layer of 6oz fiberglass in between is extremely strong and doesn't bend easily at all.

That's what we used on both the Delta III and the Pershing II. The former flew on a small Q cluster, the latter on a small Q.

Unless you're going for ultra-high performance, a single layer of 6 oz glass in between two sheets of quality 1/4" plywood is more than adequate. No need to tip-to-tip fiberglass it, unless you want to.

-Kevin

[MarkM]

For my L3, which used a 6" airframe, I used 3/8" birch plywood fins laminated with 6oz fiberglass. I installed the fins, did internal and external fillets and that was it. No tip to tip. No problems on an AMW M1350 for the cert or a Gorilla M1665 since the cert. I'm sure it will take the motors in the 75/7500 case from AMW or Gorilla as well. My L3 fins have a fairly large span, too; the rocket is an upscale of the Estes Astron Bandit.

[Handeman]

I like the ease of working with fiberglass fins, but when you get into big rockets they price jumps.

I am making a 10 1/2 foot tall 7.5 inch rocket for my level 3 attempt and can't decide whether to use 1/2 in Baltic birch or .125 or .187 inch fiberglass for the fins.

I saw Vern K used .125 fiberglass for one of his big rockets.

Whats the proces and cons Other than .125 fiberglass sheet is about $80 (3/16 or .187 is about $130) versus $20-$30 for the plywood.

Reading through the replies, the thing that seems to come through is that you don't need the big stiff fins unless you're going to push to Mach. Plywood fins will do just fine with huge motors if you stay below Mach. Once you get into the high performace range, then requirements change. I haven't done a L3, but what I've found on my L2 and below is that Mach and landing are the two things that destroy fins. I've never had or seen a fin ripped off from motor thrust.

Don't know how helpful that is, but maybe you can take another look at your rocket and get a better handle on what limits you will have and what fins will hold up to those limits.

[knarfster]

What is Luan?

[knarfster]

Thanks for all the great answers. I am now leaning toward a 6" rocket for reduced cost. I can also fly it on K's and L's after cert for more savings. I plan on going with .125 G-10 and my use carbon for a fin to fin. My design approaches mach 1, but nothing higher so I should be good to go. My local TAPs al chimed in on my club BB and they all like G-10 for the increased stiffness and overall less work.

Thanks Again

[troj]

What is Luan?

The cheapiest, cruddiest plywood I've ever seen. I'd never rely on it to provide any real structure.

-Kevin

[bobkrech]

What is Luan?

Known to have caused at least 1 L2 cert failure.

Luan is an inexpensive grade of mahogany plywood. It is used for floor underlayment and door faces. It is not very strong in the versions I have seen. A good material to stay away from in structural applications.

Also all plywood of a given thickness is not the same. Don't forget that plywood is a composite. You can get 1/4" plywood in a variety of grades and with 3, 5, 7, 9, and 12 plies, and the strength is roughly proportional to the number of plies. The best 12 ply aircraft grade Finnish Baltic Birch is phenomenal stuff. (And @ $6 per sq.ft. it should be.) Probably as strong and stiff as regular 3/4" plywood with 1/3 the weight and thickness.

Stiffness is proportional to thickness cubed for uniform materials. This is important because rocket fins fail due to flutter which is an oscillatory motion that is speed dependent. Fins should be lightweight to minimize the requirement for nose weight. You can accomplish this using FG or CF skins over a foam, honeycomb or plywood inner structure. The use of foam or honeycomb makes the fin lighter since the strength and stiffness is derived from the FG or CF skin.

One layer of CF is as stiff and strong as 3 layers of FG, however CF is about 3 times more expensive so it's a wash as far as price goes, however FG is heavier than CF and you get a much lighter fin from CF than from FG.

Bob

[hardinlw]

I flew an upscale Fat Boy on a K-550 at NYPOWER-13. The fins were 1/4" aircraft plywood with one layer of 4oz carbon fiber. No problems. I'm currently building an L3 certification project. The fins will be 3/4" poplar plywood with one layer of 10oz fiberglass and one layer of 2oz fiberglass. The choice of poplar plywood was made because it is one of the lightest available that is flat and has no voids to speak of. The strength is coming from the fiberglass which is vacuum-bagged to the fins and wrapped around the leading and trailing edges. The second layer of 2oz cloth is to give me a smoother surface to work with during finishing and to make sure I don't sand into the 10oz layer that is giving me my strenght. The choice of glass over more carbon fiber was driven by cost as Bob indicated.

[Diosces]

Known to have caused at least 1 L2 cert failure.

Luan is an inexpensive grade of mahogany plywood. It is used for floor underlayment and door faces. It is not very strong in the versions I have seen. A good material to stay away from in structural applications.

Also all plywood of a given thickness is not the same. Don't forget that plywood is a composite. You can get 1/4" plywood in a variety of grades and with 3, 5, 7, 9, and 12 plies, and the strength is roughly proportional to the number of plies. The best 12 ply aircraft grade Finnish Baltic Birch is phenomenal stuff. (And @ $6 per sq.ft. it should be.) Probably as strong and stiff as regular 3/4" plywood with 1/3 the weight and thickness.

Stiffness is proportional to thickness cubed for uniform materials. This is important because rocket fins fail due to flutter which is an oscillatory motion that is speed dependent. Fins should be lightweight to minimize the requirement for nose weight. You can accomplish this using FG or CF skins over a foam, honeycomb or plywood inner structure. The use of foam or honeycomb makes the fin lighter since the strength and stiffness is derived from the FG or CF skin.

One layer of CF is as stiff and strong as 3 layers of FG, however CF is about 3 times more expensive so it's a wash as far as price goes, however FG is heavier than CF and you get a much lighter fin from CF than from FG.

Bob

Bob, Just keep in mind that it's all about the application. Steve Eve's history making Saturn V rocket was made with Luan plywood Steve Eve's Saturn V project

"He built the tubular skin from Luan plywood—nearly 300 square feet of it, according to Rockets magazine—and then coated it with fiberglass. "

[troj]

Bob, Just keep in mind that it's all about the application. Steve Eve's history making Saturn V rocket was made with Luan plywood Steve Eve's Saturn V project

"He built the tubular skin from Luan plywood—nearly 300 square feet of it, according to Rockets magazine—and then coated it with fiberglass. "

Just because it worked once doesn't make it a Good Idea.

I'd never recommend someone use that stuff in a rocketry application. Floor underlayment is readily available, is inexpensive and is significantly better.

-Kevin

[bobkrech]

Troj

I would not have a problem using thin luan for a skin that is glass coated. This is a large area application that has relatively low stress levels. Glass coated luan should take distributed stress fairly well, but it don't take concentrated stress very well.

I probably should have been a bit clearer in that cheap luan plywood should not be used for high stress applications such as bulkheads as I've seen recovery eye-bolts ripped clear through luan bulkheads make from hollow core doors covered with a layer of glass.

Bob

[sandmantoy]

I think if you are confident in your application and craftsmanship you can make anything work. If you are going to wing it you are looking for trouble anyway. If you are learning as you go then you need good advice and it is better to stay with stronger heavier materials until you are confident in building something large. I am more worried about hurting someone or damaging property rather than cut corners, just putting the effort in to doing it right has to account for a lot of the strength to a build.

[troj]

I would not have a problem using thin luan for a skin that is glass coated. This is a large area application that has relatively low stress levels. Glass coated luan should take distributed stress fairly well, but it don't take concentrated stress very well.

To me, even then, I wouldn't use it or recommend it -- it's not much cheaper than floor underlayment, and the latter is significantly higher quality.

-Kevin

[dlb]

Known to have caused at least 1 L2 cert failure.

Luan is an inexpensive grade of mahogany plywood. It is used for floor underlayment and door faces. It is not very strong in the versions I have seen. A good material to stay away from in structural applications.


Bob

Well, thats not the stuff I used, it was a underlay for flooring, but it was 5 ply Baltic Birch plywood(aa on one side a c on the other), not an inexpensive grade of mahogany plywood, hey just maybe it was not Luan, but I was told it was. When it comes to strength, after the fiberglass insertion, I could stand on it and with a 2 foot X 2 foot fin hardly had but a 1/4" warp from tip to tip and after 3 good launches, (2) M's and (1) N I see no cracks or stress problems at all. not even in the paint (cracks ). Granted this was maybe not for everone , but is strong, very strong and half the weight and cost of a fiberglass fin of the same size. Heck it was a budget flight at $200 dollars, minus the motor and the airframe was heavy card board shipping tubes from Yazoo with no glassing. Even with all I have learn since then, I still would have done the same thing. Heck I guess already did with a "P" motor project.

The whole build can be seen here:http://www.rocketreviews.com/reviews...ty_kills.shtml

or here:

http://www.wolfstardesigns.com/gravity_kills.htm

She still lives but need to be launched soon, it's calling out to me!

People here will help as much as they can, but ultimately up to you, and test or verify everything, thats just a norm. for rocketry!

[Binder Design]

I've flown masonite fins past mach 2.....FWIW. I also have several rockets with luan and underlayment fins that have broken mach.

Binder Design kits come with aircraft ply fins but I save the good stuff for the customers and only use the cheap stuff for my own fins. Makes it more exciting.

Mike Fisher

[dlb]

I've flown masonite fins past mach 2.....FWIW. I also have several rockets with luan and underlayment fins that have broken mach.

Binder Design kits come with aircraft ply fins but I save the good stuff for the customers and only use the cheap stuff for my own fins. Makes it more exciting.

Mike Fisher

Thats a fact Jack, errr Mike

[Chrisn]

it is the outer layers that do all the work.

Why?

[kramer714]

With a rocket fin there are three main issues, strength (will it break under flight loads), stiffness (will it flutter causing the flight loads to go WAY up), and damage tolerance (what happens when it hits the ground).

Without going to far into the math (I would be happy to do that if some one wants), lets hit the strength and stiffness.

Stresses
Rocket fins can be considered a cantilevered plate, cantilevered because the e fin is not free to rotate at one end. When it flexes, one face is in compression, and one face is in tension. Take a stack of papers, keep them from sliding at the ends and flex them, one side is being pulled tight while the other side is trying to buckle in compression. in the center of the stack of papers, there will be one piece of paper that isn't in tension or compression, this is called the neutral axis of the beam.

The stresses in each piece of paper is proportional to the distance from the neutral axis. The farther away from the neutral axis the higher the stress is. I'm simplifying quite a bit but what the math says is that I can use a weaker material in the center and a stronger material in the outer layers without sacrificing the overall strength of the beam. In a fin, wood is weaker but lighter, fiberglass is stronger but heavier, use the fiberglass on the outer layers where you need them and save the weight and cost in the inner layers.

Stiffness
Stiffness is as important as strength for rocket fins, this gets worse as the speeds go up. Again simplifying the math, there are two factors that are used to determine stiffness of a cantilevered beam (fin), Modulus, how stiff the materials is (the symbol E is used for this), and a geometry factor ( in beams the symbol I is used).

If I change the thickness of my fins without changing the materials, the stiffness (and resistance to flutter) goes up as the square of the thickness, double the thickness the stiffness goes up by 4, triple the thickness and the stiffness goes up by 9. If I use a less stiff material for the middle of my fin (wood) and a more stiff material on the outside (fiberglass) i still get most of the benefits of having a solid fin without the weight or cost.

Corrugated Cardboard
The best example of this that i pint out to rocket builders is corrugated cardboard like the side of a box. It is really light for its thickness. The box uses a less stiff center, and stiff outer skins. If it was solid it would weight a lot more. When you bend it one skin wants to fail in compression (buckle and (put a slit in the tension face to see this better) the other side wants to fail in tension.

I hope this helps understand what is going on. Tim VanMilligan from Apogee has a good explanation of this in one of his news letters, im sure you can find this on his site.

[Diosces]

I've flown masonite fins past mach 2.....FWIW. I also have several rockets with luan and underlayment fins that have broken mach.

Binder Design kits come with aircraft ply fins but I save the good stuff for the customers and only use the cheap stuff for my own fins. Makes it more exciting.

Mike Fisher

The stuff Mike puts in his kit is top of the line stuff! and the custom stuff is even betterer!

As for the naysayers on luan, again it's all about the application. Like I originally posted, Steve used luan for the skin of the Saturn V and reinforced with f/g. None of the East Coast rocket gods of MDRA indicated it was a "Bad idea".
Like Bob said, an example of a bad application would be using luan for a bulkhead...

[kramer714]

Back to the whole idea of core and skins, Luan would be fine for a core, skin it and away you go.

As a side note, I have used 'cheep plywood' as cores, laminate with 1 or 2 plys of glass and use a piece of mylar as a 'caul plate' squeege out the excess resin with the mylar and you get a smooth surface.

[Graham Orr]

to add to the engineering discussion:

let's not forget dynamic behavior of materials.

the resonant frequency can play an important role in high velocity flights. while g10 may be stronger that wood, the resonant frequency is lower...

typically if one is switching from wood to fiberglass they make the fiberglass thinner than the wood to cut down on weight and because the safety margin is larger with the "stronger" fiberglass. however, in this case, a few things are wrong with this approach:

1) density of fiberglass is higher
2) fiberglass is thinner
3) modulus of fiberglass isn't THAT much higher


resonant frequency is proportional to sqrt( EI / rhoA )

rho (density) is increasing a lot
E (modulus) is increasing slightly
I (second moment) is decreasing a lot
A (cross-sect area) is decreasing slightly

net effect: resonant frequency decreases --> more prone to aeromechanical resonance failure at lower speeds than wood.


BEST APPROACH: COMPOSITE STRUCTURE
Use light plywood as core material and do a few layups of fiberglass or carbon fiber over the outside. the laminations will add little weight and area, and increase the second moment incredibly.