When to use something else instead of balsa for fins...

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Bill S

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I'm contemplating an upscaled version of my Space-X Enforcer rocket (50% or so, going from an BT-60 to 80 tube), and one thing that concerns me is the fins. The original version used 1/8" balsa, and an upscaled version would use 3/16" (or 1/4" for extra stiffness). I'd like to a) reduce the amount of puttying and sanding ad nausem, and b) keep the weight down so as to minimize the amount of nose weight required to ensure stability.

At what point should I be looking at alternative materials instead of just continuing to use balsa fins? I considered something like the fins that the Estes Super Big Bertha uses, a lightened core piece with thin skins on it, and that may help a bit with weight, but not my other concern.

This upscale will be mostly be using Estes E/F and F composites, as I don't have a level 1 certification yet (have no plans for such right now).

Thoughts?
 

afadeev

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I'm contemplating an upscaled version of my Space-X Enforcer rocket (50% or so, going from an BT-60 to 80 tube), and one thing that concerns me is the fins. The original version used 1/8" balsa, and an upscaled version would use 3/16" (or 1/4" for extra stiffness). I'd like to a) reduce the amount of puttying and sanding ad nausem, and b) keep the weight down so as to minimize the amount of nose weight required to ensure stability.
Do you paper your fins?
CA-them?
For me, un-protected balsa fins are way too fragile for transportation, never mind flying. The paint on them becomes the strongest part, and fins get nicked and damaged before they reach the launch rod.

One way to maximize balsa fin strength is to paper and CA soak them.

At what point should I be looking at alternative materials instead of just continuing to use balsa fins? I considered something like the fins that the Estes Super Big Bertha uses, a lightened core piece with thin skins on it, and that may help a bit with weight, but not my other concern.
If you are going to approach Mach 1 (> 0.85), or otherwise worried about fin integrity during storage or transportation.
The next step up would be 3-plywood material. Or 5-ply:

Then Fiberglass or CF (will outlast the airframe).

HTH,
a
 
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neil_w

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I think that when you're considering 1/4" thick fins on a BT80 rocket, that is an indication that you should look into a stronger material that won't need to be so thick. 1/4" fins on a rocket that size seems crazy.

1/8" plywood would be an obvious choice, if the weight is OK. 1/16" plywood might even be enough... although they might look too thin for an upscale.

Otherwise, use some sort of sandwich with balsa core and a harder skin. Like, say, 1/8" balsa core with 1/32" basswood or plywood skins, which would add up to 3/16".
 

dr wogz

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Balsa also comes in various grades or densities. going thinner in balsa, but upping the grade / density will add strength.

The next 'upgrade' some do is to go with 'bass wood'. It is stronger, but also heavier than Balsa. And Ply is likely the strongest, but also the heaviest (but not by much)

You can also go with 1/32" ply on both sides of a built-up 'rib & spar' fin frame (like they do for planes / the larger Estes kits with 'hollow' fins)

Papering adds a tremendous amount of strength for what it is, and gives a fairly smooth & consistent surface.


And, experiment! try the few suggestions provided and determine which is the best for you, for your build..
 

Bill S

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I'm liking the idea of a 1/8" balsa core with thin skin of a stronger material on the outside, particularly if it reduces the need to fill in the balsa grain, etc. I don't mind some smoothing out, but I need a break from putty, sand, reputty, sand, etc. :)

I guess I'll have to play around with some numbers in Rocksim, looking at 1/8" balsa with either a 1/32nds basswood or plywood skin.

Is there a general rule of thumb as to how much material you can take out of the core layer to reduce weight (like areas cut out)?
 

Bill S

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I ran some numbers in Rocksim, using various materials from the materials database. Results:

1/8" balsa fins = 1.28 oz.
3/16" balsa fins = 1.92 oz
1/4" balsa fins = 2.56 oz

2 x 1/32nds fins of basswoord: 1.94oz
2 x 1/32nds fins of aircraft plywood = 1.66 oz

1/8" balsa fin with skin of 2x 1/32nds basswood = 3.22 oz
1/8" balsa fin with skin of 2x 1/32nds plywood = 2.94oz

1/8" basswood fin by itself = 3.12oz
1/8" plywood fin by itself = 3.92 oz

It looks to me like all the skin options weigh more than plain old balsa, though admittedly they are probably going to be easier to smooth the surface of than balsa.

I'm not sure how much core material I can remove to save weight without actually just going ahead and making a test fin and cutting out sections and weighing it.
 

rklapp

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I recently tried Duffy's styrene technique but I used TB2 instead of spray. Maybe it will fall apart, we'll see. I did thin CA the edges.

 

dr wogz

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Is there a general rule of thumb as to how much material you can take out of the core layer to reduce weight (like areas cut out)?
Depends on the structure, the loading, the thickness & stiffness of the skin... a few variables come into play..

Some have cut a fin from 3/32" or 1/8" balsa and removed most of the middle. they've then filled the void with a piece of stryrofoam meat tray material. Then glued it all together. (with foam safe glue!)

You can likely build up a stick structure to make teh fin shape, with cross braces / trusses in between to make up the 'fin frame / core. You can likely 'swiss cheese' it with a few holes here & there..

And I wouldn't reply on the numbers from RS for the weights. Balsa weights (density) can vary wildly! But they do 'trend' in the way we all expect.. the glue you use, and the paper you use will also have varying weights..
 
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Greg Furtman

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Cape Byron

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With Estes kits being produced in China, I think we should see a lot more effective use of Bamboo based materials.
I've been doing some testing with bamboo ply for a couple of kits and it is incredibly strong for its weight. It's also really, really expensive - about 4x the cost of basswood in 1.5 mm width.

Chinese production is more likely to include Tung wood as an alternative to balsa. Already seen in some Sky Rockets kits.
 

Greg Furtman

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Chinese production is more likely to include Tung wood as an alternative to balsa. Already seen in some Sky Rockets kits.
I tried looking up the properties of Tung wood but didn't find anything, only info about the nuts & oil. Does anyone know of a source of information? Like density, grain, etc. It's not in The Wood Database.

 

Greg Furtman

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It is Vernicia Fordii. Here is the Wikipedia link.
Thanks. I read the article and it is a lightweight wood. Then I saw this: "The tung tree is poisonous in all of its parts, including the fruit and the seeds, although some parts of the tree have been used for medicinal purposes in the past. "

I wonder if handling the wood would cause any problems?
 

afadeev

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With Estes kits being produced in China, I think we should see a lot more effective use of Bamboo based materials.
Any evidence of that?
Or just a supposition?

Chinese production is more likely to include Tung wood as an alternative to balsa. Already seen in some Sky Rockets kits.
Any actual evidence of that?

Estes had shifted assembly of kits to China more than a decade ago.
If there is firm evidence that balsa parts have been substituted, I strongly suspect we would have confirmed it by now.
 

Back_at_it

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This is something I've thought about as I'm planning a number of upscale builds this summer. For me it really comes down to the size and shape of the fins. If I'm building something with long fins that stick off the back of the rocket like a Super Big Bertha for instance, I would go Plywood without question. If it's something that has the fins above the end of the tube like my upscale Bullet then Basswood is more than strong enough. In all honestly, I've gotten away from Balsa in anything other than normal stock kit builds.

I've recently finished a Clone of the Semroc Brighton and used 3/16 Plywood for those fins. After making the jump to composite motors, I don't worry about weight. Build it with a motor mount one size larger and go for it. If I'm flying here in the Chicago area, I'm limited on altitude as there just isn't room for recovery. If I get 1000' out of a flight I'm happy so weight doesn't really apply. I'd rather build for strength.
 

Lee

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Do you paper your fins?
CA-them?
For me, un-protected balsa fins are way too fragile for transportation, never mind flying. The paint on them becomes the strongest part, and fins get nicked and damaged before they reach the launch rod.

One way to maximize balsa fin strength is to paper and CA soak them.



If you are going to approach Mach 1 (> 0.85), or otherwise worried about fin integrity during storage or transportation.
The next step up would be 3-plywood material. Or 5-ply:

Then Fiberglass or CF (will outlast the airframe).

HTH,
a
One way to maximize balsa fin strength is to paper and CA soak them.

Please elaborate - are you using CA to attach paper and then soaking with CA?
 

afadeev

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One way to maximize balsa fin strength is to paper and CA soak them.

Please elaborate - are you using CA to attach paper and then soaking with CA?
Not exactly.
Step 1: paper your fins as you normally would. In my case, I use copy paper and glue it to the balsa fin core with TBII wood glue. Pic #1 in the set below, left to right.
Step 2: trim & sand the paper to the edges of the fin, CA all exposed edges, and sand the fin edges into the final desired shape. Pic #2.
Step 3: apply thin CA to the exposed paper surfaces of the fin. Pic #3.
Step 4: sand and paint the surface to perfection, or your level of satisfaction (not available in my picture set, yet ;-).

1624295593367.png


HTH,
a

P.S.: The funky fins used in the above illustration are from Estes X-wing kit.
 
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Cape Byron

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Any evidence of that?
Or just a supposition?



Any actual evidence of that?

Estes had shifted assembly of kits to China more than a decade ago.
If there is firm evidence that balsa parts have been substituted, I strongly suspect we would have confirmed it by now.

"The Condor is a competition style glider that is boosted into the sky using rocket power. It uses a molded plastic nose piece on the glider that ensures easy release of the engine pod after the rocket reaches apogee. The glider then flies gently to the ground while the engine pod descends via an aluminized plastic streamer. The glider is made from the stiff and lightweight wood of the Chinese Tung tree."

 

BABAR

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While there are differing opinions, if you are papering balsa for strength, I think most would agree that you get more strength from glue than adhesive paper. The downside is that with glue sometimes defects in the balsa if Not sanded smooth before applying may show through, and even sanded rough grain balsa may show a bit of grain with glued fins. adhesive papering is much much easier (less messy) and cosmetically at least as good and probably better than regular glue.

when I think of glue, I am thinking liquid glue, I usually use white glue. I am not sure how much strength you get from using adhesive spray or glue sticks, I know some have used them successfully with good cosmetic results. As always, YMMV.
 

Back_at_it

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While there are differing opinions, if you are papering balsa for strength, I think most would agree that you get more strength from glue than adhesive paper. The downside is that with glue sometimes defects in the balsa if Not sanded smooth before applying may show through, and even sanded rough grain balsa may show a bit of grain with glued fins. adhesive papering is much much easier (less messy) and cosmetically at least as good and probably better than regular glue.

when I think of glue, I am thinking liquid glue, I usually use white glue. I am not sure how much strength you get from using adhesive spray or glue sticks, I know some have used them successfully with good cosmetic results. As always, YMMV.
Agreed 100000%. Adhesive paper is more cosmetic than anything else. I've also seen instances where adhesive paper peels off after a while. Especially if you are flying on a hot day. The glue becomes soft and it curls right up.

I decided to try something new on a restoration a few weeks ago using notebook paper and super thin CA. It worked better than using wood glue but I want to try it again on another build before posting about it.
 

BABAR

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Agreed 100000%. Adhesive paper is more cosmetic than anything else. I've also seen instances where adhesive paper peels off after a while. Especially if you are flying on a hot day. The glue becomes soft and it curls right up.

I decided to try something new on a restoration a few weeks ago using notebook paper and super thin CA. It worked better than using wood glue but I want to try it again on another build before posting about it.
@neil_w has found that adhesive paper sticks better if your pretreat the balsa with masking tape (blue seems to be color of choice) to pull off most of the fuzzies before you attach adhesive paper.

i hadn’t thought of heat loosening white glue, I wonder if you get best of all worlds by using white glue first, the. When completely dry “painting” the fin with thin CA. haven’t tried it yet,

fast majority of my fins are polygonal (flat sided), so paper is wrapped AROUND the free (non-root) edges, and root edges are held by fillets. So I don’t encounter dehiscence frequently. Wrap around is not generally practical with curved edges.
 

MidOH

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I tried looking up the properties of Tung wood but didn't find anything, only info about the nuts & oil. Does anyone know of a source of information? Like density, grain, etc. It's not in The Wood Database.



Sorry, Couldn't help myself.

Pretty sure my recent Estes kits have balsa in them. Except for the cardstock Extreme. Which actually seem quite nice.
 
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