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CA soak fins: How, where, why?

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afadeev

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Wow, 3/8" balsa? What's this rocket look like? I can't think when I'd want thicker than 1/8" or maybe 3/16" before switching over to basswood or plywood.
There is a whole family of Estes Maxi Brute kits (now OOP) that do just that - balsa cores, with a veneer of ABS plastic or similar.
That veneering layer adds a lot of strength to the final part, and balsa core keeps weight down to the minimum, which is good for stability.

BTW, this is similar to the early surfboard construction methods, with balsa (or some other lightweight wood) in the middle, and fiberglass skin on top:

This is second hand knowledge, but everything I've read states that papering with label paper is great for making finishing easier, but it doesn't add any strength to speak of.
Papering by gluing copy-paper to the surface of a balsa fin with TBII strengthens the structure by a LOT!

At this point, I don't even dare to touch or sand naked balsa fins, since I tend to break off a piece ~25% of the time.
Thus I paper all balsa fins first (copy-paper covered with TBII on both sides, then allowed to dry under pressure to prevent warping).
Then I sand and CA the edges, to harden the areas most likely to sustain impacts, and prevent paper veneer from lifting.

If the fin is really big and I am worried about its strength, I will CA the entire paper veneer surface of the fin. However, this adds a lot more sanding work, and requires weather to cooperate. I've only done this on Estes Shuttle booster (replacement) fins, and Nike-Hercules booster fins.
 

neil_w

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The general consensus is to add thin CA to the adhered paper. I haven't tried it yet. I might start applying the CA the edges. I rarely crack a fin but I only fly LPR. Mostly, I break the fin off when it pulls the epoxy off the paper tube, usually when the parachute fails.
It the epoxy separating from the tube, or pulling the surface of the tube with it? What kind of epoxy?

Removing the glassine (or even just sanding it down a bit) can create a more porous surface that produces a better bond.
 

jqavins

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There is a whole family of Estes Maxi Brute kits (now OOP) that do just that - balsa cores, with a veneer of ABS plastic or similar.
That I understand. Balsa core with 1/16" or 1/32" birch ply face sheets would work too. I understand surf boards nowadays use a styrofoam core. The spacecraft I used to work on use honeycomb core and either aluminum or CF face sheets. All are the same idea.

But 3/8" naked balsa still seems odd to me. You're papering it, so it won't be naked, but I got the impression that that's your addition to a design without the paper. Perhaps that's where I've done wrong.
 

rklapp

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I rewatched the eRockets presentation and he said it's water based. The eRockets website says don't breath in the vapors, we'll see if it's different than wood filler.
Got the sanding sealer today. It’s very watery and kinda smells like mineral spirits. It warped the balsa quickly so will need to place it between wax paper and heavy weight to dry. Still probably easier to work with than thin CA.
 
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neil_w

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Got the sanding sealer today. It’s very watery and kinda smells like mineral spirits. It warped the balsa quickly so will need to place it between wax paper and heavy weight to dry. Still probably easier to work with than thin CA.
Do you have evidence that it adds strength?

I’m still not convinced whether these various schemes are adding significant strength beyond glue-papering. And at some point, why not just glass the fins and be done with it?
 

kuririn

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In the late 60's we used sanding sealer and dope to finish our rockets.
The reason most builders aren't still using them now is that there are better alternatives.
CWF fills balsa wood faster, is cheaper, and no solvent fumes.
Technology marches on.
As for hardening fins, any adhesive that soaks into the wood and hardens will stiffen the fibers.
Wood and white glue, CA and finishing epoxy.
Downside is glues are harder to sand than wood sealers and fillers.
And the cost.
As mentioned at some point it would make more sense to use a stiffer material like basswood, lite ply, fiberglass or phenolic. Tradeoff is strength v, weight.
If using sanding sealer, make sure to stir/mix the container so the filler is evenly distributed. Otherwise you will be painting your wood with solvent instead of filler.
I still use a water based sanding sealer occasionally on basswood and lite ply.
BTW there is a product in home improvement stores called Minwax wood hardener.
It is used to repair spongy wood caused by wood rot.
I tried it on spongy balsa fins to see if they stiffened the wood.
I checked to see the amount of flex in the fins before and after treatment.
I could see no appreciable difference.
For me this was a fail.
Your results may be different.
Laters.
 

afadeev

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rklapp said:
I rarely crack a fin but I only fly LPR. Mostly, I break the fin off when it pulls the epoxy off the paper tube, usually when the parachute fails.
It the epoxy separating from the tube, or pulling the surface of the tube with it? What kind of epoxy?
I've seen that happen repeatedly with scouts' Estes Wizard rockets - 3 long fins, trailing beyond the aft end of the rocket, short root cord, attached to the surface of a small paper tube.
What happens with about 1/4 of the kids' rockets, including my own, is that a fin becomes the first point of contact with the ground as the rocket is rotating under the chute (or tumbling without a chute ;-), and the torque just peels the outer layer of the paper from the tube.

Solution - ditch Wizards, and order BMS's scout rocket!

As mentioned at some point it would make more sense to use a stiffer material like basswood, lite ply, fiberglass or phenolic. Tradeoff is strength v, weight.
True.
Alas, the context of these inquiries is, usually: I am building a kit, and would like to make fins stronger and/or make them look better.
Making an investment to replace balsa fins with any of the above (stronger) alternatives is usually outside the scope of ambition of most builders.
Not impossible, just expensive and requires additional prior experiences and/or skills.
 

neil_w

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I've seen that happen a lot with scouts' Estes Wizard rockets - 3 long fins, trailing beyond the aft end of the rocket, short root cord, attached to the surface of a small paper tube.
What happens with about 1/4 of the kids' rockets, including my own, is that a fin becomes the first point of contact with the ground as the rocket is rotating under the chute (or tumbling without a chute ;-), and the torque just peels the outer layer of the paper from the tube.
I have seen that as well, but I've also seen a partial where part of the fin root pulls the paper and the rest lifts cleanly. Was curious which version of problem the OP was having.

If the paper peeled off, then it's a BT structural failure and not a glue failure, and the only solution would be TTW fin mounting.
 

rklapp

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I agree that I don't see a discernible difference with or without the sanding sealer. The balsa flexes just the same but has more of a clean break instead of splintered break. I only gave it one coat however. It was only $5 so decided to give it a try since I was already ordering the Adepter. At this point, my preferred choice is paper adhesive since it removes the grain and doesn't require a lot of messy sanding like wood filler.

I believe the fin separation is pulling of the BT layers. With the Bull Pup XL, I gave it a good sanding of the glassine. The fins didn't break off completely. I was able to get epoxy under the fins, then cover the crack with paint. The hard part is making sure the fin stays on straight. The main problem was the shredding of the parachute causing the hard landing.

 

lakeroadster

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The general consensus is to add thin CA to the adhered paper. I haven't tried it yet. I might start applying the CA the edges. I rarely crack a fin but I only fly LPR. Mostly, I break the fin off when it pulls the epoxy off the paper tube, usually when the parachute fails.
If you're joining wood based parts... use wood glue.

Using CA, or epoxy, on wood based parts for structural attachments makes no sense. Wood fins... wood pulp based paper.. attached with wood glue. What does the CA do other than make the fins brittle?

The edges, that aren't papered, sure that makes sense just to seal them so they don't absorb primer / paint.
 
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rklapp

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If you're joining wood based parts... use wood glue.

Using CA, or epoxy, on wood based parts for structural attachments makes no sense. Wood fins... wood pulp based paper.. attached with wood glue. What does the CA do other than make the fins brittle?

The edges, that aren't papered, sure that makes sense just to seal them so they don't absorb primer / paint.
That’s what we’re discussing, adding thin CA to the balsa to make it strong but not to adhere it to something. I was saying that I paper the fins but haven’t added CA to the paper but might with the edges
 

jqavins

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Lake, the idea is to bond the wood fibers to each other more strongly that the natural lignin does, and also coat them with the cured CA. That is supposed to make the wood stronger and stiffer (mainly due to bonding) and harder (mainly due to coating). Which makes sense and seems from experience to be true. Wood glue doesn't do this as well because it doesn't penetrate nearly as thoroughly as thin CA glue.

Of course papering with wood glue is another way to accomplish strengthening and stiffening. I have to wounder if thin CA penetrates the wood glue barrier that is created by the papering process, but it should harden the paper I guess. A controlled experiment might be in order.

My attitude remains that if I need more strength etc. than papered balsa gives me than I'll use basswood or plywood, but I'll probably be (willingly) paying a weight penalty.
 

neil_w

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I remain unconvinced that CA-soaking a papered fin increases its strength very much; rather, it mostly provides surface hardening and sanding protection for the paper. I am experimenting now with using thin CA (now that I have my SuperGold and can use it willy-nilly indoors) to seal and coat the edges of my label-papered fins, whereas previously I used wood glue. So far so good. I believe it will save me from needing a doing a filler/primer layer on the fins, which mostly addresses messiness at the edges.

Couple more observations:
1) I did some measurements and label-papering adds about 50% additional weight to 1/8" medium balsa. That is significant but still somewhat less than basswood.
2) My unscientific impressions of basswood are that it is very strong against the grain, but not really so great with the grain. If I were in a situation where I really needed stability in both dimensions, I'd stick with papering or plywood or fiberglass.
3) On Blackfish I felt like I needed a *lot* of fin strength so I glue-papered 3/32" basswood and boy oh boy those *really* felt strong. I can't imagine ever needing anything stronger than that for any reasonable LPR build.

Papered balsa is still my go-to.
 

lakeroadster

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That’s what we’re discussing, adding thin CA to the balsa to make it strong but not to adhere it to something. I was saying that I paper the fins but haven’t added CA to the paper but might with the edges
FWIW: I papered the TTW fins on my Hammer Head.... and soaked the edges with CA.. Papered Fins

015.JPG
 

jqavins

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Neil, I may have bought this up before; have you ever consider or tried balsa plywood? Take two pieces of 1/32" or 1/16" balsa, apply and squeegee glue on one or both, lay them cross grain, and apply weight while the glue dries? Basically like glue papering but with different material. I keep meaning to try it but haven't yet.
 

neil_w

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Neil, I may have bought this up before; have you ever consider or tried balsa plywood? Take two pieces of 1/32" or 1/16" balsa, apply and squeegee glue on one or both, lay them cross grain, and apply weight while the glue dries? Basically like glue papering but with different material. I keep meaning to try it but haven't yet.
Yes. It is fair but not great. Comparing against equal thickness single sheet, it's stronger in one direction and weaker in the other. Has its place but not a general solution to this problem. Two layers just isn't enough. I think papering a single piece of balsa is better, and easier to finish as a bonus.
 

jqavins

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Curious. How can cross grain lamination make it weaker in either direction? How does it know which direction weaken?

Y'know the old joke about a thermos? It keeps hot things hot and col things cold. How does it know?
 

neil_w

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Curious. How can cross grain lamination make it weaker in either direction? How does it know which direction weaken?
Compared to a single 1/8" sheet, two cross-laminated 1/16" sheets have more strength in one direction and less in the other. Total 1/8" thickness either way.
Y'know the old joke about a thermos? It keeps hot things hot and col things cold. How does it know?
:D
 

lakeroadster

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Yes. It is fair but not great. Comparing against equal thickness single sheet, it's stronger in one direction and weaker in the other. Has its place but not a general solution to this problem. Two layers just isn't enough. I think papering a single piece of balsa is better, and easier to finish as a bonus.
I've made home made balsa ply for numerous components... it's always stronger in both directions.

Logic and proportion dictate..
 

jqavins

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I got what you meant, but I don't get how it's possible. Which direction is weaker? Like the thermos, how does it know? But unlike the thermos, for this it's a valid question.
 

neil_w

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OK, I'll try one more time.

Comparing 2 plies of 1/16" balsa vs. 1 ply 1/8" balsa. Each is a total of 1/8" thick.

The two-ply has roughly equal strength in all directions, roughly equal to the "strong" direction of a single piece of 1/16" balsa.

The one-ply has the strength of the "strong" direction of a 1/8" piece of balsa in one direction (stronger than the two-ply), and the strength of the "weak" direction of a 1/8" piece of balsa in the other (weaker than the two-ply).

Does that make more sense?
 

jqavins

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Yes. I sounded like you were saying the two ply had two strengths, one better than x and the other worse. But no. I now see that you meant the two different directional strenghts of one ply, with the isotropic two ply being better that one direction of the one ply and worse that the other.

So 1/8" two ply is stronger than the weaker direction of 1/8" one ply. That's a win in many situations, i.e. when the strong direction of 1/8" one ply is more than enough and only the weak direction is a concern. It certainly eliminates the issue of grain direction in a complex fin shape.
 

neil_w

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If it works for your application, great. For me, once I'm using 1/8" of wood, I want more strength than the 2-layer laminate gives. Once I'm gluing layers together, might as well just paper.
 

Philip Tiberius D.

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I have not seen a suggestion or response on using epoxy... say 1088 epoxy, on fins to strengthen them. Mix, squeegee (two ee’s?) on, hang to dry. Much more controllable as far as flow - just use tape to keep it off the root edge and peel before dry. Might have to sand a bit but should be nice and hard.
 

Steve Shannon

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I would just use wood hardener instead or better yet just laminate those fins with glass or carbon fiber. As Joseph said, that’s really thick balsa. People who are trying to optimize their designs sometimes use balsa cores and then vacuum bag a couple layers of glass or CF on both sides of the fin, but there’s no real advantage to using balsa that thick.
 

Philip Tiberius D.

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I would just use wood hardener instead or better yet just laminate those fins with glass or carbon fiber. As Joseph said, that’s really thick balsa. People who are trying to optimize their designs sometimes use balsa cores and then vacuum bag a couple layers of glass or CF on both sides of the fin, but there’s no real advantage to using balsa that thick.
I’m actually building a EZI-65 for my L2 and did the 1088 last night - mostly on a whim. Trying to decide (and not trying to highjack the conversation with this but willing to move it) what Rocket To use - the EZI and use a 38mm Baby J or get a Mach1 Momentum EXT in 75mm.
 

rklapp

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Careful, the epoxy will drip, especially the 30 minute set. Let it dry flat.
 

jqavins

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I'm alegey building an EZI-65 for my L2 as well. I've been building it for going on 10 years. But I do think it will eventually prove to have been a good choice.
 
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