A simple balsa/fiberglass fin idea

One of the common issues when scratch building rockets is how to construct a light, strong and STRAIGHT fin. I've come up with a solution I'd like to pass on. It's pretty straight forward and may have been posted here before, but if not, I'll do it now.

Most of my rockets are 3" to 4" body tubes, weighing from 2 to 10 pounds and motors ranging from Gs to K's. I've used this on all of them.

One of the problems is always finding material that is REALLY straight and rigid. I gave up trying to find inexpensive AND straight plywood. So, I decided to make a simple 3 layer fin with 1/8" balsa sheets sandwiching 6 oz fiberglass cloth soaked in resin. The resulting fin is 1/4" thick incredibly strong and perfectly straight. When the resin has cured, the rigid cloth makes for the perfect center-line for sanding your taper on the edges. I'll include pics and explanations, although it's pretty self explanatory.

1) Print fin template from program. I use Open Rocket.
2) Trace onto a piece of card stock or 1/16" balsa, which is what I did.
3) I use 1/8" x 6" x 36" balsa sheet purchased from Hobby Lobby with their 40% off coupon found on their app. If you have larger fins, I just butt glue them together with instant glue.
4) I cut the long sheet into section large enough to border the template on all sides. Make two of these per fin.
5) Cut a piece of cloth ( I used 6 OZ in these photos) the same dimensions as the two piece of balsa sheet
6) On a very flat work surface ( I buy a 2'x 4' piece of sheet rock) cover the work area with waxed paper in case any resin oozes out the side.
7) Mix resin. In these pics I mixed 1 Oz. Pour half of it on the lower balsa sheet and use a piece of scrap balsa to spread it around evenly.
8) Lay the cloth in the wet resin. Using the same piece of balsa or spreading tool work the resin through the cloth weave.
9) Pour the rest of the resin on the saturated cloth and again spread it evenly over the cloth.
10) Lay the second balsa sheet on the top with the edges even. Using light pressure, slowly move the top sheet in circles to work the resin evenly.
11) I used a 1" x 8" wide pine, very straight, board to lay on top of the top balsa sheet and used a 5 Lb. weight to apply even pressure on the top board.
12) I let it cure about an hour minimum.
13) I then check to see that the resin is somewhat hardened. I then trace the outline of the fin, using the fin template on the sandwiched assembly.
14) You can cut on the lines any way you want. When the resin is still soft, I've used a single edge razor blade and straight edge. In my case I have a small table top band saw which works perfectly.
15) You can sand the perimeter of the fin any way you want, and the best part is the hardened resin center make a perfect center-line that the sanding bar stops at, making a razor sharp, perfectly straight tapered leading edge.
16) You can finish the balsa anyway you want. Any kind of a sealer fills the grain really nicely, then just paint. These are super straight and strong.

The next post shows the sanding bar and how it will taper nicely to the center hardened cloth.

Here's the sanding technique and a pic of the Alpha I built using this type of fin.

Nice work; balsa is a great core material.

Nice work. The fin would be stiffer if 2 skins of fiberglass were used, and it would be about .135" or so thick if 1/8" were used or about .26" if 1/4" balsa is used, however it would be harder to shape.
Very nice looking Alpha upscale btw.

THANK YOU for this!!!:clap:

I've been searching and pondering on such a course of action for the construction of the wings on my SA-5 Gammon build.
This method should allow me to make the bevels on them nicely, and maintain the rigidity that I require.

I have a nice Granite surface to press them on.

Thank you Rich. You are correct. This offers a good starting point. SO far, I've used this on all sizes and weights of rockets. Most of the time, I use it as shown, with just filler and primer on the balsa, but of course there are many ways to beef it up. ON my up-scaled 4" bigger Bertha, I added 2 Oz. cloth on both sides of each fin using peel ply. The fins were super strong and had a glossy finish. Fo smaller rockets, I use the paper/wood glue trick. Not only do they paint nicely, but are incredibly strong.

Thanks on the Alpha compliment!..I upscale everything...LOL.I have a 4" Big Bertha and a 4" Patriot.

Thank you Topramen!...Granite would work perfectly. These are super lightweight and strong.

Thank you John...I've been using it in RC planes and rockets for decades and I always come back to it after trying other materials.

sunderll said:

Thank you Topramen!...Granite would work perfectly. These are super lightweight and strong.

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I'll still glass them on the outside as usual, but now I know how they can be rigid enough to be shaped BEFORE the outer glass.
I'll use 1.25oz. or 2oz. cloth on the inside, shape them, then cover in the .75oz.

Yeah, that Granite slab is awesome, I have many more in my yard, but that one was the most convenient.
Granite is a standard part of everyones landscaping aroung here, as Rock of Ages Quarry is only 10 minutes away.
There are also plenty of places to go get it for free as seconds and such. Even with the nicely polished surfaces.
The granite sheds here make Surface Plates for NASA and the like.

https://www.rockofages.com/en/industrial/products/surface-plates/

Awesome, I will be trying this out with balsa now. Used carbon tissue/weave on ply for years. Never thought of this! Great idea.

Thank you Rob!...... ....These fins work great pretty much as they are shown here without additional covering/strengthening on the outside. If I want to strengthen, I use lightweight fiberglass cloth or printer paper and wood glue.. I use these as shown with no additional covering on my 4" diameter rockets with motors as big as I211s with zero problems. On another subject, there are always new ideas with what techniques to make fillets at the base of the fins. I've tried everything and by far the best that I've found is a simple Window & Door Siliconized Acrylic Caulk. It comes in a toothpaste like tube, is inexpensive, super easy to apply (I use my thumb), PAINTABLE, is lightweight, is water clean up, is flexible so there are never any cracks along the fin tube seam like used to happen all the time with epoxy or resin. It comes in either clear or white. When dry it's quite strong and adheres extremely well to balsa and all types of body tubes. I usually do it right before quitting for the night to give it overnight to really dry good before painting.

I hope you have good luck with the fin building!!

I finally got to try this method, and while my sanding skills are definitely going to need some improvement, the test fin I made generally looks like I want it to, and the method will allow me to achieve the results I require.

I used 2oz. glass, but will try the 3, 4, and 5 too, as the rocket I'm building now, an SA-5 Gammon, is still in the stage where I am making practice parts.

Sorry for the crappy pics' but the camera I have now sucks.

Michael's Mid Power Rocket kits used this technique on their 29mm Black Brant kit. Fiberglass core with basswood. You could get a nice bevel on the Black Brant fins with out a lot of trouble. Easy finish to a high level and very strong. Flown many times. Landed in rock canyons, tangled chute landing hard on packed dry ground, no problem, just small paint chips.

He made fins like yours with balsa for a V2 - WAC Bumper project we did. Not quite as strong or durable as basswood but boy did they look nice when shaped, embossed and finished. Just like a real V2 skin over frame. After a few nice flights we had some real good wrecks and although the balsa was damaged the core remained in tact and the balsa could be repaired. The light weight was a big plus, you want to keep the weight off the hind end of you V2!

The LPR WAC on top used thin Baltic birch laminated with balsa for perfect shaping and strength on it's four fins. Oh the good old days of WAC bumping!

Top Ramen, very cool!! After a few times, the technique becomes second nature and really seems to turn out pretty strong straight fins.

Daddyissaber, great to hear that. My 4" Bertha has fins designed to break every landing. But with this technique and also glassing the outside, they are amazingly strong. My son and I launch most of our rockets on a large cleared area that was going to be a subdivision of homes, but the developer went Bankrupt. So, we have an area that is about 4 square miles with a grid pattern of roads. My estimate is that the roads account for about 5% of the total area, the reast is mowed grass. YET, my Big Bertha lands on the asphalt about 80% of the time. I use DD on it and have a massive parachute to at least make a soft hit when it hits the roads...LOL

sunderll said:

Daddyissaber, great to hear that. My 4" Bertha has fins designed to break every landing. But with this technique and also glassing the outside, they are amazingly strong. My son and I launch most of our rockets on a large cleared area that was going to be a subdivision of homes, but the developer went Bankrupt. So, we have an area that is about 4 square miles with a grid pattern of roads. My estimate is that the roads account for about 5% of the total area, the reast is mowed grass. YET, my Big Bertha lands on the asphalt about 80% of the time. I use DD on it and have a massive parachute to at least make a soft hit when it hits the roads...LOL

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With light glass on the inside and outside they will be bulletproof! Now I could just stay away from the "light cheese" in those deep fried wantons at the Chinese restaurant!

...a little late to the party, but how would fins built with this technique compare to plywood of similar thickness?

For example: Balsa/FG/Balsa vs. FG/Balsa/FG/Balsa/FG vs. Plywood

sunderll said:

Top Ramen, very cool!! After a few times, the technique becomes second nature and really seems to turn out pretty strong straight fins.

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I'm getting the hang of it. I did it on my Star Trooper rebuild, and they turned out real nice, but I can't find the memory card for my camera, so I can't share any pics'.
Either way, I'll be using this method from now on on most of my builds, and really enjoy doing it, so thanks again!!!

The practice fin that I shared a pic of actually needs to be a lot thinner to be scale, so the actual ones I'm making for that rocket won't be as tricky to get right. I ordered plenty of the thinner balsa from BMS in my last order to do them.

I'm intrigued by this method... It might be something that I can accomplish here in the PRC (presuming I can find/afford epoxy, and balsa). I wonder if it would work with foam (of which there is no shortage of here).

@ OP, Very Nice looking Alpha