In Search of ... Symmetrical Fins

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GregGleason

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One of the most elusive pursuits for me personally in rocketry has been the desire to create nice and symmetrical airfoiled fins.

One of the ways to do this is through a lot of math and then converting it into 3D geometry.

After coming up with a fin profile, I then created the fin geometry I wanted in Excel.

I exported the fin geometry into scripts that could be imported into 3D CAD (Rhino), and from the airfoil profiles created the surface of the fin. After joining the surfaces of the fin into a unified, closed polysurface, I subtracted that fin volume from a rectangular box that would represent the mold.

Below is the result of the above process.

Fin.Detail.jpg

Fin.Mold.Perspective.jpg

I don't know how long this process will take, but I expect to learn some things along the way.

Greg
 
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One of the most elusive pursuits for me personally in rocketry has been the desire to create nice and symmetrical airfoiled fins.
I've made Nike-style fins by sanding balsa (see my video). But for my Aerobee, I used a 3D router, which turned out spectacularly well (if I may say so myself).

boosterfincan.jpg
 
Nice work John.

This is where CAD/CAM really can shine and do the tedious things with dutiful precision.

Greg
 
It's remarkable that the Easter Islanders mastered this technology 1,000 years ago, but then the technology was lost.

In Search of...

(the Ancient Aliens of the 70's...)
 
Those rock formations in the background look suspiciously like those found on Easter Island in the quarry areas. Eh???
 
Those rock formations in the background look suspiciously like those found on Easter Island in the quarry areas. Eh???
It's actually one of the unreconstructed ahu along the coast (Akahanga).

But, back to the topic: I have never tried molding for profiled fins so I'm curious to see how this works out. I assume you're planning to lay up fiberglass in two mold halves and then join them?
 
... But, back to the topic: I have never tried molding for profiled fins so I'm curious to see how this works out. I assume you're planning to lay up fiberglass in two mold halves and then join them?

That's correct. I will need to make a left and right mold half. After the layup is cured, then join the halves together with an epoxy/microballoon/fumed silica concoction.

I don't know if I will go so far as to vac bag the layup. I've done it before when I have made fin plate. We will see. It's all new, so a lot of theory that's been percolating in my mind will get put to the test.

Greg
 
It's remarkable that the Easter Islanders mastered this technology 1,000 years ago, but then the technology was lost.

In Search of...

(the Ancient Aliens of the 70's...)

TB ... you got the reference. Bonus points if you can remember the narrator.

Greg
 
After the layup is cured, then join the halves together with an epoxy/microballoon/fumed silica concoction.
Why wait until it cures? If you join the pieces before full core, you can get a chemical bond (although the fibers will still have been severed).
 
Why wait until it cures? If you join the pieces before full core, you can get a chemical bond (although the fibers will still have been severed).

I did consider that John, but here is the rationale I came up with. If it's not fully cured, it's at risk for some level of deforming. So on picking the "lesser of two evils", my preference is to go with perhaps a slightly weaker bond and be assured that the profile is still intact.

But like I said before, this is a learning process and I may change if things don't work out as expected.

Greg
 
Once the mold is in this format, all it is is just pretty electrons. It needs to get from this point into something solid.

For the next step we need another bit of software to make the jump. For that jump, we need to output the model into a format the next software can read. That format is a stereolithography file or .stl file.

The .stl file is then imported into software used for rapid prototyping called DeskProto.

Leviathan.Fin.in.DeskProto.jpg

Greg
 
I then need to inventory the available bits for milling and pick the one I think is best for the operation. The mold material I plan to use is MDF.

The first pass is a roughing operation. The purpose is just like it sounds, it is designed to provide a rough cut of the finished project.

The cut paths are similar to what you would see on a contour map, and provide a visual reference of the increments of the depths as well as the travel of the bit.

Leviathan.Fin.in.DeskProto.Rough.1.jpg

Greg
 
I took a break from this thread since the initial milling did not work out so well. That and I wanted to get my DD Wildchild up and running.

So, the concept is the same but the fins have changed. This is a fin plan that is fairly popular on a modroc so I wanted to see if I could do something simple but learn the fundamentals, too.

Extra points if you can tell what rocket these fins will go on.

DRM.Fin.Basic.Dims.jpg

Ok, you really don't have to be Sherlock Holmes to figure out what fins these are for.

Greg
 
Can you elaborate on what the problems were with the first milling?

Funny, at first I thought Blue Bird Zero, one of my faves. Then I saw the obvious clue. Elementary, really.
 
Can you elaborate on what the problems were with the first milling?

Funny, at first I thought Blue Bird Zero, one of my faves. Then I saw the obvious clue. Elementary, really.

Well, the problems were "guy behind the mill didn't know what he was doing" type stuff. I was trying to do to much, which resulted in many facepalms. I should have broken down the milling operations into smaller sets. Still doable, but the learning curve is tough when your only mentor goes by the name of "Mr. Learn as You Go".

Back to the fin, here is the solid model.

DRM.Fin.Solid.jpg

Greg
 
Yep, that's a good summary John of what these fins are. The fins are 1/8" with the TE and LE having a 1/16" radius. Really nothing fancy and that's ok. I've got an Estes Patriot sport scale waiting in the wings that I can do the diamond profiling on. The milling portion is just one part of the whole operation. It's the mold process that I want to learn as well. I wanted something simple to start with so I can understand the process for future projects.

Greg
 
It's the mold process that I want to learn as well. I wanted something simple to start with so I can understand the process for future projects.
I'm curious how your molded profiled fins turn out when you get back to them. Making the mold using a CNC mill seems natural, but it will be interesting to see how you get a smooth-enough surface for a mold.
 
That's exactly right John.

I ordered a few 0.500" balls (brass) from McMaster-Carr for geometric registration of the two halves. The pockets are about 0.001" larger than the upper end of the tolerances of the the ball. So that's the theory anyway. I'll see if it works in practice.

Greg
 
Thanks Brian.

It's interesting that he uses vacuum as his method to hold down the work piece. I use clamps and that is ok for what I am doing.

Corian is a great material for moldmaking, but I don't have any as of yet. I may just go with MDF. Not as good as Corian, but it's better than natural wood.

Greg
 
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