Super Big Bertha - Fin Filling, Fillets, and F- er, uh, Rail Guides

 

I moved on to writing up the Estes V2 build and nearly forgot to finish the Super Big Bertha series!

After the fins were assembled and the edges sanded square, I moved on to filling in the wood grain.

Brodak sanding sealer is my favorite way to fill balsa wood grain. Traditionally, you apply sanding sealer after gluing fins on. But I often apply a few coats and sand beforehand. It's easier to sand them smooth without the body tube in the way.

Before anyone chimes in, yes, you do need to take care to keep the sealer off the root edges, as they can impede a good glue bond. But I've literally never had a fin break off because of using this method.

If I accidentally get some sanding sealer on the root edge, I quickly wipe it off with a paper towel and give the edge a quick sanding. And you know what? It's fine.

Furthermore, with these fin tabs, there's no problem. And you can hold the fin by the tab and seal the whole fin at once, without getting sealer on your fingers.

Brodak is expensive, but I find the results worth the price. It's more cost effective to buy a large can, rather than several smaller ones. You can buy it online HERE (Note: Looking at it as I write this, Brodak sanding sealer is a lot more expensive today than it was when I bought two cans two years ago. Inflation is pretty high right now. That said, if I needed some, I would probably still spring for it. That's how much I  like the stuff. But there are definitely much cheaper options for balsa filling out there!)

If you're worried about getting a large can, because you've had sanding sealer quickly go bad once you open it if you don't use it all in a short period of time, don't worry. I've had a can open for a year and it's still as fresh as new. This is quality stuff.

I brushed three good coats on, then sanded them smooth before attaching the fins to the rocket.

Once the fins were glued on to the Bertha, fin fillets were applied.

These are wood glue fillets, using Titebond Molding and Trim Glue, 3 layers deep at this point. The molding and trim glue is less prone to sagging and shrinking, and makes a great fillet.

I put an Elmer's pointy cap on the bottle for better fillets and more precise gluing - the bottle necks are identical, and School Glue is always on sale for 99 cents. Not good for rocket building, but the caps are perfect replacements for the wide Titebond caps. I got this tip from Chris Michaels' Model Rocket Building blog, and I wouldn't do otherwise now.

The Bertha will get conformal rail guides instead of the 1/4 inch launch lugs it comes with. Since I almost always fly with a club, a launch rail will almost always be available when I feel like flying this rocket. A 1010 high power rail is less likely to whip when a rocket lifts off, and less likely to get bent.

A friend gave me a tip that I tried. Take a rail guide from Aerospace Specialty Products and cut it in half with a razor saw and miter box. You get 2 for 1 that way. The original rail guide is a couple inches long, and I decided this would be a good rocket to try that with.

It worked, but I probably won't do it again. Holding the guide still while cutting was difficult, because while the rail guide is long, half a rail guide is not. The saw kind of shredded the ends of the 3D printed part, which required a lot of cleanup. And a short segment of this guide is kind of hard to get on the rocket perfectly straight. While the guide does conform to the BT-80 body tube, the curve is very slight, and with a layer of epoxy between the guide and the tube, there's a bit of play, and I had to keep checking to see if I really got it on there straight. A short piece is harder to get on straight than a long piece, and I found lining up the second little bit of guide 11 1/2 inches away more difficult.

Besides, these rail guides from ASP are cheap and lightweight. I'll probably just use full length guides in the future. You can get them HERE.

After I put the rail guides on, my eyes kept playing tricks on me. Were they straight? Were they in alignment with one another?

I got out my camera and backed it as far as I could in the Rocket Room and zoomed in to see if I could better tell. I couldn't really tell what I was looking at, because it was pretty dark.

In this picture, the two guides don't look lined up, but that may be because the camera and rocket were off-center of each other. In the daylight, they look alright.

There is a chance that the upper guide is *slightly* crooked. Like, ever so slightly. I'm hoping it's fine. I guess we'll see when we get it on the pad.

I suppose I should get myself a length of 1010 rail so I can check these things at home.

It's probably fine...

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