Thursday, June 14, 2018

Magical Weight-Reducing Balsa Filler (???)

I recently wrote about a test of various balsa grain fillers and sealers and the weight they add.

The clear winner of that test was the Brodak butyrate dope sanding sealer, which added 0.3 grams of mass to a piece of 3 inch by 3 inch balsa, sealed on both sides. In other words, the sanding sealer added only 0.033 grams per square inch or 0.005 grams per square centimeter of fin area. That's pretty light!

Well, it's one thing to test out something in the abstract - on square pieces of balsa. I was curious how much I would add to an actual rocket I was building.

I'm trying to finish all the rockets I started last year before starting anything new. It's been tough! I've got so much on the build pile, and I'm itching to build everything. But experience has taught me that I should limit myself to one or two projects at a time, focus entirely on them, and I will actually be more productive - and enjoy building much more.

I started to build the Estes Hi Flier XL. Sometimes, when I start a kit, I'll cut some spare fins, and practice shaping them into airfoil or streamlined shapes, so I feel confident when I move on to the kit fins. Sometimes, if those fins turn out well, I go ahead and make a clone of the whole rocket - if I have the parts, which I often do. Usually, the clone ends up finished long before the original kit.

This is what happened with the Hi Flier XL. I built a clone, added a payload section, since it's long enough to require two body tubes, and painted it like my original little Hi Flier. It looks great, but it feels a little heavy, and doesn't fly as high as I'd thought it would.

I filled the fins on the clone with Elmer's Carpenter's Wood Filler. I also think my paint job was a bit heavy. And there's the payload section which included a 1/4 inch thick basswood bulkhead, screw eye, and a small dab of epoxy.

So, now that I'm building the kit, I'm weighing it at each stage, just to see where the mass is coming from.

On the kit, I filled the grain with Brodak sanding sealer. I weighed all three fins together, and they were 33.1 grams.

Then I used two coats of sealer, followed by a good sanding. Then I did a third coat and sanding, and a final fourth coat.

Then I weighed the fins. They came in at 34.1 grams before I sanded off the final coat of sealer.

So, with no sanding, the whole set gained 1 gram - and these are large fins! That was really encouraging.

I sanded rigorously, until the fins were glass smooth, then wiped off all sanding dust, and weighed the fins again.

33 grams even.

What?? The fins lost a tenth of a gram after sealing and sanding! Is this possible? Did the sealer allow me to sand off a bit of extra weight while maintaining a smooth finish? Is my scale acting up?

Instead of adding a tiny amount of weight, my fins lost weight! At the very least, they didn't gain weight.

I have a history of owning things that don't work very well - cars, appliances, etc. So when I see a result like this I tend to be skeptical that I got the right answer. I tend to think there's something up with my equipment or with the way I'm using it.

So, maybe the fins lost weight after filling. Maybe not. I guess this means I'll have to weigh future builds carefully to be sure.

Why am I so obsessed with what my rockets weigh, and where the mass comes from? Two reasons. First, I think readers of the blog will find it interesting, and I hope some will find it useful. The second is that when I started out, my rockets were all lighter! As my paint jobs have gotten prettier, my rockets have gained weight, and their altitude has probably suffered.

My Big Bertha - not smooth and shiny, but very lightweight!

I used to paint very light coats, moving the spray can quickly, until I got full color coverage. Doing this, though, I rarely got a nice, shiny gloss coat on a rocket. To do that, I needed to paint a little heavier, so that the atomized droplets of paint could run together to form a nice shiny shell of paint. It's a tricky thing to get - you don't want to go so heavy with the paint that it runs and sags!

My Estes Goblin. It looks nice, but that shiny coat of paint is on the heavy side.

I've got the wet coat pretty well down, but the rockets have gotten heavier. I want to see if I can have pretty rockets that also don't weigh a ton. So I need to be rigorous with my testing. Maybe I can save weight another way. Or maybe I need to change my painting method.

In any case, it does fit with the original mission of this blog - I learn about this stuff, then I share it with you - and any other Rocket N00bs out there can benefit.

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1 comment:

  1. I think your conclusion that the sealer allowed you to sand off enough fin material so the final sealed and sanded fin weighed less than it did “in the rough” is correct. Another possible reason could be how the sealer interacts with water - if the sealer is hydrophilic and absorbed water from the fin, water that subsequently evaporated as the sealer cured - that plus sanding could account for the weight loss. However it happened it is a pretty nifty way to loose a bit of weight from the tail end of your Hi-Flier!