Thursday, April 9, 2015

Mid Power: Building the Quest Big Dog (Part 6)

Click here for Part 1

Often, when I taper or airfoil a fin, I use some thin cyanoacrylate, or CA - craft superglue, basically - to reinforce the trailing edge. When you sand balsa to a thin point, it gets much weaker. A tip can break off when the rocket lands - or even when you're transporting it to the flying field, if it bumps up against something.

CA wicks into the wood fibers, making them harder. But these fins are so strong, I decided not to bother. Actually, it would have been a good idea regardless. CA not only hardens the wood, it seals it against moisture. I'll come to that in a minute.

Using Chris Michielssen's ratio of 2.5 parts Carpenter's Wood Filler (CWF) to 1 part water, I painted CWF onto all four fins of the Big Dog.

The Big Dog fins are on the right. On the left are the fins for an
Estes Cosmic Explorer I'm building with an E-sized motor mount.
CWF contains moisture, and moisture can cause wood fins to warp. The cells of the wood soak up water and swell, and if the cells on one side of a fin swell up, the fin curls to the other side. If you evenly and quickly paint both sides with CWF, many builders say that you shouldn't get any warping. While this makes sense, I still get warp on my fins some of the time with CWF.

Of course, to avoid warping, you want to allow air to get to the whole fin so that it can dry evenly. Some people stick a pin or wire into the root edge of the fin, fill the fin with CWF, and then stick the pin into some foam to keep the fin from touching anything while it dries. When I tried this, I shoved the pins through the face of my fins, and it generally didn't go well.

Usually, I press my fins under a book after filling them. Many people sandwich them between sheets of wax paper when they do this. I prefer parchment paper, because it wicks some of the moisture away from the CWF, allowing them to dry a little faster. But as it does, it causes the paper to form ripples, and the ripples cause the CWF to bunch into ridges, which, when dried, require more sanding. They can also leave low spots in the CWF that leave little imperfections when sanding them off, and it usually take me about three coats to get the fins where I want them. Fewer than that, and I can still see wood grain when I paint the rocket. Regardless of what paper you use, pressing fins makes the drying time take a lot longer - which, in turn, makes build time take longer.

So, I came up with a compromise.

The surface of CWF starts to dry pretty quickly. After filling the fins, as you can see in the picture above, I laid them on a cooling rack. I got a couple of these at Bed, Bath & Beyond for about six bucks, and I now use one for cooking and one for rockets.

After a few minutes, I flipped the fins over, just to assure that both sides got an equal amount of airflow to allow drying. If, after close examination, I saw any signs of warping, then under a book they would go.

The Cosmic Explorer fins dried just fine with no warping. About half the Big Dog fins were OK, except for two - and it was the tapered trailing edge that had begun to warp. They had to go under the Riverside Shakespeare.

The Big Dog fins are so large, both Shakespeare and Chaucer were required.
Here's where I thought it might have been a good idea to wick some thin CA into those trailing edges - not so much to keep them from breaking upon landing, as these fins are really tough. More because if I'd used CA on them, the trailing edges would have been waterproof. I could have let the fins air dry completely, and started sanding an hour or two later.

But, because I'd let them start drying before pressing them, the CWF didn't bunch up from the parchment paper. They dried more quickly than they would have with waxed paper, and because they'd got a decent start for about ten minutes in the air, they dried more quickly than if I'd put them under a book immediately.

When you sand the CWF down, it always feels so smooth, and each time I do it, I think this time, it's a breeze! But when I wipe off the dust, I always see little pits, divots and imperfections in the surface.

These pictures are in extreme closeup, so the details here are slightly exaggerated:

But I have had those pits show up in paint jobs before. This is a detail a lot of people who look at my rockets might not notice, but I can see it. More sanding is necessary, but I can almost never totally get rid of these with one coat. It's probable that a good coat of filler primer (not now, of course - later, after the fins are on and you start the painting process), followed by some good detail sanding and perhaps another coat of primer, would take care of these. But for good measure, I sanded as much as I could out of these fins. But usually if I do this and stick with only one coat, the fins look and feel very smooth. It's only after I begin painting that I realize that I've sanded some of the filler completely off, exposing the wood grain.

When I first started using CWF, this baffled me. I thought the CWF was supposed to fill in all of the wood grain. It's really more for scratches, holes and gouges. And I thought, I thought using this stuff was supposed to be easy!

And it is easy. But to get a really nice-looking rocket, it requires some patience and some care.

One of my as-yet unstated goals is the Perfect Build. Now, there's no such thing as a perfect rocket. But on every build, I've done something - sometimes something small, some times repairable, and often not - that I would consider a mistake. Something I didn't mean to do - either through a moment of carelessness, or impatience, or through trying something new I didn't know how to use yet. That last one is a good thing. Most stuff has a learning curve on it. And most of my "mistakes" are things only I would notice, unless I point them out. But some of them are obvious. Let's take a close look at the Cosmic Explorer:

Torn decals, with chunks missing

Instead of using decals for the stripes, I painted them on - and made some critical errors.

Compared to some craft mistakes, tiny little pits in the fins are minor, and won't be noticeable to anybody else. And I want to make this clear to other rocket n00bs - your rockets won't stay perfect. They will get dinged up in flight and landing. Wear and tear happens. But I like my rockets to look as good as possible, even if it's only for the maiden flight. When I take them out to the field, I like to be able to say, "Look, guys! I built this!" Besides, rockets spend a minute or two in the air at a launch. They spend a lot more time on a shelf at home, and I like to show them off.

As it turned out, Chad and my other friends liked the paint problems on the Cosmic Explorer. They said it made the rocket look "weathered." It sure did...

OK, enough digression. Back to the Big Dog.

Once I got the fins sanded to where I liked them, I decided it would be best to do a second coat of CWF. I nearly didn't, but I knew if I did not, I'd be disappointed with the results. The Big Dog was a little more expensive than a lot of Estes models, so I'm taking a lot of care with this rocket.

The good thing about a second coat of CWF is that you can thin it a little more with water than you'd do the first coat. You're really just trying to cover the parts that you sanded completely off, and fill any imperfections you missed the first time. So there's less to sand off the second time, and it's just easier.

Again, I had to turn to Shakespeare and Chaucer for help. When I was done, I had smaller ridges to sand off:

And using 400-grit sandpaper, I got the fins nice and smooth.

The fins are filled. The longest part of the build is finished.

Next time, I'll introduce any rocket n00bs reading this blog to the concept of rail buttons - tiny ones, for this rocket, but something commonly used in higher power rocketry - and we'll get those fins on the rocket.

Click here for Part 7

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