Tuesday, October 7, 2014

Building the Big Bertha - Part 4 (For N00bs)

[Click here for Part 1]

Once you're happy with the smooth texture of your fins, it's time to set them aside for now, and focus on the body tube, with a couple of Rocket Pro Tips.

I got some of these tips from various places online, including Chris Michielssen's Model Rocket Building blog.

First thing I always do, even though it is not written in the instructions, is to reinforce the ends of the body tube. To do this, I use cyanoacrylate - commonly called CA. This is the same stuff as Super Glue, but you can find specially made bottles at hobby shops which come in different viscosities.  This stuff is useful for a lot of rocketry applications (though not generally recommended for gluing on fins or motor mounts).

For this, you want thin CA, which is pretty watery. This stuff bonds instantly to skin, as I learned when I glued a bit of paper towel to my finger, so you'll need to be careful. With thin CA, you should also wear eye protection.

Protective eye gear, thin CA (pink bottle) and CA un-bonder,
in case you glue your fingers together

Open the bottle of CA and run a very thin bead of the glue around the inside of very ends of the tube. Using a bit of paper towel, wipe off any excess - though don't use your fingers. Roll up a bit of it into a kind of wand or baton, hold it from one end, and run the other end lightly around the inside of the tube. Throw the paper towel away immediately, so you don't accidentally grab it by the wet part later and end up with stuck fingers.

Thin CA will wick into the paper fibers, and when it dries, the ends of the tube will be stronger and harder - good if your rocket has a hard landing.

It also makes it less likely the end of the tube will wear out from having the nose cone inserted and removed launch after launch.

Now, these low power rockets are all made with tubes of rolled paper - cardboard, essentially, a little like what you find at the center of a roll of paper towels or toilet paper, but better constructed, a bit thicker, and usually with some kind of waxy, smooth coating on them.

But they do all have that spiral groove winding their way from one end to another. That groove will show up through the paint job, so when people look closely at your rocket, they can tell. "Oh, yeah, looks like a paper towel tube." But you can fix that, and hide that groove, so that people may not be able to tell what it's made of.

Before I go any further, let me say that you can totally skip this next step if you're not into it. A lot of people build tons of rockets and don't bother. I didn't even know it was an option when I built my first rocket, Der Red Max, and I was really happy with the way that turned out - though now I can see the spiral.

A visible tube spiral on Der Red Max. Spirals are sometimes hard to see on a bare
body tube, but they're really visible once you prime and paint the rocket.

 But a lot of people prefer it, and I try to do it with every rocket I make.

There are a few options for filling a tube spiral (technically, it's a helix), but by far the most common solution is to use more of the carpenter's wood filler - CWF - we used to fill the wood grain in the fins. This doesn't take nearly as much time as the fins did, and if you get it right, the results are rewarding.

I admit, I've had trouble getting this right, and it's only with this latest build, the Big Bertha, that I finally did it to my satisfaction. More about that in a minute. The other option, which I've read about but not tried yet, is to skip the filling, and when you go to prime the rocket for painting, use automotive filler primer instead of mere sandable primer. Filler primer is used to fill in deep scratches in car paint jobs, so it actually has some structure, and you can sand it smooth, filling in the seam as you go through the priming process.

When I heard about this, it sounded much easier, but I'm stubborn, and wanted to learn the CWF technique, so I kept at it.

You'll need some CWF - thinned, but perhaps not as much as you used to paint onto the fins - and a razor blade. You'll also need something to apply the CWF to the spiral seams. Chris Michielssen of the Model Rocket Building blog uses the tip of a used (dulled) hobby knife to apply CWF right to the seams, and he's a real pro at getting rockets to look great. But I've tried this, and I personally don't have a lot of success with it. So I use a fine tipped artist's paintbrush. You'll want a fresh razor blade. You might also want a freshly-sharpened pencil.

There are actually two seams. One of them is easier to see than the other, especially on white tubes (they're usually white or brown, and they're a little different, but I don't know enough to tell you what the technical difference is). Often, that one is actually filled in - the seam is on the inside, and you're seeing the filled outside of it, so it looks darker. Run your thumbnail around the tube until you feel the other seam - it may be invisible. That's the one you're going to fill, because that is the one you'll see when you paint the rocket.

Run the tip of the pencil down the whole length of the seam, from the top of the tube to the bottom. Now you can clearly see the seam, and you'll be able to get CWF exactly where you need it.

Next, apply the CWF to the seam, going about an inch or so at a time at most. Use Chris Michielssen's knife blade technique if that works for you, or paint it on. I try to tap it into the seam with the tip of the paint brush, then brush over that, so I get CWF all the way into the seam.

Next, grab your razor blade, and scrape off all the excess. You're going to hold the razor so it is lined up with the body tube, perfectly straight, and so the blade is perpendicular to the surface of the tube. Be careful not to cut into the tube - you're just using this blade as a super fine putty knife to push the putty into the seam, and scrape off all the excess, so you'll have less sanding to do later.

Don't hold it at an angle like this - this was just me trying to use one
hand to hold the blade, and the other to take a picture.

 Wipe the edge of the blade off often, on the edge of the container of CWF. Go inch by inch, all the length of the tube. When you're done, you should have a fully-filled spiral, with not much excess to sand off.

Once the CWF on the tube is dry, you're going to sand it off. Now, this is supposed to be really easy, but I admit for the first few rockets I built, every time I tried sanding the CWF down to the surface, I ended up scuffing the waxy coating on the tube and ended up raising all of these hairy paper fibers from the tube. I could not for the life of me stop doing this, and it ended up making my rockets have some weird fuzzies on them.

Paper fuzzy bits poking through the finish of my Crossfire ISX. This
was driving me absolutely crazy for my first few builds!
When I asked about this on The Rocketry Forum, I got a lot of responses, ranging from "You need to sand in little circles" (I did) to "Did you use a sanding block?" (sometimes yes, sometimes no, always with the same result) to "I don't bother to do that on tiny paper rockets" or even "Why bother? Nobody's going to notice when it's rotting in a tree!"

What nobody ever said was "Yeah, I used to have that same problem myself. Here's how I solved it..."

I tried fine grit sanding paper, rougher grit sanding paper, wet sanding (bad idea with no paint on it), dry sanding, etc. So, I was beginning to think that this whole CWF spiral filling thing was either beyond me, or that maybe it was some kind of rocket urban legend, and that the secret was something the real craftsmen weren't sharing with the rest of us. It was really frustrating me, and I felt like it was holding me back, because regardless of the size of the rocket or the fact that it might end up in a tree or a pond, I wanted to craft a nice rocket. So I kept experimenting.

Then I cracked it.

I now use a sanding sponge. It's like sandpaper, but in sponge form, and it comes in various grits. The finest I could find was 320. This is why it is important - for me - to use the razor blade method and get as much of the excess CWF off the rocket body before I sand, because I want to sand as little as possible. Something about the sponge being softer and conforming to the body of the rocket makes it less likely that I raise those nasty paper fibers, and I end up with an acceptable surface for painting later.

My sanding sponge, just before sanding off the CWF. You can rinse these
out, and they last a lot longer than a piece of sandpaper.

Chris Michielssen - whom I've mentioned on this blog numerous times, and I recommend you check his blog out if you want to see some expert builds - sands CWF off seams by going parallel to the body tube, thereby going across the CWF. I have to be really careful, so I lightly sand with a corner of the sponge in tiny circles, trying to touch only the CWF. I go slowly, and stop if it looks like I'm going too far.

You might just try using sandpaper - I've seen some people using grit as coarse as 150 or even 100. Maybe it's just me.

Anyway, when you're done, there should be no raised bits of CWF, and when you prime your rocket, those spiral seams will be invisible.

Now, I had said in Part 3 that we'd put the fins on in this post, but this body tube prep post has gotten longer than I'd anticipated, so I'll save that for Part 5. We'll figure out where the fins go, attach them to the rocket, and I'll show you a cool, very useful tool that you can make yourself for less than two bucks.

Click here for Part 5

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