Tuesday, October 14, 2014

Building the Big Bertha - Part 6 (For N00bs)

[Click here for Part 1]

We're getting closer to finishing our Skill Level 1 rocket!

Once the fins are on, attach the launch lug. That's the short tube that looks like a piece of drinking straw.

First, I like to scuff it up a bit, so the glue holds better. I do this on one side with rougher grit sandpaper. Then I do a double glue joint, putting a bit of glue onto the roughened surface of the launch lug and attaching it to the rocket for just a moment, as perfectly straight as I can along the launch lug line we've already marked. I remove it, and allow the glue to dry a little, then apply more glue and attach it to the rocket.

Scuff one side of the launch lug
Apply a thin layer of glue the entire length of the launch lug

Place it on the rocket where you have marked the launch
lug line, getting some of the glue on the rocket

Allow the glue to dry for several minutes. These reverse-
action tweezers aren't necessary, but they are a nice little tool!

Add a bit more glue and attach the launch
lug to the rocket - make sure it's straight!

You must make sure it is straight - the launch lug will slide down over a metal rod called the launch rod (when we actually take the rocket out and launch it). The rod keeps the rocket flying straight for the three feet or so it takes to gain enough speed for the fins to keep it stable in flight. If the launch lug is askew at all, the rocket can grab hold of the rod, and either not take off, or fly in a weird direction, or even try to take the rod with it! Site down the length of the body tube to make sure the launch lug is straight and true. You can even (carefully) slide a launch rod into the lug and verify that it follows the line you marked on the tube - then you'll know it's straight. Just (carefully) remove the rod so it doesn't weigh down on the glue and push the lug out of alignment as it dries.

Site down the length of the rocket to verify the launch lug alignment. Be sure to adjust it
while the glue is still pliable if it's crooked. Work fast - a double glue joint sets quickly!

Let the glue on the launch lug dry before moving on.

The fins are on, so let's reinforce the joints. We're going to make fillets. A fillet is a rounded joint in a corner, often found when two pieces are welded together at a right angle. In model rocketry, this makes the joint where the fin connects to the rocket stronger, and it also reduces interference drag, making the rocket more aerodynamic.

For our rocket, we'll use more wood glue.

The simplest way to do this is to apply a bead of glue at the base of the fin at the leading edge, then take your finger and run it along the base of the fin, dragging the glue with it. You should get smooth, even line of glue connecting the fin to the body all the way down the root of the fin.

Instead of your finger, you could also use a craft stick (popsicle stick) or coffee stir stick, or even a narrow dowel rod.

I always get extra glue on the fins if I do it the simple way, which either I can't sand off, or don't have the patience to. So I mask off my fins with a little low-tack masking tape. I've been told by some real rocket craftsmen that this isn't necessary with glue fillets, but I find it makes mine a lot neater, so I always do it.

I start by masking two fillets - two joints between the fins and the body - at a time. You'll do two fillets in one go on each side of the rocket, and by side, here I mean the space between the fins. So, if you have a four-fin rocket, like the Big Bertha, you'll have four sides. Three fins, three sides. Etc. Each side will have two fillets each, one for each fin.

Here, let me just show you...

This is what I'm talking about. I've got a tiny bit of tape on the body tube at the leading edge of the fin, then one piece of tape running the length of each fin, and tape on the tube itself, with a little gap between the tape lines where the glue fillet will go. If you're building a rocket where the fins are not flush with the base of the body tube, I'd put a piece of tape there as well.

You'll want to lay the rocket on its side with the joints you're working on facing upwards, so you can work on the fillets, and so they'll dry evenly. Here's one of the many little things I love about rocketry - coming up with a creative solution to problems. There are big problems and little problems in building rockets, and they all require creative solutions.

Here's a few ways you can do this.

You could lay the rocket on its side, with the tube supported by a thick book.

Could be just fine. But you might not want to put any lateral stress on those fins before you reinforce the glue joint. And if you're building a three-finned rocket, this won't work.

The joints aren't facing upwards!
 You could lay a dowel rod on a table, place something heavy on it, and slide the rocket body over it.

You could place the rocket down on a table (or in this picture, a thick book) with the fins hanging over it, and lay a heavy blanket over the body of the rocket. This will hold the rocket down, but won't squash the body.

In my case, the problem is solved by the Guillotine Fin Jig. The long metal arms are for holding the fins when you attach them to the rocket. But once you're done, you can slide the rocket through the jig the other way to hold it gently but firmly in the horizontal position for working on it.

Whatever solution you come up with for this little problem, just make sure the rocket is secure, that it won't fall or get crushed, and that it can remain undisturbed in a horizontal position while you work and while the glue dries.

OK, enough of that digression - back to fillets!

Drop a healthy bit of glue in the gap between the tape lines. If you're building a small rocket, a bead may be enough. The Big Bertha is pretty big, and I find if I don't put enough glue, I can't get the fillet to run the whole length of the fins, so I put a bigger blob. Don't put too much, though. You are going to pull most of it off, and the more glue you use, the more of a chance you'll get a mess.

First, notice that I don't put the bead right at the fore of the fin.  With a craft stick or coffee stir stick (depends on the size of the rocket, and therefore, the radius of the fillet I'm making - bigger rocket, fatter stick), I push a little of the glue forward to the leading edge, then run the glue back down the length of the fin, smoothing it as I go. Do this quickly, so it spreads evenly, and if there are any bubbles when you're done, smooth them over with the stick.

Once the fillet is smooth and covers the entire seam between the fin and body tube, remove the tape. Don't wait long to do this - fresh glue will smooth out and make a nice smooth seam, but once the glue has started to set, it will leave a sharp ridge.

Remove all the tape, from the fins and also the little bits from the body tube.

When you remove the tape, you should have smooth, even fillets.

Let these set and dry a bit before moving on. I can't tell you how long - 20-30 minutes, maybe? I dunno. Go have a snack or something. When the glue looks dryish on the surface and the color has changed from off white to more yellowish, you're fine to move on. Turn the rocket to the next side and do those fillets. Repeat until you're all done.

Oh, and don't forget to put a fillet on each side of the launch lug! The launch lug takes some stress from the launch rod, so you want it to be secure!

We're nearly done building our first Skill Level 1 rocket! Next up, we'll prime and paint this sucker, then we're almost ready to fly!

Click here for Part 7

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