Friday, March 30, 2012

Building A Model Rocket - part 2

This is a series of posts where we're building a basic model rocket online. Each post shows part of the process step by step, including pictures and passing along tips and tricks I've learned along the way. To learn more about what model rocketry is about, see this Q&A.

I hope you follow along because when we get done you'll have built and flown your first model rocket. Questions asked from previous posts are answered too, so if you have questions, please leave them in the comments or email me.

The instructions for most model rocket kits are wonderful. Estes has been doing this for years, and their experience shows. Let me stress one point right up front: always, Always, ALWAYS follow their suggestions for glues to use. You can sometimes use an alternate (I have almost 20 different kinds of adhesives for various situations), but their recommended glue will give you the strongest bond.

Looking at the Parts

Almost everything that needs assembly tells you to lay out the parts and make sure you have everything, and also to read through the directions first to understand things. This is a simple kit, so do it if you'd like, but it won't be a problem if you don't. For more complicated kits, I do recommend doing it.

Lets look at the various parts, most of which are obvious. The biggest tube with the slots cut in one end is the body tube. In a simple rocket like this, it's main purpose is to hold all of the important bits in their correct places. The nose cone is straightforward, as are the fins. The parachute is the plastic sheet with the strings attached. So much for the obvious bits.

The tiniest tube (it looks like a drinking straw) is the launch lug and it's used to steady the rocket on the launch rod. The length of elastic is the shock cord, remember I recommended replacing it with a longer piece bought at the store. The two cardboard disks, the medium sized tube and black ring will be put together with that little metal strip and become the motor mount.


Using your x-acto knife, carefully cut the fins out of the balsa sheet. They're die cut and held in by just a few short bits of wood. If you want to, you can gently sand the fins (with the grain) with the fine sandpaper before freeing them. Make the same kinds of cuts to remove the smaller middle circles from the cardboard disks. Finally, you may need to open the inside of the squared-off loop at the bottom of the nose cone (see the picture). Do all of these carefully, and watch for the sharp knife.

Make sure the fins fit the slots in the body tube. Sand them lightly if needed to ensure a smooth fit.

The following steps are completely optional.

Using the sandpaper, sand the seam on the plastic nose cone until it disappears. This isn't a quick process, but it does make for a much nicer looking end result.

Lightly sand the entire body tube until you've scuffed the shine off. Don't sand too much, the purpose here is to remove the glassine layer, which will make for a stronger bond between the glue and the paper tube underneath.

Take some of the Fill'n'Finish and thin it with water until it's about the consistency of pancake batter. Slather it on the body tube (I use my finger) and work it into the spiral groove. You won't need much, and most of what you use will be sanded away. Let it dry (it's pretty quick) and then lightly sand. The Fill'n'Finish sands easily, and when you get done there should be no spiral groove left. Repeat if you need to.

Use the same thinned Fill'n'Finish to fill the grain of the balsa fins. Keep the coats very very light, and sand between coats when dry. When you do the fins, do both sides at once, because the balsa will warp slightly and this will help even it out. The warp will straighten out when both sides are dry.

The reason for all this filling and sanding is because the smoother the surface, the less drag which makes for a higher flying rocket. I don't do it for every rocket, but I do take the time for most of them. The paint job looks much nicer on the smooth finished surface too.

Questions Answered (from previous posts)

What is a "fishing swivel?" Also known as snap swivels, they're used to prevent the fishing line from twisting. They have a small loop on one end and a large loop on the other end that opens like an old fashioned safety pin. Here's a (not great) picture of a few, and like I said, you'll only need one, and it's optional. Also in the picture you can see the package of sewing elastic, the glue and Fill'n'Finish, and an x-acto knife.

This is our rocket so far, after sanding the nosecone seams smooth and filling the spiral grooves on the body tube. Total sanding time was maybe 20 minutes. If you did those steps, you'll notice that the tube is a little fuzzy. Don't worry about that, because we'll smooth it out when we spray primer. I also had to spend a few minutes sanding the tabs on the fins so that they would slide easily into the slots. Nothing is glued together yet.

You may have noticed that the fin tabs have a small slice trimmed out at the bottom. This shallow notch fits over the black ring of the motor mount. This particular rocket boasts a nice bit of engineering because everything fits together and reinforces itself, making for robust construction. In fact, although we'll be flying this bird stock on B and C motors, I've seen the same kit strengthened and modified to fly on I motors (128 times more powerful)!

Now, I'd like to talk about what actually happens during the flight, and some of the basic aerodynamics involved.

Model rockets are set up on a 'launch rod', which ensures that the rocket stays straight until the rocket is moving fast enough for the fins to keep it stable. A good way to picture how the fins work is to compare it to a weathervane, and how it always points into the wind. When a rocket is moving through the air, its flight through the air provides the 'wind' that the fins work with.

Everyone has stuck their hand outside a car window at speed and felt the rush of air. When you keep your hand flat to the ground, the air moves smoothly past it, but if you try to cup your hand against the wind, then the wind pushes against it. The fins work in the exact same way, and it's this push that causes the rocket to stay straight.

The main effect of this is that the straighter the flight, the less drag the rocket has to overcome and the higher it will go. I'll go into other aspects of this as we go.

Next time, we start gluing things together!

And as always, leave questions in the comments. Thanks!

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