December 08, 2003

Build It - 3

This is a series where we build a model rocket step-by-step. You can find the rest of the series here.

The main part of the post is in the extended entry so you donít have to deal with it if you donít want to, but I hope you follow along because when we get done youíll have built and flown your first model rocket. Questions asked from before are answered too.

If you have questions, please leave them in the comments or email me.

This is my 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)!

I said I'd be building the motor mount this time around, but I'll save that for the next one. Instead, 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.

Posted by Ted at December 8, 2003 06:41 AM | TrackBack
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