Building the (scratch) Au-198 (part 1)

Yes, I know, I still haven’t built the booster for the flexwing glider, but this is higher priority and doesn’t involve sanding airfoils. The Au-198 is my NARTREK Gold rocket. This NARTREK level is very different from Bronze and Silver: It requires you to design, build, and fly a single rocket — making estimates of the center of mass and center of pressure, predicting the altitude, measuring the actual altitude, and comparing prediction to reality.

My design is pretty simple. I contemplated some more unusual features but then remembered I’m supposed to sim this thing. So I drew up something that I think is fairly compatible with the assumptions behind the Barrowman equations. I started with a nose cone I have in my stash and worked from there. The airframe is BT-50 at the front widening via a paper shroud transition to BT-55 at the back. Actually the BT-50 continues inside the BT-55 all the way to the rear end, becoming a 24 mm motor mount tube. The diameter change is partly to avoid minimum diameter complications, and partly to give it a little more visual interest. The forward few inches of the airframe are a payload compartment for an altimeter.

I decided I wanted to use a screw on motor retainer, but not an Estes one. I like the Estes retainers. They’re cheap, they work. But they’re also kind of chunky. The 24 mm ones are decidedly larger in diameter than a BT-55 tube, and if I used one it would look kind of like a big knob hanging off the back end. So while at eRockets I picked up a Rocketarium retainer, considerably more svelte than the Estes.

The fins are, well, fins. Three of them, quadrilateral though not trapezoidal. I started construction with them. They’re 3/32″ balsa, papered, non root edges rounded. I don’t know how much good it’ll do to put them through the wall but I figure it’s not going to hurt, and given the 24 mm motor mount I feel better doing it that way.IMG_8222

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