Building a Bad Cholesterol (part 1)

The Syracuse Rocket Club has an informal contest at each monthly launch; nothing much like a Pink Book competition, very low key, with a model rocket kit as a prize. One of my goals is to put an entry in each of the contests this year. The first, at the May launch, will be a D dual egglofter duration. The only egglofter I have is a single (Estes Eggscalibur) so I’m going to build a dual. Nothing much like a Pink Book competitor! Just a simple, inexpensive bird that’ll put two eggs in the air — I hope! — and possibly even bring them back intact.

I could buy a commercial egg capsule, but what fun is that? Besides, I’m cheap. I like the prices better on the egg capsules available at drug stores and craft supply stores this time of year. Some assembly required.Rockets 2014

I found plastic eggs of several sizes and materials. Possibly the best one to use would’ve been the Etch-a-Sketch egg, but I didn’t find that one until I’d started building around one of the eggs from the package of six. I might build another egg capsule around it later on.

I’ve got a preliminary design in OpenRocket done, and even a decal. I’m calling this rocket Bad Cholesterol.

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I’m going with a BT-60 tube, so I can use a big chute. My design has a 24 mm motor mount, although after trying a few sims I think I’d like to use an 18 mm Aerotech D21-7 D10-5 for the contest flight. I’ll come up with an adapter if so.

I’m starting with the egg capsule and then building the rest of the rocket for that. To make the extension tube I measured the diameter of the egg, marking it on a strip of paper:IMG_2483

Then I used the marks to lay out lines on a 2.75″ long piece of BT-70, and I cut a strip out of the tube:IMG_2486

I taped it together, checked the fit, and then glued a piece of paper around the tube, shown here before trimming to length:IMG_2493

Finally I cut a couple strips of cardboard from a tissues box and glued them inside the tube, one of them near the back end and one sticking out the front. They both reinforce the tube, and the one in front provides a shoulder for the top half of the Easter egg:IMG_2494Here you can see the bottom half of the egg taped to the extension tube. I’ve drilled a hole for the shock cord anchor, and glued on a BT-60 coupler using epoxy. I’ve peeled off a layer of paper from the outside of the coupler to make a slip fit into the body tube.
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And here’s the anchor. I was originally thinking of an eye bolt but decided to use a button instead. Novel!

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The idea is to spread the shock cord force over a wide area of the egg. I poked toothpicks through two of the holes to make sure they stay open; I’m going to thread 100 lb Kevlar through them and make a loop the shock cord will attach to. The one thing that worries me is how much force the button will withstand; of course, as I’ve learned, estimating the force exerted on a shock cord is no easy matter. I’ll definitely use elastic, though, to keep the maximum force down.

Thinking about it just now, I realized I could put two Kevlar loops through all four holes, spreading the force a bit, and perhaps if the button breaks it’ll only be between two of the holes and the loop through the other two holes will keep everything together. I didn’t keep epoxy out of all four holes, and in fact I’m pretty sure there’s plastic egg behind two of them, but a little drilling would fix that. So maybe I’ll do it that way.

And that’s the progress up to now.

 

 

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