Building the (scratch) B3S2 (part 1)

I’ve started on my (first?) rocket for the club’s ongoing B streamer spot landing contest this year. I started designing something a while back, and after a while I looked at it and said, huh, that looks like a Baby Bertha, doesn’t it? So I revised the design a bit to make it a Baby Bertha kitbash. Hence B3S2: Baby Bertha B Streamer Spotlander.Screen Shot 2016-02-07 at 6.38.10 PM wm

The fins might not look that Bertha-ish to you, but look again. They’re Baby Bertha fins, cut off flush with the end of the body tube. That moves the CP forward, of course. Big deal. I’m expecting to fly this with a lot of nose weight anyway. The apparatus in the nose cone is something I was considering for adjustable nose weight. I think I’ll do it differently, though.

So this thing is heavy, coming in under streamer. Rough landings anticipated! But notice the chute is in the back, on top of the motor: This is a rear eject model, which reduces the chance of breaking a fin. A characteristic of this rocket is that I’m building it tough and not worrying about weight. (No fiberglass or carbon fiber, but strong paper construction.) Rear ejection also helps keep a steadier trajectory at deployment for a more predictable landing.

Heavy, fast descent, nose first: You don’t want to be standing under it. Hopefully no one will. But that’s why the design had an elliptical nose cone from the first, as a safety precaution.

Inside the front of the body tube are three coupler tubes as stiffeners. I don’t want the body tube to crumple on these rough landings. There’s also a bulkhead just ahead of the motor mount. The forward half of the body tube then is effectively a payload section, and I can fly an altimeter to check performance against the sim and tweak as necessary.

The Baby Bertha’s 1/8″ launch lug will get replaced with a 3/16″ due to the weight of the model and the undesirability of rod whip.

I drew this with the fins canted, for spin stabilization, but I haven’t decided if I’ll actually do that.

So, the build. I started with the fins. Man, this balsa had some tough sections to cut across the grain.
IMG_7863I’m saving the cutoffs, they’d make good fins themselves. I rounded the leading edges and papered the fins for strength.

Anticipating the adjustable nose weight, I cut the bottom off the nose cone.IMG_7869 I built the motor mount more or less standard, and didn’t glue it in.

I trimmed about 1/8″ off one of my couplers and glued in the plywood bulkhead. (Yeah, “in”. I was expecting it to fit inside the body tube, instead it fits inside the coupler. Well, fine.) I also tacked a piece of Kevlar on the forward side.IMG_7875 While that was drying I marked the tube for fins. Twice, because the fin marking guide that came with the kit was hopelessly inaccurate. I also treated the airframe ends with thin CA.IMG_7881

Last step for today, I epoxied the stiffeners inside the body tube, with the Kevlar captured between the stiffener and the airframe. I also painted epoxy on the rear (motor-facing) surface of the bulkhead.



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