Tuesday, September 29, 2009

Doin' Some work: Twofer update

Pyramid: currently mostly stalled until I can actually get something done on cutting the sides.

The Twofer is doing somewhat better. I got the fins (two Baby Bertha fins, 1/8" thick) glued on and fillet, the engine mounts glued in and plugs put in to direct the ejection gasses forward, and the shock cord mount attached. I then swing tested it, but it wasn't quite stable. I finally added more clay and some metal bits to the nose cone, and then I tested it again. It was stable even at pretty low speeds, and I was able to let out about 20 feet of string to test it.

It's about 21" long and roughly 1.3oz unloaded and without a chute inside. The CP is about 15" behind the nose tip, right near the front of the fins. Loaded with two 13mm motors, the CG is somewhere near 13" aft of the nose tip, so stability is actually pretty good now.

I put on a 3/16" launch lug primarily because I'll need to launch it off the away pads at CATO in case it's unstable. The lug is between the fins, so there's plenty of access for hooking up the cluster on the side away from the rod.

Simulations, using modified RASP-93 running on my TI-84 with custom motor files. Equivalent motors are based on actual thrust, not designation.
2x 1/4A3-3T: 63 ft / 62 fps / 1.8 optimal delay. Ejection at roughly 10 feet. no good. (=1/2A4-3)
2x 1/2A3-4T: 213 ft / 124 fps / 3.3 optimal delay. Ejection just after. First flight. (=A5-4)
2x A3-4T: 526 ft / 212 fps / 4.6 optimal delay. Ejection just before. (=B4-4)
2x A10-3T: 513 ft / 206 fps / 4.6 optimal delay. Ejection early. (=B5-3)
1/2A3-4T + A3-4T: 369 ft / 155 fps / 3.8 optimal delay (A3), 4.4 optimal (1/2A3). Ejects just before. (=B3-3)
1/2A3-4T + A10-3T: 364 ft / 154 fps / 3.8 optimal (A10) 4.3 optimal (1/2A3). Ejects just before. (=B4-4)
A3-4T + A10-3T: 520 ft / 205 fps / 4.5 optimal. Ejects early. (=B4-3)

I also simulated it with a pair of Apogee C6s. 1800 ft / 570 fps / 7 optimal delay.

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