wfcook
Mayhem Rocketry, LLC
- Joined
- Nov 15, 2013
- Messages
- 1,028
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- 11
So, how about a 3-way canted cluster of mellow? 3 J150s should be enough to lift about a 50 lb rocket plus 6.5s of burn time = fun!
Nate
In addition to the math issues already pointed out, the problem is not so straightforward.
First, to fit three 15-deg canted 38mm 6GXL motors in a rocket, it needs to be about 10" in diameter, maybe more. Of the 20 or so pounds that the rocket can weigh and still have a reasonable thrust/weight ratio, 6.3 pounds is taken up by the motors themselves. Most of that weight is of course in the extreme aft end of the rocket, and unless you go up to about an 11" body tube a good portion of that must stick below the aft end of the airframe. The fins need to protrude aft of the motors to protect them on landings. Now you have a really intense CG problem, because you have only 13.7 pounds to build a 10+ inch diameter rocket long enough to be stable. And then you've still done it wrong.
The focus of the motor mount needs to be approximately halfway between the CG and the CP. And So you usually need to add a little nose weight to the thing to get it to do that. But with a motor that long you're going to have a fairly forward motor mount focus, so then the CG has to be WAAAAAY forward relative to the likely Cp, especially given base drag considerations for such a fat rocket with motor tubes sticking out of it. So it becomes a real challenge to get the rocket to bee stable under two motors, especially given that you were scraping the lower limits with all three motors lighting.
It ain't gonna happen.
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