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Old 11-01-2007, 12:02 PM
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CPMcGraw CPMcGraw is offline
Join Date: Mar 2005
Location: Mobile, Alabama
Posts: 5,357

Originally Posted by snaquin

I really like the booster and your choice of fin design. Looks great!

There is however a limitation that I haven't been able to figure out in RockSim that is throwing your margin of stability out. Margin without engines in RockSim shows 32.28 & I think it's probably closer to 6.43

RockSim is throwing it out somewhere in your Nose Cone Tip, Transition or Fin used for the camera shroud estimation.

To determine this I eliminated those three items from the design file and replaced those items with a single ogive BC-12561 Semroc cone, same as used on the SLS Javelin. I then added a single mass object to the true CG of the Semroc cone at 4.595" to bring the total rocket weight to 3.3085 to match your designs weight and I get a 6.43 margin without engines.

RockSim just doesn't estimate the CG accurately in this case (and I'm not sure why it doesn't).


Jay Goemmer emailed me with the heads-up. Change the setting on the first tab "Rocket Design Attributes" where it says "Static Margin Reference" from the default setting of "Nose Cone Base Diameter" to "Maximum Frontal Diameter". Everything else being equal, this should bring a loaded margin (with a D12-5) to about 5. The base diameter of the nose cone is actually larger in this case than the body tube, which is what RockSim normally defaults to. Empty margin comes to 8.

Jay put this info in another thread not too long ago, and I just forgot to make this change. I used his NC drawing method to produce the AV shape.
Craig McGraw

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