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Old 09-05-2008, 10:47 AM
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CPMcGraw CPMcGraw is offline
Join Date: Mar 2005
Location: Mobile, Alabama
Posts: 5,357

Originally Posted by CoachJT
...The Goony Flare. An 18mm two stage Goonybird Payloader...

I took a look at your RockSim file. Nice design overall, it really captures the goony philosophy. I do need to draw your attention to the "Margin" value on the 2D line drawing. "Margin" here represents the stability, or "calibre" (caliber), of the model. When you load up the model with a C6-0/C6-3 combination [which is the right combination for this bird] your stability gets whacked. Notice in your file that the margin falls below the "magic number" of 1.00 when you add the motors. As shown in your file, the rocket will cartwheel and go everywhere else but up.

Or, more graphically, this means it'll go "butt-up"...

It was a simple fix to get this number back into our "greater than 1.00" ballpark. The model just needed 1/4 oz of ballast at the inside tip of the nose cone. A couple of tiny fishing sinkers epoxied into the plastic nose cone should be enough.

I added a launch lug to the model, placing it at the CG. It does contribute to the drag value, but not enough to kill the performance.

I also 'tweaked' the model in the ringtail area by setting the "Pylon Count" to zero, instead of using the '3' you left in place. This removes those extra 'fins' from the simulation (which add a measure of drag, no matter how thin they are). When you want to use custom fins with a ringtail, set this value to '0', set the pylon thickness to 0.063" (1/16") so the program doesn't bark at you, and draw in your fins as you like.

I'm attaching the 'repaired' file below. Again, welcome to the goony side of life!

Addendum: And now, here's the real value in using RockSim. Finding out the model doesn't have enough OOMPH to get off the launch rod. After posting the above changes, I failed to notice the flight performance section of the full-stack simulation.

Launch guide data:
Launch guide length: 36.0000 In.
Velocity at launch guide departure: 36.0432 ft/s
The launch guide was cleared at : 0.249 Seconds
User specified minimum velocity for stable flight: 43.9993 ft/s
Minimum velocity for stable flight reached at: 74.0976 In.

This is a show-stopper issue. It took a full quarter-second to reach the top of the 36" rod, and still isn't going fast enough to be stable in free-flight. See that last (bold) line? You need an 80" launch rod before the model reaches a safe flight velocity.

The problem is that C6-0 booster motor. It just doesn't have the thrust spike needed to kick that model hard enough to get airborne properly. This is also one of the reasons you'll hear us talking about the elusive "B14" and "C5" motors, which would more than likely have done the job.

You might need to change the design to a 24mm mount in the booster, and use a D12-0 instead.
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Craig McGraw

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