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Old 01-13-2013, 06:37 PM
Les Les is offline
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Join Date: May 2008
Location: Johnson City, NY
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Default Conical rocket

I am trying to build a rocket that will be conical based on a plastic piggy bank.

I went to the Apogee site and in their newsletter 162 found a method on simulating a pyramid style rocket by creating a massless transition at the rear. I tried this and it states the rocket is over-stable, but when I simulated a launch it flips around and crashes. Can someone let me know what I am doing wrong?

I attached the RockSim file. It is not 100% complete (like I have some centering rings missing), but it should have been good enough for the simulation.

Attached Files
File Type: rkt Number 2.rkt (15.8 KB, 57 views)
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Old 01-13-2013, 08:44 PM
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bernomatic bernomatic is offline
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The difference between static stability and dynamic stability probably. In other words, sitting on the launch pad it is stable. when interacting with the real world it becomes unstable. Without studying the file or getting into real deep, because I'm not on this forum anymore the usual suspect is the wind (tip off upon leaving the launch rod) or causing a shift in the CG CP relationship .
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Old 01-14-2013, 07:05 PM
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Ironnerd Ironnerd is offline
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I don't think the software can correctly simulate a flight with cones or other short/fat rockets. I am limited to OpenRocket, and it cannot accurately sim dynamic stability - RockSim may have similar limitations.

The swing tests I have done seem to confirm the "Massless Cone" method of finding the in-flight stability of a squat rocket.

What I normally do is:

1: input the massless cone and find the dynamic (in flight) stability.
2: Remove the cone, and reduce the mass of the rocket to "0"
3: Make a mass item the same weight of the total rocket weight, and position it so the CG/CP relationship is the same as was with the massless cone attached.

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