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Center of Gravity Question

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jy2370

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I have a question about the center of gravity. Here I have attached a design for a rocket. The center of gravity in this rocket appears to be very low, although it is still a good distance from the center of pressure. Will this have any negative impacts on my rocket? Thanks.

Screen Shot 2017-03-02 at 8.43.20 PM.jpg
 

noffie79

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Did you physically measure the CG and does it match this sim? If so, I think you'll be just fine.
 

OverTheTop

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It really does not matter where the GC is on the rocket. All that matters is that the CP is behind it. That means as the rocket changes angle relative to the airstream, as it rotates around the CG, that the forces on the fins will point the rocket back in the straight-forward direction. If the CP is forward of the CG then the rocket keeps trying to swap ends after it leaves the influence of the rail. Behaviour known as "skywriting" results.

Skywriting is dangerous. If things are marginal then as the motor burns the CG moves forward. At some point the rocket attains stability again (with motor still lit :eek:) and accelerates quickly in the direction it was when that happened. This could be towards the crowd. Really dangerous as you don't have time to get out of its way. :(
 

Bat-mite

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Did you physically measure the CG and does it match this sim? If so, I think you'll be just fine.
What he said. The model can give you an accurate CP based on the shape of the rocket, but it can only guess at the CG. No model is perfect, and there is always a little something left out (like epoxy or glue, for instance). measure the actual CG in the loaded, ready to fly rocket and make sure it is at least one BT diameter forward of the modeled CP. It is helpful to mark the CP point on the rocket itself, as the CG changes with every motor.
 

gna

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If you haven't built the rocket, can you attach the OR ork file? People could do some checks then. For instance, what is on top of the motor? Are there 2 parachutes? Is this a TARC design?
 
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