Yet another question about rail button placement...

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Bill S

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I'm getting ready to mount a pair of rail buttons on a scratchbuild. I was under the impression that one should mount the lower button at or near the center of pressure, and one close to the center of gravity. I realize this is a question with no good answer... but on this rocket, the CP is 9" from the base, and the CG is 16. So if I mounted the buttons at the CG and CP, they would only be 6.5" apart. I suspect that is too close together. The other option is right in front of the rear motor mount centering ring (so it'd be about an inch from the rear of the rocket), and one at the CG.

See picture. The green squares are the tentative rail button placements.
buttons.png
 
You will get a lot of opinion here. But this is what I do. I put the aft button as far aft as I can make it work. I put the forward button one caliber forward of the center of pressure. Because nearly everything I build is fiberglass, I’m not looking for a centering ring to put the screw into. Remember that rail buttons only do their job when both are still on the rail. If the forward button is too far forward, the aft one doesn’t do anything after the forward one leaves the rail. On the other hand, if they are too close together on a long, heavy rocket, the stress on both buttons increases. I use one caliber forward of the CP for the forward button just so I have a really easy reference point for doing a final stability check. Mounting a button at the CG doesn’t make sense because the CG changes as you change motors. Having said all that, putting buttons where your green boxes are looks great.
 
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