OVER STABLE

A simple question, at what point would a rocket be considered over stable? I am rebuilding an Aerotec G force and am considering making it about 8 inches longer than originally designed.

THOUGHTS.

I got a minute or two and nobody has replied yet so here goes.
What do you mean rebuilding? Fixing a broken fin, zipper or completely gutting it? Did you use the hook/engine block? That limits the motors you can fly and thus the flight profile. Are you staying with the 29mm mount? Are you adding a payload bay? These things are dogs in altutude performance as they are wide and heavy. Don't get me wrong i have two I love! 4" give you a lot of bandwidth to play with. Remember, the stock build lands on either a fin or the bottom of the rocket. I cut a coupler to fit to reinforce from landing injuries. I aslo made an adapter to fly 2 airstart 24mm with mods to the 'extra tubing' for timer/altimeter use. My buddy just completed his 38mm upgrade, should fly nice with those loads. your stability will come with design intent and any extra wieght you add, either front, rear or both. Double check what motors you can fit and your liftoff speeds and altitudes. Your Cg/Cp can be good,but mite be too heavy to start the fins flying while on the rod/rail. Throw your stuff on a scale and then go to ThrustCurve.org and run a sim with your motor(s) for a rough estimate of performance. OpenRocket and Rocksim are two of the most popular design programs and OR is free! Have fun with you bird- these are really cool to fly!

I wouldn't presume extending it would make it much more stable. There's probably some tendency for the CG to move forward more than the CP but you're probably talking 1-2" on a big rocket.

Overstable means the center of gravity (CG) is a long way forward of the center of pressure (CP). This is usually wanted to be 1-2 calibers (rocket thickness) ahead, at least when using best-case CP (which varies with angle of attack to the air). The only problem with overstable is increased weathercocking, turning into the wind as it comes off the rod. That and the G-Force has been known to come down sideways when ejection fails, slowing down and reducing damage, because worst-case angle of attack (90 degrees) it's not stable.

Another issue is any increase of the volume the ejection charge has to pressurize will reduce that pressure, and that can be a problem on that rocket already. Increasing the length of the forward section would be better.

^ good point-forgot to mention that! An H-185 takes an almost bone stock G-force up to a little over 1200' It wouldn't hurt to add a little to the ejection charge.

Thanks for your input thus far, a little busy now will add more info soon. Please keep the advise coming. I am listening !

This G force was originally built stock minus motor hook and block. I had planned to get a level 1 cert with it . It became a lawn dart on first flight. The fins and motor mount is all that was left.

Have almost finished rebuild, it is as close to stock as was possible. I will reinforce rear as you suggested, also will add to ejection charge. I will put this on open rocket before finishing. Thanks for your advise.