I'm working up a 4" Prion for my Level 2. When simulating a L935 I am getting a large drop in stability right before apogee. It doesn't make sense that the CP would change as the rocket was slowing down. I'm having trouble understanding. Thanks for any help!

CP is somewhat variable as it is a function of airspeed. As airspeed changes, the CP will move. This gets pretty wild when you start getting well above mach. There is also the chance that the equations used to plot that graph start to break down at low airspeed as well. That's my favorite graph btw. Those guys who just look at the corner of the main screen are ignoring one of the best features in the program, if you ask me. XD

Nothing to worry about. The ability of the fins to provide correction is based on airflow over them. If you are approaching apogee what is the velocity of the rocket?

Speed is not a factor in CP location. Angle of attack is. The basic static margin uses zero AOA and is the best case. As the rocket approaches apogee it is turning thanks to gravity. So it is likely that the AOA increases. Plot AOA and compare.

Yeah, I should have worded that better. CP is relatively consistent until you reach mach. Out of curiosity, I looked deeper into this. The equations of calculating CP are heavily dependent on the functions describing aerodynamic pressure across the length of the rocket, relative to some reference line. Any factor which effects that distribution is critical. Velocity doesn't seem to be such a large factor until supersonic flight due to compressible flow at those air speeds, and AOA's importance is related to its magnitude lol. So I think we're both right (you more than I *brofist*).

I say fire the rocket and see what happens. Me thinks the flight will be nominal as long as all the planned expected events occur on time. Kurt

Rocket slowing down at apogee, arcing over, AOA increases, stability decreases. All normal. Nothing to worry about.

Cp moves with velocity. Yes, it certainly will move if you alter the angle of attack, but increasing the velocity will move it as well.