How do you find grain parameters for L645 Cesaroni Pro75 simulation

[bxrne]

Currently working on simulating a HPR in RocketPy for EuRoC.
RocketPy needs a .eng/.csv/.rse thrust curve to simulate the motor which is easily found.
The problem arises with the specific grain parameters which I can't find solid numbers for (guess-timations only - dry inertia)

Here are the parameters (some I think are fine)

PRO75L645 = SolidMotor(
    thrust_source="assets/motors/Cesaroni_3419L645-P.eng",
    dry_mass=1.608,  # kilograms (launch mass=3.752, wet=2.144)
    dry_inertia=(0.245, 0.039, 0.0015),  # kg*m^2
    nozzle_radius=37.5 / 1000,  # of nozzle exit in meters
    grain_number=3,  # Number of propellant grains
    grain_density=2563.3825,  # kg/m^3
    grain_outer_radius=37.5 / 1000,  # meters
    grain_initial_inner_radius=29 / 1000,  # meters
    grain_initial_height=157 / 1000,  # meters
    grain_separation=5 / 1000,  # meters
    grains_center_of_mass_position=0.343,
    center_of_dry_mass_position=0.231,  # formula: lenngth of the aft part of the motor / (length of the aft part of the motor + length of the forward part of the motor)
    nozzle_position=0.0,  # relative to the combustion chamber
    burn_time=5.33,  # seconds
    throat_radius=12.5 / 1000,  # meters
    coordinate_system_orientation="nozzle_to_combustion_chamber",  # Orientation of the coordinate system -> EXTREMLY IMPORTANT
)

I think its related to a weird stability dip between rail departure and burn out caused by an aerodynamic spin moment.
It does not get reproduced in OpenRocket simulations so I am wondering if this motor configuration is to blame or is there something else I am missing. Any help appreciated!

 

[mikec]

AFAIK this motor is simply a BATES grain so there is no way I can think of for the thrust in simulation to be anything but along the thrust axis.