The Muzzle Exit module of PRODAS allows the user to estimate the dispersion of small, medium and large caliber spin stabilized projectiles, as well as the dispersion of partially spin decoupled, medium and large caliber long rod projectiles via closed form techniques. The dispersion estimates are based on the “jump equation” and consider only two sources: angular rate from in-bore principal axis tilt and cross velocity of the projectile center of gravity at muzzle exit.

The Muzzle Exit module is typically run early in the development cycle of a projectile to determine if the design, as configured, will meet the dispersion requirements, given typical in-bore clearance and center-of-gravity offset conditions. Since it uses closed-form equations to estimate dispersion, it provides an unbiased assessment of expected dispersion from normal interface dimensions.

The angular rate component is caused by a tilt of the projectile inertial axis with respect to the bore centerline, multiplied by the exit spin rate. This “tilt” of the projectile inertial axis relative to the bore centerline can arise either from manufacturing defects or in-bore clearances between the projectile bourrelet and the barrel lands, or both sources.

The cross velocity component of the jump equation is caused by a radial offset of the projectile center of mass with respect to the bore centerline, multiplied by the exit spin rate. Again, the radial offset of the projectile center of mass can be caused by manufacturing defects, in-bore clearances, or both sources.

The dispersion estimates of the muzzle exit module are valid for short ranges, target distances over which shot-to-shot variations in muzzle velocity, total drag, and external factors such as winds, are insignificant. A target distance limit of 25000 calibers for small caliber systems, 50000 calibers for medium and large caliber systems appears to be appropriate.

Prior to accessing the Muzzle Exit module of PRODAS, the user must have performed the following and saved the file:

• Model editor to define the shape of the projectile and density of components.
• Mass properties to estimate the projectile center of mass location and the moments of inertia.
• Aero Coefficient predictions to estimate Drag, Normal Force & Pitching Moment coefficients.
• Stability Evaluation to set muzzle velocity and define exit twist rate.

The user should be aware that the PRODAS Mass Properties calculations compute the projectile bourrelet length based on projectile parameters if that parameter is not "locked" in the Muzzle Exit module. In this instance, the bourrelet length will be recomputed if Mass is rerun.