For a radial charge geometry in a solid rocket, is there an equation or rule of thumb to determine the minimum hole size in the solid propellant charge? I suspect the answer is "no" since an infinitesimally small hold diameter would result in an axial burn geometry, OR that it is relates to the Web Fraction: bf = b / r, where Web Thickness (b) is the minimum thickness of the grain to burning surface or casing, and r is the grain radius.