Overview

Ammonium perchlorate for rocket motors usually has been specially processed to create very round particles. These round particles decrease the viscosity of the propellant so that it can be easily degassed and poured. The problem with spherical particles is that the packing efficiency is very low. Due to the low packing density, the spheres require a larger volume of binder to wet them than a tighter packed group of spheres.

One potential solution to this is to use Bi-Modal, Tri-Modal, or Multi-Modal particles. The idea is that if you have large spheres, there is sufficient volume to pack smaller spheres between the large particle and increase the overall packing density.

The ratio between large and small particles to obtain the best packing density is obtained by applying the equation r1 = 2.14√(r2 β)