mechanical enhancement of the burning rate of solid propellants is achievedy the incorporation of limited percentages of heat - expandable beads into the solid propellant matrix . when the flame front reaches an individual bead , the bead which contains an expanding or blowing agent , etc ., expands to several times its volume and ruptures . bead expansion or rupture causes a disruption of the propellant &# 39 ; s surface , and the flame can penetrate into the propellant . this penetration results in a major increase in burning rate .