Increasing Burning Rate and Motor Thrust by Expandable Graphite Additives

Controlling rocket thrust may be done via propellant burning rate catalysts and enhancers. This paper presents an experimental investigation on increasing the thrust of hybrid and solid motors by adding a small fraction of expandable graphite (EG) within the binder matrix to enhance burning rate. EG is a form of intercalated graphite flakes that upon heating change their appearance to elongated fibers/strings of substantially larger length and volume. The elongated EG strings at the burning surface are hypothesized to conduct heat from the hot surroundings to the bulk, thereby increasing the burning rate. High-speed photography of the surface phenomena of fuel slabs containing EG additive subjected to flame supports the greater effect on burning rate enhancement (up to twofold) for polyester versus hydroxyl-terminated polybutadiene or paraffin wax fuels in hybrid motors. Similar investigation on the burning of ammonium perchlorate–polymer solid propellant strands revealed different surface phenomena and substantial burning rate increase (60% and more) for hydroxyl-terminated polybutadiene versus polyester binder with 5% EG additive. It can be concluded that EG can serve as a novel burning rate and thrust enhancer without deterioration of the mechanical properties of the polymeric fuel/binder for hybrid (including solid fuel ramjet) and solid propellant motors.