Thermal characteristics of expandable graphite as a burning rate enhancer in hybrid propulsion

Abstract

AbstractHybrid propulsion, consisting of solid fuel and liquid oxidizer, exhibits major advantage regarding safety, development costs, energetic performance, and controllability. Hence, it has been considered for space and other propulsion programs. One of the characteristic features of hybrid motors, that may be of major disadvantage, is their relatively low thrust due to the characteristic low regression rate of the typical polymeric solid fuels. This research group had revealed that a small fraction of an expandable graphite (EG) additive could double the burning rate and thrust of hybrid motors. EG is an intercalated form of graphite, elongating and swelling, getting a worm‐like string shape at elevated temperatures. In an earlier work it was hypothesized that enhanced heat transfer via conduction through the EG strings protruding from the hot surface, is a significant parameter increasing the burning rate. In the present investigation, thermal analysis revealed that the ignition temperature of polyester fuel containing 5 wt% of EG, is substantially lower than that of the pure polymer, which may promote higher burning rate as well. High‐speed photography of the surface of EG‐containing solid fuels as well as that of individual EG particles during controlled heating, demonstrated the dynamic behavior resulting in the formation of EG strings longer by an order of magnitude compared to the original particles. The different phenomena were related to the observation of enhanced burning rate.