Hybrid propulsion can be a good alternative to the solid and liquid propellant systems; recently it has been the preferred choice for space tourism missions. In comparison to the solid and liquid propellant systems, hybrid rockets have improved safety and controllability. However, one of the major drawbacks of hybrid motors is its slow regression rate leading to a low thrust. The main objective of this research is to experimentally and theoretically investigate the influence of an additive called expandable graphite (EG) on the fuel regression rate.
Expandable graphite (EG) has a unique property of forming accordion-like strings which are several times longer than the original particle size, when heated to high temperatures. EG is known to have a high thermal conductivity. When mixed with a fuel matrix, these strings can increase the heat transfer into the fuel through conduction, thus increasing the regression rate. Static firing tests were conducted employing polymeric and liquefying fuels, with and without EG, using gaseous oxygen as the oxidizer. The experimental results revealed an increase of 30%-200% in the fuel regression rate due to the addition of EG into the fuel matrix. A larger relative effect was observed for the polymeric fuel in comparison to the liquefying fuel. Theoretical model predictions showed good agreement with the experimental results, within approximately ± 10%.
