Sol-gel supports for catalytic ignition in energetic green monopropellant thrusters

Abstract

This research focuses on the development of catalysts with superior thermal resistance for the catalytic ignition of green monopropellant thrusters to replace current hydrazine monopropellant systems. Silicon doped alumina supports are prepared via sol-gel, pelletized using an oil-drop method, and dried as xerogels and aerogels. The spherical supports are then impregnated with platinum, calcinated and reduced. Following activation, the catalysts are fully characterized in terms of crystallite size, specific surface area and pore diameter as well as crush strength and crystal phase. Thruster tests are conducted using high concentration hydrogen peroxide in a 5N-class monopropellant thruster. The performance of the catalysts are then compared with standard commercial alumina, and another doped catalyst support, LHA, in terms $C \ast$ efficiency, pressure stability and in maintaining catalyst properties. The goal of the study is to provide a thorough analysis of sol-gel supports and examine their practicality and performance in actual rocket thrusters. Results show stable thruster operation using aerogel supports calcinated at $1400 ^\circ C$, which relies on the superior thermal stability of the aerogel alumina and good mechanical strength despite a lower crush strength than xerogels or commercial alumina. Characterization of post-firing xerogel catalysts show a decrease in the content of active material and a small fraction of catalyst pellets are fractured, explaining the unstable thruster operation. The practicality of sol-gel pellets for catalytic ignition in green monopropellant thrusters with high decomposition temperature was demonstrated and this research paves the way for a new line of research in improving the thermal stability of the catalysts by testing different doping agents with higher pre-firing calcination temperatures and the catalytic reactivity by enhancing active material dispersion.