Advanced Hybrid Solid Fuels

Enabling hybrid rocket propulsion to compete with solid and liquid propulsion is the target of the renewed international interest in hybrid rockets. From the performances standpoint, conventional hybrids may compete with conventional liquids such as LOX Kerosene and could be much better than solids but they have 2 major drawbacks; A low burning rate of current solid fuels represents the main one to overcome for successful operations of hybrid propulsion on large scale. Combustion problems with a low combustion efficiency, instabilities and a high level of residuals (difficult mastering of the combustion with a multiport motor) Moreover, to be really attractive, new formulations have to bring a decisive advantage with a much higher specific impulse. nevertheless these new formulations have to keep the 3 major ones of hybrids; a safe operating mode due to a very high level of mechanical properties, a cheap manufacturing process due to a pure fuel solid grain and the capability to be stopped on demand. Some formulations appears really attractive with a high burning rate and a good combustion with or without a large increase of the specific impulse The present paper describes the efforts, by a joint team of investigators, to reach this objective by promoting the use of novel energetic ingredients in hybrid solid fuels and will give an example of application. A variety of new formulations was tested keeping in mind that both combustion behavior and mechanical properties of solid fuel grains are important for applications. Thus, a systematic experimental investigation was planned to determine the relevant properties of several candidate formulations. For this purpose, a micro-sized hybrid rocket motor test bench was implemented for quasi-steady solid grain regression rate measurements, CO 2 laser for radiative primer charge ignition, and exhaust gases dump system. The solid grain is shaped as a traditional fuel cylinder with one central perforation. Air or mixtures of oxygen and nitrogen, injected at the head-end of the motor, were used as gaseous oxidizer. This apparatus allows, on a relative scale, a quick classification of fuel regression rates and their sensitivity to operating conditions. Three main directions were explored for developing advanced solid fuel compositions. The first one resorts to nano-sized energetic particles cast in HTPB solid fuel grains. The second direction resorts to fuels characterized by the presence of a liquid surface, resulting in droplets entrainment. Paraffin-based fuels were investigated revealing, for the investigated compositions, severe structural problems due to poor mechanical properties. The third investigated direction specifically addresses to metal hydrides compositions. In particular magnesium and aluminum hydrides formulations were analyzed, showing increases in the solid fuel regression rates depending on the hydride mass fraction. The incorporation of metal hydride in HTPB-based fuel induces also an energetic increase. For aluminum hydride, the expected specific impulse (vacuum, ε= 40) augmentation is 32 s.