Performance enhancing additives for hybrid rockets

Abstract

A comprehensive assessment of fuel additives for a paraffin-based hybrid rocket fuel and hybrid rocket test firings are presented in this thesis. The reason for the selection of paraffin wax fuel binder is discussed as well as the expected performance gain by the addition of energetic materials to the fuel. Al, Mg, LiAlH4 and NBH6 are selected by assessing the thermochemical calculation results and material availability. An experimental study with liquid nitrous oxide oxidizer is concluded which showed Al and LiAlH4 are promising materials for future studies. They increase the c* which in turn increase delivered Isp and decrease the nozzle erosion rate. Also, ammonium borane, which is a promising material because of the rich hydrogen content, is studied, but because of the problems in its procurement detailed tests are postponed in a future study. If the availability and cost problems are solved, ammonium borane is the best choice for theoretical Isp performance. However, it needs to be tested in real operating conditions to better understand its characteristics. First chapter of the thesis shows that there are improvements of the hybrid rocket regression rate, Isp and combustion efficiency with the energetic material addition. However, the most noteworthy improvement is the nozzle erosion rate reduction. Therefore, it is decided to study this characteristic in more detail. Due to its relatively low cost and wide availability, carbon graphite is one of the most widely used ablative nozzle material in hybrid rocket propulsion. The erosion characteristics of this material has paramount importance, since it directly influences the Isp performance. This is especially the case for upper stage or in-space rocket motors operating with very long burn times. In this study the effect of aluminum added fuel on the graphite nozzle erosion is studied. In the experimental studies, a high regression rate paraffin-based fuel is loaded with micron size aluminum powder for nozzle erosion reduction. In our approach, aluminum is added at high concentrations as a fuel ring in front of the main paraffin-based fuel which contains no aluminum. Based on the motor tests conducted with gaseous oxygen as an oxidizer, it is shown that aluminum addition decreased the nozzle erosion rate up to 45% and increased the nozzle erosion onset time by 1 to 3 seconds. The new method of introducing an energetic powder in a fuel ring positioned at the fore end of the motor offers an easy and scalable way of reducing the nozzle erosion and improving the Isp performance of the rocket motor. As pointed out using Al as additive for hybrid rocket motors, substantially reduce the nozzle erosion rate which increase the Isp performance. It is widely available, cost effective and easy to use material. The novel addition of the Al material as a high concentrated ring to the hybrid rocket fuel, makes this method highly scalable for larger rocket motors. In future, ammonium borane additive could be studied, but its cost and availabilty is a problem to be solved.