Evaluation of fuel additives for hybrid rockets and SFRJ systems

Abstract

A comprehensive assessment of fuel additives that are expected toimprove the performance of hybrid rockets and solid fuel ramjet (SFRJ) systems is reported in this paper. A ranking of the selected fuel additives and binders have been made based on performance and other practical considerations including cost, availability and handling properties. One of the important conclusions of this study is that incorporating fuel additives into a binder is not an effective method to enhance the regression rate of hybrid rocket or SFRJ fuels. The key driver should be specific impulse and density impulse improvement. For low energy oxidizers such as nitrous oxide, aluminum turns out to be an excellent additive that strikes a good balance between the performance gain and key practical considerations such as availability/cost. Aluminum or other metals included in this study are not effective in improving Isp when they are used with oxygen. For hybrid rockets, AlH3delivers the best performance benefit among all of the additives considered in this study. However its poor availability in the Western world diminishes its usefulness. Boron as an additive did not have a major performance impact for hybrid systems. It is believed that at the current energy and unit cost levels of the performance additives, they are not expected to generate an economical benefit (i.e. cost/payload mass) even for applications demanding high Isp’s such as orbital launch vehicles. For SFRJ systems, boron and its compounds deliver significant performance improvement. Paraffin ranks as the best binder for both hybrids and also SFRJ’s due to its high regression rate, good availability, hydrophobic nature, easy handling and low cost.