Development of High-Performance Ammonia Borane Based Rocket Propellants

Since their discovery in the early 1930s, hypergolic propellants have been a mainstay of liquid rocket systems. This class of propellants consists of combinations of fuels and oxidizers that, upon contact, self-ignite through heat generated by chemical reactions between the fuel and oxidizer. This can simplify the overall complexity of rocket systems employing these propellants due to the elimination of a dedicated ignition source. While a wide range of fuel and oxidizer combinations exhibit this property, only a select few have offered the physical properties and performance necessary to make them viable in fielded rocket systems. While these propellants can be reliable and improve the versatility of systems using them, they are generally toxic and highly hazardous to handle. As such, there remains a significant drive to develop novel fuels and oxidizers that offer comparable (or better) performance without the drawbacks of the standard hypergolic propellants. Unfortunately, few viable alternatives have been found due to the complexity of the reactions governing hypergolic ignition making it difficult to predict the hypergolic characteristics of fuels/oxidizers based on their physical/chemical properties.
In recent years, amine-borane compounds (i.e. amine molecules that have been complexed with borane adducts) have begun receiving increased attention as potential alternatives to the more conventional hypergolic fuels. The simplest of these materials, ammonia borane (AB), has been shown to be highly hypergolic with white fuming nitric acid (WFNA) and other common hypergolic oxidizers, with ignition delays as short as 0.6 milliseconds being observed in some cases. Additionally, thermochemical equilibrium calculations performed with AB-based fuels predict net gains in specific impulse relative to the hydrazine-based fuels (e.g. hydrazine, monomethyl hydrazine, etc.) traditionally used in these systems. AB is also relatively stable and safe to handle without the use of complex safety precautions and equipment. As such, AB-based hypergolic fuels could potentially serve as a safer alternative to the more conventional hypergolic fuels while maintaining the high levels of performance associated with these rocket systems.

Given these potential benefits associated with AB-based fuels, the primary objective of the work performed over the course of this fellowship was to develop and characterize high-performance ammonia borane based solid fuels for use in hypergolic hybrid rocket motors. This was achieved by progressing from small scale hypergolic ignition experiments to lab scale hybrid rocket motor testing. Additionally, as ammonia borane is the simplest fuel in the broader amine-borane family of fuels, obtaining a better understanding of its general combustion behavior and hypergolic ignition dynamics may provide valuable insights into the behavior of this broader family of fuels. This could potentially help inform the development of future novel hypergolic fuels/fuel formulations to achieve levels of performance that surpass the fuels investigated here. As such, an additional objective of this work was to develop and adapt various diagnostic techniques (e.g. phosphor thermography, multi-spectral imaging, etc.) to offer new insights into the combustion behavior and ignition dynamics of the AB-based fuels of interest here.
Over the course of the broader research effort undertaken through the financial support of this fellowship, five major areas of research were successfully undertaken to help achieve the objectives discussed previously (*see final report for further info).