Ammonia borane based fuels for use in hybrid rocket systems have the potential to be high performing while at the same time mitigating many of the issues associated with conventional liquid fuels (e.g. long term storage, toxicity, low density, etc). In addition, when used with a hypergolic oxidizer (e.g. white fuming nitric acid), the proposed fuels have the potential for use in systems requiring reliable restart capabilities. The goal of the proposed research is to investigate the behavior and performance of ammonia borane based propellants. While the primary focus of the proposed work would be related to ammonia borane as a potential fuel for hybrid systems, studies investigating its behavior as a fuel additive in solid propellants will be conducted as well.
To characterize the burning rate behavior of the proposed hybrid propellants, regression rate analysis will be performed using a conventional opposed flow burner configuration as well as with a custom optically accessible hybrid rocket motor developed at Purdue University. Once the regression rate behavior has been adequately characterized, the information obtained will be used to develop hybrid fuel grains for testing with a conventional hybrid rocket motor. The restart capability of the ammonia borane based fuels will be investigated by performing hypergolic ignition experiments on prepared fuel samples. If the ignition delays observed are short enough to prevent “hard starts”, small scale hybrid motor tests will be performed with nitric acid based oxidizers. For some tests, the oxidizer flow rate will be intentionally interrupted followed by an attempted re-ignition of the motor.
To date, there are very few publications in open literature related to the combustion behavior and flame structure of hybrid propellants. As a means of studying both the proposed fuels and hybrid propellants in general, laser diagnostic techniques like planar laser induced fluorescence as well as a novel temperature measurement technique based on phosphor thermography (currently being developed at Purdue University) would be utilized. Obtaining a more fundamental understanding of these propellants could help inform future fuel formulations and fuel grain designs to further improve the performance of hybrid rocket systems.
The In-Space Propulsion technology roadmap published by NASA (specifically TA 184.108.40.206) lists hybrid propulsion systems as a technology that could enhance a number of different proposed mission types as well as enabling a variety of explorer class missions. Predicted performance of the proposed ammonia borane based fuels meets the technology performance goals listed for hybrid propulsion systems while also having restart capability when used with typical hypergolic oxidizers. In addition, the stable solid state of ammonia borane makes it viable to store long term without the need for cryogenic cooling systems or storage tanks.