Overview of the development of hydrazinium nitroformate-based propellants

Recent improvements in the quality of hydrazinium nitroformate (HNF) and binder polymer developments have shown that it is possible to produce HNF-based propellants with hydroxyl-terminated polybutadiene (HTPB), glycidyl azide polymer (GAP), and poly 3-nitromethoxy-3-methyl oxetane [poly nimmo (PNM)] as binders. Especially when considering space applications, HNF-based propellants show results that encourage further research and development. An alternative method to cure GAP has been evaluated. This curing method eliminates the use of isocyanates, which always require special attention when used together with HNF. Combustion studies, experimental and theoretical, led to a good understanding of the steady-state combustion of HNF/GAP sandwiches and HNF/GAP propellants. Burning-rate modifiers have been identified, which lower the burning-rate exponent in GAP/HNF propellants by at least 25%. HNF/GAP and HNF/PNM propellants have a low temperature sensitivity. Liquid HNF monopropellants look attractive as a hydrazine replacement. They offer the same or a substantially higher I-sp, combined with a 30-40% higher density. The progress made in the manufacture of HNF and the HNF-based propellants (solid and liquid) was possible through extensive international cooperation.