Decomposition and combustion of monomethylhydrazinium nitrates

Energetic nitrate compounds, monomethylhydrazinium nitrate (MMH•HNO₃) and monomethylhydrazinium dinitrate (MMH•2HNO₃), were synthesized from hypergolic pair monomethylhydrazine (MMH) and nitric acid by mixing their diluted aqueous solutions carefully with an MMH:HNO₃ molar ratio of 1:1 and 1:2, respectively. The overall ignition and combustion behaviors of these two nitrate-based monopropellants were examined by using a high-pressure strand burner with optical access, coupled with a high-speed video camera for capturing and analyzing the condensed- and gas-phase processes as well as measuring the regression rates of the solid strands. MMH•2HNO₃, which has a stoichiometric F/O ratio, has a burn rate of 0.56 mm/s in 1 atm of N2. Its burn rate increases almost linearly with pressure up to 800 psi at which the burn rate is about 14 mm/s. However, the burn rate of MMH•2HNO₃ increases sharply to about 400 mm/s at 1000 psi. MMH•HNO₃, which is a fuel-rich compound and highly hygroscopic, is very difficult to ignite. Thermal decomposition of a small amount (~1.5 mg) of MMH•2HNO₃ and MMH•HNO₃ was investigated by a confined rapid thermolysis setup with heating rates on the order of 2,000 K/s coupled to rapid-scan Fourier transform infrared (FTIR) spectroscopy of the evolved gases. Both MMH•2HNO₃ and MMH•HNO₃ start to decompose rapidly at around 120°C, and the important IR-active species evolved from the decomposition of these salts include HNO₃, CH₃ONO₂, CH₃N₃, HN₃, H₂O, N₂O, NO, NO₂, CH₄ and CO₂. Possible decomposition pathways are also discussed based on the rapid thermolysis studies.