Mixed Oxides of Nitrogen - Nitric oxide (NO) is a low-boiling cryogenic gas. Both the liquid and the solid are blue. Solutions of NO in nitrogen tetroxide sharply depress the freezing point of the high-melting oxidiser. The mechanism of depression is believed to involve the formation of N2O3, which is soluble in nitrogen tetroxide. Solutions are called mixed oxides of nitrogen (MON), and have been used as oxidisers for liquid-rocket engines. Various concentrations have been considered. However, the high vapour pressure of MON limits the concentration of NO in N2O4 to about 30 per cent. Aside from the high vapour pressure of MON, the material is quite similar to nitrogen tetroxide.
Monomethylhydrazine (CH3NHNH2) is a storable liquid fuel that found favour in the United States for use in orbital spacecraft engines. Its advantages in comparison to UDMH are higher density and slightly higher performance. Monomethylhydrazine (MMH) is 95+ per cent pure, while the normally expected impurities are methylamine and water. MMH is a clear, water-white hygroscopic liquid which tends to turn yellow upon exposure to air. MMH is a toxic, volatile liquid which will react with carbon dioxide and oxygen. MMH has the typical sharp ammoniacal or fishy odour of amines. It is completely miscible in all proportions with hydrazine, water, and low molecular-weight alcohols. MMH is not sensitive to impact or friction; it is more stable than hydrazine on mild heating and similar to hydrazine in sensitivity to catalytic oxidation.
Monomethylhydrazine may be produced by a modified Raschig process; methylamine is substituted for ammonia in the reaction with chloramine. In general, substituted hydrazines may be prepared by the reaction of an alkylsulfate or halide with hydrazine. In 1959 the price for MMH was $ 15 per kg. It was projected that this would be reduced to $ 2.00 per kg in mass production. By 1990 NASA was actually paying $ 17.00 per kg due to stringent environmental protection regulations.