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.
Unsymmetrical Dimethylhydrazine ((CH3)2NNH2) became the storable liquid fuel of choice by the mid-1950's. Development of UDMH in the Soviet Union began in 1949. It is used in virtually all storable liquid rocket engines except for some orbital manoeuvring engines in the United States, where MMH has been preferred due to a slightly higher density and performance. Unsymmetrical dimethylhydrazine (UDMH) is 98 to 99 per cent pure and is described by Military Specification MIL-D-25604. The normally expected impurities are dimethylamine and water. UDMH is a clear, hygroscopic liquid which yellows on exposure to air. It absorbs oxygen and carbon dioxide. UDMH is a toxic volatile liquid. It exhibits the sharp ammoniacal or fishy odour which is characteristic of organic amines. It is completely miscible with water, ethanol, and most petroleum fuels. It is not shock sensitive. The vapours are flammable in air over 2.5 to 95 per cent concentration range. UDMH can be produced commercially by nitrosation of dimethylamine, to N-nitro-sodimethylrtmine, followed by reduction of the intermediate to UDMH and subsequent purification. UDMH can be prepared, also, by a modification of the Raschig process (see discussion of hydrazine), in which the chloramine intermediate is with dimethylamine rather than with ammonia. The price in 1959 for tank-car quantities was under $ 1.00 per kg. Engineering studies indicated a price of $ 1.00 per kg with large scale sustained production. But due to its toxic nature, production and transport costs soared in response to environmental regulations. By the 1980's NASA was paying $ 24.00 per kg.