Drawing on the German World War II Wasserfall rocket, nitric acid (HNO3) became the early storable oxidiser of choice for missiles and upper stages of the 1950's. To overcome various problems with its use, it was necessary to combine the nitric acid with N2O4 and passivation compounds. These formulae were considered extremely secret at the time. The propellant combinations WFNA/ JP-4 and later IRFNA/JP-4 were the first storable systems given serious consideration in the United States. Problems which caused the abandoning of these propellants were the absence of reliable hypergolic ignition and unstable combustion. IRFNA/UDMH and IRFNA/JP-X finally did prove satisfactory.
By the late 1950's it was apparent that N2O4 by itself was a better oxidiser. Therefore nitric acid was almost entirely replaced by pure N2O4 in storable liquid fuel rocket engines developed after 1960. The composition of propellant-grade nitric acids is covered by Military Specification MIL-N-7254. The nitric acids are fuming liquids which vary from colorless to brown, depending on the amount of dissolved N2O4. The vapours from these acids have a characteristic pungent odour. They are highly corrosive, toxic, oxidising agents and attack most metals. They react with most organic materials violently enough to cause fire. The acids are soluble in water in all proportions, with an accompanying evolution of heat. They cannot be made to explode. Approximately 90 per cent of the nitric acid is made by the catalytic oxidation of ammonia with air or oxygen to yield nitric oxide (NO). The latter is oxidised to N2O4 which, when treated with water, yields nitric acid (HNO3) and may be concentrated by distillation with sulphuric acid. Red fuming nitric acids may be produced by passing gaseous N2O4 into nitric acid, a slight modification of the above process. Production of nitric acid was estimated at 3 million tonnes in 1959. The price of RFNA was $ 0.20 per kg in drum lots; IRFNA was slightly higher. The varieties of nitric acid propellants include:
Hydrazine (N2H4) found early use as a fuel, but it was quickly replaced by UDMH. It is still used as a monopropellant for satellite station-keeping motors. Hydrazine marketed for rocket propellant contains a minimum of 97 per cent N2H4, the other constituent being primarily water. Hydrazine is a clear, water-white, hygroscopic liquid. The solid is white. Hydrazine a toxic, flammable caustic liquid and a strong reducing agent. Its odour is similar that of ammonia, though less strong. It is slightly soluble in ammonia and methyl-amine. It is soluble in water, methanol, ethanol, UDMH, and ethylenediamine. Hydrazine is manufactured by the Raschig process, which involves the oxidation of ammonia to chloramine, either indirectly with aqueous sodium hypochlorite or directly with chlorine, and subsequent reaction of chloramine with excess ammonia. Raw materials include caustic, ammonia, and chlorine; these are high-tonnage, heavy chemicals. The cost of anhydrous hydrazine in drum quantities in 1959 was $ 7.00 per kg. The projected price, based on large-scale commercial production, was expected to be $ 1.00 per kg. Due to environmental regulations, by 1990 NASA was paying $ 17.00 per kg.