Encyclopedia Astronautica
Nitric acid/Ammonia

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. 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. Ammonia (NH3) is a colourless gas and liquid with a strong irritating characteristic odour.

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.

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:

  • WFNA - White fuming nitric acid is based on anhydrous nitric acid (HNO3), a colourless corrosive liquid which fumes in moist air. They contain a maximum of 2 per cent water and 0.5 per cent nitrogen dioxide, and decompose to yield amounts of water, nitrogen dioxide, and oxygen which are in chemical equilibrium.

  • IWFNA - Inhibited white fuming nitric acid. Since container materials are attacked by WFNA and equilibrium products, 0.6 per cent HF is added for passivation by deposition of a protective metallic fluoride coating.

  • RFNA - Red fuming nitric acid. Since WFNA or IWFNA exhibit excessive equilibrium decomposition pressures, reaching 75 bar at 700 deg C. To suppress the high pressure through a mass-action effect, some 13 per cent N2O4 and 3 per cent H20 are added, in order to reduce equilibrium pressures to 2 bar at 700 deg C. The colour of the resulting red fuming nitric acid is imparted by N204.

  • IRFNA - Inhibited red fuming nitric acid. Addition of 0.6 per cent HF to RFNA produces inhibited RFNA (IRFNA). The IRFNA specification was published in 1954 and thereafter Russian rocket engines using the same fuel appeared.

  • AK20 - Russian formulation consisting of 80% nitric acid + 20% N2O4 (AK = Azotna Kislota = Nitric Acid)

  • AK20F - Russian formulation consisting of 80% nitric acid + 20% N2O4 + fluorine passivant

  • AK20I - Russian formulation consisting of 80% nitric acid + 20% N2O4 + iodine passivant

  • AK20K - Russian formulation consisting of 80% nitric acid + 20% N2O4 + unknown additive

  • AK27I - Russian formulation consisting of 73% nitric acid + 27% N2O4 + iodine passivant

  • AK-27P - Russian formulation consisting of 73% nitric acid + 27% N2O4 + unknown additive

It is a relatively high-boiling gas with a vapour pressure of 8.7 bar at 20 deg C. Ammonia. is toxic, and will dissolve easily in water. It will form flammable and explosive mixtures with air. Although ammonia itself is toxic, the exhaust gases from the combustion of ammonia and oxygen are not. Ammonia is produced by a Haber-Bosch process, in which the elements, nitrogen and hydrogen, are united at a temperature of 500 to 600 deg C and a. pressure of approximately 200 bar in the presence of a promoted iron catalyst. It is estimated that 4 million tonnes of anhydrous synthetic ammonia were produced in 1959 in the United States, at which time the price of tank-car quantities of refrigeration-grade anhydrous ammonia was $ 80 per tonne.

Oxidizer: Nitric acid. Fuel: Ammonia. Propellant Formulation: IRFNA/Ammonia. Optimum Oxidizer to Fuel Ratio: 2.1. Temperature of Combustion: 2,690 deg K. Ratio of Specific Heats: 1.24. Density: 1.10 g/cc. Characteristic velocity c: 1,610 m/s (5,280 ft/sec). Isp Shifting: 260 sec. Isp Frozen: 255 sec. Mol: 21.00 M (68.00 ft). Oxidizer Density: 1.510 g/cc. Oxidizer Freezing Point: -42 deg C. Oxidizer Boiling Point: 86 deg C. Fuel Density: 0.604 g/cc. Fuel Freezing Point: -78 deg C. Fuel Boiling Point: -33 deg C.

Location: 1610.
Specific impulse: 255 s.

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