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Red fuming nitric acid (RFNA) is a storable oxidizer used as a rocket propellant. It consists of nitric acid (), dinitrogen tetroxide () and a small amount of water. The color of red fuming nitric acid is due to the dinitrogen tetroxide, which breaks down partially to form nitrogen dioxide. The nitrogen dioxide dissolves until the liquid is saturated, and produces toxic fumes with a suffocating odor. RFNA increases the flammability of combustible materials and is highly exothermic when reacting with water.

Since nitrogen dioxide is a product of decomposition of nitric acid, its addition stabilizes nitric acid in accordance with Le Chatelier's principle. Addition of dinitrogen tetroxide also increases oxidizing power and lowers the freezing point.

It is usually used with an inhibitor (with various, sometimes secret, substances, including hydrogen fluoride; any such combination is called inhibited RFNA, IRFNA) because nitric acid attacks most container materials. Hydrogen fluoride for instance will passivate the container metal with a thin layer of metal fluoride, making it nearly impervious to the nitric acid.

It can also be a component of a monopropellant; with substances like amine nitrates dissolved in it, it can be used as the sole fuel in a rocket. This is inefficient and it is not normally used this way.

During World War II, the German military used RFNA in some rockets. The mixtures used were called S-Stoff (96% nitric acid with 4% ferric chloride as an ignition catalyst

Compositions

  • IRFNA IIIa: 83.4% HNO<sub>3</sub>, 14% NO<sub>2</sub>, 2% H<sub>2</sub>O, 0.6% HF
  • IRFNA IV HDA: 54.3% HNO<sub>3</sub>, 44% NO<sub>2</sub>, 1% H<sub>2</sub>O, 0.7% HF
  • S-Stoff: 96% HNO<sub>3</sub>, 4% FeCl<sub>3</sub>
  • SV-Stoff: 94% HNO<sub>3</sub>, 6% N<sub>2</sub>O<sub>4</sub>
  • AK20: 80% HNO<sub>3</sub>, 20% N<sub>2</sub>O<sub>4</sub>
  • AK20F: 80% HNO<sub>3</sub>, 20% N<sub>2</sub>O<sub>4</sub>, fluorine-based inhibitor
  • AK20I: 80% HNO<sub>3</sub>, 20% N<sub>2</sub>O<sub>4</sub>, iodine-based inhibitor
  • AK20K: 80% HNO<sub>3</sub>, 20% N<sub>2</sub>O<sub>4</sub>, potassium-based inhibitor
  • AK27I: 73% HNO<sub>3</sub>, 27% N<sub>2</sub>O<sub>4</sub>, iodine-based inhibitor
  • AK27P: 73% HNO<sub>3</sub>, 27% N<sub>2</sub>O<sub>4</sub>, phosphorus-based inhibitor

Corrosion

; Hydrofluoric acid content of IRFNA: When RFNA is used as an oxidizer for rocket fuels, it usually has a HF content of about 0.6%. The HF forms a metal fluoride layer on the surface of the storage vessels, inhibiting corrosion.

; Water content of RFNA: To test the water content, a sample of 80% HNO<sub>3</sub>, 8–20% NO<sub>2</sub>, and the rest H<sub>2</sub>O depending on the varied amount of NO<sub>2</sub> in the sample. When the RFNA contained HF, there was an average H<sub>2</sub>O% between 2.4% and 4.2%. When the RFNA did not contain HF, there was an average H<sub>2</sub>O% between 0.1% and 5.0%. When the metal impurities from corrosion were taken into account, the H<sub>2</sub>O% increased, and the H<sub>2</sub>O% was between 2.2% and 8.8%

; Corrosion of metals in RFNA