Red fuming nitric acid
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Red fuming nitric acid (RFNA) is a storable oxidizer used as a rocket fuel. It consists mainly of nitric acid (H{{#if:{{{1|}}}|{{{1}}}}}N{{#if:{{{1|}}}|{{{1}}}|}}O{{#if:{{{1|}}}|{{{1}}}|}}3), also containing 13% dinitrogen tetroxide and 3% water. The color of the resulting red fuming nitric acid is due to the dinitrogen tetroxide, which breaks down partially to form nitrogen dioxide.
It is usually used with an inhibitor (with various, sometimes secret, substances, including hydrogen fluoride; any such combination is called IRFNA) because nitric acid attacks most container materials.
It can also be a monopropellant; with substances like ammonium nitrates dissolved in it, it can be used as the sole fuel in a rocket. It is not normally used this way however.