Red fuming nitric acid (RFNA) is a storable oxidizer used as a rocket propellant.

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.

Addition of dinitrogen tetroxide also increases oxidizing power and lowers the freezing point.

[2] It is usually used with an inhibitor (with various, sometimes secret, substances, including hydrogen fluoride;[3]: 62  any such combination is called inhibited RFNA, IRFNA) because nitric acid attacks most container materials.