nitrogen tetroxide (N2O4)
Nitrogen tetroxide (N2O4) is a yellow-brown liquid that is among the most common storable oxidizers used by liquid-propellant rocket engines today. Like nitric acid, it is hypergolic (reacts upon contact with) hydrazine, MMH (monomethyl hydrazine), and UDMH (unsymmetrical dimethyl hydrazine). It is used, for example, with MMH in the Space Shuttle orbital maneuvering system. Although it can be stored indefinitely in sealed containers, its liquid temperature range is narrow and it is easily frozen or vaporized.