Unsymmetrical dimethylhydrazine

Unsymmetrical dimethylhydrazine (UDMH) (1,1-Dimethylhydrazine) is a hypergolic rocket fuel ingredient, often used as a bipropellant in combination with the oxidiser nitrogen tetroxide and less often with IRFNA or liquid oxygen. UDMH is a derivative of hydrazine and can be called a hydrazine also. As a fuel, it is described in specification MIL-D-25064.

UDMH is a toxic volatile hygroscopic clear liquid, with melting point -57 °C, boiling point 63 °C, density 0.793 g/cm³, and a sharp, fishy, ammoniacal smell typical for organic amines. It turns yellowish on exposure to air and absorbs oxygen and carbon dioxide. It mixes completely with water, ethanol, and kerosene. In concentration between 2.5% and 95% in air, its vapors are flammable. It is not sensitive to shock.

UDMH is storable and can be kept loaded in the rocket fuel system for long periods. However it is toxic and tends to absorb through skin. It has high density and high cost. It is used in virtually all storable liquid rocket engines. In some applications, eg. some maneuvering engines, monomethylhydrazine is used instead for its slightly higher specific impulse.

UDMH has higher stability than hydrazine, especially at elevated temperatures, and can be used as its replacement or together in a mixture. UDMH is used in many European, Russian, Indian and Chinese rocket designs. The Proton, Titan, GSLV and Delta rocket families use a mixture of 50% hydrazine and 50% UDMH, called Aerozine 50.

UDMH is a contaminant and a metabolite and breakdown product of daminozide.

Health effects
UDMH is toxic, a carcinogen and can explode in the presence of oxidisers. During the 1980s there was concern about the levels of UDMH in various foods being a cancer risk, especially for apple juice.

Other uses
UDMH is also used as a nitrogen source in metalorganic vapour phase epitaxy thin-film deposition.