Aerozine 50
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Aerozine 50 is a 50/50 mix of hydrazine and unsymmetrical dimethylhydrazine (UDMH).[1] Originally developed in the late 1950s by Aerojet General Corporation as a storable, high-energy, hypergolic fuel for the Titan II ICBM rocket engines, Aerozine continues in wide use as a rocket fuel, typically with dinitrogen tetroxide as the oxidizer, with which it is hypergolic (ignites on contact). Aerozine 50 is more stable than hydrazine alone, but has a higher density and boiling point than UDMH alone.
By cutting straight hydrazine, hydrazine's inconveniently-high freezing point of 2 C is lowered through freezing point depression. In addition, UDMH is a more stable molecule; this reduces the chances of straight hydrazine decomposing unexpectedly, increasing safety and allowing the blend to be used as a coolant in regeneratively-cooled engines.
Hydrazine may also be mixed with monomethyl hydrazine (MMH). Because MMH is slightly denser, net performance is increased slightly.
This type of fuel is mainly used for interplanetary probes and spacecraft propulsion systems, because rockets and spacecraft using it can stay fueled indefinitely. This is not true with most rockets. In some cases, it is used in ground launched rockets, such as the entire Titan rocket, or as part of a rocket upper stage, such as the Delta II rocket. In addition to the Titan rocket family (Titan II, Titan III, and Titan IV), it was also used by both components of the Apollo Spacecraft (the Apollo Lunar Module and Apollo CSM). The Ariane 1 through Ariane 4 family used a related fuel, a mixture of 75% UDMH and 25% hydrazine hydrate called UH 25.
Aerozine is not used as a monopropellant. The extra stability conferred by the methyl groups hurts reactivity and thrust.
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