Aerozine 50 is a 50/50 mix of hydrazine

Hydrazine

Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...

 and unsymmetrical dimethylhydrazine (UDMH). Originally developed in the late 1950s by Aerojet General Corporation as a storable, high-energy, hypergolic fuel for the Titan II ICBM

Intercontinental ballistic missile

An intercontinental ballistic missile is a ballistic missile with a long range typically designed for nuclear weapons delivery...

 rocket engines, Aerozine continues in wide use as a rocket fuel, typically with dinitrogen tetroxide

Dinitrogen tetroxide

Dinitrogen tetroxide is the chemical compound N2O4. It is a useful reagent in chemical synthesis. It forms an equilibrium mixture with nitrogen dioxide; some call this mixture dinitrogen tetroxide, while some call it nitrogen dioxide.Dinitrogen tetroxide is a powerful oxidizer, making it highly...

 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

Spacecraft propulsion

Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the...

 systems, because rockets and spacecraft using it can remain fueled indefinitely, which is not true with most rockets. In some cases, it is used in ground launched rockets, such as the core stages of the Titan-II/III/IV

Titan (rocket family)

Titan was a family of U.S. expendable rockets used between 1959 and 2005. A total of 368 rockets of this family were launched, including all the Project Gemini manned flights of the mid-1960s...

 rocket, or in an upper stage, such as the Delta II

Delta II

Delta II was an American space launch system, originally designed and built by McDonnell Douglas. Delta II is part of the Delta rocket family and was in service from 1989 until November 1, 2011...

 rocket. It was also used by the Apollo Lunar Module

Apollo Lunar Module

The Apollo Lunar Module was the lander portion of the Apollo spacecraft built for the US Apollo program by Grumman to carry a crew of two from lunar orbit to the surface and back...

 and the Service Propulsion System engine in the Apollo CSM

Apollo Command/Service Module

The Command/Service Module was one of two spacecraft, along with the Lunar Module, used for the United States Apollo program which landed astronauts on the Moon. It was built for NASA by North American Aviation...

. The Ariane 1

Ariane 1

Ariane 1 is the first version of the Ariane launcher family. Ariane 1 was designed primarily to put two telecommunications satellites at a time into orbit, thus reducing costs. As the size of the satellites grew Ariane 1 gave way to the more powerful Ariane 2 and Ariane 3 launchers.- Vehicle...

 through Ariane 4

Ariane 4

Ariane 4 was an expendable launch system, designed by the Centre National d'Etudes Spatiales and manufactured and marketed by its subsidiary Arianespace. Ariane 4 was justly known as the ‘workhorse’ of the Ariane family. Since its first flight on 15 June 1988 until the last, on 15 February 2003, it...

 family used a related fuel, a mixture of 75% UDMH and
25% hydrazine hydrate called UH 25

UH 25

UH 25 is a fuel mixture for rockets. It was developed for the European Ariane 2-4 launch vehicles.UH 25 was developed after a disaster during flight 2 of the Ariane 1 rocket. During launch, one of the four Viking engines on the first stage developed a combustion instability which lead to an engine...

.

Aerozine is not used as a monopropellant

Monopropellant

Monopropellants are propellants composed of chemicals or mixtures of chemicals which can be stored in a single container with some degree of safety. While stable under defined storage conditions, they react very rapidly under certain other conditions to produce a large volume of energetic gases...

. The extra stability conferred by the methyl groups hurts reactivity and thrust.