Monomethylhydrazine (MMH) is a highly toxic, volatile hydrazine derivative with the chemical formula CH6N2.

[5] MMH is a hydrazine derivative that was once used in the orbital maneuvering system (OMS) and reaction control system (RCS) engines of NASA's Space Shuttle, which used MMH and MON-3 (a mixture of nitrogen tetroxide with approximately 3% nitric oxide).

This chemical is toxic and carcinogenic,[6] but it is easily stored in orbit, providing moderate performance for very low fuel tank system weight.

MMH and its chemical relative unsymmetrical dimethylhydrazine (UDMH) have a key advantage that they are stable enough to be used in regeneratively cooled rocket engines.

The European Space Agency (ESA) has attempted to seek new options in terms of bipropellant rocket combinations to avoid using deadly chemicals such as MMH and its relatives.