The fuel used in the OMS and RCS engines is monomethyl hydrazine and the oxidizer [link to glossary] is nitrogen tetroxide. Both are liquid at room temperature, which means they are much easier to store than the cryogenic [link to glossary] fuel used in the SSMEs [link to info]. They are ÒhypergolicÓ propellants, which means that they ignite as soon as they come into contact with each other. This makes it easy for the engines to start and restart.