The Orbital Maneuvering System allows for the orbiter to make substantial changes to its orbit.  This ability is vital for rendezvous with another object in space, and more importantly is able to slow the orbiter's speed down enough so that it can reenter the atmosphere.  Without it, the orbiter would be stranded in orbit, its crew consuming its limited oxygen supply and eventually dying of asphyxia.

    Cryogenics are not the optimal propellant due to the long duration of time that the vehicle is designed to stay in orbit.  Also, given the vitality of the OMS for completion of the Mission Objectives, the system needs to be as simple and reliable as possible.  In turn, a subsection of storable propellants, called hypergolic propellants, serves this purpose best.  Hypergolic propellants ignite the moment the fuel and oxidizer meet, making a separate ignition system unnecessary.

    Hypergolic engines have been used during many missions, including the main propulsion system for the Apollo missions, and the OMS on the Shuttle.  Given the nature of the project, it would make sense just to take the Shuttle's OMS system and place it in the orbiter, since they have an excellent history to date.  Unfortunately, the Shuttle's two OMS engines are too large for the orbiter and greatly exceed what is necessary for the vehicle.  It would be like putting a V8 engine into a jeep.

    For further redundancy, I have decided to use two OMS engines, like the Shuttle but unlike the single engine used during Apollo.