**4.1 Phobos and Deimos payload missions**

A range of payload masses were included in the comparative orbital transfer cases: 1, 10 and 50 metric tons (MT). In general, the initial masses of the NEP OTVs are higher than the O2/H2 OTVs initial masses. However, the propellant masses of the NEP vehicles are generally significantly lower that most O2/H2 vehicle propellant masses. Thus, the propellant resupply masses for the NEP OTVs offer a substantial resupply mass benefit over chemical propulsion OTVs.
