where m(dry, stage, NEP) = NEP dry mass (kg). alpha = NEP reactor specific mass (kg/kWe). P = NEP power level (kWe). 0.05 = tankage mass coefficient (kg/kg m, p). m(p) = NEP usable propellant mass (kg). m(fixed) = NEP fixed mass (kg).

The OTV sizing was conducted for a wide range of power levels: 0.5 MWe to 30 MWe. Three nuclear reactor specific masses were used: 10, 20, and 40 kg/kWe (kilograms per kilowatt, electric) [15]. The OTV propulsion fixed mass, apart from and in addition to the reactor mass, was 20 MT, and the propellant tankage mass was 5% of the mass of the required propellant.

The specific impulse (Isp) and efficiency of the electric propulsion systems were 5,000 seconds with overall thruster-propulsion efficiencies of 50% for each design. These design points are typical of advanced designs of either magnetoplasmadynamic (MPD) or pulse inductive thrusters (PIT). While hydrogen is suggested for both propulsion system thrusters, the possibilities of the higher Isp option using inert gases (xenon, krypton, etc.) are also viable. The low thrust OTV delta-V value varied based on the destination of the Martian moon missions.
