System concepts for powering an unmanned undersea vehicle (UUV) with a solid oxide fuel cell (SOFC) power generator are developed and evaluated through modeling studies of variations in system configurations and design parameters. System concepts that meet the continuous electrical loads for sustained operations and mission selectable capabilities for a 21-inch (53.3 cm) diameter UUV platform with 500 W to 1500 W power requirements were analyzed. Two primary system concepts fueled with a liquid hydrocarbon fuel (dodecane) and employing either liquid-oxygen (LOx, Concept I) or a mixture of hydrogen peroxide and water as the cathode gas oxidant (Concept II) were generated and optimized to obtain maximum mission endurance within the volume constraints allocated for the power generator. Concept I (LOx) 'best' and 'intermediate' designs achieve system efficiencies of 47% and 42.2%-LHV, and mission durations of 90-h and 81-h, respectively, at 1000 W power with hull-integrated oxidant storage. The Concept II system manages an efficiency of 45.5%-LHV and a mission duration of 46-h.