a theoretical design is described for the condenser/boiler of a space-based dynamic power system. the base system is a binary rankine cycle with mercury and toluene as the working fluids. the system output is 75 kwe (kilowatt electric) with a combined efficiency of 41.4 percent . the toluene flow is supercritical to avoid two-phase flow difficulties, and the mercury flow is designed to match with the highest heat transfer coefficients achieved during previous experiments. whereas previous estimates indicate an achievable specific mass on the order of 0.11 kg/kwth, the present design has a specific mass of 0.016 kg/kwth, including toluene ducting and protective shell. the resulting design utilizes pure molybdenum for all heat transfer surfaces, transfers 137.46 kilowatts of thermal power, can operate at varying mass flow rates, and has efficiencies in heat transfer of 0.96. the effect of varying mass rates is also investigated to determine the part load operation