This thesis presents a thermodynamic analysis of a novel Rankine cycle aluminum/steam combustion power system being developed for use in Unmanned Underwater Vehicles (UUVs). The analysis is performed using a system modeling tool developed by the NASA Glenn Research Center called Numerical Propulsion System Solver (NPSS). Thermodynamic models of the individual components are created and linked together in NPSS, which then solves the system by enforcing mass and energy conservation. Design and off-design conditions are simulated and predicted performance is compared with predictions made by two other research groups. The simulations predict that this power system could provide at least five-fold increases in range and endurance for the US Navy's 'Sea Horse' UUV. A rudimentary sensitivity analysis is used to identify the factors which most strongly influence the performance of the design. Lastly, recommendations for future work and possible model improvements are discussed.