With the number of spacecraft launches increasing each year, so does the search for more efficient and greener propulsion solutions. Electrical propulsion offers high specific impulses, but the provided thrust is lower than that of chemical propulsion. The novel concept of Solar Thermal Propulsion (STP) is a potential solution that offers a higher specific impulse than chemical propulsion and a larger thrust than electrical propulsion. The Green SWaP mission aims to make space exploration more sustainable and implements STP for the secondary propulsion system. This thesis presents the design of a 1.0 N STP system incorporating a Thermal Energy Storage (TES) to enable operation during eclipse periods. Several STP concepts are evaluated, and the selection is supported by thermofluidic analysis. A transient thermal model is developed to improve radiation loss estimations and minimise TES material mass. The resulting propulsion system achieves a predicted specific impulse of 870 s.