Within the framework of the European Green SWaP (Green Solar-to-propellant Water Propulsion) project, Solar Thermal Propulsion (STP) is proposed as a sustainable propulsion technology for future space applications. The project aims to develop an in-space mobility architecture capable of converting water into hydrogen (H2) and hydrogen peroxide (H2O2) propellants using solar energy. This thesis focuses on the preliminary design of the Heat Exchanger (HEX) of the 1 N hydrogen-fuelled Solar Thermal Thruster (STT), where Thermal Energy Storage (TES) and Heat Exchanger (HEX) capabilities are integrated within the same component. The objective is to develop a methodology for identifying geometrically feasible HEX configurations, capable of heating hydrogen from 300 K to at least 1130 K while maintaining acceptable pressure losses.