CubeSats and SmallSats have transformed the space industry by offering cost-effective, modular platforms for various space missions, such as communications, Earth observation, and technology demonstration. Most of these missions demand propulsion systems for orbit maintenance, formation flying and space debris mitigation. Conventional propulsion systems usually rely on toxic or pressurized propellants, which introduce hazards during procurement and satellite operations. For these reasons, Capsule Corporation s.r.l. - the company at which this thesis work was conducted, has developed WaterCube, a plug-and-play water-based resistojet propulsion unit, which offers a safe, sustainable, and efficient alternative to conventional propellants. This thesis work focuses on the design, implementation and testing of embedded real-time flight software for the WaterCube Avionics Board (AVB).