Feasibility of an Integrated Solar Thermal Power and Propulsion System for Small Satellites

There is an increasing demand on small satellites to provide greater mission flexibility and capability while reducing their associated launch costs. Solar thermal propulsion (STP) is an attractive candidate to meet this requirement. STP provides specific impulses between 200 and 1000 s and thrust-to-weight ratios of 10-4 to 10-1 which fall in-between that of conventional chemical and electric propulsion systems. The unique coupling of an STP system with a thermal-to-electric energy conversion system has the potential to offer mass and volume savings by sharing on-board resources. The feasibility of using a micro-Organic Rankine Cycle (ORC) to generate the low on-board electrical power required by a small satellite and its associative design challenges are also discussed. The expected benefits of including a micro-ORC system are to increase the electric conversion efficiency, life expectancy, and cost-saving compared to traditional photovoltaic (PV) systems. The design consideration of a 100 W micro-ORC power system is also detailed focusing on both cycle thermodynamic performance and turbine characteristics. Results highlight the importance of considering the minimum blade height that can be manufactured with current state-of-the-art technology so that realistic operating conditions are selected. The cyclic siloxane D5 working fluid was selected for the micro-ORC based on a compromise between feasible turbine characteristics and cycle efficiency. The proposed system employs the unique coupling of using a single receiver to heat both the propellant and working fluid of the propulsion and electrical power system respectively. This system is shown to be a potential candidate for deorbit missions providing a delta-V of 540 m/s to microsatellites. Ammonia and water are promising propellant candidates for this system. The report highlights the benefits of an integrated solar thermal system capable of co-generating electrical power and propulsion for high delta-V missions to increase the capability of microsatellites.