## 1 Introduction
A typical solar-thermal engine is composed of three primary components or subassemblies:  
* A solar concentrator, which concentrates and focuses solar energy onto the receiver;
* The receiver itself, which acts as a heat exchanger for the propellant;
* The propellant storage and feed system, which includes tankage and feed lines for routing the propellant to the receiver. * A fourth component, control electronics, is not considered here but it is required to retrieve system telemetry, open and close valves, and (potentially) control both the concentrator and receiver position. A working principle scheme of a solar-thermal propulsion system [21], further called STP, is presented in Figure 1.  
A benefit of using STP as a spacecraft propulsion system is that it uses solar radiation directly to heat the propellant, therefore improving the conversion efficiency over resisto-jets that require converting solar radiation into electrical energy and then into thermal energy to heat the propellant. The operating temperature is limited by the materials available. A cold gas propulsion system (CGP) relies on the process of controlled ejection of compressed liquid or gaseous propellants to generate thrust. Due to the absence of a combustion process, a CGP system requires only one propellant (without an oxidizer), and hence can be designed with minimal complexity. The schematic of a typical CGP system is shown in Figure 2, and the main components include a propellant storage and a nozzle [22]. The simpler design of a CGP system leads to a smaller system mass and lower power requirements for regulation purposes. However, these advantages come at the cost of a monotonically decreasing thrust profile over a period of time. The thrust produced is directly proportional to the pressure of the propellant inside the tank (propellant storage) and over the course of the mission, the tank pressure decreases (due to propellant usage) resulting in a decrease in the maximum thrust generated by the system. There are many studies and applications about cold gas systems, CGPs [23; 24; 25; 26]. The novelty element of the present paper consists in the fact that it proposes a solution which combines the solar-thermal propulsion and cold gas propulsion concept, so that the solar energy which is concentrated from a concave mirror will directly heat up the propellant tank. The propellant tank is placed in the mirror's focal axis. The present paper tries to solve the operational life issue by developing a propulsion system for satellites which will increase the operational life of the satellite on the orbit. Tests at low temperature (specific for cold gas propulsion) and tests at high temperature (specific for hot gas propulsion) will be carried out. A comparison between the forces obtained in both cases for different pressures will be carried out. The aim is to see what improvement in performance will the hot gas propulsion system bring, given the same amount of mass of N\({}_{2}\) used in both the cold and hot gas propulsion systems. The new type of solar-thermal propulsion system developed by COMOTI in the frame of a national space research program STAR--project STRAUSS--can be a solution for the satellite's operational life increase. Figure 1: Schematic of a solar thermal propulsion system. Figure 2: Schematic of the cold gas propulsion system.