Development of novel green monopropellant thrusters

Abstract

Space propulsion systems that are used for spacecraft maneuvers can be grouped into cold gas, monopropellant, and bipropellant thrusters. Monopropellant thrusters are widely used in space propulsion due to simplicity, reliability, and precision capabilities. Monopropellant thrusters commonly use a propellant that can be decomposed using catalysts. In the last few decades, hypergolic storable propellants such as hydrazine (N2H4) have been the most used propellants in space applications, however, due to their highly toxic nature, concerns about environmental and operational safety are increasing. Hydrogen peroxide (High Test Peroxide, HTP) has emerged as a promising candidate for green space propulsion applications due to its lower toxicity compared to conventional liquid propellants such as hydrazine and nitrogen tetroxide (N2O4). This study aims to optimize the performance and reliability of HTP monopropellant thrusters, focusing on catalyst bed stability, efficiency, and durability during both extended steady-state and pulse mode operations. To this end, a comprehensive thruster development process was undertaken. A series of thruster prototypes, including 10 N and 1 N thrust levels, were developed. Additionally, a monopropellant test bench, including a propellant tank, valves, and sensors, was assembled. The first phase of the study included catalyst characterization tests. During these tests, catalysts with various chemical and mechanical properties were analyzed using the pressure loss across the catalyst bed as an indicator of catalyst deterioration. Following the selection of the suitable catalyst, key operational and design parameters, including catalyst bed packing, pellet size, bed load, and HTP concentration, were investigated for their impact on steady-state performance. Results indicate that an optimal pressure drop of 1–1.5 bar across the catalyst bed provides optimal stability and durability. To evaluate transient characteristics, the effects of bed load, HTP concentration, and pre-heating temperature on thruster response times were investigated. Following the optimization process, a lifetime test consisting of six consecutive firings with an HTP throughput of 6 kg was conducted to monitor performance variations over time. Additionally, the blowdown characteristics of the thruster were analyzed to assess performance under end-of-life conditions. The experiments in this study demonstrate that HTP monopropellant thrusters are viable candidates for reliable space missions, particularly for long-duration operations such as station-keeping maneuvers. A key aspect of monopropellant thrusters is the pulse mode operation, since pulse mode is crucial for attitude control maneuvers. To analyze pulse mode characteristics, repeated short burn tests at frequencies between 0.25 Hz and 2 Hz at various duty cycles were conducted. During these tests, pulse mode characteristics, including settling time, peak thrust, peak pressure, impulse bit, and mean thrust, were measured. A smallest impulse bit of 0.16 Ns was achieved. The next phase of the study was the numerical modeling of the thrusters. A one-dimensional, adiabatic, reacting, porous media flow analysis was conducted to model the flow within the catalyst bed. The model included key aspects of the catalyst bed, such as porosity and activation energy of the catalyst. The model was validated both experimentally and using a benchmark study from the literature. To investigate the variation of catalyst parameters during the operational lifetime of the thruster, the long-duration lifetime tests of the thruster were analyzed using the numerical model. According to the results, the porosity decreased linearly over time, while activation energy increased quadratically as a function of time and propellant throughput, due to the accumulation of chemical and mechanical damage within the catalyst bed. The temperature and pressure distributions obtained from the model showed high consistency with the experimental data. The final phase of the study was designing a propulsion system, considering the storability of hydrogen peroxide within the tank. A material compatibility investigation was conducted by exposing various materials used in the propulsion system to hydrogen peroxide at elevated temperatures, as higher temperatures accelerate the decomposition reaction and shorten the required experiment duration. Results showed that the tested aluminum alloy exhibited good compatibility with the propellant. The decomposition rates obtained from the tests for each material were used for the numerical modeling of the decomposition reaction, which determined the pressure and concentration variations over a space mission. The model was further improved by incorporating the accumulation of deterioration within the catalyst bed, such as the reduction in porosity and the increase in activation energy. A significant result of the propulsion system model was the semi-self-pressurizing characteristic of hydrogen peroxide in long-duration missions. This characteristic helps maintain the thrust level over the course of a mission. A sensitivity analysis was performed to analyze the pressure, thrust, and Isp variations for different mission profiles. Hydrogen peroxide has emerged as a promising green propellant for space propulsion applications. Its high density and ease of decomposition using a catalyst pave the way for its wider adoption in space propulsion systems. The evaluation and optimization of hydrogen peroxide-based monopropellant thrusters in this study contribute to the development of propulsion systems capable of being used in a wide range of space applications.