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In general, a space propulsion system has a crucial role in the normal mission operations of a spacecraft. Depending on the types and number of propellants, a monopropellant and a bipropellant thrusters are mostly utilized for low thrust liquid rocket engines. As the plume gas flow exhausted from these small thrusters expands freely in a vacuum space environment along all directions, adverse effects of the plume impingement onto the spacecraft surfaces can dramatically reduce the function and performance of a spacecraft. Thus, the purpose of the present study is to investigate and compare the major differences of the plume gas flow behaviors numerically between the small monopropellant and bipropellant thrusters. To ensure efficient numerical calculations, the whole physical domain was divided into three different subdomains depending on the flow conditions, and then the appropriate numerical methods were combined and applied for each subdomain sequentially. With the present analysis results, the plume gas behaviors including the density, the overall temperature and the separation of the chemical species are compared and discussed between the monopropellant and the bipropellant thrusters. Consequently, the present results are expected to provide useful information on selecting the appropriate propulsion system, which can be very helpful for actual engineers practically during the design process.

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Satellite attitude is usually controlled by plume exhaust from thrusters into the vacuum of space. To study the plume effects in the highly rarefied region, the Direct Simulation Monte Carlo (DSMC) method is usually used, because the plume flow field contains the entire range of flow regime from the near-continuum near the nozzle exit through the transitional state to free molecular state at the far field region from the nozzle. The purpose of this study is to investigate the behavior of a small monopropellant thruster plume in the vacuum region numerically by using the DSMC method. To obtain more accurate results, the preconditioned Navier-Stokes algorithm is introduced to calculate continuum flow fields inside the thruster to predict nozzle exit properties, which are used for inlet conditions of DSMC method. As a result, the plume characteristics in the highly rarefied flow, such as strong nonequilibrium near nozzle exit, large back flow region, etc., are investigated.

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