Recent developments in satellite technology can be characterized by two notable trends: miniaturization and clustering. The emergence of microsatellite constellations is making a significant impact on both commercial and national defense sectors. The satellite internet market has experienced rapid growth, surpassing $9 billion, and the application of real-time ground observation has proven crucial in recent armed conflicts. As efforts intensify to operate satellites in very-low-Earth-orbit (VLEO) below 450km, lower altitudes offer advantages such as improved satellite communication latency and optical resolution. While very low-Earth orbit (VLEO) offers numerous technical advantages, it is accompanied by a significant obstacle in the form of aerodynamic drag force generated by the sparsely populated upper atmosphere. Employing space propulsion systems can be a paramount solution for drag compensation. However, the requirement for large amounts of onboard propellant presents another challenge, related to the launch cost and structural capability.

The concept of atmosphere-breathing electric propulsion (ABEP) system emerged in the early 2000s as an innovative solution to counter atmospheric drag force without relying on onboard propellant. ABEP is designed to intake the upper atmosphere as a propellant, which is then ionized and accelerated via an electromagnetic field to generate thrust. Despite being in the developmental phase, ABEP has garnered significant attention and undergone active research in recent years. The Non-equilibrium Gas and Plasma Dynamics Laboratory (KNGPDL) at KAIST, led by Professor Eunji Jun, has conducted a conceptual system analysis on ABEP. This analysis employs a numerical approach to examine three key aspects: the performance characterization of the intake process, estimation of the flight envelope, and evaluation of the environmental interactions resulting from the exhaust plasma plume. The calculations are carried out using the computational resources provided by the National Supercomputing Center, ensuring scalability and accuracy.

The numerical simulation of the intake flow is performed using the direct simulation Monte Carlo (DSMC) method, which accurately captures the stochastic nature of the rarefied gas flow. The freestream conditions are obtained via the NRLMSISE-00 global atmospheric model. A parametric characterization of the intake design-performance correlation is conducted, considering the effects of geometry, surface temperature, aspect ratio, and grid duct configuration. The results of this parametric analysis, along with the proposed comprehensive design (depicted in Figure 1), are presented. Furthermore, the composition of the captured propellant is kinetically analyzed, revealing a 10% decrease in the atomic oxygen fraction and highlighting the significance of non-equilibrium kinetics. These findings provide crucial insights for the design of the ionization process.


                                                      Figure 1 Comprehensive intake design


The feasible altitude range where the ABEP system can effectively counteract atmospheric drag force is defined as the flight envelope. To assess the ABEP's performance within this envelope, the KNGPDL has developed a satellite design incorporating a comprehensive intake device. The corresponding drag force is calculated using the DSMC method, as illustrated in Figure 2. A zero-dimensional analytic model is established to evaluate the thrust performance of ABEP, taking into account the radio-frequency ion thruster mechanism. Various factors, including solar activity, solar panel area, intake performance, and gas composition, are considered to account for environmental and systematic variabilities. The flight envelope of the ABEP is determined to be within an altitude range of 200 km to 250 km under moderate environmental conditions. The envelope can be influenced by space weather and expanded through structural optimization, as demonstrated in the study.



Figure 2 DSMC simulation result of ABEP equipped satellite 
      (up: particle number density, down: axial velocity).


The ABEP system utilizes ionized propellant, which is accelerated and expelled in the form of a plume flow. However, within the plume flow, charge-exchange collisions (CEX) between plasma particles can result in ions backflowing. These backflowing ions may interact with the satellite’s surface, potentially leading to contamination issues such as erosion and charging. To model the plume flow and the associated ion backflow phenomenon, the particle-in-cell (PIC) method is combined with the DSMC method. Accurate modeling of intermolecular collisions is crucial for estimating the current of backflowing ions. A generalized formulation for collision cross-sections, considering various chemical species and energy, is derived and utilized in the plume flow modeling. Figure 3 illustrates the spatial distribution of ion number density, revealing the presence of a diffusive ion stream perpendicular to the plume flow, known as the 'CEX wing,' which serves as evidence of ion backflow. Due to the high ionization potential of the atmospheric propellant, ABEP exhibits a lower ionization rate in the exhaust plasma compared to conventional xenon-based thrusters under the same thrust operating conditions. Consequently, the density of neutral gases in the exhaust becomes higher in ABEP, potentially resulting in an increased occurrence of the CEX process and a greater number of backflow ions. Additionally, interactions between the plume and the background of the same chemical composition can lead to an increase in the energy of individual backflow ions.

                             Figure 3 O+ Ion number density of plasma plume