Sannino, Antonio (2026) Numerical simulations for satellite aerodynamics in VLEO. [Tesi di dottorato]

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Abstract

The Very Low Earth Orbit (VLEO) region, conventionally defined between 150 km and 450 km in altitude, is attracting growing interest for future satellite missions. Its advantages, which include higher resolution for Earth observation, reduced latency for telecommunications, and the potential to mitigate the space debris problem through faster orbital decay, are nonetheless counterbalanced by significant aerodynamic challenges. At these altitudes, the atmosphere, although extremely rarefied, exerts non-negligible forces that dominate spacecraft orbital dynamics, limit operational lifetime, and introduce complex fluid-structure interactions. This doctoral thesis explores the dual nature of VLEO aerodynamics: as a perturbation to be understood and managed, and as a resource to be harnessed for orbital maneuvers and for innovative propulsion. The work is structured into two main areas: 1) satellite aerodynamics, focusing on atmospheric re-entry, drag-based orbital maneuvers and stability; and 2) the aerodynamics of intakes for Atmosphere-Breathing Electric Propulsion (ABEP) systems. The common thread of the research is the use of numerical simulation as a fundamental tool for investigating the physics of hypersonic and rarefied flows. Given the nature of the flow regime in VLEO, where the Knudsen number typically ranges from 0.1 to values well above 10, continuum fluid-dynamic methods (Navier-Stokes) are no longer valid. Consequently, the entire work is based on the Direct Simulation Monte Carlo (DSMC) method. DSMC is a particle-based computational approach for solving the Boltzmann equation, in which a gas is represented by simulated particles whose motion and collisions are treated statistically. This method is essential for accurately capturing non-equilibrium phenomena, and, most importantly, gas-surface interactions (GSI), which are determining factors for the accurate estimation of aerodynamic coefficients and collection system performance. The first part of the thesis is dedicated to the aerodynamics of small satellites, particularly CubeSats, addressing three key aspects that together form a coherent narrative: first, how aerodynamic forces drive re-entry; second, how they can be harnessed for maneuvering; and third, how the satellite can be designed to remain passively stable while doing so. The study begins with atmospheric re-entry, investigating the design of a mission for payload recovery. Two distinct scenarios are compared: a purely aerodynamic re-entry, which exploits an aero-brake to increase drag and accelerate orbital decay, and a propulsion-assisted re-entry, which uses, in addition to the aero-brake, a ΔV to de-orbit the vehicle instantaneously. The role of DSMC in this context is to characterize the aerodynamic performance of the spacecraft, obtaining a trend of drag coefficient (CD) with the altitude in folded and deployed configurations. An error analysis, incorporating uncertainties in aerodynamic parameters and atmospheric density, is conducted to quantify the landing dispersion, highlighting the sensitivity of the mission design to the accuracy of aerodynamic force modelling and propulsive maneuver execution. Building on the characterization of aerodynamic drag, the research then explores its exploitation for performing propellant-less orbital maneuvers. A formation scenario with relative motion between two CubeSats (a chief and a deputy) at the same altitude is conceptualized. By modulating the deputy's exposed area, it is possible to generate a differential drag (ΔCD) relative to the chief. This differential drag, although small, induces a difference in the decay of the semi-major axis, which translates into controlled relative motion along the orbital track (in-plane phasing maneuver). The analysis of such maneuvers, supported by orbital dynamics simulations using the General Mission Analysis Tool (GMAT), allows for the evaluation of the feasibility of completely fuel-free formation orbital maneuver, based solely on aerodynamics. This approach represents a paradigm shift, transforming a perturbative force into an actuator for formation flying. Finally, to enable such drag-based maneuvers without relying on complex active control, a systematic analysis of the pitch stability of a 3U CubeSat equipped with lateral panels is conducted. DSMC is employed to calculate the lift, drag, and pitch moment coefficients over a wide spectrum of angles of attack. The analysis focuses on identifying aerodynamically stable attitude configurations. It is demonstrated that a reasonable displacement of the center of mass (on the order of 1.5 cm) is sufficient to guarantee stability over a wide range of angles, a fundamental finding for the design of passively stable satellites that reduce dependence on expensive and complex active control systems. The second part of the thesis moves from mitigation to the active exploitation of the atmosphere, addressing the design and analysis of intakes for ABEP systems. ABEP is a revolutionary technology that promises to enable indefinite orbital operations in VLEO by using residual atmospheric gases as propellant. The intake is the key component of this system, responsible for collecting and compressing the incoming hyperthermal flow to a density usable by an electric thruster. The DSMC methodology proves again indispensable for evaluating the intake's figures of merit: the collection efficiency, the compression ratio, and the generated aerodynamic drag. One of the most critical aspects emerging from this study is the profound influence of the gas-surface interaction (GSI) model. The thesis systematically compares the widely used Maxwell reflection model, valued for its simplicity, with the more physically refined Cercignani-Lampis-Lord (CLL) model, which can describe a partially diffuse re-emission. The results show that the choice of GSI model has a significant impact on the prediction of performance, influencing the mass flow rate delivered to the thruster and the total drag. This underscores the necessity of using accurate GSI models, supported by experimental data, for reliable ABEP system design. Furthermore, the research explores the scalability of intakes, demonstrating that their dimensions can be reduced to fit nanosatellite platforms without significant performance degradation, thereby paving the way for ABEP missions on CubeSats. The dependence of performance on altitude is also analyzed, highlighting how, at lower altitudes, collisions between particles within the intake become non-negligible and must be correctly modelled to avoid substantial errors in efficiency estimation. Finally, the critical sensitivity of intake performance to variations in the angle of attack is investigated. Even small angles of incidence (on the order of 5°) can drastically alter the internal flow within the duct, reducing collection efficiency and potentially leading to insufficient mass flow for the ignition and sustenance of the electric thruster. In conclusion, this doctoral work provides a comprehensive methodological and computational framework for aerodynamic analysis in VLEO. Through the systematic application of the DSMC method, the thesis advances the understanding of the aerodynamic stability of small satellites, demonstrates the feasibility of propellant-less orbital maneuvers, outlines strategies for re-entry, and identifies the critical parameters for the design of efficient and robust ABEP intakes. The research path underscores the importance of accurate physical modelling, particularly of gas-surface interactions, to transform the challenges of the VLEO environment into opportunities for more innovative, sustainable, and long-lasting space missions.

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