Study of interactions of highly rarefied gases with in-motion solid bodies, and application to spacecraft at low earth orbit

Abstract

This thesis investigates the interaction between solid bodies and highly rarefied gas flows, a physical regime in which classical continuum assumptions break down and molecular effects dominate the flow behavior. Such conditions are characteristic of spacecraft operating in Low Earth Orbit (LEO) and during the early phases of atmospheric reentry, where the mean free path of gas molecules becomes comparable to or larger than the characteristic dimensions of the vehicle. The study focuses specifically on the aerodynamics of a simple geometry (a sphere), and the Apollo reentry capsules, which constitute a well documented and representative geometry for blunt body reentry in rarefied environments. The analysis is conducted using the Direct Simulation Monte Carlo (DSMC) method, implemented through the numerical software SPARTA. This approach allows for an accurate representation of rarefied gas dynamics by modeling the motion and collisions of simulated particles at the molecular level, rather than relying on macroscopic continuum equations. The simulations account for gas/surface interactions, molecular collisions, and non-equilibrium effects that are essential for accurately describing flow behavior in high Knudsen number regimes. A series of numerical simulations are performed to evaluate aerodynamic quantities such as pressure distribution and force coefficients at different altitudes within the rarefied flow regime. The obtained results are systematically compared with analytical formulations and reference data available in the scientific literature, enabling validation of the numerical methodology and assessment of its physical consistency. These comparisons demonstrate good agreement and confirm the suitability of the DSMC approach for the study of rarefied hypersonic flows around reentry vehicles. By addressing a flow regime that is often overlooked in traditional aerodynamics education, this thesis contributes to a deeper understanding of rarefied gas dynamics and its practical relevance in aerospace applications. While classical fluid dynamics and continuum based models are widely known and extensively studied, rarefied flow phenomena remain less familiar despite their importance in modern space missions. This work helps bridge that gap by providing a clear numerical and physical analysis of gas/surface interactions in rarefied environments, therefore widening the knowledge base in this specialized but increasingly relevant field.