Dynamic Gas-Surface Interaction Modeling for Satellite Aerodynamic Computations

Drag coefficients are a large source of uncertainty when predicting the aerodynamic forces on orbiting satellites. Accordingly, the focus of this research is to improve the fidelity of drag modeling by investigating the nature of gas-surface interactions in low earth orbit. The author has investigated to what extent oxygen adsorption can influence the parameters of drag coefficient models, most notably the energy accommodation coefficient. To accomplish this, several analysis techniques are applied. Fitted drag coefficients for 68 objects were provided by Air Force Space Command Drag Analysis Office and are analyzed using analytical and numerical aerodynamic models. Gas-surface parameters are estimated by comparing the model results to the observed coefficients. The results indicate that a successful and predictive relationship of the energy accommodation coefficient can be obtained with gas-surface models incorporating Langmuir adsorption. Good agreement with data has been obtained by using a cosine reflection model below 500 km. Furthermore, it is found that satellite accommodation coefficients can be explained by a model in which atomic oxygen binds to the surface with an energy of approximately 5.7 eV. Multi-axis accelerometer data from the CHAMP and GRACE satellites has also been analyzed to derive measurements of lift and drag which are compared to model predictions given different gas-surface assumptions. The results indicate that diffuse reflection is appropriate for CHAMP near 400 km and that the accommodation coefficient before 2008 ranges between 0.86 and 0.89. CHAMP accelerometer data is also combined with remote sensing estimates of density to arrive at values of drag coefficient which do not depend on empirical atmospheric models alone. This dataset conforms the predicted drop in accommodation with decreasing atomic oxygen pressure. The culmination of this work is an enhanced energy accommodation and drag coefficient model applicable between 100 km and 500 km altitudes for satellites in both circular and elliptical orbits