The current objectives of the United States space program have instigated a renewed interest in the study of spacecraft atmospheric entry. This thesis introduces the concepts of radiative heating and the various mechanisms responsible for radiation emission and absorption. The three primary categories of radiation known as bound-bound, bound-free and free-free transitions are discussed in conjunction with the reaction rates and probabilities. Radiation models coupled with continuum and rarefied flowfield simulations are considered and common approximations are analyzed with respect to their varying degree of accuracy and efficiency. Select models are employed in an effort to compare theoretical radiative spectra and heat flux with experimental data at the same conditions collected from the Fire II and Apollo 4 flight tests. The basic concepts and configurations of shock tube facilities are presented in addition to the current issues and challenges associated with ground-based radiation testing.The main purpose of this thesis is to demonstrate the necessity for a novel approach to modeling radiation mechanisms in order to more accurately predict the integrated radiative heat flux incident on a spacecraft during lunar and Mars return trajectories. First, a sensitivity study is conducted for various input parameters into a computational fluid dynamics simulation and radiation model. The details of quantum statistical mechanics are outlined and exhibited in the form of radiation properties for both diatomic and atomic air species. Lastly, it is shown that plasma effects exist within hypersonic shock layers and may lead to important physical phenomena that substantially influence radiation mechanisms.
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