Spectrum Modeling for Air Shock-Layer Radiation at Lunar-Return Conditions

Anew air-radiation model is presented for the calculation of the radiative flux from lunar-return shock layers. For modeling atomic lines, the data from a variety of theoretical and experimental sources are compiled and reviewed. A line model is chosen that consists of oscillator strengths from the National Institute of Standards and Technology database and the Opacity Project (for many lines not listed by the National Institute of Standards and Technology), as well as Stark, broadening widths obtained from the average of available values. Uncertainties for the oscillator strengths and Stark broadening widths are conservatively chosen from the reviewed data, and for the oscillator strengths, the chosen uncertainties are found to be larger than those listed in the National Institute of Standards and Technology database. This new atomic line model is compared with previous models for equilibrium constant-property layers chosen to approximately represent a lunar-return shock layer. It is found that the new model increases the emission resulting from the 1-6-eV spectral range by up to 50%. This increase is due to both the increase in oscillator strengths for some important lines and to the addition of lines from the Opacity Project, which are not commonly treated in shock-layer radiation predictions. Detailed theoretical atomic bound-free cross sections obtained from the Opacity Project's TOPbase are applied for nitrogen and oxygen. An efficient method of treating these detailed cross sections is presented. The emission from negative ions is considered and shown to contribute up to 10% to the total radiative flux. The modeling of the molecular-band systems using the smeared-rotational-band approach is reviewed. The validity of the smeared-rotational-band approach for both emitting and absorbing-band systems is shown through comparisons with the computationally intensive line-by-line approach. The absorbing-band systems are shown to reduce the radiative flux by up to 10%, whereas the emitting-band systems are shown to contribute less than a 5% increase in the flux. The combined models chosen for the atomic line, atomic bound-free, negative-ion, and molecular-band components result in a computationally efficient model that is ideal for coupled solutions with a Navier-Stokes flowfield. It is recommended that the notable increases shown, relative to previous models, for the atomic line and negative-ion continuum should be included in future radiation predictions for lunar-return vehicles.