Radiative Heating Methodology for the Huygens Probe

Abstract

The radiative heating environment for the Huygens probe near peak heating conditions for Titan entry is investigated in this paper. The task of calculating the radiation-coupled flowfield, accounting for non-Boltzmann and non-optically thin radiation, is simplified to a rapid yet accurate calculation. This is achieved by using the viscous-shock layer (VSL) technique for the stagnation-line flowfield calculation and a modified smeared rotational band (SRB) model for the radiation calculation. These two methods provide a computationally efficient alternative to a Navier-Stokes flowfield and line-by-line radiation calculation. The results of the VSL technique are shown to provide an excellent comparison with the Navier-Stokes results of previous studies.It is shown that a conventional SRB approach is inadequate for the partially optically-thick conditions present in the Huygens shock-layer around the peak heating trajectory points. A simple modification is proposed to the SRB model that improves its accuracy in these partially optically-thick conditions. This modified approach, labeled herein as SRBC, is compared throughout this study with a detailed line-by-line (LBL) calculation and is shown to compare within 5% in all cases. The SRBC method requires many orders-of-magnitude less computational time than the LBL method, which makes it ideal for coupling to the flowfield. The application of a collisional-radiative (CR) model for determining the population of the CN electronic states, which govern the radiation for Huygens entry, is discussed and applied. The non-local absorption term in the CR model is formulated in terms of an escape factor, which is then curve-fit with temperature. Although the curve-fit is an approximation, it is shown to compare well with the exact escape factor calculation, which requires a computationally intensive iteration procedure.

Analysis of Nonequilibrium CN Radiation Encountered During Titan Atmospheric Entry

“…There has been a great deal of research into several different pieces of the puzzle aimed at solving the CN radiation problem. These analyses include the development of reaction schemes for various atmospheric compositions [9], developing vibrational state specific reaction rates [10][11][12][13], the development of software to integrate spectra and calculate radiation intensity [14], the development of collisional-radiative models [1,2], and work related to producing simplified assumptions to efficiently integrate spectra and calculate the radiation in atmospheric entry environments [5]. Furthermore, the analysis presented in the literature has also highlighted several shortfalls in the prediction of the experimentally measured emitted radiation during shock tube testing, such as overestimating the peak level of emitted radiation (approximately within a factor of 4-12, depending on the condition) [3], significantly underpredicting the radiation decay rate [3], and the rise time of radiation intensity just behind the shock is also generally slower in previously developed models than what was measured experimentally [1].…”

“…This has led to investigations into the heating environments encountered during entry into various atmospheric compositions [1][2][3]. During high-speed entry into atmospheres such as Titan, the CN molecule is formed in concentrations above equilibrium [1,2,4,5]. CN has been identified as a strongly radiating molecule and one of the main sources of radiation during Titan entry.…”

A Comparison of EAST Shock-Tube Radiation Measurements with a New Air Radiation Model

“…For 0.1 and 0.3 Torr conditions, these band systems are optically-thin. The SRB model is therefore accurate at predicting the wavelength-integrated intensity to within a few percent 35 . Before the comparisons between the predictions and measurements are presented, two significant caveats of this analysis must be mentioned.…”

Influence of Ablation on Radiative Heating for Earth Entry

“…As a result, Figures 5 (b) and (c) show a reduced flux contribution from this spectral range relative to the pure air case in Figure 5 As mentioned in the previous section, the HARA code applies the smeared rotational band (SRB) model for treating the molecular band systems. Although this model is known to be accurate for optically thin emission 74 , its ability to treat optically-thick absorption is questionable. The significant absorption from VUV molecular band systems shown in Figures 1 and 2 indicates that significant optically-thick absorption is encountered in an ablating or non-ablating boundary layer.…”