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Description
The Mars Sample Retrieval lander mission was designed to be the first stage of NASA's Mars Sample Return program. It would land a stationary payload to process samples collected by the Perseverance rover and place them on board a rocket for their journey to Earth. In total, the lander would be the largest entry mass and volume ever placed on the Martian surface. Furthermore, the entry speed, in its final iteration, was approximately eight kilometers per second. Most Martian landers previously landed in the five kilometer per second regime.
One of the many new design challenges of the lander was the increased radiative heat load. As one moves from a five km/s entry to an eight km/s entry, the shock layer contains enough thermal energy to both dissociate the carbon dioxide in the atmosphere and electronically excite the resulting CO molecules. This transitions the radiative heating from an infrared signal from the CO2 vibrational bands to a strong ultra-violet signal from CO (the 4th positive band). This radiative heating surpassed the convective heating of previous missions like Mars Science Laboratory (MSL) and Mars2020. This new physics required validation to certify the mission design.
The Electric Arc Shock Tube (EAST) is NASA's workhorse facility for ground-based measurements of radiative emission from high-speed flows. Simulations against EAST optical emission spectroscopy data formed the core of this validation effort. The simulations were performed using DPLR and NEQAIR, NASA's hypersonic CFD code and spectral solver, respectively.
The baseline simulations showed a large underprediction of VUV radiation, as high as a factor of two at some conditions. The primary cause was attributed to the lack of heavy particle excitation in the baseline model. However, even with these corrections, one could not bound the experimental data. In fact, using a Boltzmann model of excited states, one could not produce a conservative simulation at peak heating conditions.
This discrepancy was hypothesized to be the result of deceleration effects in EAST rather than a systematic problem in the model. A new technique was developed to account for deceleration effects. The method relies on the assumptions of Shock Tube Informed Bias (STIB) to, instead of mapping shock tube data to CFD flow solutions, map CFD flow solutions onto a deceleration shock trajectory. This was shown to result in a large change in predicted radiative heating bringing predictions more in line with experimental data.
Even with these adjustments, there continued to be discrepancies between model predictions and experimental data from EAST. For mission design, some margin factor must be calculated to provide a useful upper bound. This calculation, if done rigorously, is extremely computationally expensive or intractable. In this work, three methods were presented, starting with the methods developed for other NASA missions such as Dragonfly and ending with an approximate Bayesian inversion. These methods all produced different estimates of the radiative heating uncertainty, varying from 100% to 30%.
Summary
This work summarized the radaitive heating modeling, validation, and qualification for the NASA Mars Sample Retrieval Lander