Advanced Computational Center for Entry System Simulation (ACCESS)

The Advanced Computational Center for Entry System Simulation (ACCESS) is a comprehensive team of world-leading experts from five U.S. universities (Colorado, Illinois, Kentucky, Minnesota, New Mexico) and three international collaborators (Oxford University, National Research Center-Bari, Instituto Superior Tecnico-Lisbon). Our vision for ACCESS is to radically advance the analysis and design of entry systems through development of a tightly integrated interdisciplinary simulation framework employing high-fidelity validated physics models, driven by quantified uncertainty and reliability, and enabled by innovative algorithms and high-performance computing.

A NASA Entry System (ES) involves the Thermal Protection System (TPS), including both the heat shield and backshell, along with the supporting structure. An ES is essential to many of NASA’s highest priority space exploration missions, including lunar return to Earth (Artemis), Titan entry (Dragonfly), sending people to Mars (Mars Human Lander), and return of Mars samples to Earth (Earth Entry Vehicle, EEV). Based on the key attributes of these missions, the critical physical processes that drive ES design involve flow phenomena (e.g., chemistry, radiation, turbulence), material response (e.g., ablation) and structural response (e.g., fracture). The ACCESS research plan includes analysis of Dragonfly, Mars Human Lander, and the EEV.

Entry System analysis and design capabilities currently employed by NASA and its contractors are workable for Artemis, but have critical limitations for the more challenging environments of future missions. A first significant limitation with state-of-the-art (SOA) analysis capabilities is that the uncertainties associated with predicting key quantities of interest are so large that it is not always possible to close on a design cycle. For example, a margin of 100% for turbulent surface heating augmentation is typically employed for Mars entry, and a margin of 40% was used for radiative surface heating for lunar return. Such large uncertainties arise directly from limitations in the accuracy of modeling the key physical phenomena and represent a significant challenge for meeting design requirements, e.g., EEV has a reliability requirement of less than 1 in 106 that cannot be met by SOA analysis capabilities.

A second significant challenge for the design of ES for NASA reference missions concerns the currently available analysis tools. NASA and the contractors employ a number of computational codes for analysis of ES. However, these tools are labor intensive to apply, their computational performance is limited in part by not taking advantage of emerging computer architectures, and they do not integrate uncertainty and reliability.

To address these challenges, the ACCESS research plan involves four tightly coupled tasks:

Task 1: Kinetic Rate and Physical Processes
Task 2: Integrated Simulation Framework
Task 3: High Fidelity Modeling of TPS Features, Damage, and Failure
Task 4: Uncertainty Quantification and Reliability.
ACCESS will drive down design margins and quantify uncertainty through an innovative, multidisciplinary research approach. The entry missions targeted involve an enormous number of gas-phase and radiative processes. For example, an ablating hydrocarbon TPS can require chemistry mechanisms with about 40 species and 150 reactions. Backshell heating from radiation can also be significant. To reduce the margin, rates for all key reactions must be estimated using reliable experimental data and scalable statistical inference techniques, and the resulting uncertainty must be quantified. In Task 1, theoretical chemistry will identify the key reactions and determine new rates as needed including those for production of electronically-excited states that radiate. The overall kinetics mechanism, including both ground-state and excited-state reactions, will be evaluated through direct comparisons with experimental data generated in world-class facilities. The quantification of uncertainty associated with the rates will be established in collaboration with Task 4. The rates, along with the quantified uncertainty, will be integrated into the overall simulation tool in Task 2. In Task 3, models for gas-surface kinetics, constructed from molecular beam experimental data, must first be applied at the mesoscale for material response modeling. Our novel approach uses simulations of representative volume elements (RVEs). The RVE simulations will use detailed kinetics information (Task 1) and specific meso-structures (Task 3) as inputs, and will quantify each of the mesoscale modeling components required by the material response model; namely, oxidation evolution, porous flow trends, and thermal, structural, and radiative properties. The RVE simulations will provide natural variability in these models and associated parameters (distribution functions), which is crucial to model a full TPS including uncertainty and reliability. The novel stochastic material response framework (Task 3) will be directly coupled to the overall simulation tool (Task 2) and will be developed within the proposed UQ framework (Task 4). This comprehensive approach spans all of the Tasks and all of the ACCESS universities. Such innovative and multidisciplinary integrated research is absolutely essential to achieving the Vision of ACCESS of reducing the overall margins and improving the reliability for the analysis and design of an ES.

The primary product of ACCESS is the Integrated Simulation Framework (ISF) that will completely change the paradigm in comparison to SOA capabilities for the analysis and design of ES. The ISF will be developed in Task 2, will integrate the key products of all other Tasks, and will take as its starting point the widely used US3D computational fluid dynamics code. As a fundamental construct in its design, US3D allows the integration of simulation capabilities for a broad range of physical phenomena through specification of plugins. The use of plugins with well-defined interfaces makes it possible to transfer capabilities developed in ACCESS for US3D into other simulation frameworks of NASA and its contractors. Task 4 addresses UQ at the level of individual phenomena in the flow and TPS areas (Tasks 1 and 3) and for overall simulations through the ISF (Task 2). The UQ for Tasks 1 and 3 will break new ground for detailed quantification of uncertainty through close coupling between modeling and experiments. Instead of “validating” the physics models, the contribution of inaccuracy and uncertainty of individual processes to overall risk in the ES design will be quantified and transmitted through the system level simulation. One significant challenge in Task 4 for UQ and reliability is the high computational cost of each full ISF simulation, which may limit the number of sensitivity data points that are generated. To address this challenge, novel algorithms will be explored, such as Discontinuous Galerkin methods and meshless techniques, that have the potential to significantly reduce the time to set up and execute large-scale simulations. Also, key ISF algorithms will be adapted for execution on Peta/Exa scale computer architectures to reduce run time. Emerging UQ approaches will be employed that make careful use of lower fidelity physical models to achieve results consistent with more expensive higher fidelity models but at drastically reduced cost. The successful outcome of the overall Vision for ACCESS will deliver an integrated simulation framework for the comprehensive and affordable design of ES with quantified uncertainty and reliability estimates that will be ready for adoption by NASA and its contractors.