Protecting a spacecraft during atmospheric entry is one of highest risk factors that needs to be mitigated during the design of a space exploration mission. At entry speeds from space, air turns into high-temperature plasma, and spacecraft Thermal Protection Systems (TPS) are needed to protect the vehicle payload and manage the heat from the freestream flow. For extreme speeds such as return from the Moon or Mars, sacrificial materials such as ablators are the only option as TPS materials.
Modern successful material architectures of spacecraft shields use porous substrate impregnated with phenolic. The substrate is designed to be highly porous to minimize weight and heat conduction, but these properties make it susceptible to penetration of hot gases. The pores of the substrate are filled with phenolic to mitigate the rush of hot gases into the protective substrate; in addition phenolic decomposes when heated, generating gases that block incoming plasma and carry heat away from the spacecraft.
The process and background for building the roadmap towards physics-based (Type 3) models for the phenolic impregnated carbon ablator (PICA) is reviewed and summarized. We start with observations from MEDLI (MSL Entry Descent and Landing Instrument) data and SEM (Scanning Electron Microscope) images of core samples from the Stardust flight data, and build the case for a Type 3 model. The requirements for building a Type 3 model are then summarized with place holder models for each element required to build a Type 3 PICA model. Efforts in academia supported under NASA’s STMD Early Stage Innovations (ESI) funding are then summarized and placed within the requirements for a comprehensive model.
Efforts to build a Predictive Material Modeling framework from the micro-scale to the macro-scale will be presented. Developing new material architecture involves an extensive design and test cycle. Several material properties are key to a successful design of a spacecraft heatshield. These include conductivity, permeability, material recession rate, etc. To accelerate the design cycle process and reduce the need to extensive testing, NASA is developing modeling and simulation tools that enable characterizing material properties and response to hot plasma. We start with a realistic digital representation of a substrate generated either synthetically or from high-resolution tomography of a built material. The digitized representation is then used to compute fundamental properties of the material, and the response of the material to high-temperature plasma. In this context, the PuMA (Porous Microstructure Analysis) code has been developed for computing properties of porous materials from micro-CT (Computed micro-Tomography) images.
Several numerical methods and techniques will be summarized that use voxelized images to compute geometrical properties of the carbon based substrate of PICA, called FiberForm. These computed properties include porosity, specific surface area and tortuosity that are otherwise indirectly measured through experimental techniques. The micro-CT reconstructed volumes are used to build computational grids for numerical simulations of the ablation of fibers. By modeling the diffusion of oxygen through the porous medium using Lagrangian methods and the oxidation at the carbon fibers' surface using a reactivity model, the ablation of the carbon fibers is simulated for different regimes. It is shown that in the diffusion-limited regime, the ablation of the fibers occurs in the near surface region of the material. In the reaction-limited regime, the oxygen penetrates into the fibers, resulting in volumetric ablation and high material spallation. Direct simulation Monte Carlo (DSMC), a particle based method for approximating the Boltzmann equation, is used to compute the permeability of FiberForm based on its digitized representation. The method provides an accurate model for the entire boundary layer including the flow within the microstructure, where the size of the pores may approach the mean-free-path of the flow.
Finally, a Type 3 model implemented in PATO (Porous-material Analysis Toolbox based on OpenFOAM) is discussed, and some examples of ablative material response using the code are presented including for the first time 3D simulations of the full heat-shield for the Mars Science Laboratory capsule.
