NASA@SC19

Human Space Flight

Microscale Analysis of Spacecraft Heat Shields

Overview

Imagine entering Earth’s atmosphere after returning from the outer solar system. A heat shield less than two inches thick protects you from temperatures up to 2,900 degrees Celsius (5,252 degrees Fahrenheit)—the actual conditions experienced by NASA’s Stardust capsule during reentry in 2006. The capsule was protected by a heat shield composed of Phenolic Impregnated Carbon Ablator (PICA), a lightweight material invented by NASA to withstand these high temperatures.

PICA and most other ablative heat shield materials consist of multiscale composites with complex microstructures. Evaluating these materials is difficult, requiring precise knowledge of their properties. To support this effort, NASA scientists are developing research codes to compute material properties and simulate ablation at the microscale. Simulations run on the agency’s supercomputers are leading to a better understanding of current heat shields, which could in turn lead to improved materials that would allow spacecraft to survive higher temperatures. Utilizing these tools along with experiments, researchers are working to push the limits of spaceflight, allowing for greater flexibility in future missions.

Project Details

To analyze the materials in ablative heat shields at the microscale, the Predictive Material Modeling team in the NASA Advanced Supercomputing (NAS) Division at Ames Research Center has developed the Porous Microstructure Analysis (PuMA) software. This software is capable of importing sets of grayscale images representing a material’s microstructure to create a geometrically accurate computational domain.

Once the microstructure is imported, we use finite volume and particle-based methods to compute properties for the material, including thermal conductivity, porosity, and tortuosity. Ablation of the microstructure due to oxidation can also be simulated using a random walk method.

We also use Direct Simulation Monte Carlo (DSMC) methods to analyze heat shield microstructures. These high-fidelity, massively parallel simulations generate millions of molecules that interact with each other and the material microstructure. DSMC allows for more complex environments with many chemical reactions.

Results and Impact

All of these capabilities have been extensively tested and verified using numerous test cases obtained from literature or developed in house. Our findings show the computed material properties are accurate for many materials with known properties. The ablation simulations are found to be qualitatively accurate, however, a lack of experimental data has prevented true validation. Both PuMA and DSMC have been used to analyze materials in current heat shields, as well as those being considered for future missions.

Why HPC Matters

Our massively parallel DSMC simulations require around 2,000 processors and run in a few hours. The simulations are typically run using Intel Xeon Broadwell nodes on the Pleiades supercomputer at the NAS facility.

PuMA was originally developed using OpenMP for shared-memory systems. While a large multi-processor workstation is generally sufficient for computations, we have also performed parametric studies requiring many PuMA simulations to run in parallel. As with the DSMC simulations, these studies are generally run on Pleiades with each parallel simulation on a Broadwell node with 28 processors.

Work has also been done to add support for GPUs in PuMA using OpenACC. Testing is done on Electra Skylake nodes using NVIDIA Tesla V100 GPUs.

What's Next

NASA is currently investigating woven carbon fiber composites for use as heat shields, due in part to their customizability and desirable thermal properties. The added complexity of woven materials brings new challenges and opportunities for these codes going forward.

More Information

John M. Thornton, NASA Ames Research Center
john.m.thornton@nasa.gov
Nagi N. Mansour, NASA Ames Research Center
nagi.n.mansour@nasa.gov