A real-time look inside spacecraft heat shields during extreme heat conditions

When a spacecraft re-enters Earth's atmosphere at hypersonic speeds, its heat shield must endure extreme conditions, including temperatures surpassing 3,000 degrees Fahrenheit (1,650 degrees Celsius). To protect the vehicle and crew, the shield relies on specialized materials that undergo controlled degradation through a process called ablation, sacrificing the outer layers to preserve the inner components.

Designing these materials necessitates understanding their degradation at a microscopic level, yet capturing this process in sufficient detail has been challenging for engineers. They had to rely on observations made prior to and after testing to develop computational models. However, at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory, researchers have discovered a method to observe ablation in real-time.

Utilizing a sample environment on an X-ray tomography instrument, researchers can independently control temperature, pressure, and gas mixture to replicate realistic, evolving reentry conditions. A recent study conducted in part at the ALS has yielded time-lapse 3D images of this process, enhancing the accuracy of engineers' models and designs for thermal protection systems, including those for NASA's Artemis missions.

The study, published in the journal npj Materials Degradation, provides in-depth insights into how these materials degrade during heating, offering unique perspectives on the internal structural changes that occur during ablation. Researchers from the University of Illinois Urbana-Champaign and NASA used in situ X-ray micro-computed tomography (micro-CT) to examine superlight ablators, similar to those used in NASA spacecraft, heating the samples to 1,652 degrees Fahrenheit (900 degrees Celsius) and capturing images at the micrometer scale.

By studying commercial ablators SLA-220 and SLA-561V, the team tracked how high-temperature heating affects the real-time multiphase chemical decomposition and porosity of these materials. This data aids in developing and validating predictive models, reducing uncertainty in heat shield performance, and improving mission planning, ultimately contributing to the safety and reliability of future crewed exploration missions.

The researchers emphasized the importance of balancing the need for rapid image capture of large sample areas with the capture of fine microscopic details. They achieved this by employing an AI-based super-resolution method driven by generative adversarial networks, allowing them to quickly capture large-volume, lower-resolution scans while preserving high-resolution snapshots of static samples before and after heating.

The AI-enhanced imaging revealed significant differences between the two heat shield materials. SLA-561V contains cork as a structural filler, which chemically breaks down and disappears upon heating, leaving behind empty pockets. SLA-220, on the other hand, contains no organic filler; instead, its silicone matrix forms a dense, branching network of interconnected channels upon heating.

These differences in material structure impact how each material performs as a heat shield under reentry conditions.