Key Takeaways
- Through a decade-long partnership between the ALS and NASA, researchers are studying how heat shield materials called superlight ablators degrade, informing NASA spacecraft design.
- Research at the ALS offered a new detailed picture of this hidden process, one that has meaningfully improved how engineers model and design heat shields for space missions such as the recent Artemis missions.
- By combining X-ray scans with AI-trained models, the team successfully captured the microscopic breakdown of these materials in real time.
When a spacecraft reenters Earth's atmosphere at hypersonic velocities, its protective heat shield faces the most extreme conditions, including temperatures beyond 3,000 degrees Fahrenheit. To survive, the shield relies on specialized materials that absorb the heat as it degrades away in a controlled process called ablation, sacrificing the outer layers of the heat shield to protect the vehicle and crew within.
Designing those materials requires understanding exactly how they degrade - but watching that process at the microscopic level, in enough detail to capture necessary information on the materials, has been a challenge, forcing engineers to rely on pre- and post-test observations to develop computational models.
At the Advanced Light Source (ALS) at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), researchers found a way to watch ablation in real time.
Using a sample environment on an X-ray tomography instrument that independently controls temperature, pressure, and gas mixture, recreating realistic, evolving reentry conditions. Findings from a recent study carried out in part at the ALS offer detailed, time-lapse, 3D images of this process, improving how engineers model and design thermal protection systems for space missions, including NASA's recent Artemis missions.
"Nearly every major NASA ablative heat shield material has been studied with this technique at the Advanced Light Source, including those relevant to the Artemis and Mars entry missions." – Vishnu Oruganti, user of the ALS.
"Directly observing how heat shield materials degrade during heating with this technique has been transformative for atmospheric entry research since it gives us unique insights and helps us visualize the internal structural changes that drive ablation as it occurs," said Vishnu Oruganti, who was a postdoctoral fellow at the University of Illinois Urbana-Champaign at the time of the study and is now a researcher at NASA's Johnson Space Center in Houston. "Nearly every major NASA ablative heat shield material has been studied with this technique at the Advanced Light Source, including those relevant to the Artemis and Mars entry missions."
Watching materials burn
As part of a long-term collaboration with the ALS, a team of researchers from the University of Illinois Urbana-Champaign and NASA used in situ X-ray micro-computed tomography, known as micro-CT, to peer inside a class of heat shield materials called superlight ablators, the same kind used on the backshells of NASA spacecraft. The team pushed the technique further than ever before by heating the samples to 1,652 degrees Fahrenheit, the top of temperature range in which the materials begin to decompose, and imaged them on the micrometer scale over multiple time points. The researchers studied two commercial ablators, SLA-220 and SLA-561V, which are used in different parts of spacecraft backshells and are made with different compositions. Through the micro-CT technique, researchers tracked how high-temperature heating similar to atmospheric reentry affects the real-time multiphase chemical decomposition and porosity.
These measurements provide data for developing and validating predictive models, reducing uncertainty in heat shield performance, improving mission planning and ultimately helping ensure the safety and reliability of future crewed exploration missions.
"We can perform 3D imaging of samples under different extreme conditions such as heat, cold, pressure, and tension, and we can watch the internal structure of materials evolve and give a deep look into internal structure in high detail as their properties change under these conditions," said Liz Clark, ALS scientist. "This collaboration has been a perfect application of this technique. After the first Artemis mission, where heat shields didn't perform as NASA expected from computational methods, they used the ALS to examine materials from these shields to better understand how the internal structure evolves over time."
Generative imaging
To observe the ablators evolving, the scientists had to balance an experimental trade-off between rapidly collecting images of large, representative areas at lower resolution and capturing structural details at high resolution. To capture large sample areas quickly while preserving fine microscopic details, the researchers deployed an AI-based super-resolution method driven by generative adversarial networks.
The high-intensity broad spectrum flux from the light source allowed the team to quickly snap large-volume, lower-resolution scans at short intervals to track fast-moving structural changes. Meanwhile, switching to a single-wavelength beam enabled them to capture crisp, high-resolution snapshots of the static samples before and after the heating process.
By training the AI on these before-and-after images, they were able to enhance the entire sequence of real-time images collected during the experiment, producing a complete high-resolution record of how each material changed throughout the heating process.
The super-resolved 3D illustrations of the materials revealed changes in features such as the matrix (grey), fibers (red), and cork (red) when heated to extreme, high temperatures. (Credit: Collin Foster/Advanced Light Source)
The AI-enhanced imaging revealed a striking difference between the two heat shield materials. SLA-561V contains cork - the same natural material used in wine bottle stoppers - as a structural filler. When heated, the cork chemically breaks down and disappears, leaving behind open, empty pockets distributed throughout the material. SLA-220 contains no such organic filler; instead, its rubber-like silicone matrix responds to heat by forming a dense, branching network of interconnected channels.
"This is a great example of how years of working together to customize the imaging technique, and integrating AI is helping us generate high-quality data and facilitate analysis, leading to more detailed scientific insights in a fraction of the time." – Liz Clark
These are not just visual differences. Open, isolated pockets behave differently from an interconnected channel network when heat and gas move through a material at reentry conditions, and those differences affect how each material performs as a heat shield.
"This is a great example of how years of working together to customize the imaging technique, and integrating AI is helping us generate high-quality data and facilitate analysis, leading to more detailed scientific insights in a fraction of the time," said Clark.
The findings give engineers something they have not had before: direct measurements of how heat shield materials change at a microscopic level during the thermal conditions of atmospheric reentry that bolster past observational studies, improving model development and allow missions to return safely home.
The research was supported by the NASA Space Technology Research Institute and NASA Space Technology Graduate Research Opportunities program. The Advanced Light Source is a DOE Office of Science user facility at Berkeley Lab.
