X-Ray Movies Show Heat Shield Materials Breaking Down Inside as They Face Reentry Heat
Scientists at Berkeley Lab heated two NASA-type ablators to 900 degrees Celsius and imaged them in 3D as they decomposed, finding one riddled with pockets and the other laced with channels.
A spacecraft coming home hits the atmosphere so fast that its heat shield faces temperatures above 3,000 degrees Fahrenheit. The shield survives by burning away on purpose. Its outer layers char and shed in a controlled process called ablation, carrying heat away from the crew inside. Engineers have long had to design these materials without seeing exactly how they fall apart, relying on samples inspected before and after testing.
Researchers working at the Advanced Light Source at the Department of Energy's Lawrence Berkeley National Laboratory have now watched that breakdown happen inside the material, in three dimensions, as it heats up. Their results, published in the journal npj Materials Degradation, give NASA engineers direct measurements to feed into the computer models used to design thermal protection for missions like Artemis.
The team, from the University of Illinois Urbana-Champaign and NASA, used X-ray micro-computed tomography, a kind of CT scan for materials, with a sample chamber that controls temperature, pressure and gas mixture to mimic reentry. They studied two commercial "superlight ablators," SLA-220 and SLA-561V, which are used on the backshells of NASA spacecraft. They heated the samples to 1,652 degrees Fahrenheit, or 900 degrees Celsius, the top of the range where these materials begin to decompose, and imaged them at the micrometer scale at several points in time.
The two materials broke down in very different ways. The cork inside SLA-561V decomposed into isolated empty pockets, while SLA-220 developed a web of interconnected channels. That difference matters, the researchers said, because hot gas and heat travel very differently through sealed pockets than through a connected network, which changes how well a shield protects the vehicle.
Getting the images required a trick. Fast scans that capture large areas are blurry, while sharp scans are slow. The team took quick, lower-resolution scans during heating and crisp, high-resolution scans before and after, then trained an AI super-resolution model on the sharp images to sharpen the entire time-lapse.
"Directly observing how heat shield materials degrade during heating with this technique has been transformative for atmospheric entry research," said Vishnu Oruganti, who did the work as a postdoctoral fellow at Illinois and is now a researcher at NASA's Johnson Space Center. He said nearly every major NASA ablative heat shield material has been studied at the facility, including those for Artemis and Mars entry.
The work has a practical edge. After the uncrewed Artemis I flight, Orion's heat shield did not perform as NASA's computer models had predicted, and pieces of char broke away. "They used the ALS to examine materials from these shields to better understand how the internal structure evolves over time," said ALS scientist Liz Clark. She said the imaging now produces detailed data "in a fraction of the time" it once took.
