Universe Today
Heat shields are undeniably one of the most important parts of any spacecraft that intends to enter an atmosphere. And the material they are made out of is absolutely critical. Improving them has been a bit of a challenge though, as typically researchers only get “before” and “after” pictures of when the heat shield completes its job of helping a spacecraft land safely back on Earth. That changed recently, though, with a collaboration between NASA and the Advanced Light Source (ALS) located at Lawrence Berkeley National Laboratory, which allowed researchers to watch a heat shield fall apart in real time.
Falling apart is also a major part of what a heat shield does. That process, technically known as “ablation,” allows the thermal energy of reentry to transfer into pieces of the material, which then fall away from the spacecraft itself, taking that thermal energy away with it. This has long been the standard for heat shields on everything from the Mercury capsules to the Orion ones used on Artemis.
There are some obvious disadvantages to the system, though. Ablation is irreversible, and material must typically be replaced after every use. It’s also exceptionally hard to simulate - for example, the recent Artemis I uncrewed test flight exhibited unexpected charring, and even some uneven loss of ablative material that wasn’t expected based on the multitude of simulations run before the mission itself. It’s exactly that kind of unexpected, uneven physics that could result in a loss of equipment, though luckily Artemis I did successfully splash down in the Pacific Ocean.
Fraser talks about how heat shields could help save reuseable rockets.To fix that simulation gap, scientists need to understand what is happening during the 1650℃ heating the materials experience during reentry, rather than simply look at the aftermath. That is where the ALS comes in.
Researchers at LBNL designed a specialized chamber that exposes material to up to 900℃, the temperature where heat shield material begins to break down. It also allowed them to change the pressure and atmospheric gas composition to mimic some features of the reentry process, though a major one - air velocity - appeared to be missing from the chamber’s capabilities.
The big feature of ALS though isn’t the specialized testing chamber - it’s the imaging system. While the heat shield material samples were placed inside the chamber, they were subjected to X-ray micro-computed tomography (micro-CT). In other words, the team was able to capture 3D structural images of the heat shield material as it was actively breaking down.
According to Vishnu Oruganti, formerly a PhD student at the University of Illinois at Urbana Champaign and now at NASA’s Johnson Space Center, and one of the lead researchers on the project, “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.” Those included two prominent ablative materials that showed very different degradation properties.
Engineers working on the Artemis II heat shield at the Kennedy Space Center. Credit - NASA
SLA-561V includes a type of ground cork as a natural filler, and the researchers noted that the cork particles in this heat shield vaporize rapidly upon heating, creating hollow voids in the material. On the other hand, SLA-220, a type of silicone-elastomer matrix material, degrades into a set of interconnected, branching micro-channels, allowing the hot gases to vent rapidly throughout the shield.
The differences in these processes have a fundamental impact on how a spacecraft performs during reentry, but getting to that understanding wasn’t easy. In fact, the researchers had to face a classic imaging question - is it better to be quick or accurate? If they decided to take highly detailed images of the material as it ablates, the images would come slowly enough that they might miss some changes in it. On the other hand, if they decided to take pictures quickly, they would lose some of the resolution that shows how the structure changes.
To address this problem, the researchers turned to AI - specifically they trained a Generative Adversarial Network (GAN) algorithm. This allowed them to “rapid-fire” intermediate resolution scans and then convert those slightly lower-resolution images into high resolution, fast 3D records of the material as it warped, charred, and hollowed out.
What the researchers didn’t do yet is update the simulations that engineers use to actually calculate how they expect the heat shields to behave. They also did not do any analysis of the heat shields used on commercial vehicles such as Dragon (which uses Phenolic-Impregnated Carbon Ablator - PICA-X) or Starship’s reusable ceramic tiles, both of which are proprietary SpaceX technology. But, given the usefulness of this imaging system, it might be just a matter of time before the company responsible for the vast majority of reentries today comes knocking at LBNL’s door.
Learn More:
LBNL - Scientists Get Real-Time Look Inside Spacecraft Heat Shields During Extreme Heat Conditions
UT - Spacecraft Heat Shields Could Violently "Burst" When Plunging Into Alien Atmospheres
UT - Scientists Develop Test Setup For New Way To Survive Reentry
UT - An Innovative Heat Shield That Doesn't Need to Be Replaced Between Missions
