New Research Shows Nitrogen-Rich Atmospheres Erode Heat Shields

Heat shields are designed to protect the surface and cargo of a spacecraft as it enters an atmosphere. Aerospace engineers in The Grainger College of Engineering at the University of Illinois Urbana-Champaign recently observed a violently destructive difference in how heat shields function in atmospheres like Earth that contain oxygen versus nitrogen-rich atmospheres such as Venus and Titan, one of Saturn’s moons.

The surface of the heat shield is designed to breathe—expelling chemically reacting gases to slough off, or ablate, protecting the vehicle. Some of that material is eroded, flies off in chunks and particles, and redeposits itself onto the surface.

“The deposits occlude the surface of the heat shield, clogging it and affecting the performance of the heat shield,” said Francesco Panerai, a professor in the Department of Aerospace Engineering.

A series of experiments to quantify the effects of friction erosion on ablation were conducted using the Plasmatron X wind tunnel of the Center for Hypersonics and Entry System Studies which mimics the atmospheric conditions encountered in hypersonic flight. Panerai’s group studied the Phenolic Impregnated Carbon Ablator. This is one of NASA’s prime heat shield technologies, used in most Mars exploration and selected for protecting Dragonfly’s entry into Titan, Saturn’s largest moon.

“What was very surprising about the study is that, when we changed the gas, the ablation phenomenon behaved in different ways. In a classical air environment where you have oxygen present, the ablation happens in a steady way. The flow around the spacecraft erodes the surface and particles get ejected as a constant stream.

“But, when the oxygen is removed, this phenomenon becomes unsteady. Intermittent bursts of particles are ejected and, at times, the process becomes violent. I’ve been around ablation research for over 15 years and I’ve never seen this. We were all really surprised when we first observed this behavior in the tunnel.”

Panerai said despite progress, there are still substantial gaps in understanding the ablation phenomenon, particularly how spallation behavior changes in oxygen-deficient environments.

“We used high-speed imaging and particle tracking to quantify particle ejection rates and estimate spallation mass loss to the surface. Post-test microscopy and spectroscopy characterized surface morphology and chemical composition.”

Panerai gave special credit to his Ph.D. student, Ben Ringel, who graduated and now works at Varda—one of the primary flight-testing providers for the Department of Defense.

“Ben went into a deep-dive analysis. He used the facilities at the Illinois Materials Research Lab, at the Beckman Institute, and deployed a unique array of microscopes, to identify that this unsteady phenomenon is due to some deposition of carbon in the material.”

Panerai added that this study holds important information about the effect of friction, but also about the effect of the build-up of internal pressure.

“When we started understanding the experimental observations from the Plasmatron X, I immediately thought about the Artemis 1 char loss. First, because ablative heat shields never cease to surprise us, even when we think we know everything about them. And second, because of the key role of material permeability. The Artemis 1 mission reminded us of this importance in a very dramatic way. The heat shield experienced an anomalous char loss which led to an investigation. It can affect the integrity of the vehicle.”

Panerai has closely followed NASA’s investigation work.

“The material in that case was different from what we have here, but the phenomenon is somewhat similar: the material is not able to breathe. NASA found that if the material is too dense, and so non-permeable, gases get trapped inside, and the pressure increases until you have bursts that lead to a large chunk of spallation.”

Panerai stressed that this information is important as NASA prepares to fly Dragonfly into an atmosphere primarily made of nitrogen with traces of methane. The conditions tested at Plasmatron X were far more extreme that NASA anticipates for Titan entry, but particulate generated by the erosion of the PICA heat shield might have a profound effect on onboard instrumentation designed to observe the flow field during entry and measure the characteristics of Titan’s atmosphere.

“Although, this work doesn’t directly influence heat shield design, it does have very profound implications on the physics of the material — on the way the material behaves at extreme temperatures. Understanding at what conditions this phenomenon becomes prominent in flight, can help us design better heat shields.”