Entering a planet's atmosphere is a dangerous maneuver for any spacecraft, as it must withstand the intense frictional heat generated by high-speed contact with atoms and molecules.
That's why landers and rovers have heat shields. And new research from the Grainger College of Engineering at the University of Illinois Urbana-Champaign suggests that an atmosphere's composition has a big impact on how heat shields work.
A heat shield "breathes" when it hits the atmosphere. The surface of the shield begins to burn away, in a process called ablation. To mimic the hypersonic speed of a spacecraft's entry, a team of researchers led by Francesco Panerai, a professor in the Department of Aerospace Engineering, experimented with heat shields in the Plasmatron X wind tunnel at the Center for Hypersonics and Entry System Studies, which is housed a few blocks from the university's campus.
"What was very surprising about the study is that, when we changed the gas, the ablation phenomenon behaved in different ways," Panerai said in a March 12 statement. "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."
When material from the outer layer of the heat shield erodes, some of it can collect on the surface of the shield, potentially clogging up some areas and preventing the material below from "breathing." This can impact how well the shield performs. In the new study, the researchers found that changing the gases that the shield comes into contact with also changes its performance.
"When the oxygen is removed, this phenomenon becomes unsteady. Intermittent bursts of particles are ejected and, at times, the process becomes violent," Panerai said. "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."
Understanding how an atmosphere's composition affects heat shields is important, Panerai said, because NASA is getting Dragonfly, a robotic rotorcraft, ready for a 2028 launch toward the huge Saturn moon Titan. Titan has a thick atmosphere that's quite different from that of Earth: It's composed of about 95% nitrogen and 5% methane, whereas ours is 78% nitrogen and 21% oxygen.
Dragonfly will study Titan's surface, which could give scientists clues on whether or not the moon's hydrocarbon lakes and rivers hold molecules that are a precursor for life.
"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," Panerai says. "Understanding at what conditions this phenomenon becomes prominent in flight can help us design better heat shields."
The study was published Feb. 5 in the science journal Carbon.
