Protecting space crew from extreme heat during reentry

When a spacecraft reenters Earth’s atmosphere, atmospheric friction heats its surface to very high temperatures. At its peak the surface is almost as hot as the surface of the sun. To protect the crew a barrier called a heat shield is attached to the leading edge of the spacecraft. Recently,  researchers at the University of Illinois created a model to determine the optimal thickness and composition of the shield material needed.

“The material we used for the model was chosen by NASA for the Orion space capsule and was first used in the 1960’s in the Apollo missions. It’s a syntactic, silica-phenolic foam called AVCOAT. On the microstructural level, it’s comprised of phenolic micro-balloons, like small hollow plastic balls, that are mixed with glass fibers. They are fused together with epoxy glue into a solid form,” said lead investigator Abhilash Harpale, who conducted the research as a part of his Ph.D. in the Department of Aerospace Engineering.

Harpale said the material is designed to burn away and dissipate the heat. “Because of this, we need to know the exact rate at which it burns to determine the optimum thickness of the heat shield required.”

Harpale’s advisor and co-author Huck Beng Chew described the material as porous, with volume and free space. “And where there is free space, there is less heat conduction. When it burns, it gives off a gas that pushes out the hot air that’s coming in and insulates the spacecraft as it goes into the atmosphere.”

Chew said, the problem is that the more material you use, the greater the weight of the spacecraft. That’s why it’s important to find the optimum thickness.

“We know the rate at which the shield burns off from wind tunnel experiments and test data of the Orion spacecraft when it was launched into Earth’s orbit in 2014,” Harpale said. “Using this information as a starting point, we calibrated a numerical model to determine the heat shield material response for different flight profiles. We used multi-scale computations to find the rate at which AVCOAT burns. We know the heat flux on the surface for the reentry, so now we can figure out the temperature profile in the heatshield during reentry and ultimately the thickness required.”

Chew added, “In other words, we created an atomistic model to establish the chemistry of how the phenolic resin decomposes—at what temperature they break up and become gases—essentially the temperature at which it begins to burn. We used that information at the atomic-scale to create an engineering model capable of predicting the response at the structure level.”

One aspect Harpale said he didn’t expect was learning that the actual recipe for AVCOAT mattered. When he began the research, he was just interested in understanding the atomistic response of the material at temperatures ranging from 440 to almost 4,000 degrees Fahrenheit in the spacecraft’s trajectory. When attempting to recreate a data set from the 1960s, he realized that the material response is highly sensitive to the actual microstructure of AVCOAT.

“When studying the heat response of the AVCOAT composite, we also needed to consider the size and geometry of the micro-balloons, as well as the proportion of glass fibers, to determine the best recipe to achieve a specific rate of burn on reentry,” Harpale said.

According to Chew, the final product of this research is a material response model at the continuum or structural length-scale. “NASA has developed several such models in the past, but this is the first model, constructed from first principles, that accounts for the complex microstructure of the heat-shield material. In this particular model, the proportion of micro-balloons, glass fibers, and epoxy, can all be varied to determine the best heatshield response. This was not possible before. In the past, they have just mixed things together, tested it, and it worked. Now we can go a step further to design and optimize the microstructure of the material to achieve a specific spaceflight mission goal.”

Finally, co-investigator Deborah Levin noted that in between the atomistic and continuum length scales, the transport of the boundary layer and the gases created from the decomposition of the shield must be modeled using a kinetic approach. To address that, a new state-of-the-art direct simulation code, making it possible to use on both GPUs and CPUs, is currently being developed by Saurabh Sawant, an aerospace Ph.D. student and co-author of this paper.

The paper, “Ablative thermal protection systems: Pyrolysis modeling by scale-bridging molecular dynamics,” was authored by Abhilash Harpale, Saurabh Sawant, Rakesh Kumar, Deborah Levin, and Huck Beng Chew. It is published in Carbon.

This research was funded by NASA.