The Mars Science Laboratory (MSL) spacecraft that landed the Curiosity rover on Mars endured the hottest, most turbulent atmospheric entry ever attempted in a mission to the Red Planet. The saucer-shaped MSL was protected by a thin, lightweight carbon fibre-based heat-shield material that was a bit denser than balsa wood. The same material, dubbed PICA (Phenolic Impregnated Carbon Ablator), also protected NASA's Stardust spacecraft as it returned to Earth after collecting comet and space dust samples in 2006.
It is based on a family of materials that was recognised by the space agency as its Invention of the Year in 2007. SpaceX, a NASA-contracted private company that delivers cargo to the International Space Station, has since adapted the PICA material for its Dragon space capsule.
While traditional heat shields form a rigid structure, NASA Ames Research Center (NASA ARC) in Moffett Field, California, which developed the PICA material, is now developing a new family of flexible heat-shield systems that uses a woven carbon-fibre substrate, or base material.
This material's heat-resistance and structural properties can be fine-tuned by adjusting the weaving techniques. In addition, the flexible nature of woven materials can accommodate the design of large-profile spacecraft capable of landing heavier payloads, including human crews.
The new flexible heat-shield system, called ADEPT (Adaptive Deployable Entry and Placement Technology), would be stowed inside the spacecraft and deployed like an umbrella prior to atmospheric entry. Supported by an array of sturdy metallic struts, the ADEPT system could also serve to steer the spacecraft during descent.
Unlike the reusable ceramic tiles used on NASA's space shuttles to survive reentry from low-Earth orbit, the lighter PICA and woven carbon materials are designed for single use, protecting their payload while they slowly burn up during the rigors of atmospheric entry.
That's why it's critical to ensure through testing, simulation, and analysis, that heat-shield materials can survive long enough to protect the spacecraft during high-speed entry into a planet's atmosphere.
To understand performance of the system at the microscopic scale, NASA research scientists are conducting X-ray experiments at Lawrence Berkeley National Laboratory (Berkeley Lab) to track a material's response to extreme temperatures and pressures.
"We've been working on studies of various heat-shield materials for the past three years," said Harold Barnard, a scientist at Berkeley Lab's Advanced Light Source (ALS), "and we are trying to develop high-speed X-ray imaging techniques so we can actually capture these heat-shield materials reacting and decomposing in real time."
During an actual atmospheric entry at Mars, there is substantial heat load on the shielding material, with surface temperatures approaching 4,000ºF.
"We are developing methods for recreating those sorts of conditions, and scanning materials in real time as they are experiencing these loads and transitions," Barnard said. A test platform now in development uses a pneumatic piston to stretch the material, in combination with heat and gas-flow controls, to simulate entry conditions.
Francesco Panerai, a scientist with AMA Inc. at NASA ARC and the lead experimentalist for NASA's X-ray studies at Berkeley Lab's ALS, said, "X-rays enable new simulations that we were not able to do before. Before we were using X-rays, we were limited to 2D images, and we were trying to mimic these with computer simulations. X-ray tomography enables us to digitise the real 3D microstructure of the material."
The work at the ALS has already produced some promising results that show how present-day and next-generation heat-shield materials, including woven carbon fibres for flexible heat shields, gradually decompose in simulated atmospheric entry conditions. More experiments are planned to study different weave arrangements and material types.
"You can really see a lot of details" in the ALS images, Panerai said, which were produced using a technique called microtomography. "You can see clusters of fibres, and you can see that the fibres are hollow." The fibres in the studies are less than one-tenth the width of a human hair.
He added, "It's very complex to reproduce velocity and temperature at the same time in physical experiments. The ALS allows us to reproduce similar conditions to entry - very high temperatures, and we can watch how flow moves inside materials." The work could benefit future missions to Mars, Saturn, and Venus, among others.
Joseph C. Ferguson, a researcher with Science and Technology Corp. at NASA ARC, led the development of a software tool called PuMA (Porous Materials Analysis) that can extract information about a material's properties from the ALS X-ray imaging data, including details about how porous a material is, how it conducts heat, and how it decomposes under entry conditions.
All of these characteristics factor into a material's ability to protect a spacecraft. NASA developers have made this software tool available to other experimenters at the ALS for other applications. Barnard said the NASA work has pushed ALS researchers to develop faster microtomography imaging methods that capture how materials respond over time to stress.
"We have been able to push the imaging speed down to between 2.5 to 3 seconds per scan," Barnard said. "Normally these scans can take up to 10 minutes. It has been good for us to work on this project. We want better instrumentation and analysis techniques for the experiments here, and we are pushing our instrumentation boundaries while helping NASA to develop heat shields."
Dula Parkinson, a research scientist who works with Barnard at the ALS on microtomography experiments, said the faster imaging speeds, which can produce about 2,000 frames per second, are generating a high volume of data.
"The capabilities of our detectors and other instruments over the past five years have improved orders of magnitude," Parkinson said. "It has increased our needs for data management and computing power."
The ALS draws on resources from Berkeley Lab's Center for Advanced Mathematics for Energy Research Applications (CAMERA), which assists with image processing and algorithms for visualising the X-ray data, and also from Berkeley Lab's National Energy Research Scientific Computing Center (NERSC).
NASA ARC researchers also tap into ALS-produced data via the Energy Sciences Network (ESnet), a high-bandwidth data network managed by Berkeley Lab that connects research facilities and supercomputer centers. NASA scientists use NASA ARC's Pleiades, one of the world's most powerful supercomputers, to perform analysis and simulations on data generated at the ALS.
"Supercomputing is very important in this effort," Panerai said. "One of the areas we will explore is to try to predict the properties of a virtual material" based on what is learned from X-ray experiments and computer modeling of existing materials. "We would like to know the material's response before we create it," Panerai said. "We think this could help in materials design."