Only minutes before the Curiosity lander, weighing almost as much as a Mini Cooper, touched down on Mars in 2012, it was hurtling toward the surface at over 1,000 miles per hour.
And then, with a pop and a puff of smoke, a thin ribbon of nylon trailed out from behind the spacecraft and began to unfurl in the thin Martian atmosphere.
For the scientists at NASA and the Jet Propulsion Laboratory, the successful landing was the culmination of nearly a decade of work by hundreds, perhaps thousands, of people. Curiosity had traveled eight and a half months across 354 million miles of space on a journey that cost $2.5 billion to land safely on the surface of Mars.
And yet, in the final minutes of the journey, the team could only wait and hope that the parachute worked as expected and the rover descended safely. While the landing takes only about 7 minutes complete, it takes about 14 minutes for radio signals to travel from Mars to Earth. It’s an excruciating period of uncertainty known as the “7 minutes of terror.”
“For 2 of those minutes, the entire mission was riding on 100 pounds of nylon parachute and a few strands of Kevlar in its canopy,” says Charbel Farhat, a professor of aeronautics and astronautics at Stanford, speaking of the primary materials making up these supersonic parachutes.
Farhat did not work on the Curiosity project, but, in 2015, the team from the Jet Propulsion Laboratory working on Curiosity’s successor came to him with a challenge. JPL wanted to develop high-fidelity simulation tools to model supersonic parachute deployments in order to assist planning for future Mars missions. Those missions are likely to require larger payloads and therefore larger parachutes. These lightweight aerodynamic decelerators will need to deploy and perform at many times the speed of sound — several times greater than in any previous mission.
When they came to Farhat, the team from JPL had a problem: They had designed larger chutes in the past that they thought could handle the load. As part of the Low-Density Supersonic Decelerators (LDSD) Project, parachutes twice the size of Curiosity’s were designed and built to handle loads nearly three times higher than those seen by Curiosity. The chutes had performed according to plan in wind tunnel experiments. But things literally ripped apart during their real-world tests. In such tests, performed at China Lake in the California desert, the test chutes were tethered to rocket-propelled sleds and pulled down at the speeds that generate loads similar to those a parachute on a NASA landing craft would experience as it inflates in the thin atmosphere of Mars.
During two other LDSD tests the parachutes were deployed at over twice the speed of sound but failed to survive.
Slow-motion videos from the two supersonic flight tests show the chutes deploying and unfurling gracefully, but then, just as the canopies near full inflation, the fabric snaps inward and shreds in an instant. Not all the tested chutes failed, but enough to give the design team concern that their chutes were not foolproof. One design began failing well below the target threshold of 80,000 pounds.
“When they failed, they failed catastrophically,” Farhat says.
Unwilling to gamble with the Mars 2020 landing, JPL invested in performing several complicated Earth upper-atmosphere supersonic parachute deployment tests, known as ASPIRE. In tandem, JPL also wanted to invest in developing future capabilities to accurately model and predict the parachute performance.
The team asked Farhat to develop computer simulations to help them improve their chute designs. As an expert in computational fluid and structural dynamics and fluid-structure interaction, Farhat had simulated things like how turbulence roaring over an airplane’s wings causes sonic booms or how deep ocean pressure can sometimes crush submarines like tin cans. But, when the JPL-Stanford collaboration began in 2015, no one had yet simulated anything like the unfurling of a supersonic parachute, which is basically a set of nylon sheets stitched together and strengthened by Kevlar inserts, the same material used in bullet-proof vests.
Unlike airplane wings or submarine hulls, parachutes are highly flexible structures. Their lack of rigidity makes them difficult to model aerodynamically because the flow and the stress on the parachute change as it unfolds. As the simulations eventually showed, the fabric of the chute actually experiences more stress while unfolding than when fully deployed.
In retrospect, this key finding explains why the original JPL chutes succeeded in the wind tunnel tests but proved unreliable in real-world experiments. In the wind tunnel and rocket sled tests, NASA scientists had fully unfurled their parachutes in subsonic conditions and tried extrapolating those results to supersonic speeds.
During the real-world high-altitude tests, the parachutes had to unfurl just as they would on Mars with true, supersonic air ripping across the surface of the nylon, like a billion tiny whips. All it took was a few tiny tears in the nylon to begin unraveling the fabric for the chute to fail.
Farhat’s team arrived at this understanding by figuring out how to model the dynamic and chaotic forces acting upon a parachute as it unfolds. In simple terms, they created an imaginary grid of millions of interconnected points, known as grid nodes, representing the surface of the parachute. Computational algorithms then calculate the force of air pressure on each node in the grid as the imaginary fabric unfolds and, subsequently, how each node tugs on adjacent nodes until the chute is fully unfurled.
Such simulations rely on sophisticated algorithms to conceive the opening of a parachute as a series of small events — grid nodes meeting air resistance one after another — with supercomputers churning through innumerable calculations to sort through all the force calculations.
So far, the goal of the Stanford-JPL collaboration has been simply to develop new high-fidelity modeling tools for simulating the dynamics of supersonic parachute inflations. This new capability will provide a better understanding of how supersonic parachutes perform and help engineers optimize parachute designs.
Future Mars missions are likely to grow in size and will require larger parachutes that are able to deploy at increasingly higher speeds. These newly developed modeling capabilities will enable engineers to improve parachute design to ensure that future missions — some of them possibly manned — reach the surface of Mars safely.
For their part the Stanford researchers are excited that their computational insights will contribute to the hoped-for success of such important future missions.
“With this we really pushed the state-of-the-art of numerical modeling,” Farhat says.