Terminal Pressure—Designing a Parachute to Land the Perseverance Rover
By Connor Kelsay
Features
Tuesday, October 12, 2021
Photos courtesy of NASA/JPL-CALTECH/ASU.
Above: The specially designed parachute lowers the Perseverance rover safely through the Martian atmosphere.
Landing on Mars is hard. There is an atmosphere, but it is extremely thin—about 1% the thickness of Earth’s. In fact, the air density on the surface of Mars is equivalent to 122,000 feet above sea level on Earth. But that thin atmosphere is still capable of burning up a capsule on entry. Lunar landings are, relatively speaking, less of a challenge, due to the (almost) total lack of an atmosphere on the moon. That thin Mars air also complicates the design of a parachute—especially one meant to land a large object like a rover. So it was no small challenge for the teams at Jet Propulsion Laboratory and Airborne Systems North America to design a parachute that would safely deliver the Perseverance rover to the surface of Mars. The sequence of events that was required to stick that landing, which took place successfully this past February, was more complicated than any earthly skydiving stunt, and it took five years to dirt dive.
Designing for Space vs. Sport
When sport parachutes are designed, they are intended to maximize performance in areas such as opening, glide, flare and maneuverability. Those would be the performance goals for the designer of any military-style ram-air parachute. On a different planet, however, there are a different set of objectives. The disk-gap-band design of the Perseverance rover’s parachute was based on successful Martian missions dating all the way back to Viking in the 1970s, but the challenge for the JPL and Airborne Systems teams was to redesign the canopy to improve strength. At 2,260 pounds (on Earth), the rover would be the heaviest object ever landed on Mars.
The first goal of a Mars parachute is to simply survive the massive opening forces. Not only does the parachute want to explode from internal pressure, it also wants to shake itself apart due to the kinetic energy of opening so quickly. The second goal is to minimize weight. Sending things to space is absurdly expensive—sending them to Mars, even more so. On the Mars 2020 program, the maximum weight requirement of the parachute was 194 pounds. So, the key was using math and science to understand how strong each and every part of the parachute would need to be, and then designing that part of the parachute to be strong enough without being overbuilt. For example, making the skirt band of the parachute too weak is obviously a problem, but making it too strong makes it too heavy, which is also a problem.
The team also made great use of custom specification materials by working with cord and cloth vendors. A sport jumper who thinks about these things may be peeved when their slider stow cords are made of 1,500 pound Spectra when they experience only 10 pounds of tension, an example of overbuilding. But with custom material making up 82% of Perseverance’s parachute, parts were designed to be exactly as strong as needed—no more, no less. To make these predictions, Airborne Systems used advanced modeling software, simulating parachute openings and predicting the stress in each portion of the parachute. The team specifically designed each joint and seam using plenty of math, prior experience and testing. They built every component of the canopy separately and then broke them in order to determine the actual parachute strength. When the team was finally able to assemble each of those components, they had their first chute. And that was when the real fun—full-scale testing—began.
The canopy undergoes testing in the NASA Ames Wind Tunnel.
Testing a Mars Parachute
The full-scale parachute system tests were broken down into three categories: mortar fires, wind tunnel testing and sounding rocket testing.
Imagine deploying your reserve, only to have your freebag come back and hit you in the head at 400 miles per hour. Re-contact of debris is one of the bizarre considerations that normal, non-supersonic parachute systems don’t have to account for. Those parachutes, however, aren’t shot out of a mortar. One of the trickiest parts of the Mars parachute to get right was the bridle assembly and sabot capture net, which attaches the mortar tube and spacecraft to the riser, suspension lines and canopy. After the parachute is fired from the mortar, the piston-like sabot follows it out. But that sabot needs to be captured before it catches back up to the parachute and punches a sabot-size hole in the canopy after deployment.
That sabot capture net also needs to balance strength with stretchiness. Materials like Kevlar and Spectra are incredibly strong but barely stretch at all. Nylon, on the other hand, is weaker but stretches a lot. Capturing the sabot with Kevlar alone sends shockwaves back into the rover and confuses the onboard computer, so the team dedicated plenty of work to figuring out how to catch a cannonball like it was an egg. The testing to get this right included explosions, fireballs and high-speed dynamics.
A supersonic rocket touches down in the ocean after testing the parachute in Earth’s upper atmosphere.The most fun tests for the team, perhaps, were the deployment tests in NASA Ames in Mountain View, California, the largest wind tunnel in the world at 120 feet wide and 80 feet tall. The goal of that testing was to overload the parachute to see if it could survive the opening forces as predicted. Although the speed of the air was subsonic, the air was thick at near sea level, so the forces through the suspension lines, riser and bridle were actually much higher than what would be experienced on Mars.
The last set of tests were the ASPIRE supersonic rocket tests. For these, the team tried to reproduce the deployment environment on Mars here on Earth. They achieved that by sending a rocket to the edge of space, where the air is as thin as on Mars. By timing the parachute deployment with attention paid to both speed of descent and air density, they were able to replicate Martian conditions as accurately as possible. The video data from this testing phase was extremely beneficial to the team, and showed that they were ready for the next step—landing on Mars.
The Martian Landing
On February 18, 2020, the capsule containing the Perseverance rover entered the Martian atmosphere from space at around 12,500 mph. The entry, descent, and landing sequence were captured with on-board video for the first time in history and are available for viewing online. As the spacecraft heat shield hit gradually thicker atmosphere, it slowed down, but reached temperatures of roughly 2,370 degrees Fahrenheit. The drag on the capsule eventually brought the spacecraft to around 1,000 miles per hour prior to parachute deployment.
Next, the parachute deployed! Traveling at nearly twice the speed of sound, the parachute and deployment bag were shot out of the mortar with no slider or any other kind of reefing. If you think you’ve had a hard opening before, this parachute is 70.50 feet in diameter and deploys in less than half a second.
Eventually, the parachute was cut away, and rockets on a sled attached to the rover took over. The “retro rockets” were directed downward, and their guidance system actively looked for obstacles while steering the rover to a suitable landing location. Finally, the rocket sled hovered just above the ground, and lowered the rover slowly to the Martian surface. After this, the sled’s job was done, and it flew away to crash a safe distance from the rover.
Those of us familiar with the skydiving industry often refer to canopy design as an art, and it absolutely is. However, if we truly want to improve performance while understanding how parachutes work, science and engineering are key. Parachutes themselves are a beautiful combination of science, art, athletics (in the case of sport and military jumping) and fun! Jet Propulsion Laboratory’s motto—encoded into the design of the parachute in binary—is “Dare Mighty Things.” That applies to sport skydiving just as much as it does space exploration.
Connor Kelsay, D-34404, is a Project Engineer at Airborne Systems North America, the world’s largest company dedicated to the design and manufacture of decelerator systems. He designs and tests space and military parachutes, inflatable wings, unmanned aerial recovery chutes and more. He has a bachelor’s degree in mechanical engineering from Oregon State University and is currently pursuing a master’s degree in aeronautics and astronautics engineering from Purdue.
Kelsay has been a devoted skydiver since making his first jump at Kapowsin Air Sports in Shelton, Washington, while in high school. He has approximately 1,600 jumps, along with TI and AFFI experience. He is also an avid speed-flyer with approximately 1,000 flights. He lives in Southern California with his wife, Bonnie, who also skydives and designs parachutes.
Kelsay considers finally watching the parachute he helped design safely land the Perseverance rover on Mars to be one of the greatest experiences of his life.