Can you stand the heat?
It took months for the Curiosity rover to reach Mars. To survive the heat of entry, the rover needed a Bronco-powered shield.
Steven Boyd Saum
14 Oct 2012
It’s just after 10 p.m. Pacific Time on Aug. 5, 2012, and the moment of truth for Curiosity—or, as it’s better known, the Seven Minutes of Terror—is about to arrive. It’s been about eight months since NASA launched the Mars-bound craft—the biggest, most complex robot the agency has tried to land on another planet.
Curiosity enters the atmosphere at 13,200 mph and things start to heat up. The craft’s shape helps slow descent to Mach 2. But that means for two and a half minutes, friction brings the temperature on the heat shield to 2,100 degrees Celsius—past the melting point of titanium.
After catastrophic failure of the heat shield material, Beck asked, “How could we fly with this material?”
Back on Earth, millions are watching. Among the scores of engineers waiting anxiously at the Jet Propulsion Laboratory in Pasadena is Robin Beck ’77. She’s cognizant engineer for the Mars Science Laboratory thermal protection systems—the one in charge of making sure Curiosity doesn’t burn up during entry, and the one who’ll be answering questions if it does.
Tonight, every signal that pings back from Mars brings good news. But when Beck was brought onto the project in 2007, the heat shield news wasn’t good at all.
The original plan was for Curiosity to fly what’s flown on every craft NASA has sent to Mars since the 1970s: a honeycomb-structure filled with an ablator—a heat-resistant, glass-filled material—called SLA-561V. But the size and mass of the Curiosity posed challenges: The shield had to be nearly 15 feet across; and the craft was too heavy for drag alone to slow its descent significantly in the low-density atmosphere of Mars. Plus, flying in at an angle would create turbulent flow—resulting in high heating and shear forces pushing on the leeside surface.
Even so, the initial high-temperature arc jet tests on SLA-561V went fine. As in the past, when the ablator was subjected to these tests, a melt layer formed on the surface and remained in place. But when the material was tested in combined high temperature and high shear, the glassy material flowed along the surface, and gouging of the material resulted. That was a problem. At this point, Beck was brought in—with the understanding that these concerns needed to be addressed now.
The team pushed testing to further extremes, turning up the shear forces but keeping them within flight parameters. The (very surprising) result: catastrophic failure of the material—way beyond melt and flow. Basically, it disappeared from the honeycomb. “The filler was gone in three seconds,” Beck says.
Additional tests resulted in continued unpredictable catastrophic failure. Beck asked: “How could we fly with this material?”
The craft could fly with it if the flight plan were changed dramatically to include a much shallower angle of entry. But that wasn’t an option; a long, slow entry would compromise communications with the vehicle. So there they were, in October 2007—less than two years away from scheduled launch—without a shield. Which meant they had 18 months to develop, design, test, qualify, manufacture, and assemble something that worked.
“THERE ARE CRACKS.”
The heat shield team also had to work backward: Build to a weight limit of 200 kg, no more. Enter PICA (Phenolic Impregnated Carbon Ablator), developed at NASA Ames in the 1980s and used in a single piece on the Stardust craft that took samples from asteroid Annefrank in 2006. PICA was also being considered in a tiled format for the Orion craft that will carry humans into space. Benefiting from more than 100 tests already done on PICA by Orion engineers, Beck’s team began testing how PICA would perform when shaped into tiles fit onto the Curiosity aeroshell and sealed with gap-filler.
The results were good. Even better, given the lightweight nature of PICA, the team was able to build a shield 1.25 inches thick while keeping within the weight limit. That gave an extra 25 percent margin of thickness, they calculated. It also provided some ballast that Curiosity needed anyway.
With a month to spare in spring 2009, the shield was completed. Into storage it went—where it sat for longer than initially planned, because the launch date was moved from 2009 to 2011 (not because of the heat shield, though). Meanwhile, attention and funding priorities turned to other unfinished matters—including the avionics, the software that would tell the rover how to land and where to go.
Turn the calendar to May 2010. A piece of the PICA shell that underwent testing in 2008 is sitting out on an engineer’s desk at Lockheed’s Denver facility. Papers are piled up around it; perhaps it’s been knocked off the desk occasionally. A colleague walks by, admires the piece of shell—but then she sees something unexpected: “There are cracks in this material.”
Beck gets the news in California and feels a pit in her stomach. But other pieces of PICA shell that were subject have been carefully bagged and put in storage; under exam, these check out fine. The heat shield itself comes out from storage. An exam reveals micro-cracking on the surface to a depth of about .016 inches. But during arc jet testing, Beck’s team had looked at the effects of gouges three times deeper; they’d also experimented with intentional dings in the material, even cracking pieces of it apart and putting them back together. Beck is sure that, micro-cracks notwithstanding, the heat shield will do fine.
And so it is, on Aug. 5, 2012, when Curiosity begins its descent: two and a half minutes of roaring down through Mars’ atmosphere. The craft ejects six 25-kilo masses to rebalance itself—a move known as SUFR, for straighten up and fly right. A mortar fires and a 51-foot-diameter parachute deploys. It’s time to eject the heat shield; its job is done.
In about 30 seconds, Curiosity has slowed enough that it’s time to release the back shell and begin the final stage of descent: Retro-rockets fire and slow the craft further, a sky crane gently sets down the rover on the surface and flies away. Curiosity’s first photo is transmitted back from the surface of Mars.
At JPL, there are whoops and hugs. The moment is like Neil Armstrong setting foot on the Moon.
The summer of Apollo 11, a San Jose girl by the name of Robin Senigaglia was about to enter high school. Math and science were her strengths. Engineering wasn’t yet on her radar. That changed in college, after her first year at Santa Clara—and after a family friend helped her find a summer position as an engineering aide with GE Nuclear in San Jose. At the end of that summer, she changed her major to mechanical engineering.
Only one other woman was studying engineering in that class—Kristen Walsh ’77, who now also works in aerospace. But professors were very encouraging, as were the male students—for the most part. But there were enough bumps that for her senior yearbook profile, Robin Senigaglia mentioned (along with all she was grateful for) that she wasn’t sorry for “intruding” on the male-dominated field.
These days, through her position at NASA Ames, Robin Beck carries a banner for women in engineering: “Use your gifts and brains,” she says. It’s a familiar role. At Santa Clara, she literally carried the engineering banner when it was first created: for an academic procession as part of the 125th anniversary of the founding the University.
This August, getting Curiosity safely through the atmosphere of Mars wasn’t the only big item on Robin Beck’s calendar. Two weeks after the heat shield did its job, she and her husband, John Beck ’78, celebrated their daughter’s wedding.
Beck is currently working to make improvements on PICA. “It’s quite brittle,” she says. To build a big craft that can carry humans to Mars, a more flexible ablator is needed. And to be on track for a mission around 2035, “We have to be getting the materials ready now.”
STEVEN BOYD SAUM is the editor of Santa Clara Magazine.