Department of Climate and Space Sciences and Engineering in the College of Engineering at the University of Michigan

News

How Professor Renno's class helped NASA prepare for Mars landing

Posted: July 23, 2012

 How Professor Renno's class helped NASA prepare for Mars landing

By Nicole Casal Moore

The unprecedented sky crane technique that NASA will use to land the Mars Science Laboratory (MSL) is designed to keep the craft's supersonic rocket jets as far from the planet’s surface as possible, minimizing the jets’ effects on the landing site.

But they could still erode small craters into the Martian ground and kick up dust and particles in the process. It is thanks, in large part, to a group of Michigan Engineering undergrads that NASA scientists understand this.

AOSS professor Nilton Renno taught a capstone design course in 2009 in which students performed the first tests to determine how much ground erosion the MSL’s landing process could cause, and how that could affect the rover.

Over two semesters, student teams built a quarter-scale model of an MSL descent stage engine and designed an experiment to simulate the unique sky crane maneuver that NASA developed to softly set its heaviest Mars rover down.

After the parachute and heat shield separate from the craft, the descent stage---a powered platform carrying the nested rover---will use four continuously firing thrusters to continue to slow itself down. Then it will lower the rover on 21-foot tether. When the rover’s wheels touch the surface, the craft will cut the tether and the platform will blast away to crash at a safe distance.

In early 2010, then-undergrad Manish Mehta went to NASA Ames Research Center in California to do the experiments in a vacuum chamber that simulates the low atmospheric pressure of Mars, only 1/100 of Earth’s. Ground walnut shells stood in for Martian soil in the low gravity acceleration of Mars.

After the parachute and heat shield separate from the craft, the descent stage---a powered platform carrying the nested rover---will use four continuously firing thrusters to continue to slow itself down. Then it will lower the rover on 21-foot tether. When the rover’s wheels touch the surface, the craft will cut the tethers and the platform will blast away to crash at a safe distance.

Mehta went to NASA Ames Research Center in California to do the experiments in a vacuum chamber that simulates the low atmospheric pressure of Mars, only 1/100 of Earth’s. Ground walnut shells stood in for Martian soil in the low gravity acceleration of Mars.

After dragging and emptying 50 bags of ground shellsground shells into their Mars “"sandbox”,," the team fired up their model and filmed what happened with a high-speed camera. They did 23 tests over three weeks exploring the effects of altitude, soil type, and thruster throttle level.

Leslie Hall, then an AOSS undergradthen an undergrad, processed the images to calculate the amount and velocity of the sand that was lifted.

“I’d go frame by frame and track certain particles to see how fast they were moving,” Hall said. “There wasn’t an ‘Aha!’ moment as much as an ‘Uh-oh’ moment when we realized that there was going to be more dust than originally thought.

The MSL's thrusters are slightly canted in an effort to minimize ground erosion. In the students’students' test, they found that the small craters the tilted jets excavated could deflect the plume of dust and particles towards the rover, rather than away from it, said Renno and Mehta, who details the process in his Ph.D. thesis.

The team also discovered that the amount of erosion depends on the slope of the ground at the landing site and how compacted the soil is.

The rover's special thermal paint might be nicked, but it won't be stripped, the results showed. And the ground erosion would fortunately be outside the wheel-base on the rover. The findings did raise some eyebrows about a few instrument components that could be vulnerable to sandblasting, despite that nearly everything is shielded, Mehta said.

“When you do these subscale experiments, it's not a 1-to-1 match. It's a worst case scenario, but the fundamental physics of what is going on can be explored with a wide range of parameters. So the results were both groundbreaking and very useful to our engineering assessment,” said Anita Sengupta, the senior systems engineer at NASA’s Jet Propulsion Laboratory who initiated these tests and worked with the Michigan team.

Perhaps the biggest impact of this work will be on the next Mars missions.

“This test was the first of its kind to explore this regime so it is of engineering and scientific importance that goes well beyond MSL,” Sengupta said. “We created a dataset that can also be used in future to validate granular flow models that would be needed for future human landings on Mars.”

Latest Headlines