For engineers who thrive on challenge, Patrick Jordan and his team have a dream job. Their assignment: Designing and building a soda can-sized oven that can heat to a scorching 1100 ° C (2000 ° F) using only 25 watts of power.
The “pyrolysis oven” is just one of several challenging technologies now being developed for the Sample Analysis at Mars (SAM), one of 10 instruments flying on NASA’s next-generation Mars Science Laboratory (see related story). When SAM begins operations in 2010, the Goddard-developed chemical processing lab will analyze gases in the atmosphere and those that are produced when the oven heats soil and rock samples to temperatures that would melt metal. The detailed analyses of these gases are expected to answer the question that has so far eluded scientists: Did microbial life ever exist on Mars?
Since winning a spot on the Mars rover in late 2004, the Goddard team has refined and tweaked SAM’s design to accommodate science requirements. The team passed the Preliminary Design Review in March and is now preparing for the Critical Design Review in December. To assure the instrument’s delivery for a 2009 launch, the team is expected to begin building the engineering test unit this year and the flight model in 2007. Work is progressing with those milestones in mind.
“This is one of the most complicated experiments that’s ever landed on the surface of another planet,” said SAM Principal Investigator Paul Mahaffy, whose team includes Goddard and other NASA center employees as well as industry, university, and international partners.
The instrument’s complexity is in large part due to how it’s expected to carry out its job. Before SAM’s three instruments — a gas chromatograph, a quadrupole mass spectrometer, and a tunable laser spectrometer — can identify a wide range of organic compounds and determine the ratios of different isotopes, it must do a little preparatory work, particularly on the soil and rock samples.
The general idea is that the rover’s robotic arm would scoop up the soil and rock samples and a separate mechanism would grind and deliver the samples to SAM’s “sample manipulation system.” Built by Honeybee Robotics, the subsystem is a carousel-like device that contains two concentric rings holding 74 tiny tubes that each are about an inch high and a quarter-inch wide. Once the tubes were filled with the finely ground samples, the carousel would rotate and insert the tube inside the oven. Ten of the cups would contain extraction solvents and derivatization agents to bring about chemical reactions in otherwise difficult-to-analyze polar compounds, such as organic acids.
Getting a More Complete Picture
As the oven heated slowly — 90 degrees every minute — the hermetically sealed sample would begin to break down, releasing gases that SAM’s instruments could then analyze. These gases would then be carried by inert helium through stainless-steel plumbing that connects the oven to the instruments. By heating the sample slowly, Mahaffy and other scientists can get a more complete picture of the sample itself. Minerals release specific molecules at characteristic temperatures. As a result, the pattern of gases released over time as the oven heats would tell them what minerals the sample contained.
“But it’s tricky,” said SAM Thermal Lead Rob Chalmers. “You have to electronically adjust the power that gets into the oven so that it doesn’t get too hot, too quickly.” And if the gases get too cold as they travel from the oven to the instruments, they’ll condense and the instruments won’t be able to measure them, Chalmers added. As a result, “all the plumbing — about 100 linear inches of stainless steel tubing — needs to be heated to nearly 400 ° Fahrenheit. And, because of the high temperatures involved, it’s been a research project finding which heaters we could use,” he added.
But those are just a few of the challenges that Jordan and his team have encountered. Since beginning the project more than 3years ago, Jordan, a mechanical engineer who designed the oven and sample cups with a team of engineers at Goddard and the National Institute of Standards and Technology, said he’s evaluated materials that can handle the oven’s high-temperature and cleanliness requirements. He’s also identified materials that he can use for both the oven’s thimble-sized heating element that’s situated outside the oven chamber and the oven chamber itself. He’s now in the process of building an engineering unit. .But he still needs to tackle what he considers the largest challenge of all: Successfully heating the oven to 1100 ° C with a power budget of only 25 watts. Detailed thermal analysis performed by Thermal Engineer Janelle Vorreter shows that the design should use only 22 watts. In laboratory tests, he’s come within striking distance. He used 35 watts of power, and believes he can whittle down the remaining 10 watts through enhanced thermal insulation and shielding. “This is a thermal and material design challenge,” Jordan conceded, referring to the oven’s relative complexity. “If you’re working on this project, it has all the challenges you could possibly want.”
Mechanical engineer Pat Jordan heats a prototype of his pyrolysis oven. To the right is another design that will be used on the actual instrument. In the forefront is one of the tiny cups that will deliver Martian dirt and rock samples to the oven.
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