If you think engineering design today is focused only on high-volume, short-cycle, penny-critical products, you're wrong. NASA's Phoenix Mars Lander, launched from Cape Canaveral Aug. 4, carries a platform of seven analytical lab instruments that represent the culmination of years of extraordinarily detailed thinking, planning, evaluation, redesign and production, all to exacting standards, for an environment that is hostile and unforgiving.
One of them--the Microscopy, Electrochemistry and Conductivity Analyzer (MECA)--is a complete wet-chemistry lab that will analyze samples of soil scooped from the Martian surface, based on analytical techniques that are standard on Earth but had to be extensively modified and refined.
The design effort was led by Sam Kounaves, a chemistry professor at Tufts University (Medford, Mass.), and mechanical-engineering graduate student Jason Kapit, who holds an undergrad degree in physics. The work involved closely coordinated and iterative efforts among the lead designers, vendors and contractors, with the Jet Propulsion Laboratory (JPL) as overall project manager.
While every engineering design involves constraints and trade-offs, the priorities for this project were very different from those that get mass-market attention.
The wet-chemistry mini lab has four single-use beakers, each the size of a small drinking cup and lined with selectively permeable membranes or gels and their electrodes. These will assess various factors about the soil, such as pH and concentration of dissolved oxygen, chlorides, bromides and other salts and minerals. The objective is disarmingly simple: have a robot arm scoop a few cc's of frozen soil samples into the beakers, deliver and mix in the necessary reagents, and do it all at the right reaction temperature.
Unforgiving Mars environment dogged designers of the Phoenix lander's MECA soil tester (scoop shown at left). |
Adding to the challenge is the Mars environment. In the northern zone where the craft will land, temperatures range from a high (relatively speaking) of –60°C and drop down to –140°C, in an atmosphere that is absolutely dry.
The project took about four years from conception to launch. Unlike a more-conventional product design cycle, this one had innumerable iterations. The final design, as launched, bears little resemblance to the original concept, noted Kounaves. Every change, down to the most apparently insignificant, had to be verified and tested.
"The law of unintended consequences, as well as the ripple effect, plays a very large role," said Kapit.
Adding to the pressure is the fact that the journey itself lasts between seven and 10 months, depending on the launch period, and there is an available launch only every 26 months because of limits on the capacity of the launch vehicles.
Simplicity in design and thoroughness of execution are essential. Unlike a design that must work reliably over thousands of cycles, this one is based on relatively simple science and has to work just once, or a few times at most. There is no need for long-term ruggedness or repeated reliability. While that might seem to relax the design challenge, it actually has a countereffect on the test and verification process.
Wet-chemistry laboratory (WCL)
on Phoenix Mars lander
One of the wet-chemistry laboratory's four identical testing cells, about 15 cm tall (each cell can test one sample of soil). The top half of the cell includes mechanisms for receiving the soil sample from the
lander's robotic arm, and for adding water and controlled amounts of salts to the sample. The lower half includes a teacup-size chamber to hold the sample and the water added to it. The walls of the chamber hold a
variety of sensors for measuring the amounts of the different salts dissolved in the water and for
assessing properties such as acidity and electrical conductivity.
The inclination would be to test the design repeatedly to verify its proper operation, but that would be contrary to the design objective. For example, one of the key actuators uses paraffin and a piston in a cylinder. When the paraffin is heated, it melts, expands and then pushes out the piston, thus opening a drawer. It's reliable, but only for a few heating/cooling cycles, so testing it again and again does not tell you if it really will work perfectly the first time. To get around the testing dilemma, you'd need to build hundreds of identical units to verify the reliability of the "first cycle" operation, but time, cost and other factors make that approach impractical.
Kounaves noted that the "electronics are pretty well nailed down," thanks to the years of experience that NASA and JPL bring to the project. The electronic components used in the circuitry date from the 1995 era, ancient by today's technology time line. That's because older components are well-understood, have a lot of test and performance data and history, and are space-qualified. The flip side is that some of them are no longer available, and have to be specially fabricated and qualified.
Despite the relative maturity of the electronics, in this case designed by JPL, unexpected problems occurred when they combined with motors, heating elements and tough environments. Basic factors that are assumed on Earth often prove to be very different on Mars.
MECA wet-chemistry lab sensor array cell construction
The WCL is housed in an anodized aluminum enclosure; an inner beaker of cast epoxy serves as reaction vessel and platform for the electrochemical sensors on the four walls.
Measuring pH is easy here, said Kounaves, but at low temperatures, whether in transit or in operation, glass electrodes shatter and polymers don't work. The very dry atmosphere of Mars means that the reagents stick to the sides of the crucibles in which they are delivered. Condensed moisture from the wet lab is a serious problem. Outgassing of lubricants from the stirring motor causes contamination. In short, he noted, "electromechanical people don't like fluids."
It's all about materials
Every material that's used must be studied for its performance and suitability at the low temperatures and dry conditions, of course. But compatibility among materials, which engineers intuitively assume or know in detail under more-normal conditions, here becomes a major unknown that requires evaluation. Finish, treatment and coatings play a critical role in how materials wear under load, stick, slip, fracture, flake and otherwise behave by themselves and with other materials.
Even the test environment poses unforeseen challenges. A simulation chamber can serve as a first step, but that's not enough. "Fielded instruments should be tested in the field," Kounaves said, since chambers cannot capture unexpected aspects of real-world situations. For the MECA units, tests were done in Antarctica (very dry and never above –20°C) and Death Valley (also extremely dry, and above 55°C).
Testing was done from top-down and bottom-up perspectives. Subsections and individual functions were tested as the instrument was built block by block. This was closely coupled with tests of the total system and interactions, to make sure the sum of the parts worked as anticipated. In addition to signal-level issues and tolerance-error buildup, the unit's self-assessment and protection modes could have interfered with each other, for example.
It takes a village
Eight final units were built, four for the mission's wet-chemistry lab and four for test and postlaunch evaluation and investigation. This is not a project where you can provide a bill of materials and schematic, prepare the pc board layout and mechanical-assembly drawings, and go find an assembly contractor. From the start, a lot of vital and time-consuming interplay took place among vendors and partners. The development team and partners also worked closely with sources of the constituent components, such as Orion Research Inc. (now Thermo Orion Inc.; Beverly, Mass.) for the sensors.
Translating the prototype design into production documentation required space-qualified partners (among them, StarsysResearch Corp.; Boulder, Colo.) as well as JPL. Even that process went through dozens of iterations be- cause of the numerous changes that were recommended, based on experience and design rules. Each recommended change had to be thoroughly assessed.
Project management and coordination constituted a substantial part of the work, requiring frequent conversations and communications, along with ex- tensive, continuously up- dated and scrupulously tracked documentation of every project detail.
JPL handled project management, with Lockheed Martin assuming responsibility for integrating the instrument with others on the lander.
No time for rest
The long development time for the project--and the long waiting period until the lander reaches Mars on May 25--has served as an incentive to start on the next project.
Kapit and others are working on a lab that will look for life on Mars. Using just minimum assumptions, the Microbial Detection Array will mix water and sterilized soil (the nutrient) with unsterilized Mars soil, and look for any signs of metabolic activity. As with MECA, simplicity of design, optimization for single-cycle or limited-cycle use, and materials selection are critical.
For example, the main chamber--about the size of a toilet-paper roll, with modest, 15-ml capacity--must operate at 120°C and 15 psi, and must balance weight, thermal-expansion and corrosion factors. Aluminum is lightweight but may corrode; stainless steel is too heavy.
After an extensive search, Kapit found a relatively new class of partially crystalline engineered thermoplastics known as Peek (polyether etherketone) that appears promising. Although Peek is relatively expensive, that's not a dominant concern here.
For operational reliability, the chamber's piston-position control is a closed loop using optical sensors. The temperature-control loop, however, is a basic on/off cycle. That approach is simpler than the more-precise, proportional control, but it is sufficient.