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Sample Acquisition on Mars

NASA is developing its Mars mission, scheduled to launch late in 2011, to explore many of Mars' most intriguing regions for the first time. Once on the ground, the Mars Science Lab. will analyze dozens of samples scooped from the soil and cored from rocks as it explores with greater range than any previous Mars rover.

To make the most out of its trip, the rover's spectrum analyzer will rely on advanced motion control, precision bearing assemblies and a miniature turbomolecular pump as part of the process of collecting and analyzing material from the planet's surface and atmosphere. Specifically, custom bearings developed by The Timken Co. are used in the center hub of the system where the carousel is rotated to position sample cups. The application is a collaborative effort with Creare Engineering Research and Development.

The overall objective of this effort is to develop a wide range pump (WRP) to support the NASA/GSFC Sample Acquisition at Mars (SAM) instrument suite, which is part of the JPL Mars Science Lab. (MSL). The team is currently fabricating engineering model and flight model pumps, which will be delivered to NASA for integration with the instrument suite and eventual launch to Mars in December 2011.

The overarching science goal of the mission is to assess whether the landing area ever had or still has environmental conditions favorable to microbial life. Investigations planned to deliver this type of information include detecting and identifying any organic carbon compounds, making an inventory of the key building blocks of life, identifying features that may represent effects of biological processes, examining rocks and soils at and near the surface to interpret the processes that formed and modified them, assessing how Mars' atmosphere has changed over billions of years and determining current distribution and cycles of water and carbon dioxide, whether frozen, liquid or gaseous.

The suite of instruments (SAM) will analyze samples of material collected and delivered by the rover's arm. It includes a gas chromatograph, a mass spectrometer and a tunable laser spectrometer with combined capabilities to identify a wide range of organic (carbon-containing) compounds and determine the ratios of different isotopes of key elements. Isotope ratios are clues to understanding the history of Mars' atmosphere and water.

Designing for Mars

"In June 2009, Timken was approached to design and supply bearings to Creare, which manufactures miniature high vacuum pumps," says Eric Faust, application engineering group leader for Timken Aerospace, Defense and Positioning Control. "In this case, we have a history with Creare providing bearings, but this application was unique because the turbomolecular pump was going to Mars."

The vacuum pump system is primarily used during sample acquisition. The rover basically scoops up sample material and stores it in the chamber. The chamber is sealed and has to be evacuated, so gas spectroscopy can be performed on the samples to determine the composition and make-up of material collected from the surface of the planet.

The chamber itself is a carousel, and there is a suite of instruments and testing that can be done on the samples that the manipulator arm puts into the chamber. Evacuation of the chamber is achieved using an axial flow pump with a series of vanes which spins at high speeds of more than 200,000 rpm.

"The axial flow pump looks like a little jet engine with compressor blades and stators on it. It works the opposite of a compressor by pulling the atmosphere out of the chamber," says Wayne Denny Jr., chief engineer for Timken Aerospace, Defense and Positioning Control.

Faust says the challenge with the turbomolecular pump and overall system is its ability to operate in the Martian atmosphere. It's a high-speed application that requires grease with low out gassing. The speed and the nature of the pumps dictate very high precision. Timken worked with Creare on a couple different design concepts, supported them analytically, produced samples and did extensive testing.

"The engineering team reviewed the results from the testing and modified the designs," says Faust. "We have a production part that will be used on this particular mission that was optimized based on analytical and test results."

Out-of-this world Obstacles

The speed of the application is the biggest technical hurdle, since 200,000 rpm for a bearing of this size to reliably operate properly is a challenge. But the atmosphere is another issue because the vacuum level in the chamber goes to 10-7 Torr. Plus, the grease used in the application, a type of fluoropolymer grease which is less harmful to the environment but very challenging given the high application speeds required, added another obstacle in the vacuum environment.

Miniaturizing turbomolecular pumps is a challenge, in general, because the speeds must be very high to be a significant fraction of the mean molecular speed, which reduces bearing life and results in high power consumption and high stresses in the rotor.

"In this type of design, you have a vacuum and the pump is going from a Martian atmosphere of approximately 12 Torr of CO2 down to a vacuum below 10-7 Torr. There is a huge pressure differential across the bearings and pump," says Faust.

The problem is that you can have a low out gassing lubricant that operates very well for the vacuum, or choose a lubricant that works very well to achieve the speed requirement, but these goals are almost mutually exclusive. The customer selected a PTFE fluoropolymer grease or lubricant that was really designed to handle the vacuum, but is not necessarily a good choice from a speed standpoint. The challenge became optimizing the internal geometry of the bearing to operate reliably at that speed, using that particular lubricant.

"It's always interesting in these types of environments when you are looking at grease applications. You don't have the ability to have an oil flow system, so you have to do it with grease," says Denny.

Plus, he says that any time they look at space flights, there are always concerns about heavy vibration cycles as you go through launch conditions to get beyond our atmosphere, and the vibration cycles during deployment, as well. Whenever you have a precision application, the bearing has to be able to survive those vibrations and still function effectively as a precision device when it is deployed and used.

For space flight applications, there will always be extensive simulation, analysis and testing, especially with a new design. In this particular case, Timken used its comprehensive analysis tool called SYBER to study bearing performance at given speeds, loading conditions including shaft and housing fits, effects of thermal expansion and a complete suite of conditions such as bearing misalignment due to shaft and housing deflections.

"We can analyze the bearing design and how it performs under sets of operating conditions, including high speeds. The results tell us if the bearing can perform reliably and handle the loads. It's really what we consider the first layer of the design analysis," says Faust. "The next analysis comes down to a great deal of experience. Is the cage designed properly and how do other factors inside the bearing interact with the results? Some insight is gained from experience, and some from specific testing."

At the end of the design cycle, a prototype unit is developed that is optimized to the specific mission conditions. The bearing is run in the prototype, in this case by the customer, and then the pump and the bearings are taken apart, and the engineering team looks at each component and evaluates if any changes are needed to ensure acceptable performance.

In this situation, the two bearing prototypes passed the testing phase and achieved the number of hours and cycles that the customer specified would be required for a successful application or mission.

"That engineering review is also critical because it's an opportunity to look into the customer's assembly," says Denny. "The question is whether there is anything we need to change in the mounting, handling, preflight preparation or any type of vibration or loading conditions where we're seeing an amplification of one component coming back through the bearings that was unexpected. The review allows the designer to check those variables out all the way into the full assembly and make sure the system performs as required."

Experience Counts

Any time you're designing for a challenging environment, Faust says you really draw on your experience in other applications that you've done before, as well as listening to the numbers and what they say analytically. You do some calculations and simulations using the various speeds, loads and temperature ranges. But when you put the two together, you put your best foot forward and produce the best design.

During the application development, Timken and Creare tested two retainer designs; one manufactured from porous polyimide and the other from non-porous polyimide, both machined. The porous polyimide cage retains more lubricant versus a solid polymer cage, which is more structurally sound. Both passed the performance trials, but the more structurally sound cage has been selected.

"Space flight applications always provide insight into applications here on Earth," says Denny. "In the medical, semiconductor and robotic industries, we are seeing more and more applications where bearings are used at higher speeds, carrying more load, running longer and operating in high vacuum environments. Each time you take on an application like this and make a successful product, you are taking those pieces of knowledge into the next difficult application on Earth."

For more information, go to http://dn.hotims.com/27749-505 and http://dn.hotims.com/27749-506 Watch a video and animation that demonstrates how the rover will enter, descend and land on the surface of Mars at http://bit.ly/6g57dh.

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