NASA is developing its Mars mission, scheduled to launch latein 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 samplesscooped from the soil and cored from rocks as it explores with greater rangethan any previous Mars rover.

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

The overall objective of this effort is to develop a widerange 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 iscurrently fabricating engineering model and flight model pumps, which will bedelivered to NASA for integration with the instrument suite and eventual launchto Mars in December 2011.

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

The suite of instruments (SAM) will analyze samples ofmaterial collected and delivered by the rover's arm. It includes a gaschromatograph, a mass spectrometer and a tunable laser spectrometer withcombined 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 andwater.

Designing for Mars

"In June 2009, Timken was approached to design and supplybearings to Creare, which manufactures miniature high vacuum pumps," says EricFaust, application engineering group leader for Timken Aerospace, Defense andPositioning Control. "In this case, we have a history with Creare providingbearings, but this application was unique because the turbomolecular pump wasgoing to Mars."

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

The chamber itself is a carousel, and there is a suite ofinstruments and testing that can be done on the samples that the manipulatorarm puts into the chamber. Evacuation of the chamber is achieved using an axialflow pump with a series of vanes which spins at high speeds of more than200,000 rpm.

"The axial flow pump looks like a little jet engine withcompressor blades and stators on it. It works the opposite of a compressor bypulling the atmosphere out of the chamber," says Wayne Denny Jr., chiefengineer 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-speedapplication that requires grease with low out gassing. The speed and the natureof the pumps dictate very high precision. Timken worked with Creare on a coupledifferent design concepts, supported them analytically, produced samples anddid extensive testing.

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

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 achallenge. But the atmosphere is another issue because the vacuum level in thechamber goes to 10^-7 Torr. Plus, the grease used in the application,a type of fluoropolymer grease which is less harmful to the environment butvery challenging given the high application speeds required, added anotherobstacle in the vacuum environment.

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

"In this type of design, you have a vacuum and the pump isgoing 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," saysFaust.

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

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

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

For space flight applications, there willalways be extensive simulation, analysis and testing, especially with a newdesign. In this particular case, Timken used its comprehensive analysis toolcalled SYBER to study bearing performance at given speeds, loading conditionsincluding shaft and housing fits, effects of thermal expansion and a completesuite of conditions such as bearing misalignment due to shaft and housingdeflections.

"We can analyze the bearing design andhow it performs under sets of operating conditions, including high speeds. Theresults tell us if the bearing can perform reliably and handle the loads. It'sreally 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 cagedesigned properly and how do other factors inside the bearing interact with theresults? Some insight is gained from experience, and some from specifictesting."

At the end of the design cycle, aprototype unit is developed that is optimized to the specific missionconditions. 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 teamlooks at each component and evaluates if any changes are needed to ensureacceptable performance.

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

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

Experience Counts

Any time you're designing for achallenging environment, Faust says you really draw on your experience in otherapplications that you've done before, as well as listening to the numbers andwhat they say analytically. You do some calculations and simulations using thevarious speeds, loads and temperature ranges. But when you put the twotogether, 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 porouspolyimide and the other from non-porous polyimide, both machined. The porouspolyimide cage retains more lubricant versus a solid polymer cage, which ismore structurally sound. Both passed the performance trials, but the morestructurally sound cage has been selected.

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

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