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
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
"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
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
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
"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
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
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
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.