SPECIAL SECTION/ADVANCED CERAMICS: Case Study: Mission to Mars
“Imagine this scenario. The year is 2030 or thereabouts. After voyaging six months from Earth, you and several other astronauts are the first humans on Mars. You’re standing on an alien world, dusty red dirt beneath your feet, looking around at a bunch of mining equipment deposited by previous robotic landers. Echoing in your ears are the final words from mission control: ‘Your mission, should you care to accept it, is to return to Earth-if possible, using fuel and oxygen you mine from the sands of Mars. Good luck!’"1
This introduction to a NASA article sums up the challenge that faced Jim Lenihan of Gemco Valve, Middlesex, N.J, when he landed the contract to work on the space agency’s Mission to Mars project. NASA’s goal was to develop a way to extract oxygen from the oxide-rich minerals found in the Martian regolith, which is the silt-fine dust that covers the red planet. Scientists believe that the Martian regolith resulted from the impacts of massive meteorites and asteroids, combined with millennia of daily erosion from water and wind. But we’re getting ahead of ourselves.
"Short-Term" GoalsIn 2004, NASA shifted its emphasis from the idea of a direct mission to Mars to a more conservative approach. Instead of launching a spaceship from Earth on a direct route to the planet 35 million miles away, NASA would first establish a way station on the moon. The moon is a lot closer (only 239,000 miles away), and a lunar “colony” could be used for practice as a stand-in for Mars.
Astronauts could use the station on the moon to perform a dress rehearsal for many of the activities that would eventually be carried out on the red planet. Most of the relevant in situ resources of the moon and Mars are found in the regolith. Thus, NASA believes, the techniques for successfully extracting oxygen from lunar minerals could be replicated on the Martian surface.
Lunar dust is not like the dust found on coffee tables. It is diverse in size but averages 19 microns, and is jagged, high in porosity, electrically charged, and composed of nearly 60% silicon dioxide (SiO2) and aluminum oxide (Al2O3). Lunar dust has been described by one NASA official as being “like fragments of glass or coral, odd shapes that are very sharp and interlocking.” In fact, after just a short stroll on the lunar landscape, Apollo astronauts found that dust particles had jammed the shoulder joints of their spacesuits and penetrated into seals, causing the spacesuits to leak air pressure.
Valve DevelopmentA crucial part of the oxygen extraction process requires that the lunar soil be “sifted” through valves and, of course, valves are what Gemco Valve is all about. NASA presented Gemco with the challenge of developing a spherical disc valve that would handle the highly abrasive lunar ash without getting fouled in the process. In the valve developed by Gemco, the spherical disc rotates to the open position as the ash moves through the valve. That is, it swings completely out of the path of the ash, which flows freely because there are no obstacles to block its way.
To operate consistently, however, the ash needs to glide over a smooth surface that does not permit the submicron ash particles to become entrapped between the seal and the seat. Lenihan knew that his stainless steel valves would not withstand the punishment of a constant bombardment of abrasive lunar ash, so he called for the help of Mark Purington and Longevity Coatings, located in Allentown, Pa.
Purington has been in the thermal spray coatings business for more than 20 years. In his early days, he doubled as a business major at Penn State University and manager of his father’s nearby coating plant. The NASA valve, Lenihan knew, was in dire need of a high-performance protective coating. He was confident that Purington and his team had the proper credentials to develop the coating needed for this exotic application.
Crucial CoatingWorking closely with Lenihan, Purington and Longevity began work on the development of a coating formula. From the outset, tungsten carbide was the obvious choice as the major component of the coating because of its extraordinary properties. Known as a cermet (ceramic metal), tungsten carbide is vastly more dense than the average ceramic material and is composed of an approximately 1:1 ratio of tungsten and carbon atoms.
In recent years, tungsten carbide’s outstanding wear resistance and hardness (9.8 on the Mohs scale) have propelled it to the forefront for many industrial applications, surpassing traditional steel products in many instances. It is routinely the material of choice for wear parts; other machine parts; and dies that are subject to severe service conditions, such as extreme temperatures, corrosion and abrasion. In other words, it was a natural choice for the working materials (the abrasive ash) and the harsh atmospheres that characterize lunar and Martian service conditions.
In most of these applications, tungsten carbide is alloyed with < 6% of a softer metal-usually cobalt-for practical considerations like improving the malleability of the tungsten carbide. With the addition of a cobalt composite (along with necessary toughening agents), the Gemco 10 formulation was completed.
Application of the coating to the valves is performed by Longevity in a spray booth utilizing the flame spray process. In this process, billions of tiny tungsten carbide/cobalt particles are pre-heated before they are blasted onto the valve surfaces at a velocity of 8000 ft per second. “When each particle hits the substrate, it flattens like a bullet hitting a wall,” explains Purington. “Only then-after striking the substrate at high speed-do the particles melt.”
So crucial was the need to make the mating surfaces of the valve exact 3-D mirror opposites of each other that Longevity personnel took the process a step further. The surfaces of the stainless steel valve were hand-lapped with diamond paste to assure a smooth surface that would prevent submicron particles from being entrapped between the seal and the seat.
Tests on the coated valve involved the use of a "simulant,"
an abrasive powder specially formulated to simulate the
composition of the lunar regolith.
After some fine-tuning of the coating formula and application technique, a new set of coated valves was produced. These valves will be field-tested in Hawaii with volcanic ash, which is said to be a close terrestrial approximation of lunar ash.
Although the actual real-world use of the Gemco/Longevity-coated valves is not scheduled to take place for another 12-15 years, the pressure is still on for Lenihan and Purington. “NASA is in a big rush,” said Purington. “They’re pushing to get it done as quickly as humanly possible.”
For more information, contact:
- Longevity Coatings, 6047 Adams Lane, Allentown PA 18109; (610) 871-1427; fax (610) 871-1217; e-mail firstname.lastname@example.org; or visit www.longevitycoatings.com.
- Gemco Valve Co., 301 Smalley Ave., Middlesex NJ 08846; (732) 752-7900, ext. 305; fax (732) 424-2483; or visit www.gemcovalve.com.
SIDEBAR: Mining OxygenWhen the lunar base (described as the size of Washington’s National Mall) is completed sometime in the 2020s, it is NASA’s plan to use the facility as a way station for the later mission to Mars. During the intervening years, astronauts will learn to deal with the exotic and unforgiving lunar environment in preparation for the equally challenging Martian environment. A major objective of the lunar base is what NASA scientists call in situ resource utilization (ISRU), a lofty way of saying “make do with what you find there.”
The capability of extracting oxygen from Martian soil would make a manned Mars mission more economically feasible since it avoids the excessive cost of hauling weighty liquid oxygen tanks from the Earth’s surface and transporting them to Mars. In addition to providing breathable air to replenish liquid oxygen tanks used on the trip from the moon to Mars and for working astronauts on the red planet itself, the oxygen produced from material excavated on Mars could also be used to power spacecraft on the return trip to Earth.
“It’s beyond impractical for the astronauts to bring with them enough oxygen for a sustained stay,” says Purington. “And what applies to oxygen for breathing also applies to oxygen as a power source.”