Are there habitable planets, like Earth, orbiting their own sun-like stars far out in space? Five years ago, NASA launched the Kepler Space Telescope in hopes of finding out. Four exciting years of near-monthly transmissions followed, a stream of data and photos giving tantalizing views and clues.
Alas, having identified 931 exoplanets – including a few perhaps capable of being inhabited – Keplers extended mission had to be scrubbed last summer. The fault was not in the stars, but in bearings or housings in some of the vessels four gyroscope-like reaction wheels. At least three of the four fast-spinning, metal reaction wheels are needed to precisely point the $600 million spacecraft for reliable scientific data collection – and two of them no longer work.
One reaction wheel failed in July 2012, after exhibiting chaotic friction, and another began showing evidence of friction issues in early 2013. At that point, NASA tried switching off that wheel and allowing it to rest, in hopes that the lubricant inside would redistribute itself and resolve the friction. It didnt help. In August NASA gave up, and stopped trying to reactivate the wheel.
As Kepler lies inert, some 3,000 light-years beyond the reach of any repairman, it brings into focus the role played by tribology, the science of friction, wear and lubrication. Kepler has very few moving parts, yet these have failed. In fact, the short history of space travel is riddled with mechanical systems that proved unable to withstand the voyage.
Throughout space exploration, tribosystem failures have occurred with frustrating and even deadly results, points out Nikolai K. Myshkin, director of the V.A. Belyi Metal-Polimer Research Institute in Gomel, Belarus. Speaking in January to the 19th International Colloquium Tribology in Germany, Prof. Myshkin cited tragic examples such as the Soyuz I mission, which in April 1967 experienced a parachute bushing seizure. The ship crashed and caused the death of a Russian cosmonaut.
Another example: In 1991, the planetary explorer Galileo was commanded to unfurl its high-gain antenna, but the motor stalled far short of full antenna deployment. Researchers theorize that the cause was a misaligned taper bearing plus a high-friction condition. From tests, they later realized that Galileo probably had undergone classic fretting conditions while it was being transported and launched; rapid oscillatory movements and vibration had worn away the solid lubricant coating on the antennas stand-off pins and sockets, leaving them vulnerable to friction.
Nearly every country involved in space exploration – the U.S., U.K., Russia, Japan, India, China and others – has experienced a tribosystem failure in space, he went on to say. Japans space agency suffered a series of setbacks from 2000 to 2005 caused by failures of ball bearings in turbopumps, and the docking unit of the International Space Station has failed due to seizure in spherical bearings, he said. And the list is still continuing to grow.
Most space craft are loaded with tribosystems: gears, bearings, joints, locks and more, all operating in the most challenging environment. There are elevated temperatures, where even the simple rotation of the vehicle, moving from solar to shadow and back again, gives a 100 degree temperature change, Myshkin related. Zero-gravity conditions, bombardments of radiation and high-energy particles, and varying degrees of vacuum are experienced.
The challenges begin with the blast into space, during which mechanical systems undergo intense loads and vibration that can create fretting wear. Next may come long periods – years even – of standby waiting. Nevertheless, moving parts must activate instantly when called on. Solar arrays must uncoil, antennas must point, docking stations must engage, latches pivot and landing gear extend, all on demand.
Another challenge for these components, and for their lubricants, is the trend in space towards miniaturization as scientists strive to trim the size and weight of the cargo. Each kilogram of cargo sent into space costs a lot of money to put into orbit, Myshkin said. So he expects agencies to continue to emphasize the use of lightweight, wear-resistant materials and lubricants, including self-lubricating coatings.
Of course, researchers try to evaluate these materials before theyre sent into space, Myshkin said. Its easier to make a test on the ground, using accelerated tests simulating space conditions. Originally, in the 1960s, people thought it was sufficient to do a test under vacuum and radiation, but it turned out thats not the fact. And some of the factors, like weightlessness, are not easy to create, and even if you have a test, are not reproducible.
The common thread in these efforts is tribology, but friction and wear and lubrication may not interact in space as they do on Earth. If we really want to know whats going on, we have to take the questions into orbit, he asserted: Tribotesting in space is the answer.
Some progress, happily, is being seen. The first space tribometer – a device sent into space solely to gather data regarding wear – Myshkin traced to the Luna 22 mission, a 1974 effort by Russia that operated for a year and a half. The tester held pin-on-disc and bush-on-shaft friction pairs, and focused on a molybdenum disulfide solid lubricant material used in bushings.
The tests ran on the ground for 15 months, and then again in space. The results turned out to be a surprise, because the space environment turned out to be more soft, more mild than on the ground, Myshkin revealed. We believe the reason appears to be the atmosphere; that is, there appears to be a difference between having no atmosphere (as in space) and what you see in a vacuum chamber on the ground.
In the Luna 22 tester, the coefficient of friction was quite low, and the coating lasted longer in space than in the ground tests. Researchers have speculated that one beneficial factor might have been the absence of gravity, which meant that any debris from the solid lubricant was kept at the contact zone, thus improving lubrication. Overall, the ground test went 118 hours, while in space, it ran 128 hours without failure.
The next era of tribotesting in space dawned when the International Space Station launched. This was a real opportunity for accelerating the progress in space tribology, Myshkin said. He was delighted to see that on board from 2009 to 2011 was an eight-cell NASA pin-on-disc tester, unlike any seen in Earth-bound labs. Called MISSE-7, this miniature device used a 3.2 mm pin, and a load of 1 Newton, and moved at a low sliding velocity (13.2 mm/s). According to results presented at the 2013 World Tribology Congress in Turin, Italy, this low-orbit mission also experienced lower coefficients of friction than predicted by ground tests.
There seem to be two main reasons for better results in space, Myshkin surmised. First, there is no gravity. Second, the atmosphere is different than vacuum. It includes gases and particles. So tests done under vacuum on the Earths surface may be more severe than what actually occurs in space.
Other progress was also being made at the European Space Agency. ESA put into space a multi-cell tester called TriboLab, which operated aboard the European platform of the ISS in 2008 and 2009. One of its low-velocity, pin-on-disc experiments looked at the wear suffered by a solid lubricant material (MoS2 doped with tungsten carbide). In its first ISS trial, the coating lasted a million life cycles, versus 650,000 cycles that had been achieved on Earth. Some TriboLab experiments involved ball bearings and liquid lubricants, as well.
Although the body of knowledge continues to grow, Myshkin pointed out that past tests have had shortcomings. Usually, wear is not measured directly, but estimated by conductivity. The variety of test conditions is often limited, test duration is short, or tests are carried out at only low sliding speeds.
The biggest drawback though, to his mind, is the interminable waiting. The time it takes from tribology test design and preparation to actual launch into space is about 10 years, he declared with evident impatience.
The Belyi Metal-Polimer Research Institute is now working on the Tribocosmos Test, which will be housed in the Russian module of the ISS. Myshkin thinks it especially exciting that rather than being a passive passenger, this tester will have the active attention of the scientists aboard. The tester is a 15-kilo apparatus with eight test cells or units, using pin-on-disk and conformable block-on-ring configurations. Although each test cell is just 15 cm in size, the aim is to develop short-time test techniques that can accurately forecast a 15-year life for materials intended to be used in sliding bearings and gears.
The data generated will help create a database on prospective materials, coatings and lubricants for space applications, he reported.
One issue the Institute faced was to design and run tests that can be scaled up with validity. For example, on the ground, you can measure temperature at any point on the tester; when the entire test cell is 15 cm in size, that can be a brain-busting challenge.
Myshkin concluded by saying, The history of exploration has confirmed the importance of tribotests. Weve had three testers in space so far, and we hope for a real opportunity in the 2015-to-2016 timeframe to have another. The problem is not simply to launch the tribometer, but how to design the whole tribotesting module, assemble and test it on the ground, and put it into orbit. And then of course, to get the results and samples back to Earth, and to analyze and apply the lessons before the next module is launched. Once aboard the ISS, Tribocosmos is scheduled to see action for five years.
Meanwhile in deep space, Kepler slumbers in its point rest state, a stable configuration that uses minimal fuel to hold the craft steady. While its reaction wheels functioned, Keplers precision was equal to a human being able to hold his or her eye on a grain of salt from a quarter-mile away. But now, for want of a bearing – for want of tribology – Keplers full mission has ended.
As this issue goes to press, NASA has proposed a scheme to use Keplers two good reaction wheels and its thrusters to reorient the craft in a more limited mission, but its unlikely the space telescope will ever have such breathtaking scientific precision. Perhaps another mission will, thanks to greater use of space-based tribotesting, and novel lubrication techniques.