“Greases are absolutely the space lubricant of choice,” Christopher DellaCorte, senior technologist of tribology at NASA, asserted during the keynote presentation of the National Lubricating Grease Institute’s virtual technical sessions on Aug. 26.
But while greases are preferred for many space applications, solid lubricants, oils, process fluids and other types of lubricants have their place in space as well. For example, DellaCorte explained in his presentation that fluorocarbon oils are compatible with the vacuum environment in space and perform well at low temperatures, while solid lubricants such as polytetrafluoroethylene create good soft films. Molybdenum disulfide is also a viable solid lubricant option, he said, as it is vacuum-compatible. Thin metal films of silver, gold or platinum can work as solid lubricants in space applications, too.
Solid lubricants have limited lifespans, though, and often cannot be replenished, he explained, while process fluids require a closed system to operate properly and are usually poor lubricants. Similarly, traditional oils are difficult to apply in space, evaporate in a vacuum environment and often become too thick or freeze completely.
“Grease combines the best attributes of the other [types of] lubricants,” DellaCorte explained. “It lasts a long time, and because it’s a solid, it stays put where you want it. We can tailor it for specific attributes to meet the requirements of the application.”
Which greases are best for space applications? In the 1950s, silicone greases – siloxanes – were commonly used but were not the best option in terms of preventing friction and wear, he said.
Then in the 1970s, fluorocarbon greases made their way into space applications. “Fluorocarbon greases are an oddball kind of material because they contain a fluorocarbon base oil, which we thicken up with Teflon particles, and what we’ve found over the years is it’s really difficult to find additive chemistry that’s compatible with the fluorocarbons,” DellaCorte explained. “So extreme pressure additives and antiwear additives are not really an option. They tend to not be the best greases in terms of boundary lubrication and fighting wear, but they sure are good at low temperatures and for vacuum stability.”
Then in the 2000s, the space community found that greases formulated with multiply-alkylated cyclopentane base stocks were even more ideal, as these fluids allowed for the formulation of greases that are compatible with a vacuum environment and more amenable to additives. According to DellaCorte, these fluids are “essentially the backbone of a next-generation grease.”
Greases used in space lubricate ball bearings, gears, sliding bearings, latches, cables and other mechanisms, and are often subject to extreme conditions for long periods of time. Therefore, space greases require very specific properties to perform reliably, said DellaCorte.
“Space is generally described as a vacuum. That means there’s no atmosphere. There is no air or water flowing to provide cooling. We’ve got evaporation issues. You put certain liquids in a vacuum, they’re going to boil away. So we have tribology problems because of the vacuum of space.”
He explained how space applications expose lubricants to extreme temperatures.
“We’ve got extreme cold in space,” DellaCorte said. “Cold causes oils and greases to get a lot thicker. We have flow and shear problems. In space, we also have limited horsepower of our drive motors, so if a lubricant gets too stiff, then limited drive power becomes a real issue.”
Moving to the other extreme, temperatures in certain space applications can be quite high. This can occur in situations in which the lube and the mechanism lubricated are exposed to the exhaust of a rocket engine, or a mechanism is pointed toward the sun at all hours of the day. “[Heat] can cause oil separation from grease, as well as migration and degradation,” said DellaCorte.
Other difficulties lubricants face in space applications are X-rays and ultraviolet radiation from the sun, as well as severe launch vibes, which are vibrations of rockets that rattle the delicate mechanisms, bearings and gears in spacecraft. On planetary surfaces such as Mars or the moon, dust is an issue because it can easily contaminate mechanisms and lubricants.
Another roadblock, he explained, is that “you really can’t send people out there to change the grease or change the oil in your mechanism. There is little opportunity for replenishment, so everything kind of has to be lube-for-life.”
However, the problems tribologists encounter in space applications are similar to the problems that exist on earth. According to DellaCorte, lubricants in space applications can fail when the wrong lubricant is selected, the designers of the equipment do not fully understand the operating conditions – loads, speeds and temperatures – that equipment will face, and when the wrong engineering solutions are chosen for problems with lubricated parts.
According to DellaCorte, some common problems that can occur in mechanisms used in space include jamming and sticking of mechanisms, mechanisms that do not fully open and bearings that experience uneven friction, which leads to vibrations on spacecraft. The oils and greases in lubricants can also be depleted over time, he said, causing bearings to seize. Gear wear can lead to debris particles that result in jamming in gear boxes.
He provided an example of such a malfunction on the International Space Station. Operation of the entire station was once threatened, he said, by a design problem and a poorly implemented mechanism in the solar alpha rotary joint – a bearing similar in scale to a large wind turbine bearing, about 3 meters in diameter, that helps to slowly and continuously turn a solar panel at one revolution every 90 minutes so that it faces the sun at all times.
“We really want a very smooth-running, clockwork-type mechanism because the space station is a micro-gravity laboratory, and if the bearing is running rough, it’s shaking the space station and disrupting the science that goes on there,” he said. “If it’s not turning, you’re not generating enough power.”
When the joint was newly installed on the station, the starboard side joint bearing had turned for about a month when it became excessively noisy and hard to turn. In investigating this malfunction, NASA discovered a large amount of wear particles on the joint and that the surface of its race ring had been severely roughened.
NASA concluded that “inadequate lubrication of the roller-race contact, combined with a kinematic mechanism design that is vulnerable to roller tipping and high friction, led to damaging high roller-race surface forces and stresses.”
To solve the issue, astronauts added more grease to the bearing. After being properly re-greased, the joint, which is monitored around the clock, is now functioning properly and has been for upwards of 10 years, DellaCorte reported.