29/05/2013

Out of This World

Cast your minds back to the first engineering related advert you ever saw on TV. Hear the haunting notes of a solitary trumpet and a picture of a golden drop of oil tracing its way across a block of metal.  Would you get it? What about the liquid coming from a white plastic container with red and green stripes? Recall the succinct strapline: ‘Liquid engineering.’

I suspect that if you are the same age as me you will have immediately remembered the advert for Castrol GTX. I knew about liquid engineering years before I could even drive a car, let alone have to deal with the lubrication challenges of the fine motor vehicles of the Eighties! Fast forward 30 years and liquid engineering has boldly gone where no other oil has been – to the surface of Mars where it is helping lubricate no less a futuristic vehicle than NASA’s Mars rover Curiosity.

Many of you will have seen images from the rover on the news and some of you may have even accessed, via the internet, the live streaming footage showing the surface of the planet. The vehicle itself is like no ordinary car and the environment it is operating in is literally like nothing on earth. Here are some of the headline facts about this amazing machine.

• To get to Mars, Curiosity had to cross 350 million miles of space.
• This journey took 8.5 months and in the last 7 minutes, the rover had to decelerate from 13,000mph to a complete stop.
• The 1 ton, 6 wheel drive, 4 wheel steer rover can scale obstacles up to 65cm high.
• The size of a Mini Cooper, Curiosity is powered by plutonium.
• Curiosity’s mission lifespan is 2 years although its power supply is designed to last for at least 10.
• During this time it is only expected to travel 3 to 12 miles with a maximum average speed of 0.00073mph.

With the final cost of the rover coming to $2.5 billion, $1 billion over budget, it is therefore imperative to keep it operating efficiently. At the heart of this is keeping all of Curiosity’s moving parts lubricated, a task undertaken by Castrol Industrial Lubricants. The company has decades of experience working with NASA on a variety of projects and as business development manager, Keith Campbell is well placed to discuss the unique lubrication challenges facing Curiosity and how these have been overcome.

“The three environmental extremes which Curiosity has to deal with are temperature, dust, and vacuum. Operating temperatures in space can be as low as minus 70ºC, in some cases even as low as minus 172ºC. However, you can have a very wide temperature range in those environments. For example, on Mars, the average surface temperature is minus 20 to minus 40ºC, but around the equator during summer months, the temperature can reach up to 20ºC. In addition, spacecraft can be exposed to elevated temperatures during lift-off.”

As Campbell explains, a key enemy of lubricants in this environment is outgassing. “Outgassing is affected by vacuum and temperature and can be the result of the vacuum literally pulling the base oils out of the grease, or the evaporative loss of the grease itself. There are two issues with outgassing. Firstly, because the oil is the most volatile part of the grease, the vacuum can theoretically deplete it completely, leading to mechanical failure of the grease and the component it is protecting. Solids are not volatile so remain within the grease structure. The second issue is that the material that outgasses can condense in other areas of the spacecraft such as the optics. These contaminants can become a major pollutant on instrumentation and can cause components to fail or operate erratically.”

The other environmental extreme that lubricants have to face is dust. For instance, Mars experiences frequent dust storms that include a wide variety and size of dust particles. Even though the joints on the rover are generally sealed well enough to resist dust, the lubricant still has to resist dust ingestion as a second line of defence. And as Campbell adds, “The difficult challenge in space is that maintenance is usually not possible so lubricant applications are fill for life. This is very much the case for Curiosity so the lubricants need to withstand these environmental challenges for many years.”

So how do you set about designing a lubrication system that can handle these types of challenge? “As with applications on the ground, you must determine the critical characteristics of the lubricant and work from there,” explains Campbell. “Generally speaking in space the critical characteristics are the ability to operate at very low temperatures while demonstrating low vapour pressure and outgassing at high temperatures.”

For the Curiosity rover, the company drew on its heritage of working with NASA where lubricants based on perfluoropolyether chemistry have proven to be the best cold temperature and low outgassing. The solution chosen was a lubricant called Castrol Braycote 601 EF, part of the Castrol Braycote line which includes sister products 600 EF, 602 EF and 815 Z. To get the combination of viscosity over a wide temperature range and low outgassing, Castrol Braycote 601 EF is formulated from perfluorinated polyether (PFPE) oil, thickened with PTFE powder. The PFPE has a viscosity index of 350, which provides the wide temperature range required for this environment.

The grease, which has a paste-like consistency, is not ‘cooked’ like conventional greases. The grease is manufactured by mechanical mixing to distribute the PTFE particles into the oil, as PTFE is not soluble in PFPE. The grease also contains a corrosion inhibitor to protect components in the time before launch or during storage, where it can be exposed to high humidity. It is inert to acids, bases and oxidizers as well as rocket propellants. In addition, the non-toxic, non-flammable lubricant does not use any chlorofluorocarbons during manufacturing, so it is safe for the environment.

The testing of the lubricant to approve it for space use is carried out by the space agencies themselves, often in conjunction with the lubricant manufacturers. Two parameters are measured in the test: total mass loss (TML) and collected volatile condensable materials (CVCM). The amount of water vapour regained can also be measured after completing the exposures and measurements required for TML and CVCM of 0.10 per cent.

The Castrol Braycote 601 EF has been formulated and tested to operate in temperatures ranging from minus 80 degrees to 204ºC. Campbell again, “More than this, it is also able to drop well below its operating temperature parameters, to be then brought back up and still work without adverse effects on its ability to lubricate. Its use in the Apollo moon missions, the Hubble space telescope, on numerous satellites, the International Space Station (ISS) and previous Mars rovers means that it has proven its suitability many times over.” Almost as an aside, he adds that the lubricant was also used in the spacesuit of record-breaking Austrian space jumper Felix Baumgartner, which helped him safely travel from the edges of space to the surface of the earth.

He goes on to cite a specific example where a sister product solved a critical lubrication issue which prevented a threatened failure of the International Space Station. The space station is the largest artificial satellite in Earth’s orbit and serves as a long-term cosmic platform. The power needs for the range of research activities carried out by the crew are provided by a number of solar panels that rotate to meet the sun. When a moving part started to show serious anomalies in its operational data in 2007, astronauts investigated and discovered metal shavings where they shouldn’t be and severe degradation. Investigators detected a ‘joint drag’ problem with the bearings, a problem that threatened to lead to what the investigation called an ‘unrecoverable stall of the mechanism’. As the solar power supports the station’s life and all its activities, the problem put the whole ISS at risk, and it was Braycote 602 EF that came to the rescue, solving a problem that the astronauts found even pure gold could not fix.

All of which made it the ideal choice for Curiosity where at this very moment it is lubricating most of the rover’s moving parts including the robotic arm (shoulder, elbow and wrist turret), mast deploy actuator and pivot mechanism, 6 wheel drive actuators, 4 wheel steering actuators, drill mechanisms, science instruments on the turret, and the dust removal tool.

There are also business applications closer to home that can benefit from lubricants with such specific properties, typically where there are any hostile chemicals or extreme environmental conditions that would preclude the use of other lubricants, such as the chemical industry and air processing and handling. Companies with manufacturing processes that require a vacuum such as the production of semi-conductors, micro-chips, flat panel display and hard disk drives can also benefit from using this type of lubricant. Typical applications in these areas include ball and roller bearings, gears and as an assembly lubricant for O-rings and elastomers.

Looking to the future, as the space industry develops, including the rise of the private and space tourism sectors and developments by other space entities, such as the European Space Programme, the customer base for these high-performance products is expected to widen. Exploration of Mars’s higher and colder latitudes would require lubricants that could withstand extreme cold, which would also them to operate on the surface of planets such as Venus.

Returning to the present, by the time you are reading this, Mars will just about be completing its journey around the far side of the sun and becoming increasingly visible in the Eastern dawn sky. And somewhere on its distant surface will be the Curiosity rover, still running smoothly, thanks to some advanced liquid engineering.