In order to deal with this challenge, Manning’s team decided to go with a new approach. Rather than using liquid grease to lubricate actuators, as had been done on previous rovers, they used a molybdenum disulfide dry lubricant. The dry lubricant is good down to -135C, so the low temperatures would not be a problem. The actuator gears themselves would be made from titanium.
Unfortunately, during life testing, it became clear that this approach was not going to work. The particles of molybdenum disulfide apparently migrated away from key points on the gear teeth, and the thin layer of titanium oxide on the surface of the gears (which, under Earth conditions, is naturally replenished) broke down. Manning likens this to breaking away the hard candy shell of an M&M candy and exposing its “soft, chewy center.” The result was premature gear failure. The actuators could not survive even half of their intended lifetime.
This forced the team to redesign the actuators to use liquid grease. The gear material was changed from titanium to Vascomax, a high-strength maraging steel. This material is roughly twice as strong as titanium. However, it also has a thermal expansion coefficient that is roughly twice that of titanium. One consequence of this is that the gearset must have greater backlash in order to account for the increased thermal expansion. The team needed to develop strategies to deal with the greater backlash, and ultimately came up with what Manning refers to as “some neat tricks.”
A bigger problem was the fact that heaters would be needed in order to warm the liquid grease to a temperature at which it would be less viscous. Adding these heaters also meant adding thermal controls and fault protection, dramatically increasing the complexity of the system.
Most importantly, the heaters would require power. The Curiosity rover is powered by a radioisotope thermoelectric generator (RTG). At the core of this are about 10 lb of plutonium dioxide, surrounded by a number of small, solid-state thermocouples. The heat generated by the radioactive decay of the plutonium produces a voltage in the thermocouples. The generator outputs about 120W of electricity, nowhere near the thousands of Watts needed to heat the actuators.
Instead, the modest output of the generator is used to charge a lithium-ion battery. The rover design already included a battery to power other subsystems. With the added requirement of heating the actuators, the battery needed to be doubled in size.
The team worked around the clock to design solutions to these problems, to get the redesigned components manufactured, and to conduct testing to validate the new designs. Ultimately, however, it became clear that the 2009 launch date could not be met.
For Earth-bound engineers, missing a key schedule date might mean delaying a project until the next model year, but for Manning’s team, it meant waiting for the planets to align. The minimum-energy launch window from Earth to Mars only occurs once every 26 months. The launch had to be delayed until 2011.
That's what it takes. The problems in space systems are generally very difficult becuase the conditions do not occur, in general, on earth. In addition, except in a limited number of cases, once a system is launched, that's it.
Generally, the budgets are a lot bigger than for terrestial based systems. They are not unlimited, but sometimes they seem to be. The comment about missing the date is instructive. In a normal engineering situation, you might actually abandon a product development if you miss a key date. This could be a model year or a selling season (e.g., XMAS). When RIM announced that their Blackberry 10 phones would not come out until late January 2013, their stock fell. This is equivalent to the constant threat of Congress cancelling or delaying a program. It can be really frustrating for the engineers involved.
A really interesting article on what happens behind the scenes to design a better machine by learning from what didn't work in the past, and also having the patience to see it through. That it's about one of the most interesting and well-known robots to be created in the last decade also makes it a worthwhile read. Thanks for the blog post, Dave.
Good article, Dave. One detail that caught my attention was the description of the thermoelectric generator used to charge (and repeatedly re-charge) lithium batteries. I didn't realize the Curiosity was stocked with multiple Li-Ion cells, and knowing that this type of chemistry has a relatively finite life span of charge/discharge cycles (about 500-600 times) that definitely seems to dictate a finite life span. Of course, that span could be measured in decades if one cycle were several weeks long. Is that the case-?
Great article, Dave. The obvious emotion among the engineers in image #6 is a reminder of the comments from NASA engineers in the late '60s and 70s, who used to go outside and look at the moon for inspiration during late night work sessions. As you point out, this is a dream job for most engineers.
@JimT: I don't have a definite answer on the battery life, but this status report says that "the batteries are expected to go through multiple charge-discharge cycles per Martian day." The rover is supposed to last at least one Martian year, or about 669 Martian days. So it sounds like the batteries are expected to last well over a thousand charge-discharge cycles.
The Spirit and Opportunity rovers, which have similar batteries, survived for several Martian years, and as the article notes, Opportunity is still alive.
Dave; thanks for that. Yardley website indicates the Lith-Ion cells are good to 2100 deep cycles. That's unheard-of in the consumer electronics industry. I had been making vague assumption's that, while the Consumer Market and the Aerospace industries have vastly different needs, that the basic physics and chemistry of the batteries would be a common denominator. Certainly NOT the case at 2100 cycles. That's approximately 3-4 times better than the high-end expectations for Consumer portables at about 600 cycles. I'm book-marking that Yardley site, for sure. Thanks!
Rob Manning asked me to convey that Howard Eisen and his team were mainly responsible for the successful resolution of the issues described in the article. Eisen is the Deputy Flight System Manager, and like Manning, has been closely involved in all of NASA's Mars rover missions, from Sojourner in the late '90s to the Curiosity rover today.
Of course, the success of the Curiosity rover is the product of the work of literally thousands of people, at NASA and at their suppliers (including Aeroflex and Yardney, among others) and other partners.
Mechanical, electrical and power management issues are common to many designs, but not many engineers have such a hard deadline as a launch window to contend with. The men and women who work on these extra-terrestrial projects have a lot to contend with and my hat's off to them and the managment team that make these projects happen - despite the long odds for success. Thanks for the post.
Excellent post Dave. It's amazing to me how dedicated engineers always find a way to make it work. Also its obvious NASA appointed the right individual to lead this team. I wonder if Congress would be so accommodating today if delays of this type presented themselves for comparable programs.
In a world that's going green, industrial operations have a problem: Their processes involve materials that are potentially toxic, flammable, corrosive, or reactive. If improperly managed, this can precipitate dangerous health and environmental consequences.
An analysis of what’s needed to implement Design for Disassembly and Design for Recycling results in eight strategies engineers can use to design an intentional end-of-life stage into their products.
Government regulations, coupled with growing consumer sensitivity about data and identity theft, require that data storage organizations demonstrate proper protection and due diligence in protecting sensitive information stored inside datacenter enclosures.
When a crane doesn't have a monitoring system, crane owners schedule service every six months and simply scrap the parts they replace, even if a part has had little use and doesn't need replacing. This can cost thousands.
From Dell / Intel® New Paradigms in Design Work Scott Hamilton, vertical market strategist for Dell Precision workstations, 5/2/2013 5
Early in my career, I worked as a draftsman and remember the days of drawing on vellum with numbered pencils and Mylar with plastic lead. This was a fun experience in the sense that I ...
I've been using workstations for more than 10 years and love finding ways to get more performance from my system. With demanding professional applications that require more power each ...
A lasting memory from my first job as an engineer in an auto assembly plant is standing on hard concrete at six in the morning, vending-machine coffee clutched in hand, listening to ...
A quick look into the merger of two powerhouse 3D printing OEMs and the new leader in rapid prototyping solutions, Stratasys. The industrial revolution is now led by 3D printing and engineers are given the opportunity to fully maximize their design capabilities, reduce their time-to-market and functionally test prototypes cheaper, faster and easier. Bruce Bradshaw, Director of Marketing in North America, will explore the large product offering and variety of materials that will help CAD designers articulate their product design with actual, physical prototypes. This broadcast will dive deep into technical information including application specific stories from real world customers and their experiences with 3D printing. 3D Printing is
To save this item to your list of favorite Design News content so you can find it later in your Profile page, click the "Save It" button next to the item.
If you found this interesting or useful, please use the links to the services below to share it with other readers. You will need a free account with each service to share an item via that service.
Tweet This
4 
