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Wheel Damage Adding Up Quickly For Mars Rover Curiosity
An anonymous reader writes: The folks in charge of the Mars rover Curiosity have been trying to solve an increasingly urgent problem: what to do about unexpected wheel damage. The team knew from the start that wear and tear on the wheels would slowly accumulate, but they've been surprised at how quickly the wheels have degraded over the past year. Emily Lakdawalla at the Planetary Society blog has posted a detailed report on the team's conclusions as to what's causing the damage and how they can mitigate it going forward. Quoting: "The tears result from fatigue. You know how if you bend a metal paper clip back and forth repeatedly, it eventually snaps? Well, when the wheels are driving over a very hard rock surface — one with no sand — the thin skin of the wheels repeatedly bends. The wheels were designed to bend quite a lot, and return to their original shape. But the repeated bending and straightening is fatiguing the skin, causing it to fracture in a brittle way. The bending doesn't happen (or doesn't happen as much) if the ground gives way under the rover's weight, as it does if it's got the slightest coating of sand on top of rock. It only happens when the ground is utterly impervious to the rover's weight — hard bedrock. The stresses from metal fatigue are highest near the tips of the chevron features, and indeed a lot of tears seem to initiate close to the chevron features."
The things are the thinnest element in the entire lander. When I first saw those wheels, I just shrugged and figured they knew what they were doing. But the reality seems to be that they stuck with some sort of legacy design and somehow nobody ever asked the obvious question about those miserably thin wheels.
Though maybe I should instead be celebrating the fact that they didn't get their metric crossed with their imperial.
The Opportunity (MER-B) Rover landed on Mars January 25, 2004. More than 10 years later, it is still going strong even though it, too, was only expected to perform a 3 month (90 day) mission.
The success and longevity of the earlier Mars rover missions sort of sets expectations that future missions will last just as long....
We of course realise that is not possible. Plenty of missions end early, Spirit (MER-A) got its wheel stuck and got in trouble years ago but Opportunity keeps on running and sets unrealistically high expectations of Curiosity and future missions.
You can tell how powerful someone is by the magnitude of the crime they can commit and be able to get away with.
The planned mission duration was 2 years not 3 months.
http://en.wikipedia.org/wiki/Curiosity_%28rover%29
You are thinking about Spirit and Opportunity, whom both enormously exceeded their planned mission duration.
Solarpanels ? Curiosity is powered by an RTG not solarpanels.
http://en.wikipedia.org/wiki/Curiosity_%28rover%29
You are thinking about Spirit and Opportunity, whom both have solarpanels.
Spirit
Obligatory, because it's beautiful.
Curiosity is not spirit or opportunity. This is a much heavier rover. Plus, it consumes way more power and moves faster. The forces on the wheel are much much rougher than on the MER rovers.
A fool and his hard drive are soon parted.
Aluminum does not have a fatigue limit. That is, no matter how beefy you make an aluminum part, after enough cyclic stresses it will suffer fatigue failure. This is why airframes are retired after about 100,000 pressurization cycles - to avoid the fate which befell the de Havilland Comet.
Other materials like steel or titanium can be designed so it can withstand an infinite number of stress cycles and not fatigue. Given the nature of the mission and power source (multi-year if not multi-decade operation on another planet with no hope of human intervention if something should go wrong), they really should have allocated sufficient weight budget for non-aluminum wheels. This is basic materials science that every undergrad mechanical engineer learns. I was very surprised when I heard they were going with thin aluminum wheels on this rover.
Ultra low temperature silicon rubber springs to mind.
Could have bonded a couple of millimetres thickness onto each alloy wheel. It seems the wheels only break when they have no cushioning underneath them, then the point loads on the tread are too high.
Oh well, I guess they'll know for next time :-)
You are in a twisty maze of processor lines, all alike.
There is a lot of hype here.
In short: Because they aren't idiots and know enough about this field to make informed comment. The rover has reached its planned mission life, everything beyond this is a bonus. The wheels survived and will likely, with proper management, last considerably longer still. It's a great success.
Your comment on the other hand is a great example of how people who are ignorant on a field automatically assume it must be simple and that they have some valuable insight. You know when you hear people who don't have a clue say something stupid about something you know a lot about? That's you when you comment on wheels for vehicles travelling on other planets (unless you'd like to point out what makes you remotely credible in this field).
Did you care to read the link, which said things such as excellent resistance to UV and cosmic radiation?
Anyhoo, I guess it's an iterative process. Better wheels on the next one please guys.
You are in a twisty maze of processor lines, all alike.
There is a lot of hype here.
All of them ride around all day with no spare tire? Seems to me the biggest oversight here was NOT packing an extra set.
And a mechanic to change the wheel is just a phone call away because NAS had the foresight to take out full MAA (Mars Automobile Association) membership. They'll even tow it back home!
To all you idiots who think you could have done do a better job, read Emily's article. There were serious weight constraints for the wheels that effected everything from EDL to operations. Any huge engineering project is full of tradeoffs. Hindsight is 20/20.
TFA says adding 1 mm of aluminium to the wheels would have added too much weight to the wheels. Silicon rubber is about half as dense as aluminium, so a couple of millimeters of that would also have been too heavy.
There are probably lots of other ways to improve durability, like for instance by making the chevrons on the wheels slightly less pointy.
"The rover has reached its planned mission life, everything beyond this is a bonus."
I agree with the rest of your post, but this part is tricky to evaluate. From an engineering point of view, yes, it met that goal. The rover landed with a brand new technique, persisted over the duration of the primary mission, drove that whole time, did plenty of analyses along the way, and figured out some great science. However, the primary target of the mission was not the floor of Gale Crater, it was to study the stratigraphy of Mount Sharpe, the mountain in the middle of the crater. The rover is not there yet, in part because it's had to drive more slowly because of the wheel damage issue. It's going to be several more months before it gets there. While it's true that in some sense everything beyond this point can be considered a "bonus", in another sense the mission won't be complete until it gets to the place where it can finally study the rocks that were the primary scientific reason this site was chosen over the other candidates. People were always concerned about how far the rover had to drive out of the landing ellipse area to get to that target. It's turned out to be more difficult than expected.
Don't get me wrong. It's a great and successful mission even if the rover died tomorrow. It was fortunate that there were good outcrops inside the ellipse already (that was hoped/planned when it was chosen), and what's been done already has made the mission worthwhile; but it's kind of like going to a fantastic and quite expensive restaurant, enjoying the appetizers thoroughly, and then getting a little impatient waiting for the main course to start. Now that the problem has been evaluated it looks like the rover will get there, but if it doesn't, it will be a significant disappointment.
Plastics don't do very well in a vacuum like atmosphere full of radiation with wide temperature swings in the long term. Plus the low average surface temperature of -82F/-63C makes plastics less malleable and in many cases, brittle.
In the low atmosphere they can become brittle from outgassing and are susceptible to cracking and can simply shatter like glass. Nylon wire ties in a vacuum chamber simply fall apart after a few months. Though the 6 mbar (4.5 Torr) Atmospheric pressure of Mars isn't a hard vacuum, it is still 0.6% That of Earth's average sea level pressure.
Then you have radiation degrading the plastics which again makes them brittle. A friend worked on RHIC out in Brookhaven National Labs and since he was small and skinny he was tasked with changing out a lot of the sensor cables on the ring. The insulation simply disintegrated from radiation. There was nothing they could do about it save for bulky shielding which would have made servicing impossible.
In the end, metals are simply more suited to the task.
The temperatures at the landing site can vary from 127 to 40 C. So if you look at the spec you linked, it's outside the range.
It's almost like the engineers are aware of this sort of thing when they designed it..
The Kruger Dunning explains most post on
As an embedded systems (electronics/firmware) engineer, I was going to half-jokingly, half-seriously say, "Well, we'll just send a new firmware update to Curiosity to help with the problem." And then of course as I read the article, that was one of the proposed mitigations:
I've been developing embedded systems for more than half my life, and I never get bored...
The landing site was chosen because it allowed it to fulfil its primary mission AND had a more interesting secondary mission than other landing sites. The mountain was not the primary mission. The primary mission is long since complete. Please don't re-write history.