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Wheel Damage Adding Up Quickly For Mars Rover Curiosity 162

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."
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Wheel Damage Adding Up Quickly For Mars Rover Curiosity

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  • by Anonymous Coward on Wednesday August 20, 2014 @03:40AM (#47710283)

    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.

  • Re:Duration??? (Score:5, Interesting)

    by DiSKiLLeR ( 17651 ) on Wednesday August 20, 2014 @03:55AM (#47710343) Homepage Journal

    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.

  • Re:Duration??? (Score:5, Interesting)

    by Beck_Neard ( 3612467 ) on Wednesday August 20, 2014 @05:30AM (#47710599)

    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.

  • Poor material choice (Score:3, Interesting)

    by Solandri ( 704621 ) on Wednesday August 20, 2014 @05:56AM (#47710681)
    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.
  • by ColaMan ( 37550 ) on Wednesday August 20, 2014 @06:00AM (#47710695) Journal

    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 :-)

  • by MichaelSmith ( 789609 ) on Wednesday August 20, 2014 @06:13AM (#47710739) Homepage Journal

    But the wheels aren't failing. The skin on the wheels is failing but the wheels will work fine with structure alone.

  • by ColaMan ( 37550 ) on Wednesday August 20, 2014 @06:38AM (#47710821) Journal

    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.

  • by Anonymous Coward on Wednesday August 20, 2014 @09:02AM (#47711471)

    "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.

  • by Smerta ( 1855348 ) on Wednesday August 20, 2014 @10:24AM (#47712091)

    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:

    Changing driving software to reduce the forces experienced by wheels hanging up on pointy rocks. <snip> The rover can sense wheel currents, so it can sense when a wheel is sticking. <snip> By implementing a "smart controller" on the wheel current and allowing wheel rotation rates to vary intelligently in response to sensed conditions, they might be able to mitigate the damage.

    I've been developing embedded systems for more than half my life, and I never get bored...

"Never face facts; if you do, you'll never get up in the morning." -- Marlo Thomas