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Mars NASA

Wheel Damage Adding Up Quickly For Mars Rover Curiosity 162

Posted by Soulskill
from the wheels-on-the-rover-go-round-and-round dept.
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 @02:40AM (#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.

    • Pic of the wheel ...

      http://www.garrettbelmont.com/... [garrettbelmont.com]

      The first time when I saw the wheels I was wondering why the hell they spend so much money to send up a robot to Mars and then equip that thing with such flimsy wheels

      And I did post question here on /, and there were people (NASA fanbois, perhaps) defending those flimsy wheels

      • Re: (Score:2, Insightful)

        by andydread (758754)
        Would you mind posting a pic of the wheel that you have engineered that would do better? thanks. Make sure to consider launch weight, and sustained temeratures below 255deg F among other things.
        • by ColaMan (37550) on Wednesday August 20, 2014 @05:00AM (#47710695) Homepage Journal

          Ultra low temperature silicon rubber [likon.com] 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 andydread (758754)
            Yes and how does that stand up to the onslaught of UV rays on mars? its not that simple.
            • by ColaMan (37550) on Wednesday August 20, 2014 @05:38AM (#47710821) Homepage 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.

              • I don't think the UV and radiation is as much a factor as the huge temperature swings. Every silicon rubber I've seen can't handle going from cold to hot to cold repeatedly without breaking down. It could be somewhere between -150C to 20C depending on the location.
              • by Gnomaana (1698442)
                Are you sure "excellent" for use on Earth where maintenance can be done equals "excellent" for use on Mars where they can never be touched again? Also, what, if anything, would that layer of silicon do to the traction of the wheels? I'm guessing those "point loads" you mentioned are there for a reason.
          • by rasmusbr (2186518) on Wednesday August 20, 2014 @08:02AM (#47711459)

            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 biggest problem is the curvature of the wheels is concentrating all of the weight of the vehicle on the center of the wheel, and not only that, but with the tread design, all the force is concentrated on a spot about 2 square millimeters. They just need to add a rib going down the middle like bicycle tires.
          • 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..

            • by geekoid (135745)

              /. stripped the minus sign. 'minus 127' to 'positive 40' C

            • by Dishevel (1105119)
              Might be outside the range of its excellent low temperature performance.

              I would think that it would last a quite a while and offer some flexibility still at 11C under that temp.

              The flexibility that is left would likely be more than enough to mitigate damage to the wheels.

          • by drinkypoo (153816)

            Ultra low temperature silicon rubber springs to mind.

            Even Wikipedia would have let you know that this is inadequate for the range of temperatures found on Mars.

            Oh well, I guess they'll know for next time :-)

            Right back atcha

        • Re: (Score:2, Informative)

          by Anonymous Coward

          Exactly. Without knowing all the design constraints the engineers were juggling, we can't judge the design too much. Since it already surpassed its original 2-year mission, one would safely assume they hadn't tested the wheel beyond that and these problems would not have shown themselves.

          • by Anonymous Coward

            I love how the retards are bitching because the probe that got launched to Mars is starting to have problems after exceeding the design life.

        • Would you mind posting a pic of the wheel that you have engineered that would do better? thanks. Make sure to consider launch weight, and sustained temeratures below 255deg F among other things.

          http://www.harborfreight.com/1... [harborfreight.com]

        • Uh... yeah. How about 40 years ago [wikipedia.org]?

          Granted, those wheels were not exposed to as much rock, but they drove 2 passengers much farther than Curiosity has gone, at a far higher rate of speed. The astronauts even hotdogged it a little bit. No damage whatever.

          GP was correct: it was a questionable design decision from the beginning. Somebody made a bad choice.
          • by andydread (758754)
            well you said it. they were not exposed to much rock so that does not count.
            • "As much" rock. There certainly were rocks. And remember, it went much farther, faster, than Curiosity. Under otherwise harsher conditions: much hotter sometimes, much colder sometimes. The Mars atmosphere may be thin, but at least there is one.
      • by N1AK (864906) on Wednesday August 20, 2014 @05:00AM (#47710697) Homepage

        The first time when I saw the wheels I was wondering why the hell they spend so much money to send up a robot to Mars and then equip that thing with such flimsy wheels

        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).

        • by Anonymous Coward on Wednesday August 20, 2014 @08:02AM (#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 N1AK (864906)

            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.

            I don't believe that was the primary purpose of the mission. Curiosity had clear scientific objectives and MEP has clear goals, none of which include reaching a specific location. It may well be the case that the team intended to go to Mount Sharpe in order to complete its scientific missions, but it has been able to achieve it without goi

          • by Anonymous Coward on Wednesday August 20, 2014 @10:05AM (#47712403)

            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.

          • >> "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.

            This [huffingtonpost.com] says "mission accomplished".

        • by klui (457783)

          > unless you'd like to point out what makes you remotely credible in this field

          slashdotter with low id number.

      • Only someone with no brain would post that.
      • by Anonymous Coward

        Perhaps they were 3D printed and thus impervious to criticism?

      • by geekoid (135745)

        The reference for the wheel design is the specifications to meet it's goals.

        So, by an actual good measure, the wheel design is a good one. How can something that exceeded it's goals be considered flimsy?

        If you bought 50,000 mile tires for you car, and they lasted 75,000 miles would you call them flimsy?

      • Pic of the wheel ...

        http://www.garrettbelmont.com/... [garrettbelmont.com]

        The first time when I saw the wheels I was wondering why the hell they spend so much money to send up a robot to Mars and then equip that thing with such flimsy wheels

        And I did post question here on /, and there were people (NASA fanbois, perhaps) defending those flimsy wheels

        Obviously the engineers designing the wheels had just come off a Livin' The Low Life [tv.com] marathon and felt inspired.

      • There are management problems at NASA (i.e. too oriented toward manned spaceflight), but not at JPL that built and runs the rover. I just love you hindsight jocks with zero engineering knowledge of the project.
      • by danbert8 (1024253)

        They should have at least made a curved tread pattern. Those points of the chevrons are huge stress risers which makes fatigue fracture much more likely.

      • by fisted (2295862)

        Pic of the wheel ...

        http://www.garrettbelmont.com/... [garrettbelmont.com]

        ...

        Let me FTFY:
        Pic of the wheel... [garrettbelmont.com]

        Wasn't that hard, was it?

      • Well mission parameters for the Mars wheel would have mostly likely been as light and thin and wide as possible. Light and thin so that it would fit the weight and space requirements. Wide so that it could traverse the sandy, dune like conditions.
      • Pic of the wheel ...

        http://www.garrettbelmont.com/... [garrettbelmont.com]

        The first time when I saw the wheels I was wondering why the hell they spend so much money to send up a robot to Mars and then equip that thing with such flimsy wheels

        And I did post question here on /, and there were people (NASA fanbois, perhaps) defending those flimsy wheels

        I wish the wheels on my daily driver would last as many years without servicing as Curiosity's have.

  • Still unsure as to why they didn't go with polyurethane or hard plastic wheels or similar. Probably about the same weight as the alloy ones, much less susceptible to fatigue.

    Might be hard to find something that's good for those temperatures, but surely not that hard. Or were they expecting more sandy areas?

    • Don't forget the thing weighs a ton.

    • by LoRdTAW (99712) on Wednesday August 20, 2014 @08:10AM (#47711547)

      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.

      • by geekoid (135745)

        And you can't use the average temperature as a primary guide. The landing area temperature range from minus 127 C to positive 40 C

        • Nowhere on Mars has ever been +40 C.

          • by tipo159 (1151047)

            According to a JPL article, "During their exploration of Mars, the rovers have recorded temperatures ranging from midday highs of about 35 degrees C" (Source link [nasa.gov]). Making the range a round number like 40 C seems reasonable in this instance.

          • by nedlohs (1335013)

            Spirit recorded temperatures of +35C - http://mars.jpl.nasa.gov/mer/s... [nasa.gov].

            You are claiming that in say the last 10,000 years nowhere on the planet has ever managed to get just 5C higher than what was measured in an arbitrary few year window?

      • Curiosity does have a bunch of 'nylon tie' like objects on the top of the rover, holding bundles of cables together. Wonder what they're made out of. A quick search found lots of documentation on exactly how to run the cables (fun factoid - they still use knots on cord) but not much on what the stuff was made out of.

        • by mirix (1649853)

          Historically cable lacing was done with waxed cotton. Since WWII more nylon and polyester, as they wear harder and don't burn as good, don't like water, etc.

          I'd imagine NASA uses some kind of space age stuff.. polyimide or some sort of fluoropolymer, but who knows, maybe cotton has better extreme cold weather performance.

      • My guess is the metal fatigue is being cause by radiation. Mars does not have the atmosphere and magnetosphere of Earth (no news flash here). So maybe it's just a case where cosmic radiation causes metal fatigue and we didn't know that -- or perhaps it's SPECIFIC to the type of allow being used.

  • xkcd (Score:5, Insightful)

    by Thanshin (1188877) on Wednesday August 20, 2014 @03:07AM (#47710369)

    Spirit [xkcd.com]

    Obligatory, because it's beautiful.

    • by Hillgiant (916436)

      I cry every damn time I read that one. And I don't care who knows. Heck. I cry just reading the wikipedia entry for Spirit.

      That collection of nuts, bolts, and solar panels did more with less and used up every last bit of its capability in the pursuit of its mission. Yes I know I am anthropomorphising a bit (a lot), but I DON'T CARE.

  • Seriously, Spare Tires? Or spare belts for tires so that the rover can re-tread itself.
  • Simply build a wheel changing robot and launch it to Mars.

  • I am guessing that part of the reason for an all-metal milled wheel is because of the (largely) unshielded RTG power source which Curiosity uses may seriously degrade organic-based materials.

    Could someone with more knowledge of materials near RTG sources comment?

    • by animaal (183055)

      I've heard that rubber, being organic, wouldn't be allowed as a raw material for the wheels - it might interfere with the tests to find organic materials on Mars. But maybe degradation is also a factor.

    • by necro81 (917438)
      Curiosity's RTG [spaceflightnow.com], like most that came before it, is powered with Plutonium-238 [wikipedia.org]. Pu-238 is an alpha-particle emitter [wikipedia.org], meaning that the radiation is easily blocked by most solid objects (as opposed to, say, gamma or neutron radiation, which require significant shielding). The radiation levels that leave the RTG housing would, I expect, be non-significant compared to the ambient radiation on the surface of Mars.

      UV radiation would be a bigger problem as far as plastics are concerned.
  • The thousands of microscopic missiles launched at the rover to stop it anhialating more of their tiny cities is finally paying off. If one of them only had an old macbook and some way of getting to its core....
  • Poor material choice (Score:3, Interesting)

    by Solandri (704621) on Wednesday August 20, 2014 @04:56AM (#47710681)
    Aluminum does not have a fatigue limit [wikipedia.org]. 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 [wikipedia.org].

    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 MichaelSmith (789609) on Wednesday August 20, 2014 @05:13AM (#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 Kjella (173770)

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

        If that is true, why do the wheels need skin in the first place? I doubt anything on that mission is there for decoration...

        • by bgarcia (33222)
          As explained in the article, the skin is useful for travelling over a sandy surface.
    • by Ly4 (2353328) on Wednesday August 20, 2014 @06:19AM (#47710955)

      they really should have allocated sufficient weight budget for non-aluminum wheels.

      In the FA, it notes that the weight of the wheels isn't a stand-alone issue. During the landing, any extra wheel weight would significantly stress the bogies and rockers that hold the wheels, so you'd need much more strength (and weight) there.

      The article also notes that they made their decisions based on the surfaces they expected; they found many more 'strongly cemented vertical rocks' than they planned for.

    • by Gothmolly (148874)

      +Mod points if I had them. I think we were talking about this the second semester of my freshman year. Stress vs. strain and all that.

    • by Ogive17 (691899)
      Even though the mission was originally set to last just 90 days?

      I spent my first few years as an undgrad studying aerospace engineering. You don't build a lander to survive a trip to Pluto if your mission is the moon.
    • by SpzToid (869795)

      Another more recent example of airframe stress was Aloha Airlines flight 243; most, but not all people survived.

      https://en.wikipedia.org/wiki/... [wikipedia.org]

    • by Hillgiant (916436)

      ...no matter how beefy you make an aluminum part, after enough cyclic stresses it will suffer fatigue failure.

      You realize that chart has a log scale, right? It is not a matter of designing for infinite life, it is a matter of designing for "infinite enough". This is how they make aluminum engine blocks and heads. Sure it will fail in fatigue eventually. But after 50 years or so, it is time to replace the dern thing anyway. In other words, fatigue strength is just one more variable to design around. Even if the part was made from titanium or steel, they still might make the decision to not design for "infinite

      • More bad luck than bad planning because the type of terrain they landed on had not been observed on Mars before.

        It's not "bad luck", it's why you explore. If you send out a scout to the east, and he comes back with an arrow in his back, bummer for the scout, but at least you now know the east is dangerous before sending the rest of the troops to the east.

        Better we find out about stiff rocks now instead of when humans are driving a rover there, without AAA.

        Note this rover is better able to handle sand based

    • by Bogtha (906264)

      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)

      Curiosity was intended to last two years, it's been going for almost three. It wasn't intended to last this long, and it definitely wasn't intended to operate for decades.

    • Slight correction. The reason the de Haviland Comet's fuselage failed was because of the square windows concentrating stressors at the corners created a series of weak areas all along the fuselage. You will notice that any aluminum aircraft or boat has somewhat rounded corners on windows.

      Take that Apple!

      • by mirix (1649853)

        Only modern, pressurized aircraft. Cave tech like the DC-3 are made of aluminium, have square windows, and have no problems. Even ones that have been flying for 70 years.

  • by Squidlips (1206004) on Wednesday August 20, 2014 @07:50AM (#47711377)
    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.
    • ...effected everything from EDL to operations...

      To save anyone else needing to look it up, EDL is Entry, Descent and Landing. Not the right-wing nutters that originally sprang to mind.

  • Launch and land a "pep boys" on mars...

  • by Smerta (1855348) on Wednesday August 20, 2014 @09:24AM (#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...

  • by Last_Available_Usern (756093) on Wednesday August 20, 2014 @12:15PM (#47713439)
    I don't get what's so hard. Just have Jebidiah Kerman exit the rover and fix the wheel.
  • What is being done to improve future wheels on rovers and others?

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