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Wheel Damage Adding Up Quickly For Mars Rover Curiosity

Soulskill posted about 4 months ago | from the wheels-on-the-rover-go-round-and-round dept.

Mars 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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Next time: No aluminum foil (3, Interesting)

Anonymous Coward | about 4 months ago | (#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.

Someone with no brain is running NASA (2)

Taco Cowboy (5327) | about 4 months ago | (#47710471)

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:Someone with no brain is running NASA (1, Insightful)

andydread (758754) | about 4 months ago | (#47710537)

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.

Re:Someone with no brain is running NASA (0)

Anonymous Coward | about 4 months ago | (#47710585)

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.

What are temeratures? I'd just use a wheel design from a previous rover and improve upon its design.

http://www.chartgeek.com/wp-content/uploads/2012/08/mars-rovers-wheels.jpg

Re:Someone with no brain is running NASA (4, Interesting)

ColaMan (37550) | about 4 months ago | (#47710695)

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 :-)

Re:Someone with no brain is running NASA (1)

andydread (758754) | about 4 months ago | (#47710779)

Yes and how does that stand up to the onslaught of UV rays on mars? its not that simple.

Re:Someone with no brain is running NASA (3, Interesting)

ColaMan (37550) | about 4 months ago | (#47710821)

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.

Re:Someone with no brain is running NASA (-1)

Anonymous Coward | about 4 months ago | (#47711037)

too bad the uv would crumble it in a few days, you dumb fucking piece of shit.

Yet another person that thinks they aren't a moron, but in fact, is.

Re:Someone with no brain is running NASA (0)

Anonymous Coward | about 4 months ago | (#47712199)

Did you even read the link? It claims to be resistant to exactly that..

So whos the dumb fuck now huh?

Re:Someone with no brain is running NASA (0)

Anonymous Coward | about 4 months ago | (#47712375)

Still you. That silicon can't survive the -127C to 40C temperatures on Mars.

Re:Someone with no brain is running NASA (3)

rasmusbr (2186518) | about 4 months ago | (#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.

Re:point loads on the tread are too high (1)

DocSavage64109 (799754) | about 4 months ago | (#47711861)

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.

Re:Someone with no brain is running NASA (4, Insightful)

geekoid (135745) | about 4 months ago | (#47712053)

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..

Re:Someone with no brain is running NASA (1)

geekoid (135745) | about 4 months ago | (#47712071)

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

Re:Someone with no brain is running NASA (1)

drinkypoo (153816) | about 4 months ago | (#47712349)

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:Someone with no brain is running NASA (2, Informative)

Anonymous Coward | about 4 months ago | (#47711277)

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.

Exceeded Design Life (2, Insightful)

Anonymous Coward | about 4 months ago | (#47711869)

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.

Re:Someone with no brain is running NASA (0)

Anonymous Coward | about 4 months ago | (#47711575)

You sound like a petulant 12 year old. One can criticize a bad idea without having engineered a full replacement.

Re:Someone with no brain is running NASA (5, Informative)

N1AK (864906) | about 4 months ago | (#47710697)

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).

Re:Someone with no brain is running NASA (4, Interesting)

Anonymous Coward | about 4 months ago | (#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.

Re:Someone with no brain is running NASA (1)

N1AK (864906) | about 4 months ago | (#47711727)

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 going there.

To slightly correct your analogy: It's like wanting to go out for excellent food, discovering the tube station near the restaurant you had selected due to its reputation is closed and deciding to instead go to another equally great restaurant that is near an open station instead.

Re:Someone with no brain is running NASA (0)

Anonymous Coward | about 4 months ago | (#47710729)

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

Sure you did, and I bet you also knew everything NSA was doing before the Snowden revelations.

Re:Someone with no brain is running NASA (0, Insightful)

Anonymous Coward | about 4 months ago | (#47710763)

No doubt you are too young to remember TIA or Echelon but yes, yes we did, actually. Snowdens leaks were surprising in their detail, not in their scale.

Re:Someone with no brain is running NASA (0)

Anonymous Coward | about 4 months ago | (#47710965)

The details are the beef. It's not much if you only have a good guess at which scale NSA is operating.

Re:Someone with no brain is running NASA (0)

Anonymous Coward | about 4 months ago | (#47712289)

What it means is that all the rumors about the intelligence agencies which have been circulating for decades, turned out to be
TRUE.
Few people expected this, except for the paranoid conspiracy "nuts" which have been trying to tell you this shit for years.

Re:Someone with no brain is running NASA (0)

Anonymous Coward | about 4 months ago | (#47710857)

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

Ironically, that "flimsy" wheel is likely far stronger than the wheels you trust to push you down the road at 70MPH every single day.

From the sounds of it, the wheels are not the issue. Scientists have been driving cars on Earth for quite a long time now. They don't ever buy new ones? Ever? All of them ride around all day with no spare tire? Seems to me the biggest oversight here was NOT packing an extra set.

Then again, we never expected them to last this long.

Re: Someone with no brain is running NASA (0)

Anonymous Coward | about 4 months ago | (#47710901)

Ironically, that "flimsy" wheel is likely far stronger than the wheels you trust to push you down the road at 70MPH every single day.

What makes you think I drive?

Re:Someone with no brain is running NASA (4, Funny)

gsslay (807818) | about 4 months ago | (#47711175)

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!

Re:Someone with no brain is running NASA (1)

CheezburgerBrown . (3417019) | about 4 months ago | (#47712111)

No, actually their wheel would only last bout 8km on the surfaces I ride on every day.

Re:Someone with no brain is running NASA (0)

Anonymous Coward | about 4 months ago | (#47710869)

The wheels appear flimsy because they are designed to act as a major part of the suspension. A bulkier wheel would not do so and thus require a much more complex separate suspension system involving springs etc, which would have made the lander too heavy and raised its centre of gravity higher making overturning more likely.

Re:Someone with no brain is running NASA (0)

Anonymous Coward | about 4 months ago | (#47710913)

It lasted 2 years on mars, I think it did its job.

Re:Someone with no brain is running NASA (0)

Anonymous Coward | about 4 months ago | (#47711127)

Hopefully they will see their error and put you in charge next time.

Re:Someone with no brain is running NASA (0)

Anonymous Coward | about 4 months ago | (#47711163)

I wish I could remember what it was I was watching, but I saw some show (on the science channel, I think) talking about how they designed different extra-terrestrial vehicles and the R&D that went into it. Many designs failed for so many reason...tendency to get stuck, or be unable to climb, or too much weight, or susceptible to damage from abrasive dust particle or extreme temperatures, etc.

Re:Someone with no brain is running NASA (0)

Squidlips (1206004) | about 4 months ago | (#47711345)

Only someone with no brain would post that.

Re:Someone with no brain is running NASA (1)

Anonymous Coward | about 4 months ago | (#47711363)

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

Re:Someone with no brain is running NASA (2)

geekoid (135745) | about 4 months ago | (#47712101)

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?

Re:Someone with no brain is running NASA (1)

kelemvor4 (1980226) | about 4 months ago | (#47712207)

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.

Duration??? (0, Insightful)

Anonymous Coward | about 4 months ago | (#47710293)

For a three month mission, this rover is performing fantastically beyond expectations. That is is breaking down now, two years after first landing, is not exactly unsurprising.

Sure, we should do whatever we can to continue its mission -- the knowledge being learned is still impressive but let's not expect it to perform more than eight times its original mission....

Re:Duration??? (4, Interesting)

DiSKiLLeR (17651) | about 4 months ago | (#47710343)

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??? (5, Informative)

Anonymous Coward | about 4 months ago | (#47710349)

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.

Re:Duration??? (0)

Anonymous Coward | about 4 months ago | (#47710411)

For a three month mission, this rover is performing fantastically beyond expectations.

Save it, nobody's buying it. Spirit and Opportunity set a higher bar than that.

Re:Duration??? (0)

Anonymous Coward | about 4 months ago | (#47710423)

For a three month mission, this rover is performing fantastically beyond expectations.

Save it, nobody's buying it. Spirit and Opportunity set a higher bar than that.

Hey, Spirit and Opportunity went beyond expectations... way better than Hope and Change, that failed almost instantly.

Re:Duration??? (4, Interesting)

Beck_Neard (3612467) | about 4 months ago | (#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.

Re:Duration??? (1)

SpzToid (869795) | about 4 months ago | (#47710731)

You've confused your different Mars Rovers. Curiosity was launched from Earth on November 26, 2011.

Remember when the lifespan was ~180 days? (-1)

Anonymous Coward | about 4 months ago | (#47710329)

When they first landed we were all talking on /. asking why they didn't add a brush etc. to clean the solar panels and panicking over what we thought would be a short life.

Re:Remember when the lifespan was ~180 days? (4, Informative)

Anonymous Coward | about 4 months ago | (#47710355)

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.

Odd material selection (0)

ColaMan (37550) | about 4 months ago | (#47710331)

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?

Odd material selection (0)

Anonymous Coward | about 4 months ago | (#47710377)

polyurethane or hard plastic

Worried about UV damage?

Re:Odd material selection (0)

Anonymous Coward | about 4 months ago | (#47710653)

Don't forget Mars isn't a hot desert, it's mostly freezing. Plastic would crack under the heat/freeze cycle.

Re:Odd material selection (1)

ericloewe (2129490) | about 4 months ago | (#47710925)

Don't forget the thing weighs a ton.

Re:Odd material selection (5, Informative)

LoRdTAW (99712) | about 4 months ago | (#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.

Re:Odd material selection (1)

geekoid (135745) | about 4 months ago | (#47712119)

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

xkcd (4, Insightful)

Thanshin (1188877) | about 4 months ago | (#47710369)

Spirit [xkcd.com]

Obligatory, because it's beautiful.

Re:xkcd (-1)

Anonymous Coward | about 4 months ago | (#47710417)

*YAWN*

A poor man's WALL E.

Anthropomorphizing robots, I've never seen that before.

Anybody who links to XKCD should be tagged inane and banal autist needing therapy.

Re:xkcd (-1)

Anonymous Coward | about 4 months ago | (#47710505)

Yes, because shitting on someone elses sandwich is so ok. You must've suffered alot seeing this XKCD comic. Oh you, they should give you money for the horror.

Re:xkcd (-1, Flamebait)

Anonymous Coward | about 4 months ago | (#47710761)

Randall Munroe is the tech-nerd's equivalent of Dane Cook. Unoriginal, not funny, not innovative, and not entertaining except to an extreme minority of people.

Re:xkcd (1)

Hillgiant (916436) | about 4 months ago | (#47711665)

Yay! I'm an extreme minority!

And who the fuck is Dane Cook?

Re:xkcd (1)

geekoid (135745) | about 4 months ago | (#47712127)

He is the Karate of Comedy.

Re:xkcd (1)

DocSavage64109 (799754) | about 4 months ago | (#47711915)

So wait, you hate the comic and yet know the comic's author's name? I actually like the site, but have never had any idea who was drawing them. Also, hating something just to be counter-culture is a rather hipster thing to do (though hating on hipsters also seems to be hip).

Re:xkcd (1)

geekoid (135745) | about 4 months ago | (#47712163)

70+ million hits a month is an extreme minority?

Re:xkcd (0)

Anonymous Coward | about 4 months ago | (#47711555)

Shhh!!! Thanshin is a Space Nutter. This is a religion that has the human race as a doomed species and space is the only solution. Anything at all in space is worthy of the greatest emotion and admiration.

You're being a religious bigot by making fun of his beliefs.

Re:xkcd (1)

Hillgiant (916436) | about 4 months ago | (#47711651)

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.

FIRESTONE (0)

Anonymous Coward | about 4 months ago | (#47710379)

is somehow involved.

Future Design Requirement (1)

RedLeg (22564) | about 4 months ago | (#47710403)

Seriously, Spare Tires? Or spare belts for tires so that the rover can re-tread itself.

Re:Future Design Requirement (0)

Anonymous Coward | about 4 months ago | (#47710439)

Or you take the weight added for the spare tire and mechanism to change it and add that extra weight to the existing tires instead.

Just ditch the measuring equipment, the entire point of the project is to drive as far as possible on the surface.

Re:Future Design Requirement (1)

wonkey_monkey (2592601) | about 4 months ago | (#47710485)

Just ditch the measuring equipment, the entire point of the project is to drive as far as possible on the surface.

No, it's to draw willies on Mars [huffingtonpost.com] .

Re:Future Design Requirement (0)

Anonymous Coward | about 4 months ago | (#47710627)

"Seriously, Spare Tires? Or spare belts for tires so that the rover can re-tread itself."

Too expensive. Just buy a membership card of the AAA.

Just change the wheels (2)

Irish-DnB (161087) | about 4 months ago | (#47710467)

Simply build a wheel changing robot and launch it to Mars.

Re:Just change the wheels (0)

Anonymous Coward | about 4 months ago | (#47710503)

You know which word you mis-used there.

Re:Just change the wheels (0)

Anonymous Coward | about 4 months ago | (#47710639)

To be fair, he didn't say "Build a wheel changing robot and simply launch it to Mars."

p.s. Where's the kaboom? There was supposed to be an Earth-shattering KABOOM.

Re:Just change the wheels (1)

jones_supa (887896) | about 4 months ago | (#47710513)

"Simply".

Material selection (1)

Antony T Curtis (89990) | about 4 months ago | (#47710517)

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?

Re:Material selection (1)

animaal (183055) | about 4 months ago | (#47710899)

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.

Re:Material selection (2)

necro81 (917438) | about 4 months ago | (#47711149)

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.

Any direction (0)

Anonymous Coward | about 4 months ago | (#47710545)

"...how they can mitigate it going forward", and presumably backwards too!

Awesome 7hp (-1)

Anonymous Coward | about 4 months ago | (#47710631)

Vary fOr different BSD's pcodebase need to scream that One Here but now parties, but here this post up. then disappeared Inventing excuses

The Martians are making a dent (2)

PsyMan (2702529) | about 4 months ago | (#47710647)

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 (3, Interesting)

Solandri (704621) | about 4 months ago | (#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.

Re:Poor material choice (3, Interesting)

MichaelSmith (789609) | about 4 months ago | (#47710739)

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

Re:Poor material choice (0)

Anonymous Coward | about 4 months ago | (#47710883)

Until a rock that is shaped just right with enough weight behind it gets wedged into one of those holes....

What sort of hp/torque does that thing get?

Re:Poor material choice (1)

Kjella (173770) | about 4 months ago | (#47710969)

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...

Re:Poor material choice (2)

bgarcia (33222) | about 4 months ago | (#47711309)

As explained in the article, the skin is useful for travelling over a sandy surface.

Re:Poor material choice (0)

Anonymous Coward | about 4 months ago | (#47710921)

I was very surprised when I heard they were going with thin aluminium wheels on this rover.

But remember the design specification was 3 months not 10 years. Aluminium has far exceeded the design parameters. It was never intended to be "multi-year".

Steel would be more launch weight. Titanium would be more expensive. Aluminium therefore was a reasonable choice.

In addition the design is for the wheels to deform in order to act as part of the suspension, to remove the need for a much larger suspension system. Other materials might not have performed as well in that area.

Correct material choice (2, Informative)

oneiros27 (46144) | about 4 months ago | (#47711313)

My thought exactly ...

"Oh, no! The item we built is starting to fail after it's had 40 times the planned usage!"

That's not a poor design choice ... that's a *fantastic* problem to be having.

Re:Poor material choice (2)

torsmo (1301691) | about 4 months ago | (#47711601)

But remember the design specification was 3 months not 10 years

Curiosoty mission was designed to last a whole Martian year, or 23 Earth months. So it was intended to be "multi-year"

http://mars.jpl.nasa.gov/msl/m... [nasa.gov]

Re:Poor material choice (4, Informative)

Ly4 (2353328) | about 4 months ago | (#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.

Re:Poor material choice (0)

Anonymous Coward | about 4 months ago | (#47711097)

That's not true, actually it is now known there is no such thing as a endurance limit in reality. Eventually everything fatigues under cyclic loading, at any level of stress (http://en.wikipedia.org/wiki/Fatigue_limit). It might be a very large number of cyles, but it will fatigue.

Re:Poor material choice (1)

Gothmolly (148874) | about 4 months ago | (#47711223)

+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.

Re:Poor material choice (1)

Ogive17 (691899) | about 4 months ago | (#47711327)

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.

Re:Poor material choice (1)

SpzToid (869795) | about 4 months ago | (#47711359)

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]

Re:Poor material choice (0)

Anonymous Coward | about 4 months ago | (#47711449)

This is basic materials science that every undergrad mechanical engineer learns.

And that statement is the kind of arrogant second-guessing that usually comes from undergrad engineers. Real engineers know that you're making tradeoffs with every decision.

I'm sure the team would make a different set of choices and tradeoffs with what they know now. I'm also pretty sure that the current choice wasn't made because they lacked undergrad-level knowledge of material science.

Re:Poor material choice (1)

Hillgiant (916436) | about 4 months ago | (#47711959)

...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" life due to other overriding design considerations. Weight, for example.

The concern with the wheel design was not fatigue itself, but rather a higher peak load during the fatigue cycle. The wheels were not designed for the type of terrain they landed on. More bad luck than bad planning because the type of terrain they landed on had not been observed on Mars before.

In my experience, it is very difficult to successfully design for conditions that have never been observed before.

Time to Analyse ... (0)

Anonymous Coward | about 4 months ago | (#47711203)

I wish I had the time to analyze wheel design. Why don't some of you submit to Nasa? Like those Apple analysts (but who actually get paid) reality differs from theory...

I need new tires? (0)

Anonymous Coward | about 4 months ago | (#47711275)

I should have gone with the Michelins

To all you highsight experts: RTFM! (4, Insightful)

Squidlips (1206004) | about 4 months ago | (#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.

Re:To all you highsight experts: RTFM! (1)

Extreme_biker0 (1118419) | about 4 months ago | (#47711875)

...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.

Simple answer... (1)

Lumpy (12016) | about 4 months ago | (#47711445)

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

"We'll just re-flash it" (4, Interesting)

Smerta (1855348) | about 4 months ago | (#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...

Its a software issue... (0)

Anonymous Coward | about 4 months ago | (#47712201)

FTFA:

[...] It turns out that there are mechanical aspects of the mobility system that actively shove the wheels into pointy rocks. A wheel can resist the force of one-sixth of the rover's weight pressing down on a pointy rock, but it can't resist the rover's weight plus the force imparted by five other wheels shoving the sixth wheel into a pointy rock. The forces are worse for the middle and front wheels than they are for the rear wheels

[...] If the pointy rock can move, all that pushing force behind it will just shift the pointy rock to one side or another, or it can roll beneath the wheel, and the wheel will get over it without damage. The key to wheel punctures is immobile pointy rocks. If the pointy rock is stuck in place, partially buried, or if it is a pointy bit of intact bedrock, then there's nowhere for it to go

[...] the software requires all six wheels to rotate at a constant rate, even though a wheel climbing an obstacle has a longer path to travel than one traversing flat ground. 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.

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