Breaks Observed in Rover Wheel Treads | Hacker News Reader

Breaks Observed in Rover Wheel Treads

jpl.nasa.gov

210 pointsandyjohnson0183 comments

Always loved the name of the Curiosity rover, was just reading the wikipedia article, apparently it was crowd sourced from 9,000 proposals and the selected name was from a 12 year old student. Very cool

"A NASA panel selected the name Curiosity following a nationwide student contest that attracted more than 9,000 proposals via the Internet and mail. A sixth-grade student from Kansas, twelve-year-old Clara Ma from Sunflower Elementary School in Lenexa, Kansas, submitted the winning entry. As her prize, Ma won a trip to NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, where she signed her name directly onto the rover as it was being assembled.

Ma wrote in her winning essay:

Curiosity is an everlasting flame that burns in everyone's mind. It makes me get out of bed in the morning and wonder what surprises life will throw at me that day. Curiosity is such a powerful force. Without it, we wouldn't be who we are today. Curiosity is the passion that drives us through our everyday lives. We have become explorers and scientists with our need to ask questions and to wonder."

https://en.wikipedia.org/wiki/Curiosity_(rover)

I'm very happy that we're involving students and helping increase passion for space.

Is it crowd-sourcing when the result is selected? If infinite monkeys give us Shakespeare, 9000 students should generate a pretty dense set of data given student vocabulary.

Cynicism:

1. Know what you want to call it

2. Canvass 9000 people

3. Pick the best one that agrees with you (because there must have been collisions)

Sounds like the work of Kristen Wiig in The Martian.

If you know what result you want, why not just have a patsy write your suggestion in, so that you can select it?

This poetry really speaks to me. It could have gone the other way and been named Marszi McMars Rover or something else. It would have been funny, but very uninspiring.

Boaty McBoatface situations seem to occur due to "vote on a name" rather than "suggest a name" as the situation with Curiosity appears.

Voting implies an outcome defined by what is usually an easily compromisable web-poll, whereas the suggestion-route implies some curation.

This seems dramatic, but there is a ton of life left in the wheels. Some of my colleagues performed life testing on the wheels after we first discovered the cracks and found the rover is capable of driving around on the titanium support struts even after the aluminum wheels have completely disintegrated. We've got a long way to go yet!

If Kerbal Space Program taught be anything about space, it's that most problems can be solved with struts.

If only Curiosity were as fast as the KSP rovers... top speed is somewhere around 100 meters/hour, and usually more like 40!

Should have strapped an SRB on the rover.

Reminds me; I wonder just how much ion engines are overpowered in KSP compared to their real-life equivalents. Or, put differently, if one could replicate a hovering Minmus rover I made in KSP (which used ion engines to stay afloat for a long time).

Minmus surface gravity is 0.491 m/s^2; current ion drives thrust with a couple hundred millinewtons, so I don't think a drive could even support itself against that level of gravity let alone with power and a ship attached.

However, we currently design our ion drives for power-to-thrust efficiency and long-term effectiveness. I'm not sure what we could get if we designed them for maximal thrust. Especially since in real life, right now the answer to "Do you want a 'maximal thrust' ion drive?" is pretty much "Well, have you considered using... not an ion drive?", so I'm not sure I've ever seen a treatment of that question based on current tech.

> I'm not sure what we could get if we designed them for maximal thrust.

Probably something with a worse specific impulse than a chemical engine.

The problem is less our ability to make the drives themselves have a high TWR (thrust to weight ratio) so much as that the power generation necessarily required to run them is very heavy. See here:

http://hopefullyintersting.blogspot.com/2015/03/rockets-elec...

The VASIMR engine is sort of that - an ion engine designed to be able to increase thrust when needed.

KSP: 4200s Isp, 2000 N thrust

Real life: 2000-20000s Isp, 0.01-2.5 N thrust

KSP's ion engines are hilariously overpowered in terms of thrust (and thrust-to-weight ratio) and power efficiency. It's a necessary change, however, since KSP is ill-suited for transfer burns several months long.

The flip side is that Curiosity has never, to my knowledge, flipped over and tumbled at near-escape-velocity, shedding solar panels and scientific instruments as it goes.

That really is slow. My brain interpreted that as 100 m/s, and then 40 m/s, and I read the sentence a few times trying to figure out if you meant that rovers go that fast in KSP before I figured out it said 100 m/hour.

It's a completely non-realistic game. It's lacking of duct tape is inexcusable.

Darth Bubba: "I find your lack of duct tape disturbing."

There's not a duct tape mod?

The KSP subthread makes every space article twice as much fun :)

That's funny. It's taught me that if you make a rocket long enough, no amount of struts will prevent it from wobbling itself completely apart within 60 seconds of blast off.

Struts won't, but turning off automatic stability 'assistance' will!

When they break though, I could imagine them getting tangled around things immobilizing the robot.

You're right - you can see the cable harnesses to the drive actuators in the original article. A major concern would be if a big flake bends inward and manages to sever those wires.

They don't build 'em like they used to!

They used to break within days or hours.

> found the rover is capable of driving around on the titanium support struts

Anyone who has watched enough of the COPS television show could have told ya that you can go quite far on just the rims.

Heck, check out what some normal people do on /r/justrolledintotheshop...

So, I don't get it. There are obvious issues here:

Angular ribs cause stress points. Good CAD software should tell you this.

Some metals are able to flex a small amount repeatedly, forever, without cracks. For other metals, "small" is just zero. Any flexing will eventually create cracks. Aluminum is a bad metal. Titanium is a good metal. Steel varies; unfortunately the good types tend to corrode.

You have aluminum over titanium. Putting dissimilar metals in contact encourages corrosion.

Mars is known to contain perchlorate salts. This is horrible for corrosion. Note that corrosion encourages crack propagation. Oh, and Mars has other stuff to not mix with aluminum: iron oxide and chlorides.

They didn't test against pointy embedded rocks, because there where few of those where the small rovers had been.

The main problem was the angle of the front and legs: if you drag a rolling suitcase up a curb it's all good, but when you try to push it up, you can generate forces much higher than the weight of the suitecase. Same happened to the rover.

I read the biggest danger is something pointing up and cutting the exposed cable running to the central motor controller, potentially shorting it. That would stop all motors on the rover (wheels and a few more).

This appears to be a brilliant case of reliability engineering. I'm always super impressed with these sorts of tales; given how NASAs budget always seems to be concern seeing them deliver solutions which have just enough wiggle room in them to deliver the intended results without going too far into gold plating their solutions really leaves the software infrastructure engineer in me in awe. Even more so when you consider the time spans involved in this; I personally believe we've a ton to learn in the software space from practices like this.

I think we also have a ton to learn in the software project management space. Agile for example would be quickly laughed off and dismissed.

Our field is currently full of cargo-cult practices, studies and best practices without any empirical research, java schools, a strong focus on "awesome" and easy to get started instead of useful and simple; we're still a pop culture for the most part.

Solution dictates the construction process. No point in over engineering a consumer facing app when flexibility dictates who wins in the market place. Not endorsing agile or any other practice in particular.

What's wrong with Agile?

I mean, I wouldn't design a rocket, bridge, or certain kinds of software that way... But talking about most web front-ends or feature services?

Use the right tool for the job. Risk profiles between web app software and rover software are totally different.

It's harder to sympathize with "it's ok to write shit code if failure doesn't mean a literal explosion or being literally stranded on Mars" after a completely preventable identity theft...

Its especially hard when that mentality doesn't account for future feature requests or maintenance. The original developer happily moved on to botch another project while you're the one fixing up the mess.

Also, how come software is either a "web app" or a "rover". There's a huge spectrum in between. Not everyone writes CRUDs; even if these types of applications are usually the worst to maintain because they are developed so carelessly with little regard for simplicity.

To be fair, NASA software engineers don't exactly act like your average agile start-up team,because once the "product" is out, patching is really feasible. Whereas for your average webapp, it's more strategic to iterate based on feedback.

Yep. NASA can't release patches (hardware, least) and switching to "version 2" costs a solid $800,000,000. I feel confident that if the development costs of reworking a webapp were measured in years and millions, we'd see a pretty different approach to building them.

Whoops too late to edit. Should be "patching isn't really feasible".

Except they DO send out patches. Just not very often.

Well, my point was it's a bit more trouble to patch something on another planet that it is to `heroku push.`

And yet, NASA frequently updates code on launched vehicles. In fact because of schedule and orbital mechanics, Spirit and Opportunity were launched before final flight code was ready, and an OTA was performed while the vehicles where in flight:

http://spacenews.com/jan-15-2004-spirit-rover-begins-amazing...

Same thoughts here, they know how every part in their system can fail.

There's a fantastic story relating to this in Roving Mars (about Spirit and Opportunity).

The final design had both lander-departure ramps and the now-iconic foldout solar panels, but it was initially rejected as "gold plated" on budget and weight arguments. The only reason it eventually came to pass was the realization that while the foldout solar panels were overkill, their removal would be worse than anticipated and bring the mission under minimum lifespan.

Steve Squyers talks about how he was told this as "terrible news", but realized that for him it was wonderful. There was no way NASA would accept luxuries, but if the 'budget' design wouldn't work they were willing to move up a notch. So the incredible longevity of Spirit and Opportunity is largely a function of a low-cost design that broke down too late to salvage. It almost killed the whole project, but got record results once things went through.

lesson #1 is stop piping curl output to bash as part of your software installation procedure.

that probably won't work on mars.

I actually worked in reliability, albeit only for one summer and on the precursors to Curiosity. I was more on the electronics side of things though. Um, AMA?

Breaking after a mere 10 miles, to me sounded really awful, but apparently they planned on this. The construction must have been to save on weight, making the wheels no more reliable than the exact distance they wanted to travel: "...milled out of solid aluminum. The wheels contact ground with a skin that's about half as thick as a U.S. dime, except at thicker treads."

I wonder if stresses build up from the day night cycles on Mars - which I suspect would be pretty harsh on finely machined bits of metal.

Maybe that's why it's a metal and not something more traditional.

and lots of radiation

Sort of. The early wear was much faster than anticipated, so they've since been using softer ground (sand instead of rock) as much as possible. You don't want to carry around any unnecessary grams on a rover, but I suspect the next one will have sturdier wheels.

I thought NASA used metric units; would be nice if their press releases did too. I have no clue what "half as thick as a U.S. dime" means.

It's easily Google-able. A little more than half a millimeter.

Agreed. A number of millimetres would be a more helpful comparison.

It would be about .675 mm. A dime is about 1.35 mm thick

That's a description that most Americans can more easily picture than something like 0.68 mm (including people who are comfortable with metric units like myself) and they seem to be focusing a lot in recent years on making the information they provide more accessible to average Americans, undoubtedly to help drive interest in and support for what they're doing.

I'd prefer if they wrote something like "0.68 mm (about half as thick as a US dime)." That way the number is still there for people who don't want to look up how thick a dime is.

Good point, I should have added that translation in.

Are you implying you're the author of this press release? It looks like you were job hunting 6 months ago (based on your one HN submission) -- now you're writing press releases for NASA??

Edit: Nevermind, I'm a dummy.

There really was no need to go through the user's submission history. Try not to overreact next time.

I was interested to see his profile if he worked at NASA in this capacity. He had one submission -- it's not like I pored over his history. I don't think I overreacted -- I was impressed that someone could get to a point that they're authoring NASA press releases so quickly.

Like I said, though, I'm a dummy...

It's very likely that the mechanical drawings were in inches for the machinists, even for NASA. The scientists are using metric units.

No. That would not be the case.

What makes you say that? I've worked for two spacecraft companies in the US who supply parts (and whole spacecraft) to NASA. Both of them did drawings for small parts in mils (thous) for both electrical and mechanical parts. It seems like most machinists in the US are used to working in mils.

I remember on my first day (year 2012), I was shown an E-sized drawing of a spacecraft bus at 1/10 scale. The unit was inches. I was surprised it was in inches and said something. The person showing me just laughed and said welcome to aerospace.

The Wikipedia for mil is full of machinist lingo: https://en.wikipedia.org/wiki/Thousandth_of_an_inch

The mechanical specifications I've seen for newer (post-2000, say) components at the lab that built this particular robot have all been in metric units. I just checked, and the current requirement is that metric "shall" be used, unless (a) a particular mission waived that practice - I don't think MSL did; or (b) for parts from "industries that are historically in another unit of measure." Two examples I know of are gears and printed wiring boards, which are indeed in mils.

And you're right, it is possible, barely, that the wheels were an exception. I don't know if they were fabricated on lab or not.

Thanks for the explanation and glad to hear that space is slowly moving towards metric.

Maybe they should have a second, international press release.

Just a "10 miles (16km)" and "half as thick as U.S. dime (0.68mm)" - either directly in text or as on-hover/on-tap tooltips (for web) - would've did the trick just perfectly.

"half as thick as U.S. dime (0.68mm)" is ambiguous: is the dime that thick, or the wheel?

Or you could say something like:

...10 miles on a tire only 0.68mm thick, about half as thick as a US dime.

I'd give them a pass for the press release -- it was paid for with U.S. dimes.

Remember that the rover weighs almost 2000 pounds, so each wheel is responsible for carrying around 330 pounds on average over very rough terrain.

Is that the weight on Earth or Mars? Sites listing its specs are unclear on that.

Wikipedia says the dry mass is 899 kg, which is about the 2000 pounds of weight (on Earth) mentioned by GP.

https://en.wikipedia.org/wiki/Curiosity_(rover)

Which would make it about 338 kilos on Mars - wikipedia lists Mars' gravity as 0.376g (vs earth's 1)

Technically that would make it 899kg on Mars, the kilogram is a unit of mass. However the Rover would weigh ~8800N on Earth and ~3300N on Mars.

The pound is a unit of mass, same as the kg (it's defined as exactly 0.45359237kg). Just as 900kg is 900kg everywhere, 2000lb is 2000lb everywhere. The colloquial weight is literally a force, and its imperial unit is thus the pound-force.

This is incorrect. The pound is a unit of force, similar to a Newton. The pound-mass(notated lb-m) is similar to the kg. The conversation you gave above is based on Earth gravity for weight.

> This is incorrect. The pound is a unit of force, similar to a Newton. The pound-mass(notated lb-m) is similar to the kg.

That's the pound-force (lbf).

Pound and pound-mass are aliases. Indeed the official facts sheet defines the mass of the rover in kg and pounds.

> The conversation you gave above is based on Earth gravity for weight.

My comment is based on mass being an intrinsic property of matter, gravity does not enter the equation.

There are about a dozen different units that are all called "pounds," and you have to rely on context to know which is meant. When I took physics in high school, pounds were always force (abbreviated lbf) and we used slugs for mass ( https://en.m.wikipedia.org/wiki/Slug_(mass) )

It's the weight on Earth. Just look for a source that specifies in kg - there's no ambiguity about what the SI unit represents.

No, ambiguity was introduced when the measurement was switched from force to mass. The question remains: what is the weight/force experienced on the wheels on Mars? Knowing the mass won't tell us unless we also know the acceleration due to gravity on Mars.

Yes, but it looks pretty tore up and the treads seem like they're coming completely off. I was under the impression that the wheels would have been built a little more sturdy. I know it's not the same, but car tires last for thousands of miles. So you would think that a hyper efficient tire, battle tested on earth terrain, would be designed to last at least 100. This is after 10 miles.

Edit: To clarify, I know they are thin and to save on weight. I just never thought it'd be designed to only last 20ish miles. Though another comment has said it will run fine without a lot of this material.

It's been four years, the wear could be as much a function of time as of travel. Car tires on earth dry out, crack and disintegrate over time.

The skin is "about half as thick as a U.S. dime". A car tire that thin wouldn't last a 10 mile drive.

The tires are extremely thin to save on weight. Take a look at a dime and think half of that!

the wheel skin is 0.75mm thick, and was specifically designed to a goal of 10 to 20 km with the smallest weight possible: the rover mass is 899kg, the launch vehicle totals was 531000kg - by reversing the payload fraction one can see that for every gram of rover one needs more than half kilo of rocket to get it to mars, so there's that - you can get outside of your launch weight real fast.

this article has all the answers you're looking for:

http://www.planetary.org/blogs/emily-lakdawalla/2014/0819063...

short story - the wheel were tested to specification against expected rocks etc but once on the boogie suspension the front wheel had to support weight plus the force of the other wheel pushing, so there's where the extra punctures come from, this is what force was not accounted correctly.

Thanks for the reminder of scale. Without a reference, it's easy to forget how large this rover is, in pictures it looks very cute.

Here's a good reminder:

  http://i.imgur.com/3Hg9O.jpg

This shows engineering test models of Pathfinder, MER (Spirit/Opportunity), and Curiosity. It was taken in the Mars Yard at JPL, which engineered all three.

"[...] a mere 10 miles"

You must be kidding... this is a robot, which survived a hard landing, driven on exotic alien terrain, with async remote control from another planet :-)

The fact that they built it and drove it for over 15km is absolutely impressive by my engineering standards.

I think he's just commenting on the wheels, not the mission as a whole. We obviously are good at making wheels that last longer than 10 miles here on Earth.

Agreed but I would call the landing anything but hard. The precision and care with which that vehicle was delivered to the surface is just a mind boggling feature of engineering.

For anyone who missed this, they lowered the rover to the surface from a levitating rocket-powered skycrane.

I don't know if there's another true sentence from which I can derive as much joy as this one.

...which was controlled autonomously to boot!

Definitely one of the most bad-assed ways to deliver a rover to the surface of another planet. If you had told me as a kid this would be possible, I would have considered it almost impossible, except maybe as a science fiction concept.

Is the terrain any more dangerous or grueling than a desert terrain on earth though?

They drive it over desert-like terrains now to avoid such problems, but AFAIK the very first hole punched to one of the wheels was due to a small pointy rock. Martians rocks suck.

From my laymen perspective I'd say the terrain itself is less of a problem, the change in temperatures is much more extreme. This article from 2014 has many insights into prior Curiosity wheel damage:

http://www.planetary.org/blogs/emily-lakdawalla/2014/0819063...

This is a great article. Thanks for the link.

One thing it points out is that the rocks in that particular part of Mars are special: they are sharp and cemented into the terrain. There are places on Earth that have this kind of rock, but they are unusual. The wheels do fine in a rocks-in-sand scenario.

The article goes into a lot of detail, and it turns out that some of the obvious armchair science explanations for what is happening are not the case. For instance, the main constraint on the mass of the wheels was not keeping the liftoff mass of the payload under some limit.

Sharper rocks than a lot of Earth deserts - less wind and no water to smooth things down.

More broadly, though, there are some deserts on Earth that are pretty hair-raising. The NPS guidelines for driving deep in the Canyonlands (on roads, no less) warns that with a high-clearance 4WD pickup, you should still expect "considerable risk of vehicle damage". Curiosity is going a lot slower, but it doesn't have the luxury of roads or anyone to toss on a spare tire.

Depends on which trail in Canyonlands. Some of those are extremely technical crawl trails where you need a spotter and are planning every ten feet of movement, not unlike the Rubicon. White Rim is more of what I would call a road; watching a couple videos of people running it, I'd be less worried about damage there. (It's on my list for this spring.)

Honestly Jeep owners read a warning like that and get excited. Every scar in my paint is a new story, but to your point, getting the tools out in the backcountry is usually not fun.

Yeah, fair. I was specifically quoting the warnings for the Maze, which is remote enough that they desperately want to avoid tows and rescues.

I'd love to do White Rim whenever I get a chance, the overlooks onto it make it look wonderful. The Maze stuff looks much less friendly, but it's on the 'someday' list.

Oh, sure. The Maze was the area I was thinking of as well. I have a few thousand dollars of work and a lot more practice before I could comfortably consider those trails. I know my limits and I'd rather not hike out of Utah.

This is not obvious from the perspective of someone who's not familiar with rocketry or how light space structures need to be.

I'm curious(and this is in no way meant to belittle the work they've done) - if you drove a normal Earth car on the surface of Mars, how would the wheels and tyres behave in Martian conditions? Obviously we have off-road tyres here on Earth which can withstand some extreme abuse, so I'm just wondering - is Martian soil just full of sharp objects? Or is something else going on here?

Well, regardless of how well an Earth car would perform, it'd likely be far too heavy/expensive to get to Mars in the first place. It's impressive how NASA can solve problems with the resource constraints they have.

It would obviously be a dumb idea, but Curiosity is in the same ballpark as a car in both size and weight (1,982 lbs). A Smart Car is ~1,800 pounds, so it's actually heavier than some Earth cars!

Interesting, I didn't know that Curiosity was that heavy! It's a ways away from Jeep Wrangler/military Humvee territory (especially if you loaded one of those up with the required scientific instruments) but it'd be cool to see NASA go with something heavier-duty for longer-range missions in the future.

I think low temperatures alone would wreck Earth tires (and the rest of the car, but that's a different story).

It does have sharp objects, but ordinary truck balloon tires would handle those relatively well. But at night, the temperatures drop to -100 °C, and your rubber tires freeze and shatter.

Also, rubber's much heavier, and they just couldn't bring that weight. They really couldn't drop that weight from the sky crane, not without increasing the shock absorbers in the legs... which would have meant more weight, and the plan's just impossible.

It also has horrifically abrasive dust, much worse than anyplace on earth. The abrasion from that would wreck a balloon tire seal---hard to predict how quickly.

A balloon tire also goes from "fine" to "useless" with one puncture. Apparently they got surprised by the sharp rocks: http://www.planetary.org/blogs/emily-lakdawalla/2014/0819063... , not having seen anything like that from previous missions.

That is the best solution for Mars, but you might need to build the rover in a freezer. Tires that would be soft at Mars temperatures would be liquid or worse at Earth temperatures. For example, you might fill a normal tire mold with diesel, then cool it with liquid nitrogen and there you go -- a lovely tire for Mars that will melt at normal Earth temperatures.

You'd still want to go airless probably, due to a lack of repair shops.

Well, the tyre will almost instantly go soft as the inflating gas reduces in volume due to the cold. Then the tyre will go flat as the gas escapes (try driving for a couple of years without pumping your car tyres up). And the rubber will freeze and crack, and get UV perishment and low-pressure outgassing of its own.

The balloon-like tyre of choice is a springy basket mesh, as seen on the Lunar rover. But that's only needed because we wanted to drive around at human-acceptable speeds (to get to interesting places before the humans had to go home). With a teleoperated robot, we can take our time, and that means we don't need bouncy suspension. Which is good, because big wire tyres are quite heavy.

Let's also note that the top speed of the rover in optimal conditions was 0.09 mph (0.14 km/h), and it took them a full year just to reach the first kilometer of travel.

I'm sure you would have done a better job. Now please tell me: How would you have improved on this given the mission constraints?

Are you 100% sure that this isn't a case of 'why didn't they?' where laypeople not involved in a project know exactly what should have been done by the experts that were involved in the project?

Why the harsh tone? The parent poster outlines their initial incredulity and subsequent learning. Nowhere do they suggest that they could have done better.

They planned on this, but the article (and NASA) does not reveal whether the wheel damage was expected at this point or sooner or perhaps even somewhat later.

It was much worse than initially expected.

This comment from a while ago links to a really good blog about the wheels: https://news.ycombinator.com/item?id=8206378

Here's a direct link to the 2014 blog talking about damage to the wheels: http://www.planetary.org/blogs/emily-lakdawalla/2014/0819063...

Apparently the new rover - Mars 2020 - will have narrower, thicker wheels with more growsers with a different shape https://mars.nasa.gov/mars2020/mission/rover/wheels/

NASA is funded by Americans, and this is a press release not a scientific paper. US customary units and dimes is intentional for the audience.

Also very descriptive, since dimes are also noticeably thinner than our other coins.

The wear patterns on these wheels always seemed so strange to me, considering the extremely slow pace at which this thing moves. The wheels look like they've been hit by shrapnel. Obviously I have no context as to what conditions are like on Mars, but it just seems strange that driving over rock, slowly produces wear like that. Seems like they were never strong enough to support the weight, over something sharp.

I used to think the same thing, it looks pretty intense for just rolling over some rocks! But then I learned that in between the "treads" the aluminum is "half the thickness of a dime" (as the article puts it).

So if you think of it like the lid of a can of soup or something, it's easy to see how it could deform and break like that when rolled up against sharp rocks with a ton of weight on top of it.

I understand that, which is why I said it never seemed strong enough to begin with. I get you need to conserve weight, but doing so on the wheels, seems, crazy? The most important thing of an entire car is the tires. Without good tires, a race car can't do anything, no matter how well engineered the rest of it is.

I think it's a gamble. If they made them thick enough to never break, they could have blown the whole weight budget. But if they make them too thin, the whole project is a waste.

I mean the wheels are probably among the heaviest single "things" on the rover, so adding even a small amount of thickness will really increase the weight, and that would mean cutting other things.

It's a tough call, and I'm sure as hell glad I wasn't the one that had to make it, but I think all things consitered they did a good job! It's been running for 5 years and even though these look bad, apparently it still has a lot of life left in it!

Surprisingly the wheels only weigh a few pounds each! (I still can't think in metric and I don't have a real number.)

They were primarily constrained by packing volume during cruise (https://mars.nasa.gov/msl/mission/spacecraft/cruiseconfig/) and then by shock loads as the securing pyrobolts blew during landing (https://youtu.be/Ki_Af_o9Q9s?t=4m14s).

I'm curious how you came to the conclusion that the wheels are the most important part of the rover.

Without wheels, the rover can not rove. Pretty much makes it useless if the wheels are broke, correct? Just like any vehicle, the wheel/tire is the most important piece regarding performance. Without good contact, all that fancy machinery won't do a thing.

Definitely, but couldn't you argue the same about other important components? For example, without an engine/motor, even the fanciest wheels will get you nowhere. I think it's somewhat of a paradoxical argument.

So the way I see it is that all of the major components are equally important, but you want to optimize your overall design to be both light and reliable.

Of course you could, that's why you don't skimp on those things. You can get by with a crappy interior, but things like wheels/tires/transmission/engine are high priority. That's my whole point here, it doesn't look like the wheels were over engineered enough, but I'm not a rocket scientist.

That wasn't clear from your original comment though. In case of the rover, other important things include comms/nav, movement controller, power source, and sensors.

There are two major factors - fatigue (think bending a paper clip repeatedly) due to driving over tons of small rocks, and the the mechanics of having six wheels contributing additional damage to the front and middle wheels. Think of pushing a rolling suitcase versus pulling one - when hitting an obstacle it tends to dig in instead of easy popping up. The front and middle wheels are in front of their pivots, and so tend to get driven in to rocks by the other wheels.

In terms of metal fatigue, aluminium doesn't last very long at all. That's one reason why you want electric cables made of copper, not aluminium.

True! The actual induced strain is not particularly high (the wheels do not visibly deform) but it's the cycle count that will get you. The chevron-shaped grousers also act as stress concentrators, which is why the cracks are all starting at those tips.

So the rear wheels are on trailing arms, but the front and middle are leading arms, which increase the incidence angle by the angle of the arm in regards to ground? So impacting a 45degree slope would have the force of say 55 degrees, as the arm is ten below level? The trailing arm would then impact that same slope at effectively 35 degrees. Just trying to understand their conclusions.

I would think that given the lack of water temperature variation based weathering is dominant on Mars. If this is true the rover is essentially driving on a surface similar to a pile of freshly smashed glass.

Tangential, but the quality and resolution of that image is incredible.

Are the materials used in the rover wheels too exotic? I'm metallurgist, but I know that steel, though heavy, is malleable and, if correctly tempered, unlikely to break like the rover's wheels. At what cost ultralight, exotic metals?

Edit: Are they really made of aluminum? If so, while hardly exotic, definitely more brittle than steel. I know steel is heavy as hell, but of course you can use a thinner steel for an equivalent strength (still heavier I suppose).

One of the first things that I got told in my materials class was that for the strength and weight, carbon steel and aluminum are essentially the same. Steel is about 3x as dense, but also 3x as strong, and does not fatigue like aluminum.

I'm assuming that NASA knows this, and chose aluminum for another reason that I'm not aware of.

I would love to see a write-up of everything we've learned about wheel engineering from the Mars rovers. What would the team(s) responsible for Curiousity's wheels do different now? (I don't mean to imply that their design is not successful -- just that I'm sure they've learned from this iteration)

Fun little bit of trivia: those slots in the wheels are Morse code, and they spell out "JPL." The original design had the letters JPL emblazoned on the wheels but they felt this was a little too tooting-your-own-horn, so they made them Morse code instead.[0]

0. Personal communication from MSL engineer

Would be less confusion in this thread, if the title added "as predicted and on schedule for successful mission completion"

as predicted after more tests. they where surprised about the ammount of damage initially.

So ir broke after how many years of life longer than the expected utility? Like forever, iirc...

You're probably thinking of Spirit and Opportunity, the smaller rovers which landed in the early 2000s and long outlived their primary scientific missions. This is Curiosity, the SUV-sized rover which landed in 2012, and is still in its primary mission.

Not quite - primary mission was two years (one Martian year). We are in Extended Mission #2.

Oops, you're right! Thanks.

this picture definitely helps with comparing the relative scales of the rovers

https://marsmobile.jpl.nasa.gov/images/Evans_Mars_Yard.jpg

Did you know the wheels have morse code embedded in their tread? https://www.jpl.nasa.gov/news/news.php?feature=3497

Meanwhile, Opportunity keeps going after 13 years.

https://mars.nasa.gov/mer/mission/status.html

It's a shame that the trend in increasing rover size probably won't continue. If scaling does continue, we might expect to have rovers the size of monster trucks on Mars by 2030.

Am I the only one that thought they meant the tracks left on the ground?

even blizzak coatings last more than 10 miles

this doesn't bode well for their JD Power rating

I guess if you start the odometer at launch their numbers look a little better

What surprises me is that the wheels aren't covered with a honeycomb-like texture instead of these longitudinals M's

The spin machine is always on at NASA. It's not that the wheel is failing it's "nearing a wheel-wear milestone." Still it's impressive what they have achieved.

How's it possible to get all that wear in ten miles? That's just a trip to church. My car tires don't even register that.

Your car tires aren't made out of <1mm thick aluminium driving over rocks either.

Your car tires are rubber, and designed to last for a long time at speed. Curiosity's tires are aluminum, and are designed for much less and much slower driving. That seems weird, but the important point is their mass: they only weigh 3 lbs (~1,4kg) per wheel, compared to over 20 lbs for the tire alone (without the rim) on a normal car. Mass budgets are extremely tight on space missions, every pound saved can either go towards more scientific payload or significantly decrease launch costs.

Car tires also have the benefit of a nearby support system. They were looking for wheels that would do the job with 100% certainty of never being able to be replaced.

Also rubber would disintegrate given hot/cold cycles and radiation.

And they probably do like 99.9% of their distance over roads in effectively perfect condition.

Your trip is presumably not over some extremely sharp rocks.

Martians popping our tires?

In all seriousness its unclear to me how this could happen so quickly? The article does not really explain why they didn't think through this.

The tires are exactly as strong as needed to perform the primary mission, and no stronger. They are doing that easily. This was successful engineering, not some mistake.

They did think through it, but the rover has already exceeded its planned mission time by over 2.5 years! Wheels can't last indefinitely, and more than doubling the time of a 2 year mission before they see any wear is pretty incredible.

There are a combination of factors not mentioned in the article:

One, encountering terrain with unexpectedly sharp, immovable rocks.

Two, a suspension arm design that happens to put increased force on certain wheels when they encounter immovable rocks.

Three, the wheels have a tread pattern with a sharp, angular geometry, causing stress risers at the points of the pattern, where the thin metal meets the thicker raised treads. The thinner metal thus tends to crack more readily at these points.

(BTW, it is only this last point that I consider to be a legitimate design flaw, given the innumerable constraints the engineers had to work within. The rover is a marvel of engineering on every level.)

An article for reference.[1]

[1] http://www.planetary.org/blogs/emily-lakdawalla/2014/0819063...

Does anyone else think it's odd that the treads are wearing like that after TEN miles? They probably could have done better by sticking a Goodyear on it.

I assume they're titanium. I wonder though if they use titanium crystals as per jet engine turbine blades?

The article states they're milled out of solid aluminum

Turbine blades are made from inconel. And all solid metal is crystalline.

Some axial turbines have ceramic blades

You can grow a turbine blade out of one crystal of titanium rather than many which improves the strength.

Here's an amazing video about other parts of the structure and construction of Rolls Royce titanium turbine blades. It's awesome.

I've no idea about inconel...

https://www.youtube.com/watch?v=aFRdp1Js9Kc

You're mistaken about all solid metal being crystalline - look up amorphous metal. I've held a ribbon of it, it's quite impressive how it flexes. https://en.m.wikipedia.org/wiki/Amorphous_metal

> And all solid metal is crystalline.

https://en.wikipedia.org/wiki/Amorphous_metal

SpaceX "3D Prints" their merlin engine chambers from the same alloy, inconel.

http://www.spacex.com/news/2014/07/31/spacex-launches-3d-pri...

SpaceX's entire Super Draco rocket engine is 3d printed, too.

See https://en.wikipedia.org/wiki/Superalloy

"A routine check of the aluminum wheels" - first sentence of the article.

Titanium is heavier than Aluminium and only a little bit stiffer, so it's not necessarily a good choice for a weight-sensitive load bearing part even with unlimited budget.

However, the pure/single crystalline form is potentially strong enough that it would be possible to use less material than for the non-crystalline form.

The wheels are all ready only like .65 mm thick, how much thinner do you want them to be?

The tread is 0.65mm thick.

If using mono-crystalline titanium can achieve the same hardness, strength, durability, etc. at less mass then why wouldn't you choose that option?

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