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Wheel Damage Adding Up Quickly For Mars Rover Curiosity
An anonymous reader writes: The folks in charge of the Mars rover Curiosity have been trying to solve an increasingly urgent problem: what to do about unexpected wheel damage. The team knew from the start that wear and tear on the wheels would slowly accumulate, but they've been surprised at how quickly the wheels have degraded over the past year. Emily Lakdawalla at the Planetary Society blog has posted a detailed report on the team's conclusions as to what's causing the damage and how they can mitigate it going forward. Quoting: "The tears result from fatigue. You know how if you bend a metal paper clip back and forth repeatedly, it eventually snaps? Well, when the wheels are driving over a very hard rock surface — one with no sand — the thin skin of the wheels repeatedly bends. The wheels were designed to bend quite a lot, and return to their original shape. But the repeated bending and straightening is fatiguing the skin, causing it to fracture in a brittle way. The bending doesn't happen (or doesn't happen as much) if the ground gives way under the rover's weight, as it does if it's got the slightest coating of sand on top of rock. It only happens when the ground is utterly impervious to the rover's weight — hard bedrock. The stresses from metal fatigue are highest near the tips of the chevron features, and indeed a lot of tears seem to initiate close to the chevron features."
The things are the thinnest element in the entire lander. When I first saw those wheels, I just shrugged and figured they knew what they were doing. But the reality seems to be that they stuck with some sort of legacy design and somehow nobody ever asked the obvious question about those miserably thin wheels.
Though maybe I should instead be celebrating the fact that they didn't get their metric crossed with their imperial.
The Opportunity (MER-B) Rover landed on Mars January 25, 2004. More than 10 years later, it is still going strong even though it, too, was only expected to perform a 3 month (90 day) mission.
The success and longevity of the earlier Mars rover missions sort of sets expectations that future missions will last just as long....
We of course realise that is not possible. Plenty of missions end early, Spirit (MER-A) got its wheel stuck and got in trouble years ago but Opportunity keeps on running and sets unrealistically high expectations of Curiosity and future missions.
You can tell how powerful someone is by the magnitude of the crime they can commit and be able to get away with.
The planned mission duration was 2 years not 3 months.
http://en.wikipedia.org/wiki/Curiosity_%28rover%29
You are thinking about Spirit and Opportunity, whom both enormously exceeded their planned mission duration.
Solarpanels ? Curiosity is powered by an RTG not solarpanels.
http://en.wikipedia.org/wiki/Curiosity_%28rover%29
You are thinking about Spirit and Opportunity, whom both have solarpanels.
Spirit
Obligatory, because it's beautiful.
Simply build a wheel changing robot and launch it to Mars.
If it's too difficult, I can't understand it !
Pic of the wheel ...
http://www.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
Muchas Gracias, Señor Edward Snowden !
Curiosity is not spirit or opportunity. This is a much heavier rover. Plus, it consumes way more power and moves faster. The forces on the wheel are much much rougher than on the MER rovers.
A fool and his hard drive are soon parted.
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....
Aluminum does not have a fatigue limit. That is, no matter how beefy you make an aluminum part, after enough cyclic stresses it will suffer fatigue failure. This is why airframes are retired after about 100,000 pressurization cycles - to avoid the fate which befell the de Havilland Comet.
Other materials like steel or titanium can be designed so it can withstand an infinite number of stress cycles and not fatigue. Given the nature of the mission and power source (multi-year if not multi-decade operation on another planet with no hope of human intervention if something should go wrong), they really should have allocated sufficient weight budget for non-aluminum wheels. This is basic materials science that every undergrad mechanical engineer learns. I was very surprised when I heard they were going with thin aluminum wheels on this rover.
Curiosity's RTG, like most that came before it, is powered with Plutonium-238. Pu-238 is an alpha-particle emitter, 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.
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.
Build it, and they will come^Hplain.
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.
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.
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.
As an embedded systems (electronics/firmware) engineer, I was going to half-jokingly, half-seriously say, "Well, we'll just send a new firmware update to Curiosity to help with the problem." And then of course as I read the article, that was one of the proposed mitigations:
I've been developing embedded systems for more than half my life, and I never get bored...
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). Making the range a round number like 40 C seems reasonable in this instance.
I don't get what's so hard. Just have Jebidiah Kerman exit the rover and fix the wheel.