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Rough Roving: Curiosity's Wheel Damage 'Accelerated'
astroengine writes "Despite the assurances that the holes seen in Mars rover Curiosity's wheels were just a part of the mission, there seems to be increasing concern for the wheels' worsening condition after the one-ton robot rolled over some craggy terrain. In an upcoming drive, rover drivers will monitor the six wheels over some smooth terrain to assess their condition. "We want to take a full inventory of the condition of the wheels," said Jim Erickson, project manager for the NASA Mars Science Laboratory at NASA's Jet Propulsion Laboratory, Pasadena, Calif. 'Dents and holes were anticipated, but the amount of wear appears to have accelerated in the past month or so.' Although the wheels are designed to sustain significant damage without impairing driving activities, the monitoring of the situation is essential for future planning."
Dear Lord, Father in Heaven, we pray together for the safekeeping of Rover Curiosity's wheels. Although it may be a tool of science, and its discoveries a complete threat to religious doctrine everywhere, she is but a rover on a mission of Peace and Goodness. In your ever forgiving heart, please bless her wheels with durability and robustness.
Amen.
Yep, just give it to the highest bidder for equally shitty work.
Actually, most of the rover was built in-house.
The thinness of the rovers wheels isnt so much about saving money as it is about saving weight.
Every ounce the wheels dont weigh is another ounce for science equipment or batteries.
So im sure they made them absolutely as thin & lightweight as they thought they could get away with.
Yeah, let's just toss out a few instruments and batteries so we can have wheels that last 5 instead of 3 times the planned driving distance.
Gotta love armchair engineers.
As part of planning to send people to the other planets, I am surprised that they do not try to figure out a way to get something back from the landers. I would think that seeing how the materials held up to the conditions it went through would be important data to have.
John
I know weight is important and all, but .75mm of aluminium? Really? Maybe they should have less scientists over there at NASA and more people with common sense who can raise their eyebrows.
Yes, every time something goes wrong, let us point out how "stoopid" those scientists are in hindsight and claim that the "common sense" solution would have worked. Of course, it couldn't be that the people there did a lot of simulations, analysis, and decided that 0.75mm was a reasonable (not perfect - nothing is black and white) thickness and the disadvantage of thicker wheels was outweighed by the advantages of thinner wheels.
Yes, the designers took a risk - that is their job. To clearly assess the tradeoffs and come up with a good design that trades off risk and performance at an acceptable level. Something doesn't work out as you expect? Use that knowledge in the next iteration. At one extreme you have a lot of equipment with no wheels, and the other extreme you have just wheels, no equipment. You want to do the designer's job? Go ahead, show me what your "common-sense" analysis of the tradeoffs are - what equipment would you cut for thicker wheels, and back it up with a detailed discussion on how the benefits outweigh the disadvantages.
So im sure they made them absolutely as thin & lightweight as they thought they could get away with.
Missed it by that much.
The thing I'm wondering about is why they didn't use something stronger than aluminum like titanium -- lighter too!
Density of titanium: 4.5
Density of aluminum: 2.7
So no, not lighter.
In reply to the GGP, here's what I would suggest, and why:
(Note, I work in aerospace)
I would suggest a minimum wheel skin thickness of .08 inches (a little over 2mm, it's a standard sheetmetal thickness) made of structural aluminum alloy (say 2025, or 7075, whichever is most electrically compatible with the suspension, given the pesence of perchlorate in the environment. 7075 is probably the better bet between the two, but 6Al4V might be a good choice too.) With a very generous plating of titanium nitride.
To make up the weight, (which would amount to only about 100 grams on the high side, give or take) I would look at using smaller radii on the machined parts of the suspension, using lighter gauge insulation on low voltage data wires in the electrical system, and laternative solder formulations. Also, replacing components that don't experiences constant drive or levering forces with ones made of titanium. (Parts of the arm near the wrist, parts of the camera mast, parts of the outer skin, etc.)
Titanium has a strength to weight ratio of 288 kNm/kg and aluminum 214 kNm/kg. So yes, lighter for a given load and stronger for a given weight.
How far has this thing managed to go now? Couple miles?
Tires are stupid anyway. Hey, news flash, PhD eggheads... try these things called "tracks". I'm pretty sure they'll work on Mars...
There would probably be a weight issue with tracks...
Karma: Bad
And tends towards brittleness and is a PITA to machine.
I'm rather sure the nice folks at JPL thought this one through.
Faster! Faster! Faster would be better!
That they didn't even give them an abrasion resistant coating tells me that you had beancounters making engineering desicions.
It's the nature of the beast. The launch costs were just shy of $195 million. The mass of the vehicle ended up being 900 kg. That's roughly $215,000 per kg or $100,000 per pound. That's just the ante for putting something on the surface of Mars. Shaving off a mere 5 grams saves you more than $1000 just in launch costs. You then have to add in the testing to make sure the coating actually stays on and such.
Given that this decision didn't actually endanger the mission's success, it was a successful gamble too. That indicates to me that the bean counters were actually engineers.
Hey man we don't need none of yo science, we got Common Sense Internet Man here, who read 4 sentences on the topic and is gonna design him a better rover than all them eggheads!
10 PRINT CHR$(205.5+RND(1)); : GOTO 10
The rover is designed to perform a certain mission for a certain length of time. There's no point in putting tires on it that outlast the instruments. Everything is designed to have roughly the same lifespan - so yea, the tires will be worn out by the end of the rover's mission. That's all they need.
The thinness of the rovers wheels isnt so much about saving money as it is about saving weight.
Every ounce the wheels dont weigh is another ounce for science equipment or batteries.
So im sure they made them absolutely as thin & lightweight as they thought they could get away with.
...and every ounce the probe doesn't weigh is another few hundred pounds saved in fuel for the launch vehicle.
This is reflected in the amount of power the Voyager probes put out - not even enough to power a digital watch, yet we're still getting science from them. The legwork is done on Earth, with vast arrays of massive radio telescopes gathering and filtering the signals. To put out enough power for an amateur radio astronomer to be able to pick out of the cosmic background... we'd probably have had to launch each probe with a Sizewell-B sized reactor. That's 1.1GW for those not versed in "How many football fields is that?" units of measurement. Obviously not practical in terms of escaping the gravity well.
Personally, I wouldn't worry about it until two wheels on the same side develop mission-fail flat spots. This is probably why it's got six wheels - a four wheeled vehicle would be at mission end with the failure of any one wheel. This puppy can withstand two failures and keep on truckin'.
Political debates have me rolling my eyes so much I think I got optical whiplash. I should sue. - Foamy The Squirrel
how much science has gone in to building the Rover wheels? Mountain bike wheels have been using the same technology for decades - a steel hub, steel spokes, steel rim, and air-filled rubber tyres. With the Mars Rover, they had to think about:
Tyres: no good in such a rarefied atmosphere - they'd explode, that's if they didn't explode on the way. Plus there's no way to stop and repair a puncture. Solids are making a comeback (again), but you run in to weight issues (I know, I've had newtech solids and they are *heavy*). Tyres are a necessity in any vehicle expected to roll across the surface of any body, but do they have to be solid? They're only there for grip and suspension, you can get the same effect using sprung metal cantilevers.
Rims: here we have steel and several alloys, and recently carbon composites have made an appearance. Weight is again an issue with all of these, and consider the fact that carbon composites don't react well to UV, that's those out. You've got to develop light and strong alloys.
Hub: With the number of moving parts in a hub (the average bike wheel has over fifty!) any one of those failing can end a mission. Here they had to think about a fail-proof redesign of bearings that not only had to be maintenance-free, they also had to have as close to zero failure probability as possible. Something which can only be achieved by reducing the number of parts to the point where you can not only predict when a part is going to fail and under what conditions, but also what can be done to mitigate that probability.
The short of it is, this all comes down to mass. The Mars Rover was lucky to have been equipped with SIX wheels. If the launch systems engineers had gotten their way, the mission would have been equipped with four wheels and been over long ago (probably not even having fulfilled the mission parameters - MR is on its own clock now, everything it sends back is just icing) and the LSEs would be up a few hundred kg of fuel for the next launch. How inconvenient, they have to fabricate some more!
Political debates have me rolling my eyes so much I think I got optical whiplash. I should sue. - Foamy The Squirrel
That's the #1 reason. If we had infinite thrust with no fuel consumption, we could put up ... well, some really cool stuff. Spacecraft wouldn't need to be concerned with stuff hitting them, if they could put a mile of dirt around a steel reinforced concrete floating bunker. :) And we'd probably have a few Stanford Torus' or Bishop Rings in orbit already.
Looking at the design, and the images, the front left wheel is actually pretty rough. In the linked story, look at the bottom half of the 9th picture. The metal has split almost half way across. It may end up digging into the softer sand, and could catch on rocks.
They'd have a better chance driving it backwards, letting that wheel drag along. That won't work very well though, since all the gear is on the front.
Serious? Seriousness is well above my pay grade.
By my calculations, 1000kg distributed across 6 wheels is 166kg on earth, which translates to 62kg on mars. What surprises me about the damage is that a copy of the rover was stress tested in death valley, so this level of wear and tear should have been anticipated, as the force on the wheels there was much higher. I wonder if another environmental factor, like mars' extreme cold, or increased radiation due to lack of magnetosphere is affecting the materials in the wheels.
I've decided to Diversify my Holdings. I've divided my cash between my left and right pockets, instead of all in one.