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The surface of Curiosity's wheels have picked up quite a lot of damage with significant holes beyond the "JPL" holes that were designed in. By contrast the surface of Opportunity wheels have held up pretty well.

Is there any assessment of what factors have contributed to the greater damage on Curiosity? For example:

  • rover weight
  • driving technique
  • terrain
  • wheel material
  • speed
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According to this Planetary Society blog post, it's a combination of two things:

  1. Wheel design. The wheels have a different tread pattern than Spirit and Opportunity, for better traction on soft sand.

  2. Terrain. Tears in the wheel surface come from driving on bare bedrock, where the tread pattern causes excess flexing of the wheel skin, leading to fatigue failure. Punctures come from driving on pointy rocks firmly embedded in sand (think: caltrops), a type of terrain they'd never seen before.

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    $\begingroup$ From the same article: "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." Wheels can withstand pointy rocks, but not the other 5 wheels pushing into the pointy rock. $\endgroup$ – Juancho Feb 1 '15 at 15:19
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The wheels were designed with minimum weight to reduce stress on landing gear deployment motors and on total payload by 10-20 kilos; they are 20" in diameter.

The wheel material was a bad choice: aluminium has a fast fatigue fracture potential compared to other metals. It doesn't elastically rebound into shape, but instead it memorizes stress and microscopic fractures at stress points accumulate rapidly over time. The lifetime of a good steel frame bicycle is 10 times longer than that of an aluminium one. So aluminium is not a good choice for wheels. Spirit and Opportunity had 10" aluminium wheels which were obviously more rigid and less burdened.

The aluminium between the ridges is 0.75 mm; the contractors who made them must have known the wheels were not suitable for long distance rocky terrain.

The small stones on Mars are called "ventifacts" and are sharp like walking on smashed roof tiles. The design team had not expected to have such sharp ground and the rover was designed entirely for sand and pavings.

Here on earth no one would ever use 0.75 mm aluminium for a volcano/desert rover to do 50 km (2.25 mm equivalent for earth gravity). We use cable reinforced rubber or Kevlar foam, and aluminium is just about the least suitable wheel material for a cross country rover, because it memorizes stress and its lack of springiness.

Tests have proven total failure at 8 km of all rock terrain, and limitless lifetime on sand. The Opportunity rover covered 42 km in 11 years, so they will still be able to run Curiosity for many years with limited terrain choice, and it should still be a very successful mission with a very long lifespan.

The design criteria of the wheels at the time of launch were publicised to be its good traction on sand and the distance measurements using the grooves, and the JPL written in Morse code in the grooves. The design mindset seems to have misjudged the surface of Mars to be strangely softer and less earth-like than it actually is, and to have not done distance testing on rocky surfaces. That's born out by the fact that the wheels fail after only 8 km of rocky terrain. That's how long they take to be split in two all the way around.

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  • $\begingroup$ It's just possible that the reason "Here on earth no one would ever use .75mm Alu for a volcano/desert rover to do 50km" is because Earth has three times the gravity, so you should probably adjust that to be clearer that (in your view) even 2.25mm Al would be unsatisfactory on Earth. $\endgroup$ – Nathan Tuggy Dec 26 '15 at 6:02
  • $\begingroup$ Oh yes good point. I don't know what to say about that. I have worked with 2mm aluminium previously, it's somewhere in between steel and plasticine, it dents very easily, you can almost score it with chalk and wood. You could reshape the Curiosity rover wheels with your hands if they were not reinforced. I don't know how to measure the .75mm wheel VS gravity. Space rover wheels are a new science, no one had to make any lighweight weels previously. $\endgroup$ – predatflaps Dec 26 '15 at 6:18
  • $\begingroup$ I figure that NASA had so many pre-launch tests to do it didn't physically test the wheels like Pirelli wheels are tested for 50/100km, It can't have done tests in moderately rocky surface, the design was focused on the JPL morse code chevron traction system to ensure good traction and not slipping laterally on slopes. The rover weighs 300 kilos in earth weight and has 6 wheels, so in a rocky scenario it's supporting 300 kilos on reduced area of an aluminium drum. $\endgroup$ – predatflaps Dec 26 '15 at 6:38
  • $\begingroup$ I've worked with 2mm aluminum as well, and it really depends on your alloy -- nobody uses pure aluminum for engineering purposes. As for rubber, at Martian temperatures it becomes brittle, and you risk shattering your wheels the first time you hit a serious bump. $\endgroup$ – Mark Dec 26 '15 at 9:17
  • $\begingroup$ yes sorry i didn't consider that. The performance of aluminium would be different on mars even more brittle. For aircraft grade Alu, I know that you can saw it faster than a strong oak using the same saw. Cable reinforced rubber and the materials that Darpa has developed are so wonderful, it seems avant garde to use Aluminium as an abrasion/knocking element of the craft. $\endgroup$ – predatflaps Dec 26 '15 at 9:41

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