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On Earth, there are plenty of examples of vehicles that roam difficult terrain having tracks instead of wheels, while all (at least those I am aware of) extraterrestrial rovers have wheels.

Is there a special reason (like reliability) why wheels are chosen instead of tracks?

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Tracks are heavy, high-friction, and primarily useful in soft/muddy/slippery terrain where the weight distribution is essential to prevent sinking and slipping.

They take much more energy to move than wheels, and while on Earth-bound robots that's not so much of a problem, on a lander this is to be or not to be of the mission.

They weigh quite a bit. I can't imagine sensible tracks that wouldn't be at least several kilograms in the case of rubber, much more in the case of metal.

And as long as we don't plan missions to places that are swampy, six-wheeled design is already sufficient for when one wheel loses traction or encounters obstacle higer than its axis. The only scenario when that wouldn't suffice and tracks are superior is when the whole rover starts sinking in the soil. On Earth, where soil can be damp and loose, moved by erosion and rains tracks make sense for heavyweight, multi-ton vehicles. They make sense for much smaller vehicles like snowmobiles for material as loose as snow. But so far, we haven't discovered a planet (other than our own) where wheels would be so inferior to tracks to grant being replaced, despite the problems with tracks.

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  • $\begingroup$ Small addition would be that having tracks introduces a single point of failure; the vehicle becomes immobilized if either of the tracks is damaged by transport, dust or mechanical wear. Compare to Spirit which has six wheels and thus, even if any one or two of the wheels fails, it can still continue moving. $\endgroup$ – user27492 Oct 4 '18 at 5:58
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A similar question was asked on the Robotics SE.

The Wheels provide a lot of flexibility, like with the rocker bogie system. where the rover can climb over obstacles up to twice the diameter of the wheels

And Tracks are usually heavier than wheels. Making it more expensive for deployment . It's also easier to maneuver with wheels than rely on the skid turning of the tracks.

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From a logistical perspective, solid wheels have a significant benifit over tracks; Maintance. Tracks lower the ground pressure of the vehicle to something on the order of human foot, the trade off for that is two fold.

  1. You still have "wheels" inside the track, ususally lots of them, and they each have hubs and bearings and potential places to fail. Addtionally there are flex points (or one big one for rubber tracks) all of which are just another point of potential failure.

  2. Tracked vehicels turn, by dragging one track, in soft or muddy terrain they very easily fill with "ground". Bad things happen, when the tracks get full, either the track breaks or it falls off the vehicle, in either case it takes significant resources to return to functionable, none of which would be avaialbe for a lone vehicle. On side slopes, even moving forward without turning can fill the track, leading to failure.

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    $\begingroup$ In fact, if a track "fills", whatever it fills with gets milled to dust - thanks to enormous surplus of power and extreme durability achieved thanks to using many tons of metal to construct both the tracks and the engine to power it. Which would be just awesome if we could afford bringing that much dead mass off-planet. $\endgroup$ – SF. Jul 23 '13 at 21:29
  • $\begingroup$ @SF. actually no, I was a tank mechanic in the US Army. Sometimes it gets milled to dust, sometimes the final drive or something else breaks. Then you have to pull the engine (M60 for example) to make room to get the drive out and replaced. It is a task that takes several workers, additional equipment and a goodly amount of time. In theory a couple of hours, in reality in the field the better part of a day, after you get the tank pulled out of where ever it was stuck bad enough to break it. $\endgroup$ – James Jenkins Oct 30 '17 at 17:29
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I should actually add that there is a possible third option. A 4 or 6 legged walker can probably work well on any terrain.

A possible disadvantage is that it could be maintenance heavy.

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    $\begingroup$ A legged walkers, if it can be designed reliable enough for space, probably will have the most versatility compared to other options (versatility is one of the reason why most animals develops legs instead of wheels). However, most rovers are dealing with vast empty terrains not jungles, in which case the additional versatility is largely outweighed by the efficiency and simplicity of wheels. $\endgroup$ – Lie Ryan Jul 24 '13 at 15:26
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Another point is that 6 wheeled robots can still move when one or more individual wheels' motors fail (okay not as elegantly), however, a tracked vehicle is doomed at this point.

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In addition to weight as other people have mentioned, tracked vehicles are extremely maintenance intensive relative to wheeled variants. When the nearest mechanic is hundreds of thousands to tens of millions of miles away, not breaking or wearing out under use is a critical requirement.

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  • $\begingroup$ c'mon. 42km (Lunokhod II which probably holds top record on off-Earth land travel distance) through a desert is really not a mind-boggling distance for a vehicle to cover without maintenance. And 2 engines is 2 possible point of failure, assume P is probability of engine failing, 2P for track-based vehicle to go immobile. For six-wheeled, 2 wheels and out of six and the vehicle is out, so 6/2*P = 3P, that's 50% more probable to fail critically. $\endgroup$ – SF. Jul 23 '13 at 21:25
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    $\begingroup$ @SF. - suggest asking any tanker about changing tracks. $\endgroup$ – Deer Hunter Jul 24 '13 at 12:36
  • $\begingroup$ @DeerHunter: Every 25 miles? $\endgroup$ – SF. Jul 24 '13 at 12:42
  • $\begingroup$ @DeerHunter: Followed your advice. Depends. M1, for example, has two sets of tracks. One of them lighter, which has roughly 1000km of distance in them, one of them - heavier, but considerably so, increases the distance to ~3000km but is very, very tricky to replace on the field. Also, Most Russian tanks are built with 5000km of track tolerance: to potentially reach La Manche from Ukraine. $\endgroup$ – SF. Aug 23 '13 at 12:42
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Look up "thrown track".

Tracked vehicles actually quite often lose one of their tracks, especially when turning. This is a huge issue, for both civilian an military vehicles. With enough manpower, time and winches, they can be re-tracked - on Earth. For a rover, such event would end a mission.

For space use, tracks are ridiculously unreliable.

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I also like the idea of the caterpillar tracks.

For short distances, such as the distances covered by Curiosity, they don't need as much maintenance as suggested.

In addition to that, tracks don't suffer from puctures or bursts.

The only serious problem is that they can derail if not properly designed. But at low speeds that shouldn't be a problem either.

They could be made with kevlar or other materials to get light tracks. Though I know the additional weight would come from the multiple wheels needed to move it.

Regards

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    $\begingroup$ I'd like to see the references that support your view, as this seems to go against published knowledge somewhat. $\endgroup$ – Rory Alsop Sep 28 '13 at 22:29
  • $\begingroup$ The total distance traveled by Curiosity is less than 50km yet, this distance is done on Earth by much heavier tanks in few hours, every day, and they use conventional tracks and run at much much much higher speed. Oftentimes the problem with the tracks is not that they broke, but they are not allowed to operate on many roads because they damage the asphalt. $\endgroup$ – skan Sep 29 '13 at 12:41
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    $\begingroup$ That is like comparing apples to didgeridoos. Come the day we can send a tank to Mars, things may be different. Until then we need to work within mass constraints. $\endgroup$ – Rory Alsop Sep 29 '13 at 12:53

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