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This question is similar to my other question "would walkers work on airless worlds?", however even though it's the same setting and I am talking about the same battle group tactic that question dealt with the big boys of war, the vehicles so big it would scare a "Deserts Of Karakh" tank Commander (I don't know the exact size of the things), this question is about vehicles on the size order of an m1 Abrams or AT-RT (two legged walker about half the size of an AT-ST), the main purpose of these is as anti-infantry or ant-anti-Walker vehicles. So a far different discussion.

So for a bit of background on the scenario: the earth-moon system is engulfed in the greatest war it has ever seen, a war waged between powers on both worlds over the fate of the earth-moon system. That's all I have so far... I am talking about fighting on the moon in this case but it applies to any space-rock without an atmosphere.

Since I am trying to figure out the small boys I am simply trying to figure out which is better, tanks or small walkers. They're mostly anti-infantry or anti-vehicle so they require the ability to carry either weapon and to be fairly quick. I am currently for small walkers so let me list of the reasons why.

Now the tanks have the advantage that when tread is blown it doesn't normally crash, two-legged walkers do, four-legged ones similar to crab droids would become stationary. But Walkers have one huge advantage, the lunar dust! if you don't know the lunar dust is so fine that it will get into electronics if it gets stirred, the problem is so bad that any wheeled vehicle would likely stir up so much dust that it could explode because of the amount of dust in it, consider multiplying the amount of deadly dust stirred up by a tank and multiply that by the amount of dust hundreds of tanks would stir and you have the big problem of more than half of them blowing up on the way to battle, consider now how reduced the problem is when you walk on legs, and fairly high. A transport walker (like the AT-OT) could fix the problem of the en-route problem but the dust could still destroy many tanks during the battle.

Edit: Credit to TCAT117 for convincing me that in the case of small vehicles tanks are better, even on the moon. my main concern was that the lunar dust would be a big problem, but it wasn't, and that low g would force the tanks to go ridiculously slow, but war is slow. The jury's still out on number two, I'm keeping this on still because if a pro-walker person can prove to me that walkers can easily go faster than tanks the small walkers will be more important than I am currently going to make them. I'm still going to have big ones though.

Edit: I have come up with a bit more on the scenario that should give you a better idea about why these ground forces exist. First of the earth and moon are both separated between many different governments: the earth is split between a few dozen to over 300 separate nations, the moon probably has over 2,000 nations (the exact number isn't important). but the Lunar armies are for this nation on nation combat. If your attacking from earth the combat will be different.

Edit:

So far so good, I am getting alot of answers talking about the uselessness of land vehicles on the moon and if you think you have something to add then please address it, all of the discussion around that however was before the edit above so I would ask that anyone with that viewpoint fit their answers to the more detailed addition to the scenario on edit two.

But if you want to discuss the how different land vehicles compare then it might be helpful to know that the main factor I am considering is the Lunar regoliths ability to destroy vehicles in transit.

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  • $\begingroup$ Are you set in it having to be either tanks or walkers? are hovercrafts also possible? riding on a self contained cushion of air would not topple over, have low amount dust blown up & if disabled will not crash likely killing the pilots while still allowing to go just about everywhere on rough terrain $\endgroup$ – cypher Mar 11 '18 at 18:33
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    $\begingroup$ We tried to tell you in your other question why walkers stink, and the myriad ways of knocking out armor. You ignored us then, so why would you pay attention to us now? $\endgroup$ – RonJohn Mar 11 '18 at 18:39
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    $\begingroup$ Regarding dust getting stirred up: Remember that there is no air on the moon. Dust doesn't form lingering clouds like on Earth or Mars. It drops straight to the ground with 1.62 m/s² like everything else. For reference you might want to watch the moon rover videos from Apollo 15-17. $\endgroup$ – Philipp Mar 12 '18 at 15:37
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    $\begingroup$ As a rule of thumb, unless your walkers are actually armored creatures or some sort of cybernetic elephant, you're better off with tanks or drones. Wheels and tank tracks are that good. $\endgroup$ – T. Sar Mar 12 '18 at 16:31
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    $\begingroup$ if your lunar dust was that deadly to mechanical components I don't see how any ground based civilization could be formed, your civilians walking around would be lifting small amounts of it just moving about and with repeated traffic their habitats would inevitably fail $\endgroup$ – BKlassen Mar 12 '18 at 20:55
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Aerospace engineer here, I see there's already an accepted answer but want to chip in some additional points that I don't see made in any of the other answers.

Any vehicle operating on the lunar surface has to deal with several things not normally encountered on Earth:

  • Gravity is one-sixth (1/6) that of Earth
  • There is no atmosphere
  • The temperature in the sun can range up to +300F, while in shadow can drop below -300F
  • Very irregular, rocky terrain with wide craters and a thick layer of regolith
  • Lunar regolith is extremely fine and abrasive, meaning it gets into everything and damages everything it gets into

Lunar gravity being so low is actually a benefit: vehicles which would be very heavy and lumbering on Earth would be much lighter (and therefore more agile) on the Moon. Variable suspension (think the trick hydraulics in show cars) or jump jets could allow a vehicle to leap across obstacles, which would normally be impossible on Earth.

The lack of atmosphere is a significant problem for any vehicle, because it means that any oils or greases based on volatile petroleum products would evaporate within minutes, and as such would not be suitable lubricants. NASA uses a variety of dry lubricants, but these can be high cost, high maintenance, or simply not as effective as traditional oils.

The massive temperature variation is another good reason to ditch traditional lubricants in favor of dry ones. A follow-on consequence of this temperature swing is that joints or bearings requiring high precision would not function well, due to differences in thermal expansion.

Finally, perhaps the greatest challenge is the combination of terrain and lunar soil. Wheeled vehicles are mainly designed for smooth surfaces and can only handle rough terrain with advanced suspension systems like those used in the Mars rovers. Traditional rubber wheels also don't work on the Moon due to the afore-mentioned atmosphere and temperature issues, so you need to use something else like metal mesh or plastic, which limits their top speed and load-bearing capacity. Tires also can't be too skinny, or they'll sink into the thick regolith and get stuck.

Tracked vehicles are better able to handle irregular terrain and they spread their weight across a much larger footprint, so they aren't likely to sink. But the drawback with tracks is that every single tread is double-jointed (front and rear) and track systems usually require half a dozen wheels on each side, or more. That's a lot of bearings to lubricate, and a lot of places for regolith to get inside and destroy the joints. And unfortunately, there's no way to keep the regolith out of those joints since they'll all be in constant, direct contact with the ground.

Assuming you've solved the problem of controlling multiple legs (or have a decent supply of Handwavium), legged vehicles have substantial advantages over both wheeled and tracked vehicles in this environment. They have far fewer joints than a tracked vehicle (3-5 per leg) and those joints typically do not need to rotate more than 180 degrees, which means they can be covered with boots to prevent regolith contamination. The feet can be sized for different types of terrain or modes of walking: wide and flat to support a lot of weight without sinking, small and narrow for lighter vehicles or difficult terrain. A legged vehicle can also climb or jump over obstacles that would be difficult for tracked or wheeled vehicles to traverse (here, 6 or more legs is beneficial). Legged vehicles may also have the ability to right themselves if tipped over (assuming the legs have sufficient range of motion), which most modern wheeled or tracked vehicles cannot do.

The biggest benefits I see with using tank tracks are increased speed over smooth terrain, reduced chance of bogging down in thick regolith, and lower vehicle profile. The biggest drawbacks I see are extremely high maintenance requirements (due to wear on the treads and wheels), and mediocre mobility in very rocky terrain.

The biggest benefits I see with using legs are increased speed over rocky terrain, low maintenance requirements (very few of the joints are in direct contact with the lunar regolith), and self-righting capability. There is also the additional bonus of redundancy if you have a 6-or-more-legged vehicle; if one leg is destroyed, the others can reconfigure to permit continued (albeit impaired) movement. The biggest drawbacks I see are high system complexity (independent motors for joint articulation and a sophisticated computer to correctly operate the legs), a need for some precision joints, and higher vehicle profile.

Given the comparison above, personally, I'd go with a walker. Besides the performance and maintenance benefits, it would be pretty awesome to skitter across the lunar surface in an armored cockroach carrying a big gun. ;)

Additional resources on the properties of lunar regolith: https://curator.jsc.nasa.gov/lunar/letss/regolith.pdf

https://www.universetoday.com/20360/lunar-regolith/

http://adsabs.harvard.edu/full/1973LPSC....4.1159T

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  • $\begingroup$ Alright, my question really does rely on how bad lunar regolith is, I will do some research into it and my first stop will be Wikipedia. I ask that you provide additional resources for the properties of regolith if you can. $\endgroup$ – skout Mar 13 '18 at 1:37
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    $\begingroup$ Small point of contention: reducing the weight of a vehicle by placing it on a smaller planetoid does not reduce the inertia of its mass. I'd think something like a battle tank would be less maneuverable on the moon due to a lower effective friction, between the lower weight and the dusty regolith. The important characteristic is not the weight but the mass-to-grip ratio, which makes moonbound travel more difficult than earthbound travel. $\endgroup$ – Asher Mar 13 '18 at 17:28
  • $\begingroup$ @skout Sure thing, additional reading added to the bottom of my post. Asher, it is true that reduced gravity doesn't reduce inertia. I was thinking more about the vehicle footprint pressure (an important factor for how readily the vehicle will sink into the soil) and energy required to move it. But traction and mass-to-grip are both very important as you rightly point out! I think you're right that a traditional battle tank would be less maneuverable on the moon, but that doesn't necessarily extend to all tracked vehicles. $\endgroup$ – MikeB Mar 13 '18 at 23:09
  • $\begingroup$ You should be able to get all the motion you want with only to motors -- one to switch which joint you want to move and one to move it. It's not going to be pretty though. I suspect having multiple motors (2 or 3 per leg) is the better option, if more expensive. $\endgroup$ – Clearer May 16 '18 at 9:36
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I was an Anti-tank gunner for 8 years with the Marines, I'll let ya in on why walkers will never be used by a military as an armored vehicle.

First, joints are huge weak points. Not just to enemy fire, but also from a maintenance perspective, and the sensitive servos and gyros required will be infinitely more vulnerable to dirt than anything on a tank. Next, a walker is by nature of being able to walk going to expose more surface area to enemy fire. A good armored vehicle is low to the ground for a reason. Armored vehicles make heavy use of terrain as cover in an attempt to present as small of a target profile as possible. Modern anti tank guided missile weapons basically mean literally anything from infantry to a small drone aircraft can be packing enough heat to destroy a main battle tank. This is why armored vehicles are going to be trending towards smaller and faster instead of bigger and heavier. Finally there is gravity, the moon's gravity makes any sort of walking extremely difficult, this only becomes more pronounced the larger the vehicle is. A large walker could be knocked over too easily in low G to be useful.

Dust is not something that is going to make a vehicle explode. We've been fighting a war in the harshest deserts on our planet for 16 years and our modern tech can deal with sand and dust just fine. Dust will jam up the joints and servos on a walker though.

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    $\begingroup$ Regolith really isnt that special. You can find stuff just like it in helmand. Its annoying but not catastrophic. It was only a big deal to apollo because they were afraid it might abrade a space suit joint. Its dangerousness has been gradually exaggerated over the last 50 years. $\endgroup$ – TCAT117 Mar 11 '18 at 20:50
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    $\begingroup$ Dust vs. joints or bearings strikes me more as being different rather than one being better. Joints you could cover with a flexible cover (e.g. what covers your joints), versus shaft seals where you'll always have rubbing contact of some sort. I'm wondering if a vacuum would complicate things for whatever lubricant is in the bearing, or it could be just fine; I don't know. $\endgroup$ – Nick T Mar 12 '18 at 0:27
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    $\begingroup$ I would like to comment that being low to the ground does not preclude walking propulsion. It's largely a factor of how the vehicle is designed. The fact that what most people think of is stuff from Star Wars (which are generally huge, an AT-TE is 9.57m tall and that's one of the shortest walkers in the movies that's equivalent to a tank) is largely a result of culture, not a fact of engineering. $\endgroup$ – Austin Hemmelgarn Mar 12 '18 at 1:57
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    $\begingroup$ The AT-TE still sits high enough off the ground to walk around under. Its lower to the ground, but its still a ridiculously tall machine to be expected to use for armor support against other armor. Our MRAPs were considered unfeasibley tall at 12 feet and were frustrating as hell to try to use for combat vehicles. The gunner was basically in a tower holding up a sign reading "RPGs go here." Humvees were infinitely better for actual maneuver combat, MRAPs just took IED blasts better. $\endgroup$ – TCAT117 Mar 12 '18 at 2:02
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    $\begingroup$ Star Wars is also a universe where people move objects with the space magic powers that exotic bacteria gave them. Walkers are a fantastically stupid military vehicle idea that requires a factor of 10 higher maintenance for very very little gain and a whole host of detrimental features that normal vehicles don't have. Star Wars has walkers the same reason every other sci-fi series does. rule of cool. If you want them in your fictional story for the same reason that's fine, but if you want hard science then its a doubtful tactical option. $\endgroup$ – TCAT117 Mar 12 '18 at 23:44
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From first principles: if you have got big squared away, then one could argue small should be as small as possible - 1 soldier. There is a continuum in scifi between powered armor suits and small mechs / walkers. For example, this thing from District 9. Big powered armor or a small walker / mech?

https://www.youtube.com/watch?v=m7dizLdsZJw enter image description here

The nice thing about small 1-person powered armor / mechs is that they could move by jumping, take advantage of the low lunar gravity. I do not recall much jumping in the Starship Troopers movie but in the book they wore powered armor that let them travel fast by jumping great distances. That would be great for the lunar terrain.

An infantry unit advancing with a series of bounding jumps would be pretty cool. They could ride on the big walker and then explode off of it like fleas.

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  • $\begingroup$ I was also thinking of a form of jet trooper. $\endgroup$ – skout Mar 11 '18 at 21:57
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    $\begingroup$ I love the idea of jumping mechs. It's skeet shooting with bigger guns....... $\endgroup$ – Thorne Mar 12 '18 at 4:26
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    $\begingroup$ "move by jumping, take advantage of the low lunar gravity" Jump = target. Unless you are in active flight, you can not change your path while jumping. Which means that any anti artillery has a field day. Relatively slow, precalulatable vector -> dead walker. $\endgroup$ – TomTom Mar 12 '18 at 8:27
  • $\begingroup$ @TomTom In a no-atmosphere envirioment, changing direction mid-jump is somewhat trivial with small boosters. $\endgroup$ – T. Sar Mar 12 '18 at 14:03
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    $\begingroup$ @muuski any future weapon system would be able to do so for any target - tank, mech or otherwise. That's not an argument. When you have homing missiles, everything is a sitting duck. Wheeled vehicles are even more predictable (and slower) than the proposed solution. $\endgroup$ – T. Sar Mar 12 '18 at 22:04
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I would suggest something comparable to the LAV-25 used by the canadian and US military would be more appropriate than either.

Do not underestimate Regolith, Regolith is not mere dust or sand.

On Earth we have erosion effects from wind, rain, weather in general and if nothing else, simply motion. on the moon, we do not. The regolith is entirely composed of tiny abrasive chunks of silicate, razor sharp in almost every tiny grain. That stuff is horrible, it clumps and sticks together, it's statically charged and will stick to just about anything, and if you breath it you might as well be breathing knives, it will shred your lungs and kill you.

Lots of moving parts exposed to vacuum and regolith is going to be an awful maintenance headache on its own. Wheels and motors on a relatively lightweight mobile platform will reduce that enormously. the many independent drive systems mean that if anything jams up it'll be able to keep going, and the 8 wheels will spread the weight more than sufficiently in the lower gravity.

Legs have been discussed to death, the only practical reason I can see for using them on the moon is to avoid dust by literally stilt-walking through it. The terrain on the moon is not rough enough to merit legs even if the gravity makes them easier to construct and move around with.

Treads meanwhile are 100% exposed to regolith and essentially guaranteed to clog up with it. The big draw of treads is the hugely effective weight-distribution and surface-grip. but in lower gravity you don't need the weight distribution nearly as much, I couldn't comment on the grip issue.

There is very little that can't be done by a wheeled platform instead of a conventional tank when mass is a reduced issue.

For an example in science-fiction, you might look at the Mako vehicle from Mass Effect. A futuristic version of the LAV-25 I mentioned at the start.

Lunar combat is going to be a messy affair, a nightmare combination of knife-fight range direct combat and satellite guided munitions.

Your horizon is nearer than on earth, likely nearer than it should be simply because of the craters, you have no over-the-horizon radar (on earth, we achieve this by bouncing radar off the ionosphere) your only way to track things beyond the horizon or even line-of-sight is by satellites and flying craft that can directly see them.

You may be able to achieve limited long range sensors by observing surface-vibrations from ground vehicles moving around, but in all likelihood the regolith will act as a vibration absorber, another argument for avoiding legs as the pounding footsteps will alert any such sensors like the vibrating glass of water in Jurassic Park.

Explosions, particularly fragmentation weaponry will be murderous, on earth a frag grenade is intended to injure as much as it is to kill, on the moon, any suit-puncture is likely to be immediate death without some seriously clever automatic sealant technology, even then the task of getting back to a pressurised environment is going to take long enough moon-warrior is likely to die of their injuries.

Any infantry operating on the surface will definitely want some form of armoured vehicle support, if nothing else to provide a med-evac and triage.

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  • $\begingroup$ any wheeled vehicle would bring up dust to the insides just like treads. also "big powerful footsteps" aren't exactly the biggest problem, the point of walkers is so that your vehicle wouldn't die on the way to battle doing nothing other then traveling. $\endgroup$ – skout Mar 12 '18 at 20:16
  • $\begingroup$ Fewer external moving parts means you can protect the mechanisms far more easily. A tank has dozens of wheels and hundreds of interlocking linkages for the treads. A wheel'd vehicle has only the half-dozen wheels and their join to an otherwise sealed hull. Wheels are decidedly less exposed in this scenario. For the same reason, a walking machine with multiple articulated joints is going to have a lot of exposed mechanisms to jam, a lot of weak-points in combat, and simply won't be as fast in a straight line as a conventional vehicle of the same mass. $\endgroup$ – Ruadhan Mar 13 '18 at 16:23
  • $\begingroup$ But the walkers are higher up. $\endgroup$ – skout Mar 13 '18 at 17:19
  • $\begingroup$ Moon dust doesn't tend to billow in clouds, it's more likely to cake. so in that regard a walker has the advantage as the surfaces getting covered in dust are all non-mechanical (excepting the ankle joints) however being tall in war is almost always a liability. it means you don't benefit from terrain-cover, it means you're a bigger target, and it means your stability becomes an issue, especially with recoil from cannons or impacts from explosions. Your big advantage is the vantage-point for firing weapons means you effectively bring your own high-ground to shoot down at your enemy from. $\endgroup$ – Ruadhan Mar 14 '18 at 12:54
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It depends entirely on how agile your walkers are - if they're basically tanks on legs because of rule of cool, then ordinary tanks are going to be much more sensible and feasible due to reasons elucidated in other answers. In particular, survivability is a big plus for tanks - apart from incredibly thick armor, hard-kill active protection systems (APS) are becoming a de facto standard on newer models, which allows the tank to automatically detect and destroy incoming threats that would have been fatal to previous generations of vehicles. Essentially, tanks are becoming mobile fortresses that are mostly impervious to anything except aircraft and enemy tanks of equal capability.

On the other hand, if your walkers are capable of jumping, dodging, and frustrating enemies by being generally too nimble to hit with weapons strong enough to bring them down, then you have a case. The low gravity, coupled with reaction jets, would make them even more maneuverable. Armor would be minimal (protection against small-arms fire at best) because (a) one hit from a high-calibre weapon and the unit is out of the fight anyway (b) more armor would affect agility, which is this unit's most important attribute.

The walker's joints being a weak spot is a moot point because the assumption is that the enemy forces are having a hard enough time hitting the main body of the walker, let alone its limbs.

For weapons, it depends very much on how esoteric you want to get. With a humaniform walker, the conventional approach would likely involve installing a high-calibre weapon akin to a tank's turreted main gun, in place of one of the "arms".

This all assumes, of course, that the enemy up against these walkers doesn't have/isn't using the simplest way to defeat hard-to-hit glass cannons, namely radiation-seeking missiles. (Take for example today's BGM-71E TOW, which can [theoretically] be carried and launched by infantry and is capable of defeating up to 900mm of reactive armor.) Presumably the faction using walkers would be acutely aware of this vulnerability and utilise tactics to minimise it, e.g. by trying to get their enemy to fruitlessly expend the ammunition of this weapon.

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  • $\begingroup$ "if your walkers are capable of jumping, dodging, and frustrating enemies by being generally too nimble to hit with weapons strong enough to bring them down, then you have a case" A case hwich is better for a non-walker (i.e. flying drone). YOu will mostly rely on thrusters anyway (thanks to low gravity9, so why bother with all the walking apparatus on top? $\endgroup$ – TomTom Mar 13 '18 at 13:16
  • $\begingroup$ People have a vastly mistaken belief in how fast missiles and other projectiles actually travel. $\endgroup$ – Keith Morrison Mar 13 '18 at 14:15
  • $\begingroup$ @KeithMorrison Could you elaborate? $\endgroup$ – T. Sar Mar 13 '18 at 15:22
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    $\begingroup$ @T.Sar, people often talk about "dodging" and being "too nimble to hit" when it comes to weapons because they've seen too many fictional representations where the weapon is coming in slow enough that it can dodged. Consider the reality: a rocket like the CRV7, velocity of around 940 meters per second. Which means that if someone gets a launcher to within a half-kilometer of the dancing robot, the machine has about a half-second to identify the threat, calculate trajectory, and move to avoid. How far can a robot move to avoid in well under a half second (assuming very fast threat ID)? $\endgroup$ – Keith Morrison Mar 13 '18 at 22:45
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    $\begingroup$ @Ruadhan, and if there are two enemy tanks? Much of the time when I see people trying to justify something like this, they seem to assume a fair fight--one mech vs one tank, or one mech vs one helicopter--when one of the primary jobs of any military commander is to make the fight as utterly unfair as possible. $\endgroup$ – Keith Morrison Mar 14 '18 at 16:09

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