How can I explain why my mechs don't sink into the ground?

Everybody loves the concept of mechs, but there is one big practical problem with them that gets worse the fewer legs they have: ground pressure. tl;dr: big heavy object on two small thin spindly ones isn't very good at not sinking into even the hardest ground.

Currently I'm handwaving past this issue with anti-gravity type generators installed in every mech (although these could have interesting tactical implications with respect to their failure), but I'd prefer something a little more substantial and/or closer to what we know today to make my worldscape more accessible to readers.

About the only thing I can come up with is some sort of ridiculously light and strong super-alloy, but that also feels like a somewhat obvious piece of "technological magic".

16 Answers
16

The ground is pretty strong

In an answer to another question, I demonstrate that a 20,000 ton kaiju shouldn't have too much trouble moving around on land, with feet only twice as large (area wise) relative to its body as a human.

A human sprinter puts out a ground reaction force of 3000 N or roughly 150 kPa (from that link). For a 100 ton mech (heavier than a main battle tank), ground force at rest is 1 MN (roughly), which requires only ~7 m$^2$ of foot surface area to yield equivalent ground pressure of a human sprinter's foot. Triple this for safety, and divide by four legs, and you would need each of the four 'feet' of the mech to be around 2 meters square in size. Not super large, not super impractical.

Warhammer 40k features Titans, gigantic bipedal war machines ranging from 20 to 80 meters in height, layered in armour and bristling with weaponry.

They explicitly don't care about ground-pressure. Where they go, they leave meters-deep footprints and crush boulders to gravel. To a thousand ton walker, soil and rock might as well be ankle-deep mud.

However, the largest of them compensate for their extreme mass by using anti-gravity systems to reduce it. These don't so much hover as lighten the load enough to walk without sinking to the bedrock.

This toy has booms around 80 meters long and can weight up to 14,200 metric tons, depending on configuration:

It leaves some tracks on dirt, but that's it. Notice how it goes around without sinking into the ground, which would be very bad for it since it has no legs.

As long as your mechs have a weight-to-surface ratio not less favourable than the bucket-wheel excavator's, and are not walking on quicksand, they shouldn't have to bother about sinking.

I've talked about it before, when your Mech starts growing you are going to need more legs. And using small excavator arms as legs it's practically feasible to have an 8 legged Mech of 100+tons, something that tanks have trouble with as the tanks are limited by length and width just as much as a Mech is limited by the square cube law.

If you want to have as few legs as possible then the biggest problem is ground pressure. However you can use that sagging into the ground as an advantage. For example instead of wider feet you create a spiked cone at the bottom that quickly widens. The surface of the cone already is larger than the surface of the feet. The cone will drive itself into the ground and as it does so the supportive surface grows. Additionally the weight of the Mech wont just push the earth down, it'll push it sideways meaning less groundpressure for the ground directly below it. Since you are also into the ground you have a better grip, allowing you to climb steeper inclines. This is also the part where a Mech becomes useful: it wont have lots of advantages in open terrain due to lower velocities but in hilly or mountain terrain a Mech's mobility would be higher than a tank. Still, making larger legs or splitting the feet into multiple smaller "legs" with a large surface area they stand on (and the ability to stand better on uneven terrain) are additional options.

Another often tauted disadvantage is disabling the legs. While a destroyed leg is definitely worse for a Mech than a tank, a Mech would be the only vehicle that can effectively armor the entirety of the leg. Additionally anyone firing at the legs themselves with an AT weapon would be send back to training as firing at the smaller and constantly accelerating/decelerating legs is a recipy for disaster. Anyone with training would aim for the joints where the legs meet the chassis which is practically the same as with a normal tank. That would make a Mech still less vulnerable to disabling fire due to the armor on the legs but the consequences for a successful hit are larger.

Edit: for those against the spike idea, you can easily expand on it and design better. For example a cylinder with a hollow shaped like a cone on the bottom of the feet: the edges are "sharp" and push the soil to the center of the cone where it compacts. It also adds surface area and even watery mud would get high pressures to support the feet as it goes deeper.

Larger feet are what camels use to prevent sinking into the desert. Your mechs could use that technique (it reduces pressure by increasing the area of force application). They could also have rockets or boosters of some sort to lift them out if they do get stuck.

Also, the idea of mechs sinking into the ground could be a cool plot device, explaining why mechs can't be used in a certain situation. Booster fuel could be a limitation on the amount of time a mech can walk before it has to return.

From an engineering perspective, firm clay can take 10 tons or so (100kN) per square metre, without issue. That's the typical maximum loading for house foundations here in London, and they have to be reliable for decades or more.

Ignoring the massive safety factors built into that working, and also ignoring the need to be rigid (if the ground sinks a metre does the mech care?), and also ignoring the need for stability over time (gradual changes over decades don't matter), it wouldn't surprise me if you could safely load of the order of 10 times that on clay. And of course that's on a soft material.

Dynamic forces are a bit different but probably won't make much difference - if the mech can cope with a metre of ground movement as it places a foot, the dynamic force issue will probably be swallowed up within that movement, or by the mech's handling of foot placement, and feedback of ground support/resistance.

Short version - not a worry.

Don't change the properties of the mech - change the properties of the planet

Given that it is not specified where the mech action is happening, set the action on a low gravity world (or worlds), rather than using handwavium to provide artificial antigravity. Mechs are far less impractical in low gravity, where with the addition of jets/rockets they can blur the line between ground and air units.

This also has the advantage that the poor pilot/s stuck in the mech are subjected to less impact force from landings - movies love showing giant mechs making equally giant vertical leaps. This ignores the fact that without the "knees" bending much more than typically shown on every landing, for the pilot a fall of 20m in a big mech is just as damaging as a fall of 20m in your average car (less survivable if the mech pilot is standing up miming the mech's actions).

Don´t use steel bodies

Although iron and steel may be the first option that comes to our head when talking about mechs, we must not forget that actually there are many tracks of scientific investigation of new alternative materials that can be stronger than steel and weight much more less. Assuming the Mech technology is still many years in the future, it is not unlikely to suppose that Mechs could be made of a non-metallic composite material (much more lighter, and therefore with less risk of sink into the ground).

And the trick is not only in the material itself, but in the way the material is shaped. For example: a single sheet of paper cannot support itself horizontally between two separated bricks. But if you bend the sheet of paper and make a rectangular long box (a prism) you can colocate it horizontaly supported by the two separated bricks, and the same paper will support itself properly.

At a molecular level, you can play with the structures of some materials and enhance a desired characteristic. Take for example, the graphene: graphene is stronger than steel but breaks like glass. However, you can build a spongy graphene structure able to be lighter than the flimsiest plastic and 10 times stronger than steel (not commercially available yet, but works nicely in the laboratory).

With a more advanced manufacturing technology (in the same future where the mechs are fabricated) surely there will be more options like this.

Here is the link to that article about the graphene material:
https://www.livescience.com/57432-stronger-than-steel-material.html

EDIT: And just to clarify: This super-alloys are in no way "technological magic"

:)

More legs to spread the weight.

Also, you could have the legs "splay" a network of supporting struts at each step, multiplying the "feet" surface either above or immediately beneath the surface. In both cases, this has serious implications on the mech's deployability (can't walk on some kinds of terrains, and the mechs could be lamed by shooting at their feet - a very literal Achilles' heel).

If you have two or more legs that distribute the weight of the mech over a larger portion of the ground, it should be feasible.

How large are your mechs going to be? This is a large factor in how much they weigh, and ultimately how large the "feet" need to be. I'd recommend looking into the mechs from Chromehounds, or even the Mantis from Halo for inspiration regarding mech leg structure.

Have the setting be on a planet with less gravity. Overlapping a light alloy, big feet, and a lighter gravity setting would basically minimize the problem.

(An alternate to light gravity is a dense atmosphere and a mech filled with helium. idk how well it would work unless the atmosphere was super dense though.)

Triangular pyramid shaped feet, distributes the weight in 3 directions.

But the key is with hydraulic shocks.

The triangular pyramid shaped feet hold it up and allow its gyro to find its balance. While the hydraulic shocks are what truly keep it from sinking in; as the feet touch the shocks kick in and ease the weight down, unlike plopping it down.

It's like the difference between dropping a bowling ball onto styrofoam vs placing it onto styrofoam. Dropping it destroys the styrofoam while placing it sees no change to its structure. The hydraulic shocks make it more like the weight is being placed onto the ground instead of just crashing into it.

That said, these would still have to be very impressive shocks and maybe hydraulic isnt strong enough but at least that's the direction I would focus on, or at least have my scientists focus on at least.

Hope that helps.

I would suggest that in addition to increasing the size of the contact area of the feet, you also need to change the overall shape of the mech as well.

Given issues like the square/cube law and stability due to the centre of gravity, a mech won't scale up like some monster human, but rather will become wider, have "tree trunk" legs that are proportionally much wider than human legs and likely a relatively squat torso. The idea of "arms" is really a bit of a non starter as well, the changes to balance while moving multi ton arms around (much less the recoil of arm mounted weapons) would make moving and fighting immensely more complicated.

If anything, the form factor might be more like the "Huey, Dewey and Louie" robots from the 1970 era movie "Silent Running"

Sure they're cute now, but imagine if they were 20' tall and bristling with weapons

OF course even with this they will still be vulnerable to virtually everything a modern military force can bring to bear, the Mech will have to deal with everything from 500lb "Small Diameter Bombs" launched from a stealth F-35 100 km away to Infantry solders firing Javelin "Fire and Forget" ATGM's from 4750 meters. Only truly deranged or desperate people would actually approach close enough to use an RPG or Carl Gustav type anti tank weapon.

This suggests the Mech will likely have a large proportion of its carrying capacity devoted to countermeasures such as the CIWS, Trophy active defence systems and 100kW lasers, rather than massive batteries of cannons and missiles. As a mobile protective system for a land based formation, you may have found the niche for Mechs in battle.

Your mechs could just roll around on tread in their feet instead of walking. That way you only have to worry about the weight to calculate pressure and not the amount of force your mech puts on the ground when it walks/run/does taekwando.

This would easily take care of explaining how your mechs are fast, and why their feet are so big. it would also allow you to Piggy back off of tread technology.

You should also start thinking about using some un-attain-ium techno-magic material like gundam does. In Gundam 00 the mechs are built with E carbon, which is just better Carbon fiber. In wing, Gundam's are literally built with Gundanium which is essentially a special blend of titanium built in such a way that it ends up with a damn near perfect matrix - the molecules are all packed uniformly - with highly advantageous properties. You're going to want to make your mechs light since you're going for realism.

Another solution is straight out of pretty much every mech anime. Your Mechs don't walk that much, they fly. This is another popular solution I've seen to how your mech doesn't break the ground while landing after a running jump kick or something else a little dumb.

Look at how much force you put on the ground running vs walking vs standing. There's quite a difference. Just having large feet to reduce pressure isn't the only thing to consider.

In my opinion anti-gravitation devices should not be used as such an explanation. Any Society which has technology on that level would have weapons available which far exceed our imagination and definitely no needs for mechs in the conventional sense. So unless you introduce a very good explanation (e.g. they found devices from another civilization).

'Big feet' (surface area) seems to be the general solution, in addition to "They sink into the ground a bit". Largest land animal being the elephant, foot diameter of 40-50cm, area of about 2 square feet, 13,000 lbs, 2 legs, 3,500 lbs. per foot?

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