5 replies to this topic

[Vilemind]

Okay, so I see a lot of waffle around how "mechs won't work" because of ground pressure.

The figures people quote are often accurate, but the results are massively mispreresented.
Ground pressure isn't a problem when it comes to mechs.

Here is why.

There are two main ground pressure issues a mech has to watch out for.

The first is compression of soft surfaces which would result in the mech bogging down or sinking. This is usually referenced when talking about grass/rural terrain.

The second is breaking through a surface and falling into an opening below. This would stereotypically include falling through streets into the sewers.

In regard to the first problem I will use myself scaled up as an example.

I weigh 100kg at 1.8 metres tall. So I scale myself up to 10 times the height, width and depth. That makes me an Atlas pretty much. 18 metres tall. 100 tons in weight.

Here is where the misquoted science comes in. Square/cube rule. I now have ten times the ground pressure that my 1.8 metre self. This is true. What is NOT true is that this will impede me much.

Let me use an example to illustrate. I walk on soft grassy soil. On each step I sink 5mm into the soil which as the impulse energy of the step landing then rebounds to 3mm. Follow me so far?
So my Atlas Mech self sinks 10 times further. BUT - my foot is 10 times the height.
The scaled effect on my foot as it sinks into the terrain is EXACTLY the same. Actually it is actually better since you can only compress terrain so far, so my Atlas self will actually sink proportionally less far into the ground.

But what about worse terrain? Ok, lets use a bad example. Mangrove swamp mud. I sink 15cm into the mud each step. This is a problem and slows me down. My mech self with 10 times the ground pressure sinks 10 times as far - 1.5m. Again, proportionally no further, so the impact on my movement is the same. And as mentioned above, as the mud compresses further it has an increased resistance to further compression. So I am actually less effected as a mech.

Another aspect that is mentioned is footprint size. Human feet are not very large. Even wearing running shoes increases your footprint (and therefore decreases ground pressure) by around 30%. Wearing snow shoes lets us walk on snow.

So how big are mech feet? The smallest I have seen are around 25% longer in proportion to human feet and twice as wide. So 2.5 times the surface area. Many mechs have much larger feet than that. Let me be really pessimistic and say the mech has human proportional foot length and twice the width. It is now 5 times my ground pressure, but half as effected by soft terrain.

TLDR Point 1: A mech sinking half a metre into the ground is like you sinking 5cm. This is not a problem for movement.

Then we get to catastrophic surface failure. Welcome to the sewers baby.

This one also needs addressing. There are two aspects to this. One is the impact of the foot landing. How much impulse energy is being transmitted to a rigid surface. The other is load bearing when a mech is resting on the surface.

The science is the same. A moving human generates impact energies around 3 times as great as our ground pressure. Assuming the mech has twice the foot print of a human proportional figure, the impact pressure is 5 times as high. As is the resting pressure.

Despite that, mechs do not move as fast as humans do in proportion. A human top level marathon runner averages 14kph. This would mean my Atlas self would need to be doing 140kph to be moving at the same sort of proportional speed. The heaviest mechs running are equivalent to a human walking. Lighter mechs have less problems in the first place. This means their impact energy is lower in proportion than a running humans.

We can assume (but ignore) that mechs have sophisticated shock absorbers which reduce the impact. This also reduces impact energy to the ground, just like the afforementioned running shoes example.

Ignoring that, as I said earlier, the idea that a mech creates enough ground pressure to fall into a sewer is essentially the same as saying 5 big humans jumping and transferring their force to a footprint sized area can punch through concrete. Not happening.

Modern roads have load bearing capacities way beyond the levels that mechs generate. Deformation is entirely likely but falling in, not so much. A further fact against is is that modern cities don't have the cavernous sewers old cities did. Subways are a moot point - usually too far below ground and also reinforced.

The exception would be roads that have been prepared to fail - mined, excavated under or otherwise booby trapped. Or roads that have been severly damaged by artillery/rocketry.

The only potential problem a mech would have is on bridges. Long ones. Even this isn't a weight factor. Any bridge that can take a couple of 18 wheelers will be okay with a big mech. Smaller bridges might fail (the ones that have No Trucks signs by them) but that is par for the course with tanks too.

Nope the risk with bridges is the same reason the protest marches are often refused permits to march over long bridges. The impacts set up some interesting vibrations that can potentially have nasty effects on a long bridge. This is exacerbated by the tendency of groups of humans to walk in step. Even so, my Atlas self weighs the same as 1000 of me. Protest marches causing these sorts of issues are usually a bit bigger than that. So even a bridge isn't likely to be a big problem. Don't go climbing any buildings though.....

TLDR Point 2: Mechs won't fall through sidewalks or roads unless the roads have been booby trapped.

So why no mechs currently in service? Well DARPA among others have been looking in to walkers for years. The tech isn't there. But their info says they are viable once the tech is around. The biggest issue with battletech mechs would be the silhouette and sheer visibility making them incredibly vulnerable to anti-tank or air-ground weapons.

[Adridos]

Vilemind, on 29 June 2013 - 04:24 PM, said:

I weigh 100kg at 1.8 metres tall. So I scale myself up to 10 times the height, width and depth. That makes me an Atlas pretty much. 18 metres tall. 100 tons in weight.

Just a nitpick here, but 100kg*10=1000kg or 1t, not 100t.

[Modo44]

Ignoring a couple of unimportant facts, humans can live in space vacuum.

Check how much track surface area is needed for 50-ish tons tanks to not sink into soft ground. While driving, not running or balancing a towering construct. While having the entire mass just above the ground, not a couple of stories high.

[Vilemind]

Adridos you aren't nit picking. You are misreading - 100kg*10*10*10 is 100,000kg. 100 tons.
Height width and depth. Just as you quoted.

Modo44. Irrelevant vacuum post aside, your tank comment shows the danger of assuming a lot from few facts.

Height of the weight above the ground is irrelevant for ground pressure - balance only.

Driving causes a problem - not solves it.
One of the big issues with wheeled and tracked vehicles is that they deform the terrain as they move across it and have a minimal ability to move from deformed terrain.

Wheeled vehicles in particular dig themselves deeper as they move. Tracked vehicles do the same thing unless they move slowly enough that torque from the tracks doesn't deform the ground.
Once they are a bogged down a certain distance they have no way to escape.
Legged locomotion can do that easily - because the leg is picked up and put down. Sinking a tank 50cm into the ground is a bogged down tank. Sinking a foot the human equivalent of 5cm doesn't impede locomotion at all.


Lets use the easiest possible relevant comparison to disprove your tank weight & mobility = walker weight and mobility, try this.
A decent sized shire horse weighs around 1000kg. A small car weighs the same. Which one works better off road? Which one works better in rough terrain?
If you want to argue that I should have used a tracked vehicle, how about a D2 bulldozer and an elephant? At 4 tons each they work.
If you want to insist on bipedalism there are a lot of dinosaur examples in that weight range.

The comparison issue only comes up because people scale incorrectly, and because the modern versions of walkers are incapable of decent locomotion.

Big bipeds work - at least from a mobility point of view.

Adridos you aren't nit picking. You are misreading - 100kg*10*10*10 is 100,000kg. 100 tons.
Height width and depth. Just as you quoted.

Modo44. Irrelevant vacuum post aside, your tank comment shows the danger of assuming a lot from few facts.

Height of the weight above the ground is irrelevant for ground pressure - balance only.

Driving causes a problem - not solves it.
One of the big issues with wheeled and tracked vehicles is that they deform the terrain as they move across it and have a minimal ability to move from deformed terrain.

Wheeled vehicles in particular dig themselves deeper as they move. Tracked vehicles do the same thing unless they move slowly enough that torque from the tracks doesn't deform the ground.
Once they are a bogged down a certain distance they have no way to escape.
Legged locomotion can do that easily - because the leg is picked up and put down. Sinking a tank 50cm into the ground is a bogged down tank. Sinking a foot the human equivalent of 5cm doesn't impede locomotion at all.

Lets use the easiest possible relevant comparison to disprove your tank weight & mobility = walker weight and mobility, try this.
A decent sized shire horse weighs around 1000kg. A small car weighs the same. Which one works better off road? Which one works better in rough terrain?
If you want to argue that I should have used a tracked vehicle, how about a D2 bulldozer and an elephant? At 4 tons each they work.
If you want to insist on bipedalism there are a lot of dinosaur examples in that weight range.

The comparison issue only comes up because people scale incorrectly, and because the modern versions of walkers are incapable of decent locomotion.

Big bipeds work - at least from a mobility point of view.

[Surtr]

Vilemind, on 29 June 2013 - 04:24 PM, said:

The biggest issue with battletech mechs would be the silhouette and sheer visibility making them incredibly vulnerable to anti-tank or air-ground weapons.

Also the whole demo charge to the knee thing is a pretty big weakness.

[Pht]

The fact that road surfaces in the BT setting are made of ferro-crete should be brought into the equation... even though we don't know the strength/toughness of the stuff.

I figure it's safe to presume that it's a far more capable road surface/bedding/etc.