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In my world, a race of intelligent cephalopods have created mecha similar to the Fighting Machines from War of the Worlds (long story short; it makes sense for then as they evolved and continue to live on this worlds version of a highly mountains, jungle filled Australia).

What I'm wondering is if the flexable leg pattern as depicted in the original War of the Worlds is possible to build in a 1G environment? And if so, on what principle would they function?

enter image description here

Notes:

  • Any form of technology will do. Mechanical, soft robotic, active support, ferrofluid, any thing else. All far game, these guys are very tech savvy.

  • The machines typically range in size from 3 meters high to around 13 meters high. In this question, we'll focus on an average height of 8 meters.

  • The number of legs depends on the job of the machine, with ones designed to carry heaver loads having 6 or more (assuming wheels can't be used, if the terrain is lenient enough), while other machines may have fewer. This question will focus on a machine with four legs.

  • The legs are highly flexible, allowing the main body of the machine to hide behind cover in a variety of environments and to raise the machine to full height, or hid behind a boulder.

  • The main body is non-humanoid and is more oval shaped. It contains the pilot, weapon emplacements (typically either a laser, Coilgun, missile rack or some combination), smaller manipulation arms based off of the same tech as the legs, and the power supply. Armor is present but just enough to survive a few glancing blows or one frontal attack from a sufficiently powerful weapon. The Cephalopods fighting doctrine prefers speed, mobility and stand-off line-of-sight weapons over armor and raw fire power. They do have high firepower weapons, just not on these particular mecha

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    $\begingroup$ What is a flex_a_ble leg? Is it a leg with joints acted upon by flexor muscles, or is it a fancy way of spelling flex_i_ble? $\endgroup$
    – AlexP
    Commented Oct 23, 2022 at 8:12
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    $\begingroup$ IIRC in WotW, the legs were articulated, that is, finely segmented in such a way that was functionally equivalent to flexibility, but built out of hard materials. Therefore, if desired, they could be rigid for stomping trees, cutting down bridges, etc. UUULAAAAAAH! (Aloo! for purists). $\endgroup$
    – user86462
    Commented Oct 23, 2022 at 9:21
  • $\begingroup$ With Martian technology? Definitely possible! $\endgroup$
    – Joachim
    Commented Oct 23, 2022 at 14:40
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    $\begingroup$ A two or three legged creature walks by perpetually falling forward then recovering. A four legged creature is somewhat stable in walking, and can walk by shifting weight around on a stable platform. Creatures with five or more legs can walk on a completely stable platform by shifting weight around the platform. $\endgroup$ Commented Oct 23, 2022 at 16:12
  • $\begingroup$ Question: Does a centipede fall over if it lifts all of its legs at once? Answer: No, it just squats down. $\endgroup$ Commented Oct 23, 2022 at 16:18

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It could be possible in an under water earthlike context.

It would work by having a neutrally or slightly buoyant head/vessel with anchor pods attached to it via flexible umbilicals.

These umbilicals are the legs, and carry power and data to each individual pod or foot.

Submarine context allows the legs to work in traction rather than compression which allows them to remain visually slim.

Each pod can propel itself in any direction and is powerful enough to tow the whole thing.

Each pod can move explore and anchor itself on the submarine floor, collectively all three pods combine in a gait to walk the tripod.

enter image description here

enter image description here

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    $\begingroup$ That is a sweet gif. Where is it from? Your own work? $\endgroup$
    – Willk
    Commented Oct 23, 2022 at 19:48
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    $\begingroup$ @Willk own work $\endgroup$
    – user35577
    Commented Oct 23, 2022 at 22:30
  • $\begingroup$ This reminds me of the squid-like assault craft from Star Wars that the separatists used. $\endgroup$ Commented Oct 27, 2022 at 1:40
  • $\begingroup$ I love this answer so much. The issue is that the machines are terrestrial. Not aquatic. $\endgroup$
    – Seraphim
    Commented Nov 2, 2022 at 6:11
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A few possibilities:

  1. A segmented robot arm just like the more traditional ones, with a small motor in each joint, but with far more joints.
  2. Each segment has a small wire attached to it that runs up to the base where the motors are.
  3. Metamaterials that expand or contract when a current passes them, arranged in origami-like complexity such that a computer can use a few dozen circuits to make it do whatever's wanted.
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Combine Artificial “Muscles” With Nested Carbon Nanotubes.

enter image description here

Arrange them in sheets and parallel with each other:

enter image description here

along with an control system capable enough. You have a very strong artificial squid limb.

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Materials

I feel that this is more a material sciences question than a structural design question. We are talking about fully-formed legs here, which is relatively less exotic than hovering in air or teleporting or things like. So, in short, yes it’s possible if you have the right materials. Since this question is focusing on the leg design and its implications, I will assume that the power plant can achieve the necessary output.

Now, as for the materials and design basis, that’s the real challenge. You’re going to run into a couple of issues out the gate that would require some clever explanation if poked. The major ones I can think of are: ground pressure, leg sheer, and material flexing.

Ground Pressure

We are taking a relatively large mechanical device with weapons and payloads and living things in and on it, and putting that thing up on stilts, basically. Those contact points are going to have a very high pressure level, unless the feet are comically large and well designed to distribute the weight of the device well. The other solution to this is, of course, to make it as light as possible, which wouldn’t be a terrible concept given that you’ve espoused the desire for these mechs to be maneuverable and fast. Also the fact that there are multiple limbs means the ground pressure can be more spread out than if there were two. Basically, aluminum alloys as the primary body and some kind of ultra-light metal for the armor and you’ll be in decent shape to handwave the ground pressure issue.

Leg Sheer (material sheer)

Given that your mech is supposed to eventually move, the legs will need to bend at some point. This is an issue due to the fact that, while these spindly legs can hold up the weight of the mech when they are straight and bearing weight on the compression axis of the legs, once they bend, they will be bearing weight on the sheering axis. In order to counter this, the cross-section of the material used for support in the legs will need to be thick enough to resist the sheer force of the partial weight of the mech equal total weight divided by the number of legs minus one (assuming that, during travel, only one leg will be off the ground at a time). We will then need to incorporate a safety factor into that calculation because this thing will be moving, and dynamic forces are higher than static. There would be quite a bit more engineering involved here, but this is something to keep in mind with the size of the legs and materials, etc. If you want more detail on this concept, you might want to research ultimate sheer strength in a material just to get an idea of the PSI tolerances of metals.

Flexing

I don’t know if you’ve even been in a boom lift or a scissor lift or something like that, but tall, thin metal structures have a tendency to flex with very little force input. A light breeze can cause a scissor lift that’s 20 feet in the air to deflect over 5 inches from center. No matter how stable the platform is, the thin metal appendages will experience deflection with even minor perturbations in center of mass or strong winds. Also, the device will have to maintain is axial tilt very carefully. Following the earlier lift example, a fully extended lift with an axial tilt over 3.5 degrees can be blown over completely with a decent gust of wind. These mechs we are discussion will be quite a bit more top-heavy by nature, and have less internally support leg systems, given that these legs will need to move independently. Stabilization will be a huge aspect of movement on these things. Actually, that would be a interesting visual picture, with the legs moving all over the place but the bodies appearing to glide through the air on those legs.

Conclusion

I think if you find occasion with the description of these mechs to describe and counter the basic versions of these afore-mentioned issues with both design and material engineering, you will be in good shape to remain internally consistent and present these things as logically plausible in the story.

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Hydraulics.

hydraulic leg

Consider a flexible leg element with 2 hydraulic bodies inside. At full hydraulic body pressure (red) the elements are straight. Reducing pressure (violet) in one of the 2 hydraulic bodies shrinks the element on that side and the leg bends in that direction. Even more reduced pressure (blue) bends the element such that elements above are at 90 degrees to those below. Lowest pressure for both elements (blue) shortens the element.

This is depicted as a rectangular leg element with 2 interior hydraulic bodies but a hexagonal or octagonal leg element with more hydraulic bodies would allow more precise bending. More smaller elements produces more flexiblity. A leg element could still function with some damaged hydraulic bodies.

One could have the leg element default to maximum extension or maximum compresson if too many hydraulic bodies were damaged - the leg element thus becomes "dumb" but still can serve its structural functon.

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According to this discussion to just bend a 10 cm long, 9mm in diameter steel rod requires a force equivlent to approximately the weight of 20 kg.

So, suppose your tripod is 10 meters tall, which is 100 times as long as the indicated length here. And suppose it is 1 metric tonne in mass, 50 times as much as the 20 kg bending just mentioned. And suppose it needs to be able to stop with an acceleration of 1 g. It will need that to balance in Earth's gravity, at least occasionally. Plus, you have illustrated this thing with the legs at an angle, so they would need to support the bending just standing there.

The legs need an absolute minimum area of 5,000 times that of a 9 mm steel bar. That is, they will need to be about 0.3 meters in diameter solid steel. Not including the mechanism provided to move the leg. Or sense its current location.

Now here's the thing. This rather simple calculation does not include the weight of the legs themselves. The pod was assumed 1 tonne. But 0.3 meters diameter means about 0.3 tonnes of steel per meter of length. That is, the legs would be 5 tonnes each, compared to the 1 tonne cabin. They would need to be much thicker to support their own weight.

This thing is not going to look nearly so bendy if the legs are 2 meters thick to support a 10 meter height.

Hopefully you see the point here that steel is simply not going to cut it as a single, compact, flexible limb of this type and length. The legs will break every time this thing tries to take a corner.

If you require a long flexible limb type of thing, you are going to require something much stronger than steel. Probably something at least 50 times as strong as steel. The length of the lever that wants to bend the legs is just too long. It's not quite up to the range of space-tower strengths, but it's getting there.

Structures of this size are constructed out of some kind of truss. The purpose of a truss is to convert the bending force into some combination of compression or stretching. The working load limit for a 5/16 inch steel cable is typically 1 ton. Steel can handle huge compression forces, typically 20 tons per square inch. So a truss can be constructed with very much larger bending strength than a simple steel rod of similar weight.

However, that means that the legs cannot be flexible in the fashion illustrated. enter image description here

Here is Robosaurus, a 40 foot (about 12 meters) tall amusement park attraction. Notice that the legs are essentially stiff except at one or two joints. Notice that the joints are very solidly constructed. And that the thing keeps most of its weight on the wheels at the back, not on very long legs. The limbs are covered in a surface covering that hides the truss underneath.

He is about as tall as the tripods and yet his legs are already very much klunkier and stiff compared to your tripods.

enter image description here

So basically you have two choices. Give up the slender flexible tentacle-like legs. Or find a construction material drastically stronger than steel.

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