Does surface area/volume restrictions make giant tanks and mechs impossible?

Factual Questions
1

I’ve seen it argued that giant tanks or walking mechs are not possible because of surface area : volume scaling.

Basically, since a giant tank ( I mean something the size of a naval warship) is many times longer and wider than an Abrams, it has enormous amounts of internal volume and if it weighs the same amount per volume, it will be too heavy to move.

The same argument applies to giant robots such as seen in the movie “pacific rim”.

Now, I am fully aware that giant tanks and mechs are a bad idea, because you are putting too many resources into a single vehicle that is a target for modern weaponry.

However, could they be built with existing materials? My instinct is to say these arguments are wrong : the bigger a tank is (you can approximate a tank as a dome shaped object), the more internal space it has relative to the outer surface. This means that the bigger the tank is, the thicker you can make the armor and still have enough room for an engine capable of moving it. Similarly, you can fill the inside with structural bracing and lots of empty voids that do not add any weight.

2

Well, you can’t have it both ways. If you want it armored, it’ll be heavy, require a lot of fuel, and be a drain on logistics. We’ve built massive, slow vehicles:

What good is such a thing, militarily? As even the movie Pacific Rim demonstrated, a single big object is a single big point of failure. Take out its command center and the whole thing crumbles. Or take out its legs (or tracks) and the whole thing is a sitting duck. And even if you made it phenomenally well armored, it’d just sink into the ground below it – or the enemy would just go around it because it’s so slow. Essentially you’d just have made a fortress that could crawl around at tectonic speeds.

So what if it’s big and light? Well, if you want it armored, you put it on the water and use it to launch missiles. If you don’t want it armored, you might as well give it mobility and let it fly and and shoot missiles.

What good is a slow, massive hunk of armor that is barely mobile, doesn’t have a lot of range, requires an impossible supply chain to keep operating, that the enemy can detect, avoid, and easily make immobile just by altering the road around it?

And mechs are just stupid because supersized bipedal motion just means it’ll crush/sink into pretty much any sort of infrastructure it tries to walk on since its weight is so heavily concentrated in two points. And take out one leg with a well-placed missile/pothole/rope and the whole platform crumbles under its own weight.

3

Basically, even if you could build one, it’d be pointless. It would essentially be a battleship – except one that’s slower and more vulnerable and easier to immobilize and harder to resupply. And just like we phased out battleships in favor of smaller, lighter, deadlier things, the kind of engineering and resources that would be needed for this huge behemoth could be more effectively channeled into a fleet of missile-launching drones that swarm around the target. No armor is infinitely impenetrable, especially if your target is big and slow and super easy to hit.

That said, there is this huge hovercraft that moves things around:

4

I said it was a bad idea in the OP. I mean is it physically possible to build using available materials.

5

EDIT: Simulpost. Sorry, I see what you’re getting at, and my posts don’t answer that. I’ll stop.

(leaving this post intact though because it gives you at least a semblance of what it takes)

Last post for now:

We’ve built a huge digger at 14,000 tonnes, the Bagger 293. It is powered by an entire external coal power plant that is constantly fueled by the coal it itself digs out (more in this youtube video). It is pretty much mostly hollow, just a big digging wheel with a support structure. Even something like that, on tracks and not needing a supply chain since it just sits there and digs up its own fuel, can only move at the blistering pace of 1 km per hour.

Now according to this site, a Pacific Rim-sized mech would be 24x heavier.

It’s not necessarily that you couldn’t possibly build a giant thing like that, it’ll just take more and more and more energy for less and less and less practical benefit.

6

You’re underestimating the square/cube law. A 20x20x2 steel panel weighs 8 times more than a 10x10x1 one. So when you make your tank twice as big and double its armour thickness it needs an order of magnitude more power to move its own mass (since that 8x calculation applies to each of its sides). But you don’t get 40 times more power by making an engine twice as big.

The other drawback is that on top of needing enough power to move all that mass, you need something to *support *it while it moves. Regular tanks already chew roads up ; a mega-tank would find itself bogged down all the time (and could never cross a bridge)

Finally, while MonsterTank conceivably might not have been an immediately_dismissed concept at some point in military history, these days modern armies and air forces have bunker buster bombs that can penetrate half a dozen meters of reinforced concrete before blowing up. This piece of gross overkill on wings boasts two hundred meters. Think you can make any mobile thing big enough to withstand that kind of punishment ? Much like Everest, everybody would be anxious to take a crack at it just because it’s there :stuck_out_tongue:

Nevertheless, the Nazis seemed to think giant tanks were worth a shot, for a given value of giant, but AFAIK that idiocy never left the drawing board. It would have required 4 submarine engines to move that 1.500 ton penis extension at an estimated 9 mph. Hint : if people can *outrun *your combat vehicle, it is not a practical combat vehicle.
The largest tank that ever did get built is, again, a Nazi “wonder”. *That *piece of shit could race at a cool 8 mph for oh, about 100 miles. Not that it ever went that far in practice due to mechanical failures and it, too, was quickly scrapped after one and a half prototypes.

As for bipedal mechas, yeah, they’re just a bad idea as a practical concept. Quadri/hexapodal ones, however, could prove interesting due to their inherently increased mobility over tracks & wheels when traversing very rough or steep terrain ; and DARPA is in the process of testing some. Not AT-AT sized ones though. Oh no. They’re eggheads, they’ve seen Jedi.

7

Your instinct is wrong. You are failing to understand the significance of the square-cube law.

Take your tank. Now double it in size. You’ve just made it 8 times heavier (2x2x2, since you double it in length, width, and height), but you’ve only made the steel 4 times as thick (2x2, length and width, for any slice of steel going through the tank). The wider steel of the bigger tank is 4 times as strong, but has to support 8 times the weight. Your strength to weight ratio has been cut in half.

Now make your tank 10 times bigger. The tank is now 1000 times heavier, but the steel is only 100 times stronger. Your strength to weight ratio is 10 times smaller. Even with bracing, your tank collapses under its own weight.

Aha, you say, if my tank is made out of 4 inch thick steel, I’ll keep it made out of 4 inch thick steel, I’ll just make it bigger (not keeping the same ratio as it gets larger, aka bigger “voids” inside). Now consider every vertical slice through a tank that’s twice the size. Each piece of steel is holding twice the weight, but it’s still the same thickness. You’ve decreased your strength to weight ratio by 2. Again, the more you scale it up, the weaker it gets relative to its weight, and eventually it collapses.

8

Of course it can be built, if it’s only the size of a battleship or even a very large building – we build those, right?

Would it be useful? Probably not. The purpose of a battleship is basically to be mobile artillery that gets places faster than artillery can (because big as it is, it’s on the ocean, which means it can travel faster than even bitty cannons through mud and such on land). But on land, you don’t have that “speed” (meaning faster than a burro’s walk). The purpose of a tank is essentially to protect infantry, an answer to the machine gun. For that purpose it only has to stop machine-gun bullets (and these days RPGs) and other antipersonnel weapons, and be at least as fast as an infantry advance. Your land battleship wouldn’t qualify. So what other function could it provide? The only one that occurs to me as kind of a mobile fort. But as the Second World War convincingly proved, once you have good protection for your infantry (e.g. a regular tank, plus air superiority) you can just ignore forts – go around them and defeat the enemy’s field army, then starve the fort out or something. That’s why nobody has built a fort since the 1920s or 30s.

9

There was a delightful old Jonny Quest comic book (quoting only approximately from memory) where a villain forced Dr. Quest to build a giant robot for him. Dr. Quest tried to explain how difficult it would be. The bearings would have to support immense weights; the legs would have to be incredibly strong.

Eventually, he produced a big, ugly, primitive walking machine that just barely qualified as a “giant robot.”

The villain immediately asked, “What does it turn into?”

Dr. Quest looked at him blankly.

“You know, a car, a boat, an airplane?”

Dr. Quest was magnificently dumbfounded.

10

We exceedingly rarely build self-propelled ones, however ;).

11

I recall the article about the bunker-busters built during the first gulf war. 30-plus feet of hardened steel followed by a timed-explosion bomb, dropped from 50,000 feet on a deep target. they dropped one, it took out the bunker, and apparently the Iraqi command network lit up like a Christmas tree when everyone called around saying “WTF happened?”

Making the bomb was top secret, noboy knew anything - but the factory guys weren’t dumb, the delivered bomb had the name painted on it - “The Saddamizer”.

Square-cube explains why a mouse or an ant (or cat) can drop from a lot further than its height, and walk away just fine - but a human or an elephant dropped 10 feet will experience severe damage. Also explains why an elephant (pushing the limits of animal size) has legs so much thicker than a human.

Then, along with the weight-support ratios mentioned above - consider things like surface support - double the overall dimension, and the surface area of the track or wheel on the ground goes up square - only 4 times… double the per-square-foot load. Eventually it would reach the point of sinking n the dirt, breaking asphalt, not to mentin the bridge problem others have mentioned. Double the scale of the engine, the cylinders have 8 times the volume - but the piston rods, crankshafts, etc. are only twice cross-section, may not handle the load. Ditto with axles, bearings.

Every design decision has repercussions - and rarely do designs scale well. Want to put thick armour on your Cadillac Escalade? Now it weighs 3 times as much. Needs bigger stronger tires and shocks… needs bigger brakes. Needs a bigger engine, which needs a bigger transmission and differential And so on - you might as well design a whole new vehicle from scratch.

Consider the stresses on your giant tank, if you jam on the brakes (assuming brakes can work) or you hit a pothole at 10mph. Your gizmo will pancake or shear off sideways unless properly braced. Modern large ships (the limit of mobile and big) take miles to stop or turn. The Titanic broke in half once too much of the rear lifted out of the water as the bow went under. Our instinctive understanding of structure is based on human-sized devices; once you get to the size of football fields, there’s a lot less ability to avoid bucking and folding and collapsing

12

The NASA Mobile Launcher Platform is of the right scale to be a “mobile building” though, especially when carrying something like the Space Shuttle or a Saturn rocket. Of course it also demonstrates some of the problems with such a huge vehicle, such as being really slow.

13

“For the honor of the regiment!”

14

It’s interesting, though, to note that Bolos became “practical” because of unobtainium-like advances in armor and power-plant technology. The earliest Bolos were really just AI-driven heavy tanks. The Mark II, for instance, was basically equivalent to a modern MBT with enough AI to patrol and perform guard duties (see “Night of the Trolls”).

The really monstrous Bolos were self-directed self-propelled armored fortress with nearly planet-killer weapons. They were cost effective because their impossible armor (both impossibly light, and impossibly resistant) made them mobile and indestructible, and their weapons made them unopposable. The last Marks were commonly referred to as “Planetary Siege Units”, because it would only take one unit (perhaps company-sized) to either defeat an entire planet or defend it.

15

As I understand it that’s why the pro strat when using torpedoes against surface ships isn’t to hit the ship or otherwise blow holes in it but to explode under it, preferably in the middle.

This hits the ship with a one-two punch : first, there’s a column of displaced water that shoots up and lifts the ship some but focusing all of its weight on the relatively small area that is “supported” by the column. At this point the whole ship is already buckling like a banana under its own mass. Then the column vanishes but the water supporting the ship is temporarily replaced by the big air bubble + shockwave created by the explosion, the whole ship “falls” and smacks back into the water with surface support now being concentrated at each end, once again letting the boat’s own mass stress itself apart and break its spine. It’s super effective.

16

What’s the purpose of a tank? A tank design is a balance of three factors–firepower, protection, and mobility. You want the most firepower, with the best armor, with the best mobility.

The problem with superheavy tanks is they focus on firepower and to some extent on protection, and sacrifice mobility. You might think that a superheavy tank with superheavy armor would have good protection, but that is only one part of protection. A superheavy tank is a big target–literally, and is extremely vulnerable to mobility kills.

A giant cannon is no good if the AFV can’t move the cannon to the place where it is needed, and can’t protect the cannon from the enemy once it gets there. And of course a giant cannon isn’t any good if you can’t aim the thing and fire it rapidly.

And all this ignores economic or logistical issues–home much does the vehicle cost, what sort of supply chain does it require, how many crewmembers, how prone to breakdown are its components, how easy to repair is it, and so on.

So the point of a superheavy tank is to haul a giant cannon around, and protect the cannon with super thick armor. But that takes a big engine, which means a big chassis, which means everything is bigger, so you need to spread the same amount of armor over a larger area or add even more armor.

In a very real sense modern main battle tanks are already “superheavy” tanks by the standards of WWII. The have giant guns, massive armor, advanced targeting, and good mobility. And the role of medium and light tanks is taken over by other vehicles like Bradleys or Strykers.

So what is the role for an even heavier tank? What is it supposed to fight? Modern MBTs? Right now our MBTs are easily capable of defeating comparable MBTs, you don’t need a bigger and better cannon, the existing MBT weapons are capable of chewing through existing armor like tissue paper. If you really want a heavier cannon then something radical like the Stridsvagn 103 - Wikipedia might work rather than just upscaling a regular tank.

Keeping the same cannon but adding even more armor doesn’t add that much protection, modern MBT armor is as tough as we can make it. You’re just making a bigger target with more opportunities to fail. Making a tank bigger but keeping it light by adding empty space makes no sense, the tank should be as small as possible while still being able to use its weapons. Think about it, if you can eliminate the void spaces you can lower the weight by reducing the surface area of the vehicle, this allows you to have a lower armor weight but keep the same armor thickness. Making a vehicle bigger just to make it bigger serves no purpose.

17

Depends on what you mean by giant. Nature kinda already did the giant legged thing with dinosaurs. Argentinosaurus weighed about 80-100 tons and was 110+ feet long and maybe 30 feet at the shoulder, and there’s fragmentary bones that indicate other sauropods grew even bigger. There was a locomotion simulation that suggested they could walk about five miles an hour, so they probably won’t be starring in a super hero movie. Something T. Rex sized would be more flexible.

Current engineering and materials won’t be able to replicate something like that for a long time. We do some things much better than nature, other things we’re still trying to figure out. Like how the human brain runs on a paltry 20 watts.