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Ultra-Heavy tanks are a fun concept. Simply scale up a tank to weigh 1000+ tonnes, add a stupid amount of guns and armour, and you end up with something like the image below.

Landkreuzer P. 1000 "Ratte", Artis's impression

Of course, this monster was never built and if it had, it would have been worse than useless. Ultra heavy tanks would:

  • Require as many resources to build as an entire detachment of conventional tanks.
  • Be spectacularly slow and unwieldy.
  • not be mobile and splittable, like a conventional armoured division.
  • be a sitting duck for artillery barrages and aerial bombardment.
  • struggle with a number of different types of terrain that more conventional vehicles could deal with.
  • require specialised training and manpower requirements to support and operate.
  • absolutely guzzle fuel, likely requiring either a non-conventional powerplant or be a logistical nightmare to keep supplied.
  • if destroyed in battle, represent an immense and unrecoverable loss of manpower and material.

In the real world, there isn't a single thing that a Ultra Heavy Tank can do that can't be done better by the same cost of conventional armoured vehicles.

But we aren't interested in the real world.

What combination of circumstances would lead to Ultra Heavy Tanks being a sensible and efficient weapon of war?

You are free to tinker with:

  1. The overall design of the tank, although it should still fit the bill of being a 1000+ ton monster, bristling with multiple oversized weapons systems.
  2. The technology level of the conflict. I would prefer technology kept to approximately modern day or earlier. No anti-gravity or micro-fusion reactors unless you absolutely have to.
  3. The foe. Fascist nations, aliens, robots, all are fair game.
  4. The theatre of war. You are free to posit a war anywhere on or off Earth.

I'm looking for answers that stick with as realistic an interpretation of physics and engineering as possible. Solutions drawing upon Fantasy/magical themes are not in the spirit of this question.

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    $\begingroup$ > But we aren't interested in the real world. Then just assert that they exist. $\endgroup$
    – RonJohn
    Aug 16 '21 at 22:30
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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Aug 17 '21 at 14:26
  • $\begingroup$ one point - you could conceivably have something HUGE like that, but, incredibly lightweight - using miracle materials. That's really the only workable idea I fear? $\endgroup$
    – Fattie
    Aug 17 '21 at 15:54
  • $\begingroup$ perhaps for some reason it's very easy for groups to get split up, so you need single units that are bigger and scarier than what the other side has one on one? the thought that came to mind was another universe with hyperbolic geometry, but I'm sure other reasons exist - perhaps a teleport gun that sends things to a random point within a hundred kilometers, but for some reason sends an entire object and never to anywhere immediately dangerous? I can't think of anything very realistic off the top of my head $\endgroup$
    – Silver
    Aug 17 '21 at 22:10
  • $\begingroup$ "The increased efficacy of aerial bombardment, in the Second World War, made these large guns obsolete." – Second World War Weapons That Failed, (13. Schwerer Gustav – the German railway gun) iwm.org.uk $\endgroup$
    – Mazura
    Aug 17 '21 at 22:51

25 Answers 25

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A lot of people are focusing on the viability of making giant land vehicles.

I mean... we already have stupidly large land vehicles, don't we?

enter image description here

That's the BelAz. It's a huge cargo truck, to move things like dirt or ore. Sure, war isn't its purpose, but that doesn't mean we can't shove weapons on those big trucks to make then work as battle platforms.

That's not the best way of doing so, however.

enter image description here

That's a big boy by Perkins. It's carrying parts of a nuclear reactor on its back - over 400 tonnes of metal of cargo alone - and navigating regular roads while doing so. It manages to do so by spreading its weight over 400 feet worth of truck-bits, making it longer than a football field. It isn't that fast, but it for sure is large.

Keep in mind that the average modern tank weights something like 40 tonnes. The guy above is carrying ten times that.

While not exactly 1000+, as postulated on your question, the Perkins is large enough to carry an entire battle-base on its back - be it a mobile fortress/outpost, or just a giant amount of guns to fire at things, like some sort of land-based mini-battleship.

If you need to go bigger, however...

Meet Nasa's Crawler-Transporter.

enter image description here

Despite the fact that this thing is very close to a tank already and looking incredibly awesome by itself, this thing is so beefy it can carry an entire spaceship on its back. It weights over 2500 tons by itself, and it is all sorts of ridiculous when it comes to the amount of power it needs to work. It is a logistical nightmare, but it is a solution for another, even worse logistical nightmare - putting a spaceship in place.

And we made it work!

Sure, if you're making those beasts war platforms, you'll have to exchange some bits. We'll need armor, extra fuel tanks, a lot of guns. But their very existence proves it can be built, even if we have a bunch of limitations.

Now, where those beasts would be useful, in a war?

I say, where those beasts can do what they can do best - carry big things!

I don't see those used mainly as battle platforms. Instead, I see them as vehicles made to transport big stuff that can't be broken into parts. Imagine, for example, that mysterious monoliths appeared all over the planet. Huge, black pieces of stone that can generate a lot of power seemingly from nowhere, if you can take them on your hands. They can be moved, but you need them to be moved whole, in safety. What do you do?

Shove them on top of the meanest, largest vehicles you can build, and then take them home. Of course, a large truck doesn't cut anymore. Those things would need to be kept in extreme safety, so you arm those trucks up. Once they are set, they become mobile power generators, ready to juice up your energy-weapon-based troops, and able to defend themselves from the enemy if it comes to an engagement.

If the magical-space-monolith thing isn't your cup of tea, then you can use them to retrieve warheads, spaceship parts, warsats, or whatever else you fancy that is big and must be carried in one piece, or several large pieces.

They can work as bases for giant railguns, or as mobile power generators, if outfitted with miniature nuclear plants. They could work as housing for massive secret supercomputers that should be kept near the troops for one reason or another. In any case, it isn't on the frontline they will shine - it is on the back, carrying the big things no other vehicle can carry.

Ultra-tanks would fit the bill of meaner ultra-trucks, doing the very same things, but this time with a few extra inches of armor, and a couple extra tens of millions in their budgets.

Oh, and the camo paint job.

Don't forget the camo paint job.

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    $\begingroup$ an amazing post; one further detail, the BelAz only works on very well made (dirt) roads, it can't go offroad in the slightest. $\endgroup$
    – Fattie
    Aug 17 '21 at 14:22
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    $\begingroup$ @Fattie When I worked with similar equipment, sensors were used to map out the routes to find spots that would cause slightly more stress on its systems (engines, motors, tires), and then consider re-grading that area to reduce that stress. Also, not only did it require finely maintained pathways, it is not compatible with normal roadways, and will tear them up. We had some equipment that needed moved across a county road, and the method to do so was to completely cover a strip of roadway in a thick layer of dirt and sand. $\endgroup$ Aug 17 '21 at 14:56
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    $\begingroup$ @MichaelRichardson - absolutely amazing info. meant the world to me. thank you so much! this must not be the internet! :) $\endgroup$
    – Fattie
    Aug 17 '21 at 15:50
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    $\begingroup$ The scale issue has certainly been way overblown, but it's not non-existent. All those heavy vehicles all need to be driven on hard terrain. I think the concern about tanks is their need to drive through mud or other soft terrain to be versatile enough to use effectively. $\endgroup$
    – Nosajimiki
    Aug 17 '21 at 19:16
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    $\begingroup$ Power generation is especially pertinent, considering power generators tend to become more efficient as they get larger. It may well be that a crawler-sized vehicle is the smallest thing capable of independently powering an onboard anti-ballistic-missile laser defence or something $\endgroup$
    – Pingcode
    Aug 18 '21 at 3:48
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Moon tank.

  1. Even though it is huge, it does not weigh so much. Because it is on the moon!

  2. It does not matter that it wrecks the landscape as it drives over it. The moon is forgiving in that respect.

  3. You need it big to house the nuclear engine.

  4. The moon is full of loose rocks. Pile those on top for extra armor. If they get blown up, pile some more on there. They are not heavy. They are on the moon! Moon tank is functionally underground, but is moving the ground it is under along with it. Also the rocks are good camouflage.

  5. Moon tank guns easily achieve escape velocity with their projectiles (no air on the moon to slow them down) and so can shoot at enemies in orbit. And enemies farther away than that. Moon tank functions like a gun emplacement, but mobile.

  6. Because of #5, moon tank can land a shell anywhere on the moon.

  7. Moon tank is also good based on other low gravity planets and asteroids.

  8. Moon tank is pretty comfy inside for persons spending long deployments in the moon tank.

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    $\begingroup$ "Moon tank's haunted." $\endgroup$ Aug 16 '21 at 23:30
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    $\begingroup$ @Nosajimiki- space for the reactor as well as long term living quarters for the crew is why it is big. Invulnerability to 1 shot kill is because of all the boulders on top, in an invulnerable boulder basket. It is functionally now a submarine. As regards "boulder basket" you may use that as your rap name if you wish. $\endgroup$
    – Willk
    Aug 17 '21 at 14:11
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    $\begingroup$ Needs to be big because the moon enemies are immense creatures that head butt to attack. Smaller tanks would get butted away. There aren't many of these creatures so having to split up isn't an issue. $\endgroup$
    – Keverly
    Aug 17 '21 at 22:41
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    $\begingroup$ I would say an even better planet might be Mercury. There are many sci-fi worlds where humans live on mercury but the whole city or civilization is like on a very slow moving train to keep in the shadow or edge (in theory if you stay on the edge mercury is habitable). So in theory you could have whole nation states as gigantic tanks. $\endgroup$
    – Adam Gent
    Aug 18 '21 at 13:10
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    $\begingroup$ Word of warning: Weight may have gone away, but inertia has not, and you no longer have all the lovely grip that 1g provides. Don’t go too fast if you want any semblance of control over your ludicrous lunar landfortress $\endgroup$
    – Joe Bloggs
    Aug 19 '21 at 17:48
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Better Active Defenses

Active Defenses are a rapidly advancing technology that can detect incoming weapons and destroy them. Trophe for example is a new Active Defense that is installed on many American and Israeli tanks that has a record of intercepting 100% of anti-tank missiles/RPGs ever shot at it in testing or in live combat. America is already developing a next generation version of this that will be able to stop kinetic anti-tank weapons as well.

If you make an ultra tank protected by armor that is 15 times as massive as a normal tank, it would cost almost 15 times as much but only have ~2.5 times as thick of armor in any given spot. This means that you do not need to scale up a weapon a whole lot to kill a much larger target. Active Defenses change the defense equation from the linear-cube relationship of using armor to a straight linear relationship. See this related answer about plasma shields for a deeper explanation.

If a tank is using active defenses, then a tank that is 15 times as massive as a normal tank, could also be 15 times as hard to destroy since it could cover it's whole surface with countermeasure systems. It also means that large fortifications could become way too big and well defended to be overwhelmed by normal little tanks or bombers; so, ultra-heavy tanks may become necessary for overwhelming the countermeasures of shielded fortresses since one 1,000 ton tank is cheaper to make then 15 separate 67 ton tanks.

Also, penetrating Active Defenses means you need to overwhelm it in a single volley. A single tank can better fire a few synchronized massive shells, than 15 tanks can fire a coordinated volley of smaller shells. If the attack is not synchronized, then active defenses can block your attacks in sequence with much less effort.

Addressing the Size & Weight

Like modern naval ships, ultra tanks will neither need, nor want heavy armor. Following the same doctrine as modern navies, the armor should be just enough to deflect heavy machine gun fire, while leaving all other threats to the active defenses. This means that a 1000 ton tank could actually be much larger than you would expect from just scaling up a normal sized tank which means it can be taller with with same weight per square meter of contact with ground. Larger could also means it is worth while to replace its heavy fuel tank with a nuclear reactor, and all of this fuel and armor weight reduction also mean that your ultra tank can use a proportionally lighter frame. So, all these arguments about it sinking into or destroying the landscape may not apply at all.

To get an idea of how much weight you save when de-armoring a large tank, we can look at comparing the size to mass of a WWII battleship to a modern nuclear cruiser. The Yamato Class was an armored battleship that weighed about ~277 kg/m^3. The modern Virginia Class Cruiser using active defenses instead of armor weighs in at just ~152 kg/m^3

So just using active defenses instead of armor should drop the density of your tank by nearly 50%. This places the volume of a 1000 ton ultra tank at about 29 times that of an M1A2 Abrams and your weight at only 14.5 times as much. So a 1000 ton ultra tank that is 122x45x16 ft would have the same weight per square meter of ground as a normal tank.

enter image description here

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    $\begingroup$ Although I agree with the idea of active protection, the Trophe system being 100% capable is a gross exaggeration or propaganda. I also wonder how long it takes to circumvent the system. There is already talk about missiles with decoys, but I would also guess that the mirrors the system uses for sight could be disabled with a smaller charge leading the projectile to either damage it, destroy it or cause it to protect itself until the shrapnel has passed. It already has a system in place to avoid deflecting small projectiles that cant penetrate the hull, those would be ideal to go through. $\endgroup$
    – Demigan
    Aug 16 '21 at 15:47
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    $\begingroup$ @Demigan "100%" is against every live combat and test shot ever fired at it. That said, all these shots fit the profile of a single missile at a time tested up to supersonic speeds. A hyper sonic at terminal approach missile, stealth missile, or large volley of missiles could likely overwhelm it, but no country as of today has any missiles classified as "anti-tank" that fit this description. A point-blank shot should also do the trick, but if you can get that close to a modern tank un-detected, then the tank crew and it's support infantry is doing something very very wrong. $\endgroup$
    – Nosajimiki
    Aug 16 '21 at 16:00
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    $\begingroup$ This question seriously needs a mention of Bolos, and this answer favoring active defenses seems like the ideal place, since they carried significant active and passive defensive systems. $\endgroup$
    – Ben Voigt
    Aug 16 '21 at 21:55
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    $\begingroup$ And now to the "why you need it": The big enormous gun is to fire armored projectiles, which can ignore most active defenses. That's what you need such a big platform for to begin with $\endgroup$
    – Hobbamok
    Aug 17 '21 at 15:34
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    $\begingroup$ @Hobbamok exactly, a lot of answers try to say if it can be done. Active defenses say why it should be done. This is the same reason why in many videogames a larger combatant can beat two weaker ones with half the stats, because when his shields/HP/whatever hits 50%, one of the attackers is getting killed, then he has twice the remaining attack and the HP as the remaining opponent. $\endgroup$
    – Nosajimiki
    Aug 17 '21 at 15:56
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Force shields

Sci-Fi is full of huge starships which should have been sitting ducks for all of the rockets and blasters, if not for one other thing which is a staple of sci-fi - the force shield. I presume everyone knows what a force shield is and how it works ;)

Imagine in a near future there is a revolutionary development in this area, and we can develop generators which generate a "field" capable of stopping the most powerful conventional weapons. The downside is that these generators weight 100 tons minimum, and require huge power source, which, for mobile vehicles, would necessitate nuclear power generation. This, in turn, would push the total vehicle weight towards 1000 t, and even higher.

One consequence of this development would be that for stationary installations, force fields would make even greater sense, and this should change warfare tactics considerably.

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    $\begingroup$ Realistically this is the answer. You need a reason to make big tanks instead of lots of small cheap tanks and force fields are the only answer that would fit. $\endgroup$
    – Thorne
    Aug 17 '21 at 1:33
  • $\begingroup$ I almost answered something like this, but the closest thing we use to force shields in real life are plasma shields which are a type of active defense system. Passive defense force fields suffer the same limitations of scale that armor does. This is why I said active defense system because it covers every class of system including plasma shields that can beat the issues of scale, and none of the interpretations of shields that can not. $\endgroup$
    – Nosajimiki
    Aug 17 '21 at 17:09
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    $\begingroup$ Other option: You need a huge (rail)gun because the enemy has superior defenses made out of unobtanium or force shields which can only be breached by a high speed, high energy projectile. $\endgroup$
    – Michael
    Aug 17 '21 at 19:25
  • $\begingroup$ Of course, the enemy comes with a 150-tonne force field generator and out-forces your 100-tonne force field. $\endgroup$
    – user253751
    Aug 18 '21 at 13:05
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    $\begingroup$ This is really just a subset of active defenses. $\endgroup$ Aug 18 '21 at 14:45
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Closest sense

Ultra-heavy tanks do not just "struggle with a number of different types of terrain", it would absolutely destroy any terrain it would move over. That is an extra cost of just using this tank, besides all the other detriments.

To prevent the tank from destroying everything in it's path, it could be transported by rail. Have several heavy duty rails next to each other to move this colossus. Rails help spread the weight, making sure the wake of this tank isn't useless destruction, but actual infrastructure. You can still heavily debate if the rails can take the weight, but at least it's better.

Rails immediately offer extra opportunities. You can put the powerplants for it's motive power out of reach, simply electrifying the vehicle with power lines. It introduces a big weak spot, but the tank can become more manageable. As the tank requires the infrastructure, supplies are immediately easy to transport to it. Rail does need to be laid down in advance, but it can supervise it's own rail line and retreat much more quickly if needed.

You can compare it to the big railway guns in WW2, only it's a multi-track armoured behemoth that makes those guns look like toys.

With all that prepared, you can make it more than a tank. You can make it a fortress on rails. The fortresses created during WW2 were near invulnerable to the weapons at that time. Even when conquered and professional construction crews could access it, they proved difficult to destroy. So difficult that some are now still in tact, as they simply gave up on destroying them.

A ultra-heavy tank can tank most of the fire. If the power to it and the infrastructure is destroyed it'll still be a formidable fortress, which can potentially wait weeks for rescue, being a threat to anything coming in it's insane range all the time.

Construction of the railway would be time consuming, but not as much as you might think. Railways were a good target to hit, but quickly repaired as well.

Usage

The tank would just be used as a mobile fortress. A fortress is difficult to take and usually circumvented by moving around it. This one however goes towards the enemy. Any fortification against it is useless, unless it is a modern fort itself. Even then this beast can have the weapons and range to reduce those to dust. It makes most types of warfare in it's influence useless or too precarious for the enemy.

This way you have a slow moving fort as a base of many operations casting it's influence. The railway immediately allows it to be well supplied, so no direct attrition is useful. It can outgun anything and be harmed by little. It is a symbol of strength that cannot be denied, making lots of enemies simply give up when they see this crawling closer to their town.

Other problems

The list of problems is actually much larger. With big guns comes great wear and tear. Many big guns even had ammo that was designed with this wear. They became bigger until the gun needed to be replaced, which is relatively fast. Also the limits of explosive power can be detrimental.

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    $\begingroup$ The railway guns are exactly right. The Shwerer Gustav was the largest ever and weighed more tha 1300 tons (en.m.wikipedia.org/wiki/Schwerer_Gustav). The P1000 Ratte that the OP bases his design on was a 1000+ton design, giving it real-world feasibility to use a rails-based behemoth. Maybe you can handwave a system of building the track in front of it and dismanteling it behind it? $\endgroup$
    – Demigan
    Aug 16 '21 at 15:34
  • $\begingroup$ Brief reminder that tank tracks were initially invented exactly as @Demigan mentions: A tank track is a system for building railway "tracks" infront of a vehicle, driving over them, and then picking them up behind! The only difference is that a properly built railway has many tonnes of hard-packed stone beneath the rails. $\endgroup$
    – sdfgeoff
    Aug 24 '21 at 12:27
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Well, building something this huge, bristling with weapons, and terrifying in it's own part of the theater of war is fairly common....in the Navy.

We build huge ships with huge guns that can toss a projectile the mass of a Volkswagen several miles and now can house railguns. These modern day warships are often powered by small nuclear reactors. So we have a real world example for your behemth tanks, at least as far as tonnage and power are concerned.

That leaves us with one of the real problems with super huge tanks. Propulsion. Anything that size is absolutely going to destroy anything it drives over. Roads, fields, buildings, etc. Modern tanks have mud as a mortal enemy. To the Behemoth, everything is going to have the consistency of mud, except maybe bedrock.

The only land based vehicle that even comes close to the kind of mass of your behemoth is a freight train, and those require rails. Even trains are limited by the grade they can handle as well. So conventional means are probably out.

Your behemoth is not going to work unless you you can make it as light at the point of contact with the ground as a traditional tank. That means you have to resort to something like an antigravity drive to lighten the load. I would suggest that you don't have the antigravity strong enough to lift the whole mass of the tank. If it has enough mass in contact with the ground, lateral impacts aren't going to send it skittering off to the side like an air hockey puck. Maybe make the antigravity adjustable so you can deal with things like mud easier. you could also consider making the tank segmented for additional flexibility moving through cities and such.

As for what would necessitate such behemoths? Aliens is an obvious answer. Aliens that have a weapon that can scour the landscape free of organic life and burn through conventional tank armor. It just took a little longer, so a really big tank would protect those inside. Another possibility would be an environment that requires a sealed box for the squishy organic people inside. A standard tank may not be able to house the necessary life support equipment.

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  • $\begingroup$ The problem is not surface area, but the way that surface area reacts with the ground. The center of a tank track turns on its own axis, but the farther away from the center the more that track slides across the ground, which is the whole reason tank tracks NEED such low ground pressure as it would reduce their turn rate and increase damage to the track and ground if it were higher. This is one reason to split tracks in segments that can be turned individually. It is also a perfect moment to use legs with large feet to overcome many of those problems. $\endgroup$
    – Demigan
    Aug 16 '21 at 15:25
  • $\begingroup$ Adding legged-structures to the behemoth is a good idea anyway. Place the track segments, if you still want them, on extendable legs which also form the suspension system. That way you can handle terrain differences much better and dont have the tank slide off an irregular embankment. This is essentially a large-scale idea of what NASA is already doing for future exploration vehicles, as adding a legged structure to mount a wheel on lets them cross much rougher terrain. $\endgroup$
    – Demigan
    Aug 16 '21 at 15:27
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    $\begingroup$ One could also just use wheels. Tank Tracks aren't the only solution, though I think they would look cooler. Urban tanks that are often used as swat vehicles are wheeled vehicles. $\endgroup$
    – Paul TIKI
    Aug 16 '21 at 15:31
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    $\begingroup$ Re: Real world examples; Bucket Excavators and the shuttle moving platform. $\endgroup$
    – Marky
    Aug 16 '21 at 16:57
  • $\begingroup$ @Marky True, but the Shuttle moving platform does not operate on a variety of terrain and it ain't speedy. Bucket excavators I don't think get into the 1000 ton range. Good examples for a super wide tread though and what they might look like. $\endgroup$
    – Paul TIKI
    Aug 16 '21 at 18:13
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"I'm looking for answers that stick with as realistic an interpretation of physics and engineering as possible"

This is a frame challenge.

I'm a fan of cheesy Bolo novels. There's a lot of fun to be had! But that quote, above, stops everyone and everything dead in their tracks (despite @T.sar's clever answer, which I upvoted, but that doesn't address your quote... Or, perhaps more accurately, it proves the quote...).

A 1 kilo-ton tank would...

  • ...get completely and irrevocably stuck in the first river, lake, mud bog, swamp, or even saturated dirt after a good rain storm, that it encounters. The problem isn't just the weight, it's the weight-per-square-meter on the treads. It'll sink faster than Don Rickles' chin — and nothing on the planet will get it out.

  • ...be completely confined by every bridge between it and where it needs to go. There are no bridges that can handle that much weight-per-square-meter and a tank that large can't simply climb out of a ravine.1

  • ...be devastatingly limited by its logistics. A nearly constant trail of fuel trucks would be needed to keep it running — and most would need to fuel on-the-go. It's not the tank that's susceptible to attack, it's the long train of fuel trucks. Stop the trucks, stop the tank.

  • ...be nearly useless in urban combat. Most people think larger, heavier tanks are always better. That's not really true. The bigger and heaver the tank, the harder it is for that tank to operate inside an urban center. That's because debris that gets in the way of everything from turrets to treads builds up very quickly — and this behemoth would be knocking down buildings everywhere just to move forward in a straight line.

  • ...be a bomber's aerial wet dream.

  • ...Etc.

The reason the Bolo Universe is so honking fun is that all the inconvenient rules of physics that explain why the German Ratte never got off the drawing board are simply ignored.

In other words, ultra-heavy tanks make a lot of sense when you throw your quote out the window and accept the phrase, "let's ignore why they don't make sense and just assume we have them... how much fun can we have with that?"

Answer: a lot...


1OK, so after writing that I thought to myself, "could a railroad bridge hold that leviathan?" First of all, regular bridges: The answer is no, at least in the U.S. According to the U.S. Federal Highway Administration, which stops trucks from having any more than about 53 tons per axle. 1,000/53 ~ 19 axles, right? Well, 19 axles with minimum spacing to ensure the bridge load is minimized. In other words, we're increasing the weight-per-square-meter by increasing the size of the tank but not the weight. Which makes it easier to stop and harder to turn. I'm having trouble finding a similar reference for rail bridges, but one 1880's reference said they were building at 54 tons per square foot (*cough*). OK! That would hold the tank... if it actually fit on the bridge. So I'm sticking with the basic assertion of my original point.

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    $\begingroup$ The Bolos work because they have very good active defenses. Weapons lesser than their own don't do much to them. $\endgroup$ Aug 18 '21 at 14:48
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    $\begingroup$ @LorenPechtel The value of an active defense when you're stuck in a bog, unable to move, slowly sinking, is dubious. $\endgroup$ Aug 18 '21 at 19:36
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There are two basic problems with the super-heavy tank.

1. Cube-Square law

If you double the size of a tank then it sits on 4 times the ground area but with 8 times the weight, so each square meter of land it sits on has to support twice the weight. Eventually you get to the point where the tank just sinks into the earth.

The cube-square law also applies to things like suspension and drive-train; the force needed to move the scaled-up tank is 8 times bigger, so the drive shafts need to be 8 times thicker, which means being about 2.8 times the diameter in a tank only twice the size. The same logic applies to all the other mechanisms. As you scale things up, pretty soon you run out of space for everything else the tank has to carry.

The structure of the tank also has to withstand larger forces as it moves over uneven land; sometimes it is balanced on the crest of a hill, and sometimes it is suspended across a ditch. Again, these forces increase with the cube of the size but the strength only increases with the square. They also increase with the square of the speed, so going faster becomes an issue too.

So to get a super-heavy tank that can still manouver you need materials that have a very high strength to weight ratio, but are sufficiently cheap and easy to work with to let factories mass-produce them without starving other parts of the war effort. Maybe a cheap version of carbon fibre bonded with handwavium?

2. How to Kill It

There are two types of big gun in war: tanks and artillery. Artillery are the long-range weapons. Their job is to sit behind the lines and fire at coordinates given by forward spotters because the gunners can't see the targets. In this game range is everything; if you can out-range your opponent then you can hit them while they can't hit you. Big guns have longer range, so big guns are used.

Tank warfare is not like that. Tanks fire at other tanks that they can see, which means a range of 2 or 3 miles max, often a lot less. A tank's main gun therefore just has to be big enough to destroy an enemy tank at that range. There is no point in having the gun be even bigger because it just means more weight and less ammunition, and doesn't make you any more lethal.

The only way to justify a bigger tank is that it has to carry a bigger gun, and the only thing that will justify a bigger gun is an enemy tank that needs the bigger gun to kill it. So that brings us back to the tank armour and other defences.

The Almost Invulnerable Tank

Tank ammunition comes in several varieties depending on the target. For heavily armoured targets (i.e. other tanks) you use armour-piercing ammunition which either is basically heavy with a long sharp point, or uses clever explosive lenses to focus a jet of molten metal just as it hits.

So what you need is a tank defence system which can stop anything smaller than a very big, fast pointy lump of metal. Once you have that, you have a logical justification for putting bigger guns on your tanks.

Clever explosive lenses turn out to be fairly easy to defeat; reactive armour jackets blow up in just the right way to disrupt the focus, and bars or slats can also force the explosion to happen at the wrong place.

Kinetic rounds are harder to defeat, but lets assume you have the super-material I mentioned above. Make that into armour which is strong enough to absorb shots from big guns and is layered to defeat explosive lenses. You could also imagine some kind of interceptor gun that hits incoming rounds, knocking them off-course or setting off their explosives. Or any other handwavium you see fit to invent. Now the only way to kill this tank is to have a bigger gun firing bigger rounds at higher velocity, which implies the bigger tank that you want.

Incidentally, one way that kinetic rounds kill tanks is not by making holes in them, it is by "spalling"; the armour is only dented, but this happens so fast that bits of metal break off the inside and shoot through the tank, seriously ruining the day of anyone inside. Armour that is softer and more flexible is less likely to spall, and obviously you can coat the interior with something to stop the bits.

But...

The trouble with this scenario is that everything in it also makes aircraft more powerful; super-materials with high strength-to-weight ratios are just what you want for airplanes. Tanks generally have a lot of side and front armour and comparatively little top armour. That is what makes the A-10 Warthog such an effective weapon. Aircraft are also good at dropping bombs on things and acting as forward spotters for the artillery, and their rounds also come in from above. So you need to think about how to deal with air threats, because otherwise your super-heavy tanks are just target practice for the fly-boys. The P.1000 Ratte was to have had its own private anti-aircraft battery for exactly this reason.

There are also serious logistic problems. Tanks are frequently moved around on low-loaders or trains, but a super-heavy tank is going to be too big for that; it moves under its own power or not at all. If it has a maximum speed of 25mph then it can't travel 100 miles in less than 4 hours, whereas a tank on a train or low-loader can be there in under half the time. It is also going to tear up any roads it travels on, and will not be able to pass traffic coming the other way. This isn't a problem in wartime, but for peacetime manoeuvres, exercises and deployment it is a huge issue. And as others have mentioned, fuel and other supplies become a problem (that pesky cube-square law again). But if it is the only thing that can stop the enemy, these disadvantages become a price worth paying. Military logistics were a big driver behind multi-lane highways. Maybe the issues with moving these tanks becomes part of the logic for superhighways in your world; lots of very big motorways terminating at military bases.

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  • $\begingroup$ Cube-square law isn't really applicable to this feat. When dealing with engineering, we can, and often do, implement void spaces. It is relevant to bio-beings, but when we can engineer the materials, the supports, etc - not so much. $\endgroup$
    – Mermaker
    Aug 18 '21 at 14:44
  • $\begingroup$ If you have a giant tank already, who says you can't put anti-air guns or even your own aircraft on it? $\endgroup$ Aug 18 '21 at 15:21
  • $\begingroup$ When you scale up a tank, doubling the height, width and length means you have 4 times the ground area, 4 times the armour area, and 8 times the volume. If the contents are significantly less dense than the armour, then cube-squared shouldn't blow out the weight too badly. $\endgroup$
    – Robyn
    Aug 19 '21 at 4:13
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Use as symbols

Ultra heavy tanks are terrible for warfare, so the less war is happening the more useful they are. They aren't great at fighting, but they are great at existing.

When Germany built the Maus they did so at least in part due to the sheer awe inspiring nature of this giant tank. That was true of most of the so-called "wonder weapons". Merely building a giant tank with a huge gun is impressive even if it wouldn't necessarily be a formidable foe on the battlefield.

If WW2 had been lower intensity or stretched on, these giant tanks may have served more of a symbolic purpose, as icons of the great power of Germany. If one of these behemoths rolled in to an occupied town, it would be a huge deal. I can't imagine the impact it would have on the morale of either side. While their combat usage may be restricted, war isn't all about combat.

These ultra-heavy tanks could also be used in shows of force, destroying ill-defended targets with ease.

One could imagine warfare becoming further ritualized between these ultra-heavy tanks, and indeed the concept has been explored in various media such as Girls Und Panzer or Heavy Object.

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    $\begingroup$ Just what I was about to answer. The human "who has the biggest" sentiment is singlehandedly responsible for lots of great achievements, like putting people on the moon. $\endgroup$
    – JanKanis
    Aug 18 '21 at 15:29
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This can never be viable. For the simple reason that armor does not work like most people imagine, not in any universe based on our own physics. Please see this battleship armor:

enter image description here

It can be made thicker still of course, but there's no depth of armor that will protect against such. Sure, we can posit a few ideas that overcome the other hurdles... a nuclear reactor as a powerplant so it's not guzzling 500 gallons of diesel per minute, specialized ships to be able to sail one (or maybe two) across an ocean.

But, at the end of the day, armor is to protect the occupants of the tank from small arms fire. You've just watched too many Ironman movies where Tony Stark takes a 50mm shell to the chest and bounces back.

Even today, with specialized man-portable weapons designed to pierce tank armor, they don't matter as much as they used to. IEDs, rocket propelled grenade, anti-tank rockets.

What happens when they start using those tactics to scram your nuclear reactor with 150 crew inside? When they do that to the behemoth that was projected to cost \$25 billion per unit, but ended up costing more like \$42 billion per unit due to cost overruns? How many of those can you lose before you've lost the war?

You don't even get the element of surprise here, let me remind you. You can hide your latest fighter-bomber in some skunkworks lab and fly it out of Nevada late at night for testing. But your destroyer-sized land vehicle won't be kept a secret from the only guys you hope to use it against (tell me you're not thinking this will be used in minor skirmishes and conflicts).

Just to train up the crew, they're going to need to be on patrol most of the time. As much as I might relish the idea of it just rampaging through rural Alabama on its maiden voyage so that the crew's not completely green, there are senators whose constituency might balk at the idea.

And, if somehow you can handwave all that away, you've still painted a big red bullseye on it with the label "send all tactical nuclear weapons here, c/o our debt-riddled defense budget".

The fate of all such vehicles thus matches their spiritual predecessor, Futurama's Land Titanic:

enter image description here

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    $\begingroup$ Ultimately, that image of battleship armor breached by a battleship cannon shell is evidence in favor of the viability of ultra-heavy tanks, not against them! It shows that they're viable weapons against battleship-grade foes. $\endgroup$
    – nick012000
    Aug 17 '21 at 14:48
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    $\begingroup$ @nick012000 Battleships stopped being viable prior to WWII. On the naval side, everything switched to aircraft carriers just before (or maybe during). The "you need battleships to fight battleships" theory of warfare is already disproven... the other guy has none, and you don't need those to fight them if they do have such. All the image does is prove that armor is useless in modern warfare. It does not protect. What else does a mega-tank have going for it? $\endgroup$
    – John O
    Aug 17 '21 at 16:08
  • $\begingroup$ @JohnO Do what the navy's doing! With a giant tank, you can put other stuff on it (like aircraft, in the case of an aircraft carrier). Instead of just doing the job of a normal tank but bigger, you can give it entirely new roles as well. $\endgroup$ Aug 17 '21 at 18:20
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    $\begingroup$ @RedwolfPrograms - a giant tank that transports an oceanic sized body of water, effectively a sea on wheels, so that the Navy's existing assets can now operate inland! $\endgroup$ Aug 18 '21 at 14:23
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    $\begingroup$ @RedwolfPrograms Except the point of a carrier is to provide a mobile piece of dry land. If you're actually on dry land already, why do you need to build a mobile piece of dry land to move across the existing dry land? $\endgroup$
    – Graham
    Aug 18 '21 at 22:33
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Electronics became increasingly important in warfare but were never miniaturised

Modern electronics provided so many new capabilities - fire control, meteorology, radar, thermal imaging, encrypted communication, navigation, and the computerisation of logistics, asset tracking and military strategy. Unfortunately, the integrated circuit was never developed and numerous other components were never miniaturised.

Ultimately, it became necessary to bring a computer into the theatre of war.

Your tank carries an electronic computer with hundreds of thousands of thermionic valves, ten of megabytes of core memory, hard disks and a team of computer technicians all in a climate controlled environment. It scans the battlefield with radar and cryogenically cooled thermal imaging sensors. It can automatically analyse the readings it takes, and track friendly and unknown units. It carries an atomic clock which, in conjunction with other giant tanks, set up a kind of land-based GPS for friendly units. It can accurately locate enemy guns just by the noise they make, and bring in accurate artillery fire in a moment. With readings from its environmental sensors it can run weather prediction algorithms. It can intercept enemy transmissions and potentially break enemy ciphers. The tremendously accurate real time picture it has of the battlefield around it can be encrypted and automatically transmitted over radio, and at network of these tanks create a real time coherent picture of the battlefield that make allied units far more effective.

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Enemies that can only be defeated by naval cannons.

If you want an excuse for the use of giant land tanks armed with naval cannons, then the obvious solution is to throw them up against foes that can only be meaningfully harmed with naval cannons, and which are positioned in places where the traditional vehicles for mounting such weapons (naval vessels and trains) can't access. Smaller tanks or artillery are insufficient because their shells just bounce off of the naval battleship-grade armor of their enemies.

For instance, perhaps the enemy has a massive network of super-hardened bunkers with multiple feet of armor plating protecting them and bristling with anti-tank and anti-aircraft weapons, or there are nuclear rocket-powered space battleships that are capable of atmospheric maneuvers.

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Bedrock highway

One could imagine a very particular type of terrain which would defeat conventional tanks - marshy, boggy, muddy, swampy - but which still had bedrock relatively close to the surface. A sufficiently large tank could squish the soft terrain out of its way in the same way that a conventional tank squishes small puddles out of the way.

Let’s say there is a field of battle consisting of 3 metres of mud underlaid by solid rock. A normal-sized tank would sink into it, so build your tank to be 30 metres tall and 100 metres long, or whatever proportions allow you to treat the underlying bedrock as the actual road, with the mud pushed aside as a slight inconvenience.

Megatank too big a target? Give your army prodigious manufacturing capacity so they can build hundreds of them.

Megatank needs lots of fuel? Give it lots of fuel and make the military objective worth it.

Megatank is a sitting duck for aerial bombardment? Give it active defences and put a runway on top, so it can create its own air supremacy like an aircraft carrier.

Megatank would be useless in other types of terrain? That’s OK, megatank is a highly specialized weapon.

Megatank would sink into really deep mud? You need Gigatank.

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What kind of war would lead to Ultra-Heavy tanks being useful?

The kind fought on your home turf.

Imagine the opponent with enough gall to attack a nation with enough disposable income for a super tank. If this is what they're showing, then what are they not showing us?!?!

From my other answer about the practicality of dragon blood quenched swords; it's just for show.

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Ultra tanks cannot exist, because modern human technology is very good at destroying tanks and ease of destroying a tank is proportional to it's size. And bigger tank means bigger economic damage when one is destroyed.

My solution? Make your tank fight against something that's not human. Zombies, alien monsters, anything that doesn't know how to use a rocket launcher or set up a mine field, but is still an existential threat to unprotected population. Such tank would function as a mobile fortress, with many smaller guns for protection, but it's main function would be to get the hell out, when wave of the enemies gets overwhelming. It would be powered by nuclear reactor and house entire towns inside it with all the ususal facilities.

I would imagine humanity moved to living in the oceans because huge ships make more sense than huge tanks, however some of those tanks have been created to traverse the land when it is for some reason required - for example trying to reach remaining survivors on land and bring them to safety on the ship. Because of the infrastructure collapse, oil industry completely shut down and uranium became the only reliable, long lasting fuel source, which pushed humanity to construction of huge vehicles with huge capacity. If combustion engine was still a thing, conventional convoys would probably be a better options for such missions. Or maybe the monsters came from the ocean and humanity can only survive on land?

If this is for a story, it's good to show how regular fortresses fell by being overwhelmed and escape became the only option for humanity to survive.

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Any and all futuristic setting using humans already makes the leap that we somehow haven't utilized our advancing technologies. In fact as our computers become smaller it becomes easier to build smart artillery shells to destroy targets, meaning that the days of the tank are already numbered. Worse still is that most sci-fi is more of a WWII style combat despite its technology, with armored vehicles often fighting mere meters away from each other. So with that leap of logic already in, why not justify your tanks with similarly barely explained "its just the way it is" arguments and let suspension of disbelief take over?

That said some extra justification is never a bad thing: It is not just a tank, but a mobile base to protect the inhabitants from the world outside. As an example the Arctic cruiser, although this concept failed: https://en.m.wikipedia.org/wiki/Antarctic_Snow_Cruiser

The tank houses things like water recycling, air filtration and living quarters for the crew, as well as spare parts and maintenance equipment along with protective suits they can wear to maintain the tank's exterior.

The tank has weapon systems to deal with the threats outside, and some barely explained phenomena protects against high-accurate artillery and aircraft bombardment. Maybe some kind of point-defense is available capable of defeating such weapons but failing at doing the same with more direct attacks. Maybe a shield generator can only be aligned perpendicular to the direction of gravity. It doesn't matter what you choose, so long as it gives a logical solution and you are consistent. Dune for example explains that shields protect against high-speed projectiles, but not how they work. That is fine, as long as you remain consistent. Sound in space is common too, as is a WWII combat style for spacefaring craft. Those dont even require explanation for the reader to understand, so long as you remain consistent in its execution.

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A war against hyper genetically engineered animals. With claws sharp enough to get through light armour (Up to IFVs) & able to take small arms fire. If these bugs get large enough they could get through regular tank armour. As to rule out regular super heavies in the 80-200 ton range they could get up the the size of a Dreadnoughtus:

enter image description here

The largest known land animal know to have lived at 59 tons. This would leave tanks of the ultra heavy size useful. As they would be the only things able to have enough armour to resist the bugs of this size. While very big guns would not be needed. An array of smaller weapons like HMGs, AGLs & autocannons would be ideal. Alongside mortars, thermobaric rocket launchers like the TOS-1 & a few smaller (75-100mm) tank guns for the larger creatures.

At this size they could also be able to carry weeks to months worth of supplies without having to leave the tank. To not put people at risk by having to leave the tank to get supplies on a regular basis. Living quarters. Even small laboratory facilities if the vehicles is big enough. Maybe even mining truck sized vehicles with big cranes could be used to make it so that you wouldn't ever have to leave one of the big vehicles inside an infested area.

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    $\begingroup$ Pfft. But not just "Pfft"... double pfft!! You sit a kilometer away and shoot depleted uranium KE penetrator and HEAT rounds at them. Better yet, en.wikipedia.org/wiki/Lazy_Dog_(bomb). We have the technology to make them bigger, and laser guided; aim a laser designator at one of those beasts, and then shred them from the air. Hell, shred them with an A-10 and it's GAU-8. $\endgroup$
    – RonJohn
    Aug 16 '21 at 22:41
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The atmosphere of the whole planet is hostile to life. You can't set foot outside without dissolving/freezing/burning/getting irradiated and so on. Frequent nuclear conflicts have made the concept of a city unviable. The only way to survive is to keep moving and to be well protected against the environment. There are mega-tanks on the planet and pretty much nothing else. Each is its own city, powered by a plutonium generator. They keep on moving to avoid being targeted by nuclear strikes.

Alternatively, you could have mega-tanks on a planet with one giant very flat landmass and no water. The tanks would essentially act as a land replacement for aircraft carriers, deploying effective short range air units that can quickly resupply on the mega-tank.

Finally, if you're ready to give the tank a large number of very strong legs instead of treads, you can make it capable of handling a far larger variety of terrains, at the cost of potential stability issues (especially in combat).

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Ablative armor and plasma shields provide a huge defensive advantage, but only at the cost of requiring super heavy tanks. Generations of warfare have resulted in more-or-less static kill zones between the few remaining nation states. Anti-air systems are so efficient, that almost nothing flies anymore. Cheap and effective rail guns have destroyed all orbital platforms. Even weather satellites were destroyed long ago.

One of the nations, hatches a plan to change the nature of warfare. The goal is to push into territories where they can extract more minerals. It's a gamble, that they will deny each of their enemies of critical resources fast enough that they can gain sufficient advantage to finally create a tipping point. But they have to begin mining operations within what will be an extended kill zone, in order to reap the benefits of those resources and continue the push.

Their engineers have designed massive, automated, mobile mineral processing machines, but how to defend them? The answer is to make them even more massive, and turn them into walking offensive and defensive gun platforms, with sufficient armor and power to survive in the kill zone for several months. They manage to keep this hyper-heavy project a secret by constructing them underground, just a few kilometer behind the front lines, where they will be used.

But that's obviously not enough for success. They also scale back their production of super-heavies, in favor of cheaper, lighter/faster units that they can build by the millions. These will be used to race across the kill zones in areas far away from the hyper-heavies. A combination of super-heavies and ultra-lights will push towards several enemy cities. To achieve their goals, they have to weaken their lines in some places, to redistribute their forces, and they make it obvious to their enemies.

The timing of each phase of the operation is critical. First, disinformation is "leaked" to known spies, that there are military production issues reducing production rates when in fact the opposite is true. They hope at least one, if not both of their enemies will take the bait and attempt to gain more or less useless territories for their own propaganda purposes.

The fast faints towards enemy cities comes next. Starting with heavy bombardments from the static guns and super-heavies, then a slow push forward, followed a week later by the ultra-lights. The later, just race right on through the enemy lines, killing only lightly armored forces when they are in the way, rapidly routing around everything else. The goal here is to draw enemy forces away from the quieter areas on the line, to defend against what is perceived to be an obvious (counter) push for territory.

Then the hyper-heavies, flanked by a concentration of super-heavies, push towards the needed resources. Several streams of them roll out, one behind the other. They must create and defend a bulge several hundred kilometers deep and wide. Each individual hyper-heavy is capable of hitting targets up to a thousand kilometers away, making it costly for their enemies to get close enough with enough concentrated fire power, to degrade the hyper-heavies.

If they succeed, their enemies will never be able to match their new production levels. If they fail, there will be two waring nations left on the planet. It's a huge risk scaling up attacks against both enemies, but they can't take the chance that they will contribute to weakening one of them, while the other also takes advantage and has enough time to adapt to the new strategy.

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A conflict in hostile environment(s) where units have to be deployed for prolonged periods, constantly mobile, with 24/7 readiness. I'm assuming the Tank is nuclear powered in this case, and is essentially a bit like a land going Nuclear Submarine. The extra size is partly a result of needing room for the things a Tank doesn't normally have - kitchen/eating/sleeping/sanitary facilities. Just barley enough room to move about and keep apart that the crew doesn't go insane on a deployment lasting weeks. The crew would be larger too to allow for three watches.

With a nuclear reactor providing power, in a near future setting weaponry could be quite different to that we are used to. Laser point defence and railguns. The thing could have its own air wing and infantry with a number of missile armed drones and gun armed mini tanks (or walkers of whatever design you like, robotics technology permitting).

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Very large vehicle can be useful in warfare. The U.S. military does build and operate a small number of very large vehicles. These are aircraft carriers, destroyers, battle ships etc.

Instead of thinking of the super heavy tank as just a larger version of a regular tank, think of it more like a navy ship. Its a big platform where you can combine lots of really powerful but large and expensive technologies.

BETTER DEFENSE:
Small battle tanks might have something like an Active Protection System. But the super heavy tank can have something like the Phalanx close-in weapons system. This system automatically tracks and shoots down any incoming threats. Forget energy shields you just have a wall of lead.
https://en.wikipedia.org/wiki/Phalanx_CIWS

You can combine that with missile systems like Iron Dome and David's Arrow to create a complete missile defense system around your super heavy. This protects not only the super heavy but all of your smaller vehicles within the protection radius.

BETTER OFFENSE:
A regular tank might shoot 160mm shells with ranges of a mile or two. But the super heavy tank can have a railgun. Railguns typically require large capacitor banks and some sort of large generator to charge them. But the super heavy has room for that. Railguns have insane range (like 200 miles). If you park the super heavy in a wide open space no one will be able to get close enough to you to even get a chance to fire at you.

NEW TYPES OF WEAPONS:
You can't put a battery of cruise missiles on an ordinary tank, but they fit nicely on the super heavy.

USE IT TO CARRY SMALLER FIGHTING VEHICLES:
The super heavy is large enough that it can carry a crew of hundreds, and launch swarms of drones or even other tanks. Its like a land based aircraft carrier or a mobile base.

BETTER DRIVING RANGE:
Regular vehicles use gasoline. Like a navy ship, the super heavy is large enough to fit a nuclear reactor. Now you only need to fuel up every 10 years.

BETTER COMMUNICATIONS:
Communications wins battles. Smaller tanks are always starved for power and space. Their radios have limited range and bandwidth. Long range communications is either very limited or relies on satellites. The super heavy has room for large communications antennas and radio equipment. Its extra height gives it a very far line of sight. Its much harder to jam communications because your radio is now larger than the jamming equipment our enemy is using.

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  • $\begingroup$ OP mentioned that the setting is not important. I could see those naval-like land vehicles on a desert planet or on a planet with one, gigantic continent with no water-ways towards it's center. It can create fun borders and conflicts, with landlocked powers having a fleet of giga-tanks and naval powers having a fleet of warships, neither being able to invade or destroy each other. $\endgroup$ Aug 19 '21 at 9:32
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Post-Apocalypse / Hostile Planet

The scenario is either post-apocalypse or takes place on a world with a naturally inhospitable environment (like Venus, for example). The surface of the planet is so hostile to life that people can only survive there if protected with thick armor plates.

Humans live in large underground shelters or heavily fortified surface bunkers all around the world. Those shelters require resources which are found on the surface, and there is not enough for everyone. So those colonies wage war against each other wrangling for control of these resources.

That means that they are going to need some field operating bases. But those are difficult to build if you need really thick walls to protect yourself from the environment. So ultra-heavy tanks could serve as mobile bases. Their spacious interiors would serve as command posts, medical facilities, logistic centers and/or repair facilities for smaller battle-tanks.

Those tanks would usually be vulnerable to aerial attacks or cruise missiles, but the hostile atmosphere might also offer an explanation for why those are infeasible.

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The biggest land based vehicles are bucket wheel excavators. They are used to dig out vast quantities of dirt. If you look up a bucket wheel excavator (e.g. bagger 293), they kind of already look like gigantic tanks. There are a few reasons that you might want to have an ultra heavy tank which is similar to a bucket wheel excavator.

Excavation. For some reason, your military goal involves digging up a lot of dirt. This might be because you need to dig a dirt highway for logistics reasons, you need to create dirt airfields a lot, or you simply want the dirt - wars are fought over minerals and natural resources often. If you're trying to extract resources from a technologically inferior enemy nation, one way to do it is to build a titanic mining machine, cover it in armour so they can't effectively harm it with small arms fire and weapons so they can't get close enough to board it, roll up, take their ore deposits, and leave.

Moving really heavy things. Suppose you have a supergun. It can sink battleships from hundreds of miles away. It's a tactically valuable tool, but you only have one (or a few) of them. You want to be able to relocate your supergun for strategic purposes, and the normal way to do that would be to disassemble it, throw it on a fleet of trucks, and ship it. Problem is, disassembling something as big as a supergun takes a day at least, so does reassembling it, and transporting isn't speedy either. If you take your supergun offline for days and the enemy finds out, you're doomed! So instead, you make it mobile. Build a bucket wheel excavator looking thing, replace the mining machinery with your supergun, and now it can move a few miles a day (faster if that's a design priority) and still be 10 minutes away from firing at all times.

If you can keep your supergun moving at 10 miles a day, and if everything in a 100 mile radius of your supergun is captured territory, you can leave from Warsaw and capture Moscow in 2 months.

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They called it the Second Noah Flood. Sudden changes in climate led to worldwide rains like nothing ever seen. For endless months, it rained without cease. Earth turned to mud. Metal rusted. Concrete spalled, fractured, and collapsed. Hills and mountains eroded away in tidal waves of mud. Homes, towns, sometimes entire cities simply washed away and scattered across the landscape.

The earth of today is not as it once was. Forests, plains, pastures, hills - all has turned to swamp and mud.

With the destruction of farmland came widespread starvation. Billions perished. Those that remain are mired in the final conflict - the war for food.

There are no roads now, and no automobile can traverse the endless muck. Only the ultra-tanks - lumbering multistory behemoths constructed in secret facilities that remained safe in the bowels of the earth - can move across the quagmire. Carrying the last remains of humanity, they seek food. Food at any cost.

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Hmm, I think you might only have to take one thing off the table to make this within a suspension of disbelief. (Mega tanks still probably not the best solution by a long shot but being remotely feasible)

Kill manueverable airpower.

Let's say higher wavelength light sensors like infrared and visible spectrum coupled with higher computing power have made radar-defeating stealth obsolete, and the same setup has made laser point defenses and anti-air effectively impenetrable by craft and rocket propulsion.
Nothing that needs guidance can reliably deliver a payload.

Now to reach a distant enemy you need weapons with enough mass and momentum to minimize air interference with accuracy, and to make interception less relevant as the payload is still arriving with huge force whether in one piece or in several.

So you turn to magnetic acceleration: railguns. We haven't developed anything like miniaturized fusion yet, so the power supply needs to be enormous and probably a small fission reactor. And we don't have mass production of diamond hard materials, so the rails need to be very long to accelerate the ordnance at Gforces it can withstand.

You can't just keep this gun in one spot, and you don't wanna keep it on a known railway, or the enemy won't need guided weapons to counterfire. So you make an enormous platform to move it around between volleys. It has enough range that you're not worried about hazardous terrain unless the enemy wants to abandon hundreds of miles to your lighter mobile units, just go around. But you don't want it to be forced to stay ONLY on major highways, it should be able to deal with dry mild offroad terrain, so you scale up the volume and surface area touching the ground to allow the monster not to sink into EVERY offroad condition.

This makes it really, really slow. But that's fine. Tactically it only needs to move outside the impact range of accurate counter battery fire. It never approaches the front lines or gets within the horizon of the enemy.

It also doesn't really care about armor beyond shrapnel or small arms fire. No weight of armor will save it against a bullseye slug traveling at several km/s, in fact over penetration would be the best slim hope. Which means more mass allotment for power supply and delivery systems.

It doesn't sound much like a tank, but it's kinda like comparing an age of sail ship to a ww1 Dreadnaught which though it DID have heavy armor, had very little at the water line because primary concerns of sailing ships at 500 yards became irrelevant for angles at 12 miles.

And just like that comparison, I'm sure something like the torpedo would come along to sink this magtank within a decade or so of its deployment.

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