The largest self-powered vehicle in the world is the NASA crawler-transporter, a 2700 ton machine designed to transport the Space Shuttle a short distance and in a straight line.

crawler-transporter carrying a space shuttle

During WW2, German engineers designed the P1000 Ratte, a ridiculously huge thousand-ton supertank. It was never built, and likely would have been a complete waste of resources. It would have been extremely vulnerable to bombing, and would have severe difficulty traversing the landscape of continental Europe.

P.1000 Ratte art

Fictionally, the larger vehicles of Homeworld: Deserts of Kharak appear based on the aesthetic of the crawler-transporter, but scaled up to ludicrous size. Note that the dune buggy-like vehicle in the lower right is large enough to have a multi-person crew, for reference.

fictional vehicles from the game

In the real-world, megavehicles like these are militarily non-viable. They would be vulnerable to airpower and tactical nuclear strike, incapable of traversing bridges or following roads, and offer no apparent benefit over a convoy of more specialized vehicles.

Deserts of Kharak appears to attempt to address these issues through its worldbuilding- the setting is a massive desert without bridges or roads, airpower is minimal because of sandstorms, and the extreme heat of the desert requires that vehicles be reasonably self-sufficient (ie crews cannot make camp outside their vehicles, and must live onboard 24/7). The technological base is science-fictional, but not dramatically more advanced than the present day in most respects, with the exception that nuclear weapons are not available.

My naive intuition is that if armor is viable as protection, the square-cube law favors larger vehicles as they gain greater protection for an equivalent armor mass fraction. However, by the same token, ground pressure increases at the same rate, and in sandy terrain that becomes a problem. I'm assuming that any realistic take on this concept will at least need much more track surface than the crawler-transporter or Deserts of Kharak vehicles, for the sake of ground pressure.

Do these conditions actually favor very large land vehicles for military use? If not, what are the minimum changes necessary to the environment or technological base to make such megavehicles practical?

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    If you have the technology to build megavehicles, there's no reasonable reason why you haven't also developed flight (if for no other reason than to travel quickly far above the hot sand). – RonJohn Mar 7 at 22:02
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    Correction: the largest self-powered vehicles nowadays are bucket wheel excavators on land (14,200 tons), and the Seawise giant on sea (646,642 tons). That space shuttle has got nothing on them in terms of mass. – Renan Mar 7 at 22:02
  • Also, there do appear to be two fliers in the Coalition Fleet. – RonJohn Mar 7 at 22:04
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    @RonJohn To clarify, aircraft are present in the setting, but weather is sufficiently hazardous and unpredictable (eg storms with up to 200mph winds) to make it unreliable. They typically take the form of short-ranged CAS aircraft (like in the image) or long-range haulers, only usable when atmospheric conditions cooperate. – Catgut Mar 7 at 22:50
  • OK. Note, though, that such violent sandstorms would scour everything, and if they're unpredictable, no one would want (or could) live there. – RonJohn Mar 7 at 22:59

12 Answers 12

A good place to start with this would be to look at the history of naval battle, where this issue has already been addressed.

From the 1700s through to the 1940s, battleships got larger and larger, and carried heavier and heavier armour. To be sure, there were some forays into more agile, lightly armoured battleships (called battlecruisers) but for the most part, battleships were bigger and bigger, more heavily armoured, and carried larger guns.

This last point is the most important. Their size meant that they could carry guns that were more powerful, had longer range and could do far more destructive damage. This culminated in ships like the USS Missouri that could hit targets well over the horizon with shells weighing more than some vehicles.

The problem with such ships was that they needed aerial spotters for their targets, and carried SO MUCH firepower that its use was controlled, even in a battle situation. It was also vulnerable to short range attacks from much smaller boats (like torpedo boats) and of course submarines. This is why we have a concept of battle groups; small fleets of smaller boats that protect the primary offensive weapon, in this case the battleship.

That said, we don't make these ships anymore. Why not? Aircraft carriers.

Aircraft are effectively longer range weapons without recoil effects on your battle platform. A normal modern aircraft carrier can now carry between 50 and 100 planes, meaning that you can strike multiple targets, at any range, and create a far more effective defensive screen for your weapons platform than a conventional battleship. Again, it's about range and size of destructive potential.

So; if you want to build large land based weapons platforms, those platforms have to bring some military advantage that cannot be achieved by assigning the same amount of resources (engineers, metal, etc.) to a fleet of smaller vehicles. What I'm guessing is that would have to be ranged attacks of larger destructive power.

The thing is, in a modern world with cruise missiles and other long range guided missiles, this just isn't viable. You can hold a massive ranged destructive potential on an articulated truck these days, that can act as a missile launch platform. Have a couple of tanks and 50-cal jeeps protecting your ranged missile launcher and you have the same configuration as a naval battle group, only cheaper, smaller and faster. Additionally, I don't think you'll get the larger land vehicles through mountain passes and the like.

One potential option is a large land based mobile platform for landing and refuelling (and rearming) VTOL aircraft. Ironically enough, introducing aircraft in an environment with little to no permanent buildings or fixed infrastructure may be the reason for large mobile bases as such.

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    Every single battleship can fire its guns in whatever direction its crew pleases. The page for the Iowa-Class Battleship on Wikipedia clearly shows an Iowa-class battleship doing this with the image on the top of the page. The turrets are designed so that they absorb the recoil to almost negligible amounts. Yes, recoil is absurd but these are massive ships and were designed with the guns in mind. – Andon Mar 7 at 23:41
  • Alright, agreed. I was working on the simple math, not on the anti-recoil technology. I agree with the edit. – Tim B II Mar 7 at 23:46
  • Sorry if I came across a little strongly. Battleships are a hobby of mine. – Andon Mar 8 at 0:58
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    It's all good @Andon , taken in good faith I can assure you. To be frank, most of my research on battleship design in the past had focused on their history, not on engineering. I didn't know that they had these kinds of recoil solutions in place and admittedly I'd always assumed that they would take too much energy out of the projectile to make them worthwhile. Anyway, it was a good pickup and I welcome the improvement. I'm the first to admit I don't know it all. :) – Tim B II Mar 8 at 1:03
  • From my understanding, it doesn't change the energy of the projectile, just redirects the recoil elsewhere. I've never really looked into it too much in depth – Andon Mar 8 at 2:05

Its going to be a lot bigger than you think in a lot of ways you haven't thought about

I'm a former Marine Infantryman (anti tank gunner), I don't know much mechanics, but I know tactics. There is a very good reason military vehicles have been trending towards being smaller and more mobile rather than large and "indestructible." Primarily, that nothing is indestructible. The larger a heat signature and the less ability the vehicle has to move out of the way of incoming fire the less likely such a vehicle is to survive. Its better to decrease detection signature and increase mobility than it is to increase size and armor. Anything as big as what you are talking about is going to be a massive target for artillery and air-strikes. Also, there is not real way to transport the vehicle to the combat theater. The only way such a vehicle would be feasible is if it were being deployed well behind the front echelon as a defensive measure or command and control asset. It would need to be heavily screened with air cover and be far enough into the rear echelon that artillery could not reach it. This becomes a long, loooong distance when you factor in the fact that modern artillery rockets such as the HIMAR system in use by the USA can accurately strike targets hundreds of miles away. Additionally it would need air to air defenses the likes of which we could only dream of in Reagan's Star Wars program to prevent waves of cruise missiles being deployed against it from other continents.

So, in summary, for such a super-heavy "Land-Ship" type vehicle to make sense tactically it would need to be far, far away from the action, have massive amounts of resources dedicated to defending it, and posses a nearly impenetrable air defense system. I'm talking at least an entire battalion of infantry, a battalion of armor, an artillery battery or two, a squadron of aircraft, a battalion of engineers and mechanics, a logistics and supply battalion. You're basically going to be dedicating and entire force big enough to invade most small nations just to supply, maintain, and defend the thing. You are going to need efficient enough logistics to supply not only the land-ship as it moves, but also the regimental sized security and logistics detachment you have following it around. This thing would basically be a land based Marine Air Ground Taskforce (MAGTF.) To give you an idea of how big a MAGTF is, the last time I saw an actual MAGTF summoned together we all ended up invading Iraq and controlling a province the size of Massachusetts.

Its not physically impossible, or even tactically impossible enough not to be used in fiction. Its just going to be a way, WAY bigger undertaking than you think if depicted realistically.

Rather than think 'earth-based' military tactics, one needs to look at the tactics of this particular planet, and of the purpose of the vehicle. Would there be a situation where such a vehicle would be advantageous?

As described, I understand the planet to be a huge barren, flat land mass, perhaps as big as Pangaea, with no particular geological features. No colliding tectonic plates to create mountains, for instance. Populated zones are at the poles, widely separated by this huge land mass. Covering it would take a week of travel, perhaps, with no permanent pit stops except on well-established but limited transportation corridors.

Limiting air travel is not difficult, once it is recognized that aircraft engines require a specific composition of the atmosphere. Limit the oxygen, limit the air travel, for instance. A high concentration of nitrogen in the atmosphere, perhaps. Thus, along with fuel, the aircraft would also have to carry the oxidizer. They would have very limited range, or they would be extremely inefficient payload wise.

In such a case, I would see this vehicle as a 'land-based air craft carrier'. A vehicle that would carry auxiliary vehicles from temporary base to temporary base. Vertical take-off and landing craft. Given the vagarities of the weather patterns on your planet, and the limitations on air cover, such a tactic would be very useful for close support. Move the airfield to where it was needed.

Maintenance bases for ground based troops, days away from any supply depot, would be easy targets. Supply routes would be extremely vulnerable, as they would be limited and very long. Instead of having military bases spread out over the entire land mass, make them portable. Move the base to where it was needed.

On your planet, long-distance sensors and detection would pretty much span the entire land mass, so the 'enemy' could clearly be spotted before any potential engagement. Air-born weapons would have to traverse a great distance before hitting their target. Sufficient intercept time to destroy them. Under such conditions, engagements would be localized. Hit and run. Attrition.

As an aside, it is not hard to imagine a species that was adapted enough to survive in a low-oxygen, high nitrogen atmosphere. Their metabolism would not mimic ours, but earth once had such an atmosphere, and life developed. Think slow-moving creatures, low metabolic rate.

My naive intuition is that if armor is viable as protection, the square-cube law favors larger vehicles as they gain greater protection for an equivalent armor mass fraction.

Your intuition is not naive, but it is also not completely correct either.

The biggest problem a supergiant tank would face would be power. If you follow the square-cube law, as the vehicle design grows, the armor and other heavy parts will become heavier at too fast a rate for the engines to pick up.

For example, the M1 Abrams tank weights around 60-70 tons. The Honeywell AGT 1500, its engine and main power plant, weights 1.134 tons. So we can say that the power plant to vehicle weight ratio is around 1:60.

That tank is about 10 meters long. If we scale it to be 100 meters long, the whole thing could weight as much as 216,000 tons. The engine, on the other hand, would weight 1.459 tons. The powerplant to vehicle weight ratio then becomes 0.00000675. Not only the tank would be stuck in place, it would probably be unable to move its turret and other parts.

The immediate way to solve this problem is by making the engine bigger in relation to the rest of the vehicle, but that takes space and mass from other things such as guns and armor, reducing the advantage of large sizes. At some scale the advantages of a large size are negated, and over that scale everything becomes dead weight. This is not efficient.

If you got the materials to build a huge vehicle that would be unnefective, you would make better use of your resources by building smaller vehicles, or a ground base (i.e.: a bunker or a missiles silo) rather than a giant tank.


Of course, if a leap in power generation technology happens, larger vehicles become more viable. In the 1920's cars were generally bigger and most engines back then were in the 25-30 HP range. Motorcycles nowadays can easily reach 150 or more HP with much less size and weight. If the trend goes on, giant tanks may be possible within this century or the next.


For the record, the space shuttle transport in the question is far from being the largest self-powered vehicle in existence. Bucket wheel excavators easily reach 14,000 tons, and the Seawise Giant is over 640,000 tons heavy.

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    We scale up ship engines along with the ships. Likewise, we would scale up the super-tank's engines. (Megatanks are still a bad idea, though.) – RonJohn Mar 7 at 22:21
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    I'm confused, why does the powerplant not scale with the rest of the vehicle? Where does the 1.459 ton figure come from? – Catgut Mar 8 at 1:01
  • @Catgut square-cube law. Multiply a length by itaelf, and volumes and weight go up by a power of 3. 1.134 x 1.134 x 1.134 = 1.459. – Renan Mar 8 at 11:00
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    By this math, a 1-ton engine would never change weight no matter how big you made it, and an engine with a weight under one ton would grow lighter when upscaled. That doesn't make any sense. – Catgut Mar 8 at 15:12
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    You need to use Power-to-weight ratios. Weight, mass and volume can be said to scale linearly, but Power-to-Weight usually gets better as the motor gets bigger. How much better, and what the ratio is in the first place depends on the specific technology used. But that boils down to: doubling your mass, and doubling the mass of your motor, will more than double the power availabe, and therefore slightly better the ratio of motor-power to vehicle mass! – bukwyrm Apr 17 at 10:44

Ground effect craft.

enter image description here

I know this is over water but it may be possible to use ground effect over smooth ground such as deserts.

This is not an air plane it is the Caspian sea monster

https://en.wikipedia.org/wiki/Caspian_Sea_Monster

General characteristics

  • Length: 92.00 m (301 ft 10 in)
  • Wingspan: 37.60 m (123 ft 4 in)
  • Tail stabilizer span: 37 m (121 ft 5 in)
  • Height: 21.80 m (71 ft 6 in)
  • Wing area: 662.50 m2 (7,131.1 sq ft)
  • Empty weight: 240,000 kg (529,109 lb)
  • Max takeoff weight: 544,000 kg (1,199,315 lb)
  • Powerplant: 10 × Dobrynin VD-7 turbojet, 127.53 kN (28,670 lbf) thrust each

Performance

  • Maximum speed: 500 km/h (311 mph; 270 kn)
  • Cruise speed: 430 km/h (267 mph; 232 kn)
  • Range: 1,500 km (932 mi; 810 nmi)
  • Ground effect altitude: 4–14 m (13 ft 1 in–45 ft 11 in)
  • Maximum sea state: 1.2 m (3 ft 11 in)

hopefully I don't get down voted to badly, but I just love this thing.

  • "smooth ground such as deserts." Most deserts aren't flat, and OP mentions "airpower is minimal because of sandstorms". – RonJohn Mar 8 at 6:35
  • Yea, yea. But you have to admit that thing is Kool. – ArtisticPhoenix Mar 8 at 6:37
  • Even cooler than the Spruce Goose, and totally dilapidated. – RonJohn Mar 8 at 6:49
  • yea the main problem is turning it, you have to basically turn using only yaw, and at those speeds it takes forever to turn. This and the height restrictions really limited the operational envelop. But I always love that 50's-60's Jetson futuristic look. – ArtisticPhoenix Mar 8 at 7:28
  • Looking over the specs, this vehicle could easily be operated over large expanses of flat land. The problem would be dust plume kicked up by the jets. You can see the water plume in the photo, behind it. But ob hard compact sand (wet sand, like on a beach) it would be perfect. 301 feet long. The length of three residential lots. – Justin Thyme Mar 8 at 15:23

Many of the answers here address the issue of why they aren't practical, and can be summed up as: it's big, slow and hard to defend.

So what would change this?

Option 1: Add more power Add super-advanced engines (and super advanced shock absorbers) that allow you vehicle to reach 100kph. It'll still be vunerable to bombing (so you'll need AA), but should be less vulnerable to ICBM's and other artillery now. The only option I can think of here are fusion reactors.

Option 2: Add more armour Obviously this has it's own issues (such as weight), so let's run science-fiction and introduce .... shielding. If you can shield from incoming attacks, then your super-huge vehicle can now be used. Maybe shield generators need a lot of power, or are physically huge, so only large land vehicle can support them.

Option 3: Hide it Add really powerful ECM to make it hard for long range targets to target it, or cloaking, or some method to hide it. Yes, I am talking about cloaking a massive vehicle. If sci-fi has cloaking devices on spaceships, why not on massive land vehicles? (Uh, sir, the giant tank tracks just, uh, stop in the middle of the desert)

Ok, so now that it's now blown out of the water as soon as it rolls out of your production facilities, what can we use it for?

  • Destroying towns by driving over them
  • Mobile air-base/fueling depo/resupply unit. No long supply lines.
  • Manufacturing vehicles/arms on-demand
  • Standing force for enforcing control over a captured area
  • Presumably it has lots of guns, so perhaps area denial
  • A safe place for the president
  • Psychological warfare

And how do we get it places? Well, I guess you either drive it or teleport it.

  • Ships need hulls to displace water, aircraft need wings to generate lift, tanks need tracks or wheels to carry the weight. Of these, wings and tracks depend on area while ship dispacement is proportional to volume. So air and ground suffer from the square-cube relationship, water does not.
  • Ground vehicles can easily stop where they are, load ammo from a supply vehicle to a firing vehicle, or take supplies from a supply dump. Underway replenishment for water and air vehicles is more difficult. Even in the desert, if suited crew cannot step outside for loading, how can any vehicle operate there?
  • Ground vehicle crews could also sleep in camping vans or the like instead of their combat vehicles, because volume behind the thick armor will be at a premium. Volume in a merely climate-controlled shell is cheaper.

So water vehicles can and need to get relatively large, air vehicles might need to get large but cannot, ground vehicles don't need to get all that large. That being said, tanks have grown over the years, just not as much as you suggest.

  • Once upon a time, 30 or 40 tons was a medium tank, 50 or 60 tons was a heavy tank.
  • Then they dumped the heavy and designated the medium main battle tank.
  • By today, MBTs have grown to 60 to 70 tons, heavier than a M103.
  • The question explicitly specifies conditions that would seem to invalidate points two and three, could you clarify with those conditions in mind? – Catgut Mar 8 at 15:16
  • @Catgut, better now? – o.m. Mar 8 at 17:14
  • Yes, thank you. – Catgut Mar 8 at 17:31
  • +1 for the reasoning about ships' buoyancy scaling by volume while tanks and planes scale by area! – bukwyrm Apr 17 at 10:32

As military vehicles never, as extraction equipment the military needs to defend sure, mining is already pushing for bigger and bigger machines as they are more cost effective. maybe the vehicles are really mobile refineries and just carry armaments becasue the are targets. Would help explain why they are on such a planet in the first place.

Energy Shields

As other posters have said, with current technology, big vehicle = big target.

If you have to supply power to a heavy, expensive energy shield, You would want to get as much stuff inside the shield as possible.

The main issue with huge vehicles is that they lack protection against air attack. Proposed solution: Configure your world so that air based attacks are impractical (strong unpredictable storms? Only heavy energy sources available eg nuclear reactors?)

As everyone else has said, your tank's biggest enemy would be concentrated artillery/airstrikes.

Your best defense against this would be the specifics of the planet. You've already got dust storms, good!

Add some high, gusty winds to disrupt ballistic artillery, add in some particulate to the storms that can disrupt/scatter radio/IR, that way guided missiles are ineffective. Essentially, make it such a hostile environment for anything in the air and sensors that air support is impossible.

The hostile conditions could include high levels of radiation in certain atmospheric layers, nasty particulate, corrosive gas layers, etc. Make it so the only viable fighting terrain is on the ground.

Once the war is fought on the ground, add some serious point defense, use it as a mobile base, and protect it with scouting parties.

Unfortunately, due to the atmospheric conditions, targeting/visibility would also be severely reduced, so combat is kept within 5 miles, say. No longer a massive artillery platform, but more like a moving bunker/trench system.

It'd have to be one nasty planet, but it should be doable!

If driving is all you can, driving is what you'll do. You don't have to summon the worms of Dune, but you need to have a reason why staying in one place does not work (Weird alien proscriptions? Some legal loophole ('No permanent structure shall...')? Something geological (the terminator of a very slow-revolving planet makes the local lanscape go boom)? Something celestial (Pityful atmo (perhaps mostly sand in VVLEO?), no magnetic field, and some harsh irradiators in the vicinity that burn everything they face)?). This builds the economic case to design everything mobile: quarters, industry, garages, hospitals, refineries, even warehouses.Your already established reasons against flying will prevent airships popping up as solutions, going orbital is discouraged by the necessity to return to ground for replenishing handwavium stores and the associated risk in crossing the atmosphere.

So the only viable path is going mobile, and the only way to conserve energy is going mobile in as few vehicles as possible. the energetic overhead of airconditioning (make the desert cold, not hot, otherwise that relation will be reversed. Poisonous atmosphere is a bonus), radiation-proofing, and generally xyz-ing everything that favours mass over surface will automaticall lead to huge crawlers. Now if you absolutely HAVE to have military conflict on a scale that includes attacks that could endanger whole vehicles structurally (instead of just limited raiding, or boarding-attemps), you'll have to go the way of the carrier group: huge vehicles with rather small scale weaponry and some incredibly costly cargo (be it industry or attack-craft) accompanied by smaller, faster, energetically more costly vehicles with more punch. Everyone will just engage in short range attacks and mine-laying because your aerial interdiction (winds? aliens with plane-allergies?) may be extended to make artillery shelling unreliable (bad aerial reconnaissance-possiblities already go a long way to making artillery short range).

A word on numbers: Mass goes by the cube of size, while the area (both atmospheric surface and touched surface) goes by the square of size. Ground pressure on a tank can average up to around 100kPa for the heavier ones, which is easy to remember, because that is one atmosphere of pressure. If you want to keep your lawn, you should not exceed 10kPa, if you just want the ground to bear you, everything from 100kPa (loose sand) to 800kPa (dense clay-y gravel) may apply. So in a sandy desert, we should not much exceed the current heavyweight tanks? I'm not convinced - for very large surfaces, the admissible ground-pressure may well be much higher, because the material simply has nowhere to go - The Schwerbelastungskörper was a Nazi test of how much the ground would actually bear: They weighed down 100m² with 12kt of concrete (so 1200kPa)- not much sinking was observed. You might need speial treads to actually achieve full coverage of huge areas, but why not go with pressurized mats or some other area-spreading technique? Clunky steel treads don't fare well with sharp edges of unyielding material.

Also google Schreitwerk for a very cool alternative to treads...

Bucket wheel excavators weigh in the 10kt range and have a motor in the 15MW range (Abrams: 0.06kt, 1MW, so talk about scaling...) - they also have extensive booms and overhang, so the actual tread-to-mass-ratio is probably much smaller than in tanks. Sure, to generate 1MW you need about 100grams of fuel per second, but that weird landcrawlers haunting alien deserts would need some kind of handwavium was clear from the get-go, i think.

I say take a good look at bucket excavators, scale up, armor (if you absolutely can't stop yourself) and let 'em rip!

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