Doctrineless Ground Combat Platform Differentiation

Why develop the MK.24 at all?

Modular is not the same as being able to support any module. Your engine has limited power, your turret ring has a limited recoil tolerance, your electrical systems have limited power output, etc.

For starters, once you start to cross into 140mm cannons, you are no longer really talking about tanks. A 120mm cannon can kill any tank that exists today. 130mm cannons have only been considered as a possibility for certain countries that lack access to effective anti-armor ammunition, but generally gets rejected as an impractical trade-off for tank warfare. 140mm is edging more into the realm of artillery caliber weapons, you can not have a vehicle that is both rated to carry that big of a gun, and armored well enough to survive in tank-on-tank warfare without becoming so heavy that it sinks into and gets stuck in soft terrain, or becomes unable to cross most bridges.

Instead your MK.24 is more likely designed to be an armored artillery system (more akin to an M109 Paladin than an M1A series tank). It likely anticipates much lighter armor, a less robust ADS, and less room for upclose systems so that it is still light enough to function, while upscaling the size of a magazine, the turret ring, and reinforcing the chassis frame so that it does not rip itself apart.

Keeping the idea that this is armored artillery and not a tank means that it will likely have a whole different set of modules made for different purposes. While the MK.21 and MK.22 will function as tanks with optional modules to help them fight like non-tank AFVs, the MK.24 will likely have modules to help it fight like non-140mm artillery. So, it might be able to support extra large radar, ECM, and communications systems when fielded as part of the mobile command structure. It might be able to support mid-ranges SAM systems. It might be able to support an even larger 155mm howitzer, not just the 140mm. So, even if this vehicle could in theory replace the earlly MK series tanks, everything I know about tank and artillery design tells me that these two vehically families should have different roles in the grand scheme of things, even if there is a bit of operational overlap with certain configurations.

Even if it could fully replace the older design, it takes a long time for new weapons to replace "obsolete" ones

Modern tanks often take over a decade to get from concept to production, and then another decade or two to produce enough of them to be able to START to divest your older systems. For example, the United States is able to produce about 140-240 Abrams tanks per year, but it has an inventory of of over 8000 tanks in total. When the M1E3 enters service, it will take USA about 10 years to fully replace their active M1A series tanks, and another 20 years after that to have enough M1E series tanks to spare that they don't need to keep an M1A series inventory anymore.

In other words, it does not matter if the Mk.24 can replace both other models, because when your first Mk.24s roll off of the production lines, the other tanks will still fill your tank yards for a long time to come. Even if the 21 and 22 are both taken out of production, they will continue to serve the Consortium until they are so old that no threat remains that they can still go toe-to-toe with.

What you will see for a long time is the 24s only being deployed in those rare places that you need that 140mm cannon while the older tanks still do all of the heavy lifting of clearing bunkers and trenches until such point that the 24 reaches a ubiquitous status... which it may never reach if it turns out that the 140mm comes with too many unexpected trade-offs. It could play out like the M-7 that the US developed when they realized that 5.56mm ammo could not compete with newer body armor only to reach full production and realize that no country other than the USA is actually making enough body armor to need a special body armor killing main battle rifle.

FRAME CHALLENGE:

ALL armies have operational doctrines for their equipment. And if for some reason their commanders don't develop one. Individual unit's will develop them in the field via trail and error - because they want to survive!

On top of this, even if your armored vehicles have multiple functions based on module fit-out? Then you will still have 'Doctrines' BUT the point is those doctrines won't be based on the type of vehicle so much as the vehcile's function.

Caveat: What I'm about to say is based on a conversion I had with military personnel familiar with AFVs. If it's wrong others can correct me.

I will use the example of modular Armored Fighting Vehicles (AFVs) because that's what the conversation mentioned above related to and because these perform a wide variety of functions on the battlefield not just troop transport. For example Patria,the Boxer and Lynx programs offer modularity some of which can be changed in the field. The Boxer for example supposedly has something like 30 different available mission modules.

As I'm sure you know some common ones include troop carrier (APC), reconnaissance, direct fire support, indirect fire support,engineering/ARV and Command Post.

The point is that all of the types mentioned above perform different tactical functions on the battlefield & therefore will be deployed differently. It also follows that the crew of say a recon vehicle gets specific training in the tactical use and operation of their vehicle in combat which is different to the tactical skill set needed for operating say an APC. An indirect fires crew? Will be trained and will deploy differently again. And so on and so on.

So while any crew in a armored unit can in theory 'drive' any vehicle from Point A to Point B. As far as I am aware? Crews in a unit are not expected to operate every vehicle type (module) in a unit in combat even if most armies only seem to have 4 to 5 different types in service. This doesn't mean crew's don't cross train or rotate through different mission types over time but usually when they deploy in the field they crew a specific module type and that's what they operate unless they're rotated into a seat in another vehicle with an experienced commander.

YOUR SCENARIO

POINT 1: You haven't identified how many different modules you have but the crews of reference the MBT, tank destroyer and ARV roles. Those crews will have doctrines governing how they operate in combat and those will differ for each type because they different jobs. A tank destroyer is an ambush predator and defensive fighter as opposed to the offensive operations an MBT is designed for. Same thing again for the engineering role it will have another entirely different operating doctrine to the others even if they are all based on the same platform.

POINT 2: You claim your country is 'mostly' an archipelago. Without a map it's hard to judge but taking that comment literally we should probably assume that most of your nation has maritime borders with only a small proportion being land borders. You also say that 'Sytran relies primarily on naval and air assets'. That being the case don't get too caught up on tanks. Not only are there limitations on how you can deploy them across an archipelago in an emergency but also a single hit by a sea, air or land based anti-shipping missile can successfully convert the tanks in any ship borne invading force into ad hoc submarines. True, you need tanks to defend your land borders and (perhaps) a reserve offshore but if your country is not expansionist? A defensive doctrine on the land front using lots of ATGMs as part of your force model may well take a lot of pressure off the need for a large/complex tank force (expressed as a % of your defense budget). Or in other words more tanks mean fewer planes and ships.

Theatre Variants

Better armor and better weapons are always great, right? WRONG! The modularity of both systems implies that improved armor packages could be shared across both marks. Likewise there is the potential for the larger gun to be workable (though there's potential issues with the ability of the vehicle to absorb the recoil and so on). However the platforms were designed for radically different engagement zones, and as Sytran expects to fight in both zones, likely simultaneously, so both the Mk22 and Mk24 are necessary.

As an example, I'll point to the People's Liberation Army. They fielded the Type 99 MBT for most units, which expect to fight in or near China on the mainland. But China also has the Type 63 amphibious tank as part of an Amphibious Division aimed at Taiwan. Nowadays the Type 63 has been replaced by the Type 5 Amphibious Assault Vehicle which isn't really a tank-tank but the principle is the same. The Type 5 is also a good example of a "platform" which can be configured for various roles as you propose for your Mk22/24. So there's precedent for a nation needing multiple variants of the same basic thing to accomplish its strategic goals under vastly different operational conditions.

The Mk22 was developed for an archipelago nation, which to me implies island warfare. Sooo... It's amphibious! That's a tricky thing for an armored vehicle that comes with all sorts of tradeoffs, but for a vehicle operating in an environment with lots of little islands/rivers it can be a huge tactical advantage. Generally speaking, island warfare favors smaller vehicles which can be more easily transported by ships/aircraft. Smaller vehicles also have the advantage of being able to use weaker/smaller bridges common to small islands, and are more maneuverable in unfavorable terrain such as swamps and jungles. Even if it can mount the 140mm gun it can only carry a fraction of the shells for it due to its size. Plus given its opponents are ALSO operating under island-warfare constraints the 130mm is totally acceptable.

The Mk24, on the other hand, is the nations new "continental" MBT. The Mk24 is designed to fight far from logistical bases at ports against conventional forces. It's big, which allows it to carry more armor modules and mount the larger gun while still having the same ammunition carrying capacity as the Mk22. It's designed for long-term operation (think going 1,000km before routine maintenance rather than the Mk22s 500km) at the expense of having a wider silhouette. It's not fully amphibious, because it doesn't have to be, which saves all sorts of downsides.

Now all that isn't to say you wouldn't field the Mk24, say, on the Home Island where everything is well developed. Or that you wouldn't field the Mk22 somewhere on the continent. But if your nation expects to fight wars in two radically different places, it's conceivable it'd need two different "families" of MBTs et al.

I'm ignoring the sizes of the canons and the implications since "fixing" those would still mean you are left with the same question: why would they have two vehicles when one is more modular.

The answer is in your question: the mk24 is superior at the tank role. Better armor, a gun able to defeat current day tanks, better sensors etc. So you want the M24 as a tank to do the tank role whenever you can. This is actually incredibly useful since the M22 can be changed to fulfill all the non-tank roles if you do that.

However there's a ton of reasons why most vehicles on a battlefield aren't tanks. For example:

• monetary cost. If the M22 is cheaper it can be produced in larger quantities. However in favor of the M24 you don't need to build a dozen additional modules for your M22's just to make sure they can switch out whenever necessary. See how the Littoral ship failed because having all the modularity and a dozen different modules each having to be removeable instead of part of the ship made it so much more expensive.
• resource cost. If the M24 uses more rare resources or materials that require far more complex and limited industrial production it might be easier to build M22's. But again: the modularity means you need to have several tons more of modules lying around that aren't being used but need to be in position and with the gear to swap it all out relatively quickly.
• production times. Even at maximum production, the same size production line can build significantly more M22's than M24's. But again: there need to be more production lines to make sure enough modules are available for every M22 frame you have. You don't need to have one of every module for every frame, but enough to meet the needs you expect to have.
• ease of use. The M24 might be superior as a tank but require far more experienced troops to operate. The M22 can be operated by troops with less training time. On the other hand an M22 crew now needs to learn several different modules and it's uses, or you need different crews for a single frame.
• transportability. The M22 might be easier to transport. On the other hand the M22 might need more transport capacity for all the crews and modules and gear to swap out.

The end result: these are different vehicles that can share one role where the M24 is superior, but the M22 has it's superiority in other roles.

Mk 22 and 24 must be (semi) amphibious, and serve as auxiliary shore batteries.

To even have any reason to make MBTs and not rely on coastal batteries, anti-air and airstrips, an archipelago nation needs to make these vehicles able to be operated in coastal areas, so they can land with troops on islands, or to transport the vehicles along the coast and cross short stretches in good weather. Maybe the Mk 22 is a domestic design with all the amphibious gear, while the Mk 24 is a bought tank that is at best able to ford through water if you mount a snorkel array.

Due to the nation being mostly islands and atolls, the use of heavier vehicles to serve as moving, auxiliary coastal batteries means the Mk22 and Mk24 were mainly made in smaller numbers, and are used mainly in that role - they don't have a doctrinal use besides "being relocatable shore batteries" because there is little battle theater where they are used outside of that capacity in areas where actual coastal batteries with proper coastal batteries and Anti-ship-missile launch sites are still in construction.

While there might be a low three-digit amount of these tanks made, a much more numerous item that fights them for relevance will be torpedo boats and frigates with 6 inch (152.4 mm) guns. In fact, any sensible planner would suggest refitting both Mk 22 and 24 to use the very same 6-inch shell as the quick-firing navy ships, and best to swap both to a diesel that can run on ship fuel, simplifying logistics a lot.

Just dealing with the essentials here. The term 'platform' is generally used to specify a basic component onto which various modules or systems are attached. In general this would involve a power plant, a propulsion and steering system, the basic necessities (seats, consoles, controls, etc) that allow a crew to move the platform itself from one location to another, and some type of unified interface that allows modules to be 'plugged on' with minimal refitting. We can consider (for instance) a modern naval destroyer, where the hull, propulsion, crew quarters, etc are standard, but different weapon systems can be removed or added for different missions.

So at brass tacks, platforms are going to be defined in terms of:

• power loads: how much energy the power plant can output, which will put limits on its maximum load, speed, the amount of electrical power it can produce, and the like.
• interface design: different systems will have different interface requirements, and it's unlikely one could develop a truly uniform, universal interface

So there might be an extra-heavy platform (used for things like MBTs, motorized artillery, etc), with a power plant and drive capable of carrying heavy armor and weapons; a medium platform with a smaller power plant (for troop carriers, anti-tank or anti-aircraft guns, mine clearing, etc); a light platform for units that rely on speed more than armor; maybe an entirely separate platform for electronic warfare devices (where much of the power plant output is designed for producing electricity, not directed straight into propulsion).

If you find it useful, navies have been using this type of 'platform' categorization (frigate vs cruiser vs battleship vs aircraft carrier…) for hundreds of years. You might think about how they divide things up and retrofit it to land units.