I guess what I’m asking is what are the different functions of the two types of vehicles? Why can’t militaries combine them?
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A tank that can also carry people would need to be bigger than a tank that can’t carry people. A larger vehicle means it’ll both be slower and easier for enemy weapons to hit, both are bad things in a battlefield.
Because of that some vehicles are designed to carry people and other vehicles designed to carry a big gun and lots of ammo, that way they can both be smaller and focus on fighting or moving people safely depending on what they’re best at doing.
Jack of all trades, Master of None.
Any normal tank has very little space inside, because most of it is taken up by ammo storage, engine space, or armor. Increasing the space for soldiers will make it an easier target or reduce the armor, making it a worse tank.
In addition, adding heavier armor and a turret to APCs means troop transport is slower and usually requires more vehicles to transport the same number of troops.
There are a few stories of failed tanks that went that exact way; trying to fill multiple roles, but ended up being bad at all the roles.
The Israeli Merkava can do both, but when it does it reveals the weakness of the setup; it has to remove almost all of its ammo, leaving only a handful of shots in a little rack up front.
APCs are largely big empty boxes with engines and benches. If you want to turn it into a tank with tank-sized weapons, you gotta put on a turret that takes up a decent amount of space, and ammo racks that also take up a decent amount of space. Historically, people who wanted to go in the other direction and turn tanks into APCs usually did so by first ripping the turret off.
IFVs (Infantry Fighting Vehicles) bridge the gap, using smaller weapons and somewhat smaller turrets, in exchange for a smaller carry capacity in terms of soldiers in the back.
It comes down to this tending to be inefficient. It is a thing that can happen, the Merkava can remove ammunition pallets and carry infantry instead, for example. But ultimately if you’re designing something to exist on the frontline, you don’t want it larger than necessary. Those infantry are not useful inside of a tank, but you’re significantly increasing the amount of armor, hull, treads, etc. a Tank APC is going to be more fuel hungry, an easier target, and cost a lot more to make. The fuel is a crucial element as well, tanks guzzle enough of it as is, and this is a logistics lynchpin which can be exploited by the enemy (A lesson that Russia is learning painfully, since many of their auxiliary vehicles use the same engine and hull as a tank, making fuel a significant problem for them on the frontline). There are still APCs and IFVs of course, with IFVs being more heavily armed. Realistically IFVs are the balance you’re probably really looking for, and they tend towards armaments that compliments infantry more. They exist, and are in wide use, but they typically don’t want to go toe to toe with a modern tank because the modern tank is a specialist at being a tank, and an IFV by definition cannot be, since it has other functions it must specialize in.
To generalize (very generalized, it would take all day to go through the specifics)
Tanks- Kill armored vehicles, have the armor and protection systems to survive hits.
IFV- Kill lightly armored vehicles, can disable tanks, have the armor and protection systems to protect their infantry, but they can’t survive hits from tanks.
APC- Kills infantry, can inconvenience an IFV, usually useless against tanks, lighter, faster, armored enough to not be easily disabled by small arms fire. Not meant to get into a fight, drops off infantry and leaves, unless resistance is expected to be very light.
Cost, weight, design. The factors that make a successful tank don’t necessarily line up with the factors that make a successful APC or IFV.
While there have been a handful of tank to APC conversions over the years (notably, the Israeli Namer), generally speaking an APC doesn’t need to be anything more than a lightly armored box on tracks or wheels, resistant against artillery fragments and machine gun fire. That’s because APCs are usually not expected to directly engage the enemy, they exist to safely transport infantry to the fight, hence the term “battle taxi” often applied to them.
Upping protection level increases cost and weight which gives penalties to fuel consumption, transportability, maintenance, etc. Additionally the rear engine design normal to most tank platforms is counter productive in APCs which tend to prefer front engine placement. Having the engine in the rear on an APC leads to funky exit arrangements for the soldiers inside.
All in all, it’s generally not considered worthwhile to make your APC out of a tank. As an example, the US Stryker was primarily adopted because of it’s ability to be rapidly moved via strategic airlift as well as drive faster on roads due to its wheel (vs. tracks), something an M1 Abrams based APC would not be able to do.
Tanks on the other hand are expected to directly engage the enemy, and as a result require a significant amount of armor and firepower to fight and survive. They’re gonna be heavy, they’re gonna be expensive, but you gotta pay the price to have that capability.
IFVs (Infantry Fighting Vehicle) are an interesting middle ground, but while they sit somewhere in between APCs and tanks they still lean far closer to APCs and thus still wouldn’t necessarily benefit from using a tank chassis.
do you want a spoon or a fork? how about a spork that does neither well enough? now what if your life depended on it? this is called design trade offs. you can do one thing really well. or multiple things not so well. or if you wanted it to do everything well, then it becomes so impractical that it’s even more useless.
Some militaries do in fact combine them, the Israeli Merkava for example can carry a small number of troops.
Generally speaking though tanks should be heavily armored, as low to the ground as possible, and all spare room carries equipment or ammunition.
Adding troop carrying capacity to the body forces the tank to be larger and taller which makes it more vulnerable.
The Merkava makes room by removing a bunch of it’s internal stores and putting the engine at the front vs the back which is generally considered the wrong way round. Putting the engine up front makes it more vulnerable as the heat exchange and vents are the weakest part. On the flip side all that extra metal of the engine is in between the crew and the shell so it’s a trade-off. It makes the tank easier to kill but makes the crew more survivable.
The movie *The Pentagon Wars* goes into this in detail and why the Bradley, an armored fighting vehicle or APC + tank hybrid, is considered a fundamentally flawed design.
APCs should ideally be fast so weighing them down with extra armor and guns works against that.
Adding the turret and optics onto it to make it a scout makes it too tall and vulnerable. The gun turret takes up space making it carry less troops. Also having big guns and missiles on it makes it a bigger target, which is bad if you are using it to protect infantry. So it’s a bad compromise born out of a misguided attempt to save money by combining multiple vehicle concepts into one.
An analogy here:
Do you use a multitool when building something? It has all, or at least many, of the functions you need.
Pliers, knives, saws, screwdrivers…
It *can* do a lot of the work. But it isn’t good at any of them. And someone that has one of each tool of the proper size can do the job faster.
In war, faster means the other side is dead, you are not.
A dedicated tank would punch through the ‘tank/apc’ rapidly as the extra space inside means less engine, smaller weapons, less armor. Everyone inside is dead.
And the tank then pivots and hits the next one, and the next one.
The tank/apc’s likely can’t even damage the the tank enough to stop it.
Then a dedicated APC will roll up next to the tank, and let the troops out to clean up.
Armour, guns engines etc are very heavy. The ground, bridges, etc have limits limits in what weight they can handle. The longer a tracked vehicle is compared to it width the harder it is to maneuver.
The guns, ammunition, engines, fuel, crew, soldiers etc take up space. The larger you make the vehicles the more will it weigh if the armour remains the same. If the weight increases you need more powerful engines, drive trains etc for the same mobility. A heavier vehicle needs more fuel and it takes up space too
The heavy tanks already weigh about 65 tonnes and you really do not want to get even heavier. So a tank that can carry soldiers would be less well protected than one that can.
Israeli Merkava has a space that technically can be used to carry troops in the back. The problem is that the space ammunition usually carries space so if you carry a trope it is a lot less usable as a tank. It is for medical evacuation especially in low-intensity conflict and as a way to enter and exit the take more protection the exit in the back is useful.
You can build a vehicle with less armor than a tank, a lower caliber gun efficiency against everything that is not the front of a tank, and have space for troops. This is and has been done for a long time and they are called infantry fighting vehicles (IFV). IFV has replaced APC for lots of military applications. Some of the heaviest IFVs are a bit over 40 tonnes. You often have anti-tank guided missiles on them so they can take out tanks, what they can’t handle is a direct hit from the gun of a tank.
Military units are meant to cooperate not work alone. A single vehicle does not need to do everything. A tank and APC combination that could do both would cost more than a vehicle that is only one of the tasks. A tank and an IFV provide two guns and a sensor system. IFV guns will be more efficient against targets that have little or no armor and they can carry more ammunition because it is smaller and larger. So use both and let them engage the target they are best at.
A single-tank APC combination is also a single target and a single hit can take it out and kill everyone in it. Two vehicles are more survivable than one.
You’re essentially describing an IFV, it’s not very commonly used because the role of an APC and the role of a tank has very little overlap and what makes a good APC makes for a bad tank and vice versa.
APCs are meant to transport a large number of troops quickly around the combat space and are meant to prioritise mobility and troop capacity above pretty much all else. Their armour is only really designed to protect against small arms fire and should never be in direct combat as that would place their troop complement at risk.
MBTs are designed to be an armoured spearhead that can stand toe to toe against other tanks, hence prioritise armour and fire power and only secondly mobility.
They need to be heavily armoured, highly survivable and be heavily armed to do so, placing heavy restrictions on profile and available space and ideally should always be in active combat or repositioning to ambush in active combat.
Hence these two roles are somewhat incompatible, APCs should be positioning, deploying, then retreating to increase the strategic and operational mobility of infantry forces, whereas MBTs should flanking, ambushing and otherwise fighting in the thick of combat.
IFVs do have a niche in providing some heavy weapons support to infantry against other soft targets hence are typically armed with autocannons designed around anti-infantry purposes. This allows the specialisation of MBTs into a pure anti-tank role, but this is typically unnecessary as modern armaments have by and large outpaced modern armour to the point that you don’t need specialised tank destroyers like in WW2.
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