Why Can’t Tanks be Larger? Rheinmetall’s 130 mm Gun and the Future of MBTs

Rheinmetall's new "Main Ground Combat System" tank concept, which utilizes the 130mm gun.

Rheinmetall’s new “Main Ground Combat System” tank concept, which utilizes the 130 mm gun. Courtesy of Rheinmetall.

Updated 6/9/2017 to correct an error concerning the Rheinmetall 120 mm’s users.

German arms manufacturer Rheinmetall recently unveiled its new highly-anticipated 130 mm cannon design for use on next-generation tanks. Rheinmetall’s 120 mm tank gun is the most widely fielded non-Russian tank gun and holds a virtual monopoly on NATO and Western-allied tank armaments; currently, most Western main battle tanks (MBTs) are armed with smoothbore 120 mm Rheinmetall cannons, except the UK which uses a rifled 120 mm cannon, and the French who engineered their own CN120 120 mm smoothbore. The Rheinmetall gun has two variants, the L/44 and L/55, which have barrels of different lengths. In many cases, the cannons are license-produced, such as the L/44 M256A1 cannons used on M1A1 and M1A2 Abrams tanks, which are manufactured at the Watervliet Arsenal in New York and have some alterations including a coil-based recoil mechanism instead of a hydraulic one. All of these 120 mm cannons have generally similar ballistic performance and will be referred to collectively.

An M1A2 SEP V2 Abrams tank, armed with a L/44 120mm gun.

An M1A2 SEP V2 Abrams tank, armed with a license-produced L/44 120 mm gun.

Intended for use in next-generation main battle tanks, the new gun 130 mm comes at a time when tank armor is becoming increasingly difficult to penetrate. Innovations such as explosive reactive armor, high-end composites, and active protection systems oftentimes mitigate if not completely nullify the effects of anti-tank weapons such as kinetic (KE) sabot rounds and explosively formed penetrator warheads. Russia, in particular, fields Kontact-series explosive reactive armor, which is designed to, among other things, reduce the effectiveness of the of KE rounds fired from 120 mm tank cannons. Equipped with Kontact ERA, many newer Russian tanks have armor levels which approach that of advanced NATO tanks and inhibit the penetration of all but the highest-power anti-tank weapons.

This T-72 tank is covered in ERA bricks.

This T-72 tank is covered in older ERA bricks. Newer ERA such as the Kontakt-5 fitted to some T-90 tanks is even more effective.

It is in this context that the 130 mm gun becomes appealing. Because increases in bore diameter are not proportional to increases in propellant volume, the modest 10 mm increase in bore size actually yields a massive 50% increase in kinetic energy over the standard 120 mm round. This increase ensures that the 130 mm gun will remain potent for some time, whereas the ability of the 120 mm gun to counter future advances in protection is less clear.

However, it remains unseen whether or not the 130 mm gun will eventually achieve the ubiquity of its predecessor. Upping the caliber of tank cannons to increase kinetic energy is far from a novel idea. In fact, trials during the Cold War were undertaken with even larger 140 mm guns. However, a number of problems precluded the introduction of the 140 mm gun, and it is easy to see how these same issues may manifest themselves in tanks with the 130 mm as well.

This Abrams-based testbed is fitted with a massive 140mm cannon. The idea ended up being abandoned, as issues effectively integrating such a large gun into the Abrams platform arose.

This Abrams-based testbed is fitted with a massive 140mm cannon. The idea ended up being abandoned, as issues effectively integrating such a large gun into the Abrams platform arose.

The biggest issue that will face engineers trying to incorporate the 130 mm gun is its increased weight and dimensions. The 130 mm is heavier, at 3.5 tons, than the 3 ton 120 mm gun. Housing the slightly enlarged 130 mm, as well as its correspondingly enlarged equipment, will necessitate a redesigned turret, according to Rheinmetall engineers. This could be problematic because many NATO tanks are already behemoths. The American M1A2 SEP V2 Abrams weighs in at a staggering 69 tons, a weight similar to the largest heavy tank destroyers of World War II.

At this point, it is worthwhile to examine the role of the MBT on the battlefield. Unlike the highly specialized 70-ton gun carriages of World War II, main battle tanks are not slow breaching vehicles. Rather, they are all-purpose armor intended to fill a variety of roles, from engaging other tanks to exploiting maneuver opportunities and bolstering infantry operations. Thus, the “main” in “main battle tank” reflects that MBTs are intended to be versatile, which requires speed and mobility. Further increases in weight could threaten this mobility significantly.

Most obviously, there is the issue of operational mobility, or the ability of the tank to move to the battlefield under its own power. One factor affecting operational mobility is ground pressure per unit area, usually expressed in psi. This figure is obtained by dividing the tank’s weight by the surface area of its tracks’ contact patch. Obviously, a higher weight per unit area results in the tank being more prone to sinkage into soft ground as well as more rapid wearing of road surfaces. If the weight of a 130 mm-armed tank were to be increased but the track size was to remain the same, problems associated with higher ground pressure would manifest.

Alternatively, when working with added weight of the 130mm, tank engineers could work to increase the track surface area proportionally to the weight increase and thus achieve a low ground pressure. However, tanks with tracks of a larger length and width have their own issues. The larger the vehicle, the more difficult it is to fit on narrow roads, through urban streets, and over bridges. No matter the track size or ground pressure, overall weight also plays a factor: excessively heavy tanks may be unable to pass over weak bridges without risking collapse, which is problematic when there are many rivers in the area of operations.

Airmen load a M1A1 tank onto a C5M Super Galaxy airlifter.

Airmen load a M1A1 tank onto a C-5M Super Galaxy airlifter.

Also important is the effect of weight on strategic mobility, which is the ability to move the tank from where it is based to where it is needed for combat. Tanks generally do not move unassisted over long distances, because they have atrocious fuel economy and the wear incurred by such travel is expensive. And, of course, if there is a large body of water such as a sea or ocean, boats are a necessity. Strategic airlifters such as the C-5 can move tanks very rapidly, but this is by far the most expensive option and is only available to countries that possess such aircraft. Tanks are usually moved over land by a prime mover, whether it be a train or a truck, for the aforementioned reasons. Trains and trucks have payload size limits, namely the width of roads and the maximum width of a train car as permitted by rail infrastructure. Airlifters have similar limits to their maximum cargo dimensions and weight. Thus, the limits of tank size are not only established by battlefield needs but also logistical ones; for economic as well as tactical reasons, all tanks produced must fit onto some sort of strategic transport.

So, in conclusion, tank designers will have to avoid significant increases in weight and size when producing new tanks with the 130 mm cannon. Increases of more than a few tons over current-gen tanks would likely be unacceptable, and such increases could be wrought on not only by the inclusion of a new gun but also of new protective systems, drive technologies, etc. Because the 130 mm cannon will be heavier, other components will need to get lighter.

Given past trends in tank warfare as well as the trajectory of aircraft and ship weights, this seems a tall task — tanks, planes, and indeed most vehicles have tended to become larger and heavier as time passes. Whether tank designers can halt this trend while still incorporating the larger 130 mm gun remains to be seen.

The issue of increased cartridge weight is another important aspect. The cartridge, which contains the shell and its propellant, is loaded into the breech of the gun the gun before it can fire. The 130 mm cartridges are 30 kg, whereas the 120 mm cartridges are 21 kg. Because of this significant 9 kg increase in weight, Rheinmetall engineers believe that the 130 mm cartridges must be autoloaded. Autoloading is the process by which mechanical means are used to reload the gun, as opposed to manual loading, in which a human loader performs the task.

A Russian serviceman loads trays into a T-72 autoloader.

A Russian serviceman loads trays into a T-72 autoloader.

On paper, autoloading seems attractive. After all, machines tend to perform simple tasks (such as loading a gun) faster and more precisely than humans do. However, there are actually many benefits conferred by using a human loader. Counterintuitively, a well-trained human loader can potentially load rounds more quickly than current autoloaders, under ideal conditions. And, while machine loading may sound precise and reliable, autoloaders are highly complex and thus can break down or fail, which can be catastrophic in combat. Many tank commanders also like having the extra crewman for a number of reasons, including the increased manpower for odd tasks and the redundancy in case of casualties. Because of the numerous benefits of manual loading, it is the method currently preferred by the majority of NATO countries, including military heavyweights such as the US and Germany.

Autoloaders do have some advantages, the largest being their smaller size relative to a human loader (or, more precisely, the space required to house a human loader in the turret). The usage of an autoloader is one of the reasons that Soviet tanks such as the T-72 and T-90 have a lower profile than manually loaded Western tanks, thus presenting a smaller target. This decrease in turret volume afforded by autoloaders could play a role in compensating for the increased size and weight of the 130 mm gun, assuming the necessary autoloader is still smaller and lighter than a human loader and its facilities. Another advantage afforded by the autoloader is its indifference to fatigue and bumpy terrain (human loaders can be jostled around if the tank is careening through ditches or over bumps). It is also possible that engineers could produce an autoloader that is faster and more reliable than a human loader in the future.

In conclusion, there are barriers to introducing the 130 mm tank cannon. Not only will it be difficult to stop weight and size increases from impacting mobility, but it will also require some NATO members to adopt autoloaders, which they have opposed for decades. However, the 50% increase in kinetic energy is quite a dividend and could make all the engineering effort well worth it. If Russian tank armor and defenses progress much further, adoption of the 130 mm may be the only way forward for Western tank armaments. In any case, it will be around a decade before the 130 mm gun is ready for mass production and probably even longer before a tank is designed around it, so the best that can be done is to wait and see, as the progression of armor and resulting changes in NATO penetration requirements will not be revealed for some time.

8 Comments on "Why Can’t Tanks be Larger? Rheinmetall’s 130 mm Gun and the Future of MBTs"

  1. Nate Peterson | June 9, 2017 at 9:35 am | Reply

    > “currently, all Western main battle tanks (MBTs) are armed with either smoothbore (most countries) or rifled (UK) 120mm Rheinmetall cannons”

    All is a very false statement. The British L30 rifle has absolutely nothing to do with Rheinmetall. Nor does the French CN120-26/52 52 caliber smoothbore. The M256 is developed from the L/44 variant of the Rheinmetall line, but other than “on paper” ballistic performance using the same munitions, their is near nothing in common with each other. Technically speaking the M256 is a “licensed copy” according to Rheinmetall’s product line, but as Watervliet puts it “licensed variation”.

    > “thus presenting a smaller target”

    On a plain yes. In uneven terrain, a very big possible no.

    • Thanks for commenting. You are right about the guns; I’ll fix the error regarding the L30 and CN120 and clarify the explanation of the M256.

      As far as target profiles, autoloaded tanks such as the T-90 and Leclerc tend to present smaller profiles not only on flat surfaces but also slopes, from the side, etc. as they are smaller in every dimension than a tank such as the Abrams or Leopard.

      • Chris Kohler | March 23, 2019 at 5:44 am | Reply

        Not sure whether that is what Mr. Peterson meant with his comment on Russian tanks not presenting a smaller target on uneven terrain, but he most likely refers to gun depression.
        Meaning, because Russian tanks are so low and have such flat turrets, they can not point their guns down very far. Only about 3 to 6 degrees, as far as I know, compared to the 9 to 10 degrees usual on NATO MBT designs.
        That doesn’t seem like a important stat to the uninitiated, but as everyone who plays one of those tank warfare computer games like “World Of Tanks” or “War Thunder”, can tell you, it is super important. How far down you can point the gun, your gun depression angle, determines how much of your tank you have to expose to the enemy before you can point your gun at him, when coming over the ridge of a hill, or when simply driving in rough terrain. If it isn’t immediately clear what I am talking about, imagine a hill ridge and 2 tanks, one on each side of the hill, with the hill blocking sight on the other one for each. To shoot the other tank, each tank would need to drive up the bank of the hill and peek over the ridge, right? A Russian tank design, which is very flat and can only point its gun barrel down a few degrees has to drive all the way up to the apex point of the hill and even further. It actually has to come down the other side of the hill, before it can get its gun pointed at the other tank in the valley on the other side. It has to totally expose itself to return fire and has to spend a lot of time in direct line of sight and fire from the enemy tank before it got its gun aimed down on it.
        Now, a taller tank design with a lot of gun depression is way better off in this situation. It can not only get its gun on target way faster, but it even has a chance to aim at the enemy tank without even exposing itself, with most of the tank remaining covered behind the ridge of the hill and only the upper part of the turret being visible to the enemy, while still being able to shoot at him.
        That is a huge advantage and many tanks, like the American T29 or T32, or the German King Tiger, have specifically been designed tall, with a lot of gun depression and a especially tough turret front armor, in order to maximize that ability and use terrain as effectively as possible.
        So this common layman idea, championed by fans of Russian tank designs, that “flatter is better”, is just not true. It depends on the terrain. Flatter is only maybe better in very flat terrain and even then, modern tank fire control systems are so accurate that having a tank that is a foot or two shorter, doesn’t really give you any significant advantage anymore. I even heard the fact that Russian tanks are smaller makes them easier to spot for infrared optics, because they produce the same amount of heat as bigger tanks, but have less surface area to dissipate that heat and thus warm up more and stay warm longer.

  2. On a 130mm gun I cannot understand why 2 part ammo cannot be used, Chally 2 is 2 part, (well actually 3 but that is the firing charge) the heavy 12omm round is the hesh the Fin or sabot is much lighter and easier to load, like wise the charges will be of a greater dimension but not neccesariy longer. We seem able to load 2 part just as quickly as others load the 1 part round and charge. No doubt UK will go to a 130mm smooth bore eventually and do away with hesh for heat, I cannot see a problem for the loader other than needs to be a bit fitter than me.

    • Interesting take on the issue. I would speculate that it might be possible to manually load the 130mm rounds, but the increased fatigue and requirements for loader fitness may make it impractical. Of course, Rheinmetall’s opinion that it should not be manually loaded might be ignored by tank designers. Only time will tell.

    • Chris Kohler | March 23, 2019 at 5:19 am | Reply

      The biggest drawback of segmented ammunition for tank cannons is that the penetrating ability of a KE round depends on the length of the penetrating rod. The longer the rod, the more armor it can penetrate. Even the hardest kinetic penetrators mushroom at the tip and get used up on their way through the armor. The shorter it is, the sooner it runs out of material and energy. Cutting the shell in half means reducing the space you have for the rod. The ammo for that 140mm cannon that Rheinmetall developed in the 80s and 90s actually was segmented, but the main segment was still almost twice as long as current 120mm shells, in order to house a longer penetrating rod.

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  4. 1. Both Japan Type-90 and Korean K2 Panther do use smoothbore 120mm guns made on a license of 120mm L/44 by Rheinmetall. As for Abrams, why they need a license if they are making their own 95% different gun? Because it is just a 120mm L/44 gun, which was adopted to Abrams turret construction. Challenger II will get a new turret to fit 120mm smoothbore L/44 or L/55 gun inside. Some say it will be better protected while its mass will be reduced (don’t know for sure about the 2nd feature)
    2. Autoloader?
    There was some comparison test of few MBTs. Leopard 2 took first place over Abrams M1 in shooting precision, and speed + maneuverability on rough terrain. Abrams did very close to Leo2 in targeting speed and engagement. T-90 burned its propellant for so long that 125mm canon became dangerous to tank crewmembers. Leclerc jammed its autoloader for many hours with its 1st round. Aside from Russian and French tanks, Challenger II took the 3rd position due to its low speed and acceleration. Also, it didn’t perform very well in mud terrain maneuvers. Shooting precision went OK for Chally, but its rounds punched with 1/4 energy less on big range targets than those of Leo2 and Abrams did.
    3. Japan made a state-of-the-art 2nd, smaller MBT Type 10. Yet heavier, less sophisticated Mitsubishi Type 90, will stay as a Japanese core MBT force. There is also an AFV Lynx by Rheinmetall. It will likely get armor capabilities to take a frontal hit from some oldest 125mm soviet guns. There is hope for low weight tanks. Lynx will stay around 40 tones.
    4. Russian designers also made a new turret – for T-90M. Almost all ammo will be stored now as separated from the crew compartment. Blow-out pannels and 125mm gun from T-14 will be also its features.

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