Design Compromises: A Case Study

Every design is a compromise. There are no free lunches. And trying to work out the why can be very informative. So let’s take a look at one of my favorite tanks, the M1 Abrams, and look at some design compromises, and their results. Since it’s very nearly equivalent, and designed at about the same time, I will use the Leopard 2 as a point of comparison. The Leopard 2 is somewhat more conventional internally in a few subtle ways.

The most obvious difference is the engines. Both designs have 1,500 hp engines, but where the Leopard 2 uses a pretty conventional twin-turbo V12 diesel, the Abrams uses a gas turbine. This gives the Abrams better acceleration, but also necessitates a greater internal fuel capacity. Where the Leopard 2 can get away with 1,200 L of fuel stowage, the Abrams needs about 1,900 L to meet its (shorter) range requirements. More fuel means more space. We can note that the Abrams has fuel tanks on either side of the driver, in addition to in various other places. The Leopard 2 does not have fuel stored up front in the hull.

The hull front on the Leopard 2 is used to store ammo in a pretty conventional rack. There’s not much in the way of blast venting provision here, so a penetration would be extremely bad news. That said, this is a pretty common place to store reserve ammo1, and hull hits are much less likely than turret hits. Still, from a survivability perspective, this is clearly not ideal.

The Abrams designers were able to shoehorn a few (six 120mm rounds, more of the smaller 105mm rounds) into a compartment aft by the engine, because of the shape of the gas turbine power pack. This rear ammo compartment has blow-out panels and a heavy door to isolate it from the crew compartment, but it’s not a lot of reserve ammo. The Abrams carries the vast majority of its ammo in the turret bustle. On the one hand, this makes subdivision easy. It’s a simple engineering exercise to add blow-out panels to the bustle, and this makes the Abrams among the most survivable tanks in the world.

Storing 34 120mm rounds in the bustle has its disadvantages. It forces a wide turret. Turret height is determined by the desired maximum gun depression, and a wide, tall turret means the armored volume is correspondingly large. The Abrams has considerably more armored volume than the Leopard, both in relative terms (i.e. crew space), and in absolute terms. Because so much of the Abrams’ ammo load is in the turret, there’s a significant amount of armor protecting the side of the turret bustle. More volume means it takes more weight to provide the same level of protection. Or, you have to use more expensive exotic materials (like depleted uranium).

On the other hand, more internal volume is another survivability gain. Armor penetrations are less likely to cause significant casualties or destroy enough systems to score a mission kill simply because there’s more volume to deal with, and volume leads to dispersion, which is the enemy of the shaped charge jet.

To be honest, on these grounds I prefer the survivability over protection. Protection can be added, but it’s much harder to do a redesign in favor of survivability.

We can see another difference in the guns on the latest models. Since the M1A1, the Abrams has been equipped with a license built Rheinmetall 120mm/L44 gun, just like Leopard 2s up to the A5 model. Subsequently, the Germans went to a longer L55 gun for more penetrating power. The Americans have not. So what gives?

Recall that Americans like their depleted uranium. The Germans don’t. Something something environment or something. Anyway, depleted uranium makes awesome armor. It also makes awesome armor piercing rounds. The Americans have done a good job of sinking plenty of R&D funding into new depleted uranium APFSDS rounds. They’re up to a fifth iteration of the design with the M829A4 round. So when adapting a longer barreled gun proved more costly than anticipated in the 90s due to stabilization issues, the US Army quietly dropped the project and stuck with their fancy rounds.

I don’t know if the Leopard 2 didn’t have the same stabilization issues as the Abrams with the longer gun, or if the Germans were just unwilling to change round composition. Regardless, the Germans adapted a longer gun. It means they can use tungsten-based APFSDS rounds, but it also means they will have somewhat more restricted mobility in urban environments.

For this one, six of one, half a dozen of the other. I’m indifferent here, provided both are available. I do wonder if the DU rounds will also perform better in the L55 gun, or if they’re optimized for the L44.

I suppose I should also comment on the engines. I strongly suspect that the Germans made the right choice here with the conventional V12 diesel, though I would strongly prefer an air-cooled model like the AVDS-17902. It’s possible the gas turbine just hasn’t gotten enough development funds, but a diesel engine company can push research into the civilian sector to recoup costs there, in addition to the military. I also approve of forward fuel tanks, and don’t approve of forward ammo stowage. Remember, well-designed fuel tanks provide reasonable supplemental protection.

1.) It’s also used on the Leclerc, K2, and Challenger 2, among others. Doesn’t mean I like it.
2.) Early versions powered the M60 Patton, and the 1,200 hp variant powers the Namer. A 1,500 hp variant is available.

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