2009/08/31

AFV mobility

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The study of mobile warfare history and lessons learned offers hints that we should better improve the mobility of Western heavy (armour, mechanized) brigades.


The mobility of a tank is mostly defined by specific fuel consumption, fuel capacity, power/mass ratio (leading to acceleration and speed), (mean maximum) ground pressure and durability (tracks, engine, suspension). Other factors (gearbox, suspension and track design, for example) are relevant as well, but today usually not really restrictive any more.

Driving range (fuel capacity by specific fuel consumption) looks even more important than mere specific fuel consumption to me. The driving range defines the frequency of refueling. It has a huge impact on the formation's ability to maneuver freely. The tether of fuel supply can be long or short, and a long one is a huge advantage in mobile warfare ("long" not in the meaning of "overstretched").

Formations are usually awfully slow in relation to individual vehicles' speed potential. A low top speed (often used to describe a tank's mobility) is not very important for march mobility.

Acceleration is important on the level of individual tank tactics, but only so in terrain that provides opportunities for cover or concealment.

Ground pressure defines what kind of open, flat terrain is negotiable or not. Soft and wet soils can be impossible terrain for a tank if it has a high ground pressure; a common problem with main battle tanks. The "lighter" ones (like the Russian models) have usually lower ground pressures.

Durability was a major weakness of old Soviet tank models, and it's the primary reason why armies don't move their tanks much. A truck can easily run 10,000 km in a few weeks without major problems. A tank would need many expensive repairs (spare parts). A breakthrough in tank component durability could cause a revolution in land warfare because durability is the greatest constraint for the employment of tanks.
Let's take the T-72 with its relatively poor durability: track 3,200 km, engine 1,000 h, gearbox 7,000 km. This explains why their crews are usually ill-trained; much more training would not be affordable.

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A WW2 lessons learned result of the German army was a requirement for 500 km off-road range. That collided quickly with reality, as the Bundeswehr got thirsty, short-legged M-47 and M-48 (ridiculous 110 - 200 km road range; less than Pzkw IV in WW2) as first equipment and needed to compromise in the Leopard 1 development as well.

The post-Cold War Leopard 2A5 has a quite good range, around 500 km - on road (the A4 version had 550 km with a fuel consumption of only about 218 l/100 km on road, but A5, A6 and A6M are heavier and therefore more thirsty).

This nominal fuel consumption isn't really relevant, though. Let me use the often-cited results of a Swedish tank competition that led to their procurement of Leopard 2 versions: The Leopard 2A4 consumed 26,874 l diesel on 3,730 km (mixed off-road and road driving).

The results looked like this:
Leopard 2A4: 720 l / 100 km
M1A1 Abrams: 1,478 l / 100 km
Leclerc: 1,380 l / 100 km
(The French claimed that their tank was still ill-prepared for the Swedish climate; their bad.)

That resulted in practical ranges (different fuel capacities) of:
Leopard 2A4: 167 km
M1A1 Abrams: 127 km
Leclerc: 120 km.

Leopard 2 and Abrams have since become even heavier, and their effective ranges were therefore significantly reduced.

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Steps towards more durability and less specific fuel consumption seem to be incremental only. We can expect improvements if our procurement system presses for improvements, but we shouldn't expect huge improvements. Tank power pack evolution leads rather to higher power densities (more compact integrated engines) to save volume and therefore armour weight.

This drive for less volume is officially directed at saving system weight, a direction that I don't understand entirely.
The Cold War is over, and the German armoured forces have rediscovered mobile warfare and operational art; the emphasis on mobility is much greater than during the Cold War when the NATO was preparing for huge attrition battles with relatively little emphasis on mobility.

Saving weight is fine, but the change in power pack shape suggests a shorter tank hull (example Leclerc), which does neither save surface area on the most heavily armed frontal surfaces nor is it a good idea to reduce the length of a tank much (optimal track contact length:gauge ratio for good driving characteristics is 1.5-1.7).

The "EURO Power-Pack" (1995) is a well-known upgrade engine for legacy main battle tanks. The Bundeswehr didn't upgrade its Leopard 2 with this power pack. The reason was apparently a combination of costs and having no good idea about what to do with the extra volume in an old hull.

It could replace a Leopard 2's engine with a length saving of about one metre, and a volume saving of a bit more than two cubic m. That would suffice for an extra fuel capacity of about 2,000 liters.
2,000 litres diesel fuel (3,200 l instead of 1,200 l) would increase the driving range of the Leopard 2 by more than 150%.
Let's say I have an idea about what to do with the volume.

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A battalion is a formation, though. Many other vehicles would need to refuel much earlier than such upgraded Leopard 2's would need to. That's not a major problem with soft vehicles which could quite easily get additional fuel capacity. Other armoured vehicles would be a greater problem.
The new Puma IFV is said to have a road range of about 600 km, little more than a Leopard 2.
The Fuchs has about 800 km road range, while the upgraded M113's have less than 500 km.

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The tether of fuel supply doesn't seem to be officially recognized as a pressing operational problem. Neither upgrades nor new requirements lead to vastly improved ranges of our armoured vehicles. That's a bit disappointing, for it imposes avoidable logistical restrictions on mobile operations.

Sven Ortmann

Source for most data: "Kampfpanzer", Rolf Hilmes, 2007
(German standard book on tank technology, comparable to Ogorkiewicz' books)
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4 comments:

  1. I'm just shocked that a Leo uses that much less fuel than an Abrams, operationally. Even if the Leo users don't upgrade to the new engine, the U.S. should with the Abrams.

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  2. The U.S. upgraded many Abrams with an APU. The Abrams' gas turbine was worst in idle mode when it did nothing but burning fuel for electricity. I should have mentioned that.

    The quoted figures are the best publicly available figures, but they#re also already outdated.

    The difference between official road range and practical range was the point, not the exact fuel consumption.

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  3. Looking at your criteria for AFV mobility, I am struck by the one that is not mentioned, that is repairability/recoverability. To use a somewhat imprecise anecdote, while the WWII panther was a superior tank to the M18 Hellcat in as far as armor protection, it was considerably inferior in repairability as some components on the Panther ( notably the weak final track drives) would take days to remove the associated hardware from the powerpack to just get to the component. Where the the M18 this time would be much less.

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  4. Yep, maintenance time and tool requirements are important.

    I was satisfied with mentioning durability.

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