Overview of Tank Mobility
Tank mobility can be broken down into three types, tactical mobility, operational mobility and strategic mobility. Tactical mobility is a tanks ability to move about a battlefield. Operational mobility is a tanks ability to move about the Area of Operation. Strategic mobility is the tanks ability to be moved to the AO. Mobility traditionally forms one of the three corners of the tank capability triangle, with the other two corners being armour and firepower. The mobility of tanks is one of the features that has made them so important on the modern battlefield.
Tactical mobility can be broken down into two major categories, agility and obstacle clearance. Agility encompasses a tanks acceleration, deceleration, turning and top speed. Obstacle clearance includes a tanks ability to cross trenches, walls and move over rough terrain. This mobility is provided by a tanks engine and its tracks.
Tanks are universally powered by internal combustion engines. Almost all tanks from the first combat tank, the Mk. I, through the modern Leopard 2, use diesel piston engines, with most using inline engines. Some tanks have employed radial engines due to their improved power-to-weight ratios and greater reliability. Radials were included on many WWII era American tanks including the M3 Stuart and some models of the M4 Sherman and M3 Lee.
Some modern tanks employ high power turboshaft engines. The M1 Abrams mounts the Honeywell AGT 1500, a multi-fuel turbine engine that produces 1,500 hp. The only other tank to be powered solely by a turbine engine are certain models of the Russian T-80, though some models have been modified to mount a diesel engine to improve fuel consumption rates. Some tanks including the Swedish Stridsvagen have both a piston and a turbine engine, alternating between the two based on the situation at hand.
Tanks use continuous or “caterpillar” tracks for propulsion. Continuous tracks allow for improved performance on rough terrain by virtue of their lower ground pressure and higher overall traction. Tracks are made up of a chain of interlocking metal pieces which orbit around the track assembly. Track assemblies are made up of a series of wheels which keep the track running smoothly.
- Drive Wheel (rear wheel drive)
Connects to engine and moves the track.
Moves across the wheels in a continuous motion.
Keep the track aligned with the drive wheel. Some arrangements forego these entirely, instead allowing the track to droop onto the road wheels. This is known as a slack track arrangement.
- Drive Wheel (front wheel drive)
- Road Wheels
These wheels transfer the load to the ground. Road wheels are generally attached to some form of suspension which allows a smoother ride.
This small wheel is used to tension the track to marginalize the chance of the track becoming detatched from the assembly.
Many military tracks use metal links with a rubber shoe, this allows for good traction while preventing damage to both the ground that the tank passes over, and the metal links themselves. The shoes are easily replaceable. Pictured below are two new pads and two old worn pads on an M1A2 tank.
Operational mobility encompasses a tanks ability to cross larger obstacles such as rivers and minefields during movement around the AO. Engineers have invented a variety of ways of dealing with each of these obstacles.
The two main ways that tanks cross rivers without pre-exsisting crossings is by fording or with bridge layers.
Fording a river means simply wading across it. The main consideration when fording, is ensuring that the tank does not go so deep as to cut off oxygen to the crew and engines. This problem of keeping engines supplied with oxygen is familiar to submariners and a familiar solution has been employed. While many tanks can only ford rivers that are less than about 1 metre deep in their stock configuration, some tanks, including the Leopard 2, can be fitted with a vehicle snorkel to allow for the crossing of rivers deeper than the vehicle is tall. This technology was pioneered during WWII by both the Axis and the Allies for different reasons. The technology was developed by the allies to optimize tanks for beachhead invasions by allowing them to be offloaded from landing vehicles far from shore, then swim up to the beach. Pictured below are two Sherman tanks fitted with snorkels over their intake and exhaust.
The Nazis developed a similar technology out of necessity. Early in its development, the Tiger tank was found to be simply too heavy to cross most bridges in Europe and so it was almost always forced to ford when a river was encountered. The problem was that, despite the Tigers height, many rivers were simply too deep to allow for effective fording. The solution was to attach a tall tube to the turret and and seal all openings with rubber gaskets. This allowed oxygen into the compartment and prevented water from seeping in.
Many modern snorkels are wide enough to double as escape hatches in the case of a mechanical failure while the tank is submerged.
The need for armoured bridge layers became apparent during WWII when blitzkrieg forces needed to move rapidly across terrain including rivers without leaving behind other mechanized units and without concentrating forces on a few available bridges. Modern bridge layers build on this doctrine by allowing efficient operational force deployment in areas with little to no infrastructure. One such vehicle, the American M104 Wolverine, can deploy a 26 metre bridge that can support up to 70 tons in as little as 5 minutes.
Minefields, in general, pose less of a threat to modern military vehicles than they once did, however, minefields can still immobilize even the most heavily armoured vehicles and they can still pose a lethal threat to personnel and light equipment. There are two main ways of clearing minefields employed in the modern military, mechanical and explosive.
There are two primary mechanical mine clearing methods, the flail or roller, and the plow. The roller, invented during WWI acts by placing pressure across a large area in an attempt to detonate any mine it passes over. These rollers are largely invulnerable to the explosions and are placed far enough from the vehicle that carries them to keep the vehicle from being damaged. The main disadvantage of a roller is that, on uneven terrain, it may only put pressure on part of the ground, leaving some mines untouched. The solution to this problem was the flail, developed during WWII and still in use today. The mine flail consists of a large rotating drum to which lengths of chain are attached. When the drum is spun the chains whip around impacting the ground evenly and causing mines to detonate.
This method can be very effective, but in order to effectivley clear the area, the flail must move slowly. The mine plow is simply a heavy plow that overturns the earth and pushes mines to the side. This method can clear a path but may leave many mines undetonated.
In a combat situation, when mines must be cleared quickly while under fire, the Mine Clearing Line Charge (MCLC) or “Mick-Lick” is often deployed. The MCLC consists of a series of explosives attached to a chord which is, in turn, attached to a rocket. When fired, the rocket travels across the minefield laying the explosives along a path. When the explosives are detonated, any mines along the path are also detonated. This method is fast and effective and has been used since WWII.
Strategic mobility essentially boils down to transporting the tank. On a strategic scale, tanks cannot generally transport themselves. The tread system and low fuel efficiency of their engines mean that long distance travel under their own power is difficult and expensive. Instead, on long journeys, there are a variety of vehicle that allow tanks to be transported by air, land or sea.
While not the most efficient way to travel, tanks can be carried by air. As an example, the American C-5 Galaxy can carry two M1 Tanks while the CH-54 can carry a tank under slung.
The main advantage to this type of transport is its speed and versatility. Helicopters especially can deliver tanks to places where they would not normally be able to go. The biggest disadvantage is the cost. Moving 100 tanks by air is an extremely fuel consumptive operation, and moving hundreds across the Atlantic would be a logistical nightmare.
Most large scale, long range tank transport is conducted by sea and most sea transport is conducted by Large, Medium Speed Roll-On/Roll-Off ships. These ships allow tanks to be easily loaded and transported on long ocean journeys.
Another key mode of strategic mobility, in use by countries such as Japan, the US and Russia, is the Landing Craft Air Cushioned (LCAC) known colloquially as the hovercraft. These incredible machines allow insertion of tanks, infantry and other fighting vehicles onto about 70% of the worlds beaches, including those that cannot accommodate traditional amphibious landing craft, such as swamps, marshes and soft sand beaches. Their high speed and ability to transport fighting equipment across beaches and not just to them makes these craft essential peaces of equipment for any amphibious invasion force.
While tanks are able to propel themselves over land, most forces prefer to use special tank movers when covering any long distance. An excellent example of this type of vehicle is the Oshkosh M1070 Heavy Equipment Transporter. In production since 1992, this transport can carry up to 80,000 kg and is the main transport used to carry the ground vehicles of the US armed forces. There are 3 and 4 axle versions.
Trucks work well for small numbers of vehicles or in areas with little to no infrastructure. However, in many places, including inside the US, tanks are transported via train. Pictured below is a a train laden with M1A2 Abrams Tanks and Bradley Fighting Vehicles travelling through downtown Los Angeles last summer. Also pictured is me with my favorite hat and my friends dog that got tired.