Ways to retain AFV mobility in harsh terrain based on traction and engine performance restrictions

 The loss of an AFV's ability to move based on the traction characteristics of the engine/chassis is characteristic when moving vehicles on bases with low load-bearing capacity (weak soils).

The ability of the vehicle to move on terrain with a low load-bearing capacity is estimated using supporting patency indicators of AFVs. 

Terrain passability is determined by the ratio of resistance forces to the AFV's movement, over a given terrain, its traction capabilities and coupling qualities under given conditions. In the presence of sufficient traction forces, movement is possible whenever the forces of adhesion exceed the forces of resistance to movement, and movement ceases as soon as the forces of resistance to movement exceed the force of traction, even by a minimal amount. 

When testing a vehicle's ability to drive on soft soils, it is expected to move without any additional support. In extreme circumstances, however, spare parts such as spurs, wideners, snow grabbers (duckbills), etc. can be used to enhance the vehicle's off-road performance.

Studies indicate that patency is affected by both the design parameters of an AFV chassis and the conditions that reflect the vehicle's movement. However, due to the requirements that must be met for the running part of the AFV and the overall layout and design of its systems and components, there is a limit to how much support patency can be increased by design parameters. The need to fulfil other requirements for the AFV also prevents significant changes to its chassis overall and layout parameters.

At the same time, the value of the average specific pressure on the ground is most widely used as an indicator of supporting patency. For modern tanks, the average specific pressure on the ground is 0.95–0.98 kg/cm^2, which makes it virtually impossible for them to move through swampy and heavily moist soils. Reducing the specific pressure on the ground to 0.5–0.6 kg/cm^2, which would allow for limited but acceptable patency on weak soils, is almost impossible.

It may be possible to improve cross-country ability in these conditions by developing methods that affect the indicators of movement and form the parameters of the AFV's interaction with the terrain.

Therefore, it should be assumed that cases of overcoming swamps or deep snow, as well as steep and slippery icy, clay, sandy and rocky ascents, reservoirs of small depth with a viscous bottom, etc., both short and, especially, of considerable length, can only be fought when advancing to the line of contact with the enemy or on the march. At the same time, the AFV will not theoretically be subjected to intense enemy influence. This makes it possible for crews to independently take appropriate preparatory measures to solve the problem of overcoming consistently emerging obstacles. For this reason, portable devices and new ways of overcoming such obstacles should be developed.

To minimize the time for preparation and overcoming obstacles, as well as dismantling and laying devices back on the vehicle, it is important to consider a new class of transportable means to increase the patency of the AFV. These means should be pre-installed or installed independently by the crew, without the need for engineering units.

Two types of devices can affect AFV movement: those that provide additional external traction, reducing the resistance to movement of the vehicle, and those that minimize track print formation by preventing slipping of the tracks, which also results in reduced resistance to movement. The latter type has a positive impact on the weight and overall characteristics of the devices themselves.

The device of the first type is a device for increasing cross–country ability. It creates an additional external traction force applied to the drive sprocket parallel to the track traction force. The device allows a single vehicle to overcome various obstacles independently.

The device is designed to improve a vehicle's cross-country ability. It consists of two drums with a special profile, one mounted on each drive sprocket (refer to Image 1). In addition, there are two steel cables, each measuring 80-100 meters in length, with an elastic synthetic cable at the front end that is 5-7 meters long and can handle 1.5-2 times more workload than the main cable. Each cable has a synthetic core at the end.  The kit also includes rigging brackets and centring and guiding brackets that can be installed on the vehicle in advance. 

The device is portable and lightweight, and some of its components can be mounted on the vehicle beforehand. It can overcome obstacles of any length as long as the cable is long enough. However, the device's ability to overcome steep climbs is limited by the vehicle's chassis traction capabilities. For testing purposes, a BMP-2 IFV was chosen.

When used on horizontal terrain, the length of the obstacle can be significantly increased by reinstalling the cables to new support points.

The device works as follows: 2-3 turns of a cable are wound on the drum, the end of which is fixed to an object behind the obstacle, and the second is attached to a support vehicle, with which the cable is tightened to prevent slipping on the drum. (Case seen in Image 2)

The device provides the AFV with additional traction force in scenarios when the tracks cannot gain enough traction. The absence of slipping and traction of the ground by the track reduces the depth of the track and decreases the resistance to movement. This reduces the traction force required.

Image 1
1 - track; 2 - drive sprocket crown; 3 - steel cable; 4 - drum

In contrast to vehicle tracks, swamp traction is determined by constantly changing terrain interaction conditions and significant dynamic shifts. The stable operation of the drums to create traction ensures the absence of a significant dynamic load on the cable. This is to compensate for the remaining force peaks.

Image 2 - BMP-2 equipped with the prototype device
1 - Drum; 2 - leading cable branch; 3 - Guide for directing steel cable

For the experimental device, a steel cable (GOST 2688-80) with a diameter of 12mm and a breaking force of 92kN was utilized. The drum diameter was selected to be 615mm, which is equal to the diameter of the front drive sprocket. Also, the drum had a thickness of 48mm, and the cone-shaped working surface had an inclination angle of -12°.

During the experiments, tests were conducted on swamps and sandy inclines. When attempting to climb a sandy incline with an initial steepness of 25° and a length of 17.5m (as shown in Image 3), the tracks burrowed into the terrain, causing the angle of ascent to increase to 30°. The cables used to climb the incline experienced forces in the range of 35-36 kN. The depth of the track was between 35-40 mm, which is 5 to 6 times less than the track depth when traversing such obstacles in the usual way. Two pine trees, with a stem diameter of approximately 150 mm, growing on sandy soil were used to secure the cables. One of the trees could withstand a force of more than 50 kN when the cable was secured at a height ranging from 50-100 mm above the ground. To overcome a crust-type swamp about 40m wide, cables were fixed to trees on the outer edges of the swamp. The forces on the incoming branches were 20 kN, and the track depth was 300 mm. It's worth noting that the standard track depth in swamp terrain is around 400-450 mm.

Image 3
BMP-2 climbing a 25° sandy incline

Tests have shown that the device can be used in various conditions without prior terrain preparation. When overcoming several consecutive obstacles or operating the vehicle in areas where frequent use is possible, the device can be installed in advance. All preparation will consist of unwinding and securing the cable. This method reduces the amount of preparatory work and the time taken to overcome the obstacle. If necessary, the device's functionality can be expanded, for example, to ensure the movement of the machine without a track or to increase the stability of the track on the drive sprocket.

The advantage of the device is that it optimizes the mode of interaction of the tracks with deformable terrain: slipping is eliminated and the pressure plot is levelled, as a result of which the depth of the track decreases, which, in turn, reduces the resistance load on the device. 

The proposed method has an advantage over existing ones in terms of the length of cable used to overcome obstacles. It is practically unlimited when compared to using a log or a winch. However, the size of the obstacle that can be overcome is limited by the size of the winding drum. Devices of this design do not usually provide synchronization of a joint operation with the moving vehicle. As a result, the variable diameter of the winding affects the tank's slip, especially during the initial stage of movement. This leads to an increase in track width, i.e., resistance to movement. Therefore, a thick cable is required, which limits the length of the obstacle that can be overcome within the finite dimensions of the drum.

The device is suitable for overcoming obstacles of various types and has been tested with high efficiency in swamps and on sandy climbs.

To gain more traction when driving in soft terrain, another design can be used, as shown in Image 4. This design utilizes disks with crowns that are installed on the road wheels, aligned with their blades protruding beyond the width of the track. As the tank moves, the blades cut into the ground and create additional traction force without damaging the track. 

Image 4
BMP-2 equipped with protruding blades (ground crawling blades) on 1st,3rd and 6th road wheels

The disks have an asterisk shape with ground crawling blades that engage with the tracks, protruding beyond the dimensions of the track to prevent the road wheels from slipping. The introduction of these ground crawling blades significantly improves the traversability on soft terrain with low load-bearing capacity. When moving on solid ground, the devices can be removed from the road wheels and stored in a spare parts kit. Tests were carried out in a swamp with a depth of 1.5–2.5 m, and Image 6 shows the penetration characteristic, which determines its bearing capacity. A total of 14 trials were held using a BMP-2. The ground crawling blades were installed for 8 out of the 14 trials on the 1st, 3rd, and 6th road wheels. The devices allowed the vehicle to overcome soft terrain obstacles like swamps, as shown in Image 7. A vehicle not equipped with such devices failed 5 out of 6 swamp trials and could not reach its destination.

Image 5

Accordingly, these devices have shown relative effectiveness in overcoming swamps. After appropriate refinement, the device could be recommended to increase cross-country capabilities in swamps. 

Image 6
Chart representing ground pressure based on the depth of a swamp.

Image 7
Formation of additional traction force with the help of blade ground hooks

A feature of the undercarriages of modern AFVs, which determines their behaviour in swampy terrain, is the large dynamic movements of the road wheels. With a relatively small stiffness on the elastic elements of the suspensions, along with a significant (up to 10% of the weight of the machine) value of the pre-tension of the tracks with the rubber-metal hinges, due to the need to ensure its sufficient stability in the outline, there is a significant change in the plot of loads on the ground under the support rollers when moving along such a section. Moreover, the heavier the traffic conditions, the greater this unevenness.

For instance, a T-72 tank moving through a 0.6-0.7 m thick snow cover at a temperature of +20C experiences a resistance force of about 8 tons. The working tension of each trailing track section is 4 tons, including the pre-tension of 6 tons. When the suspension stiffness in the area of the static stroke of the road wheel is about 300 kg/cm, the value of the last, sixth road wheel on the rear trailing edge of the track will be about 75-80 mm. This substantially removes the load from the ground and leads to a redistribution of loads on the ground under the remaining road wheels.

Image 8

Studies have shown that when AFVs attempt to cross challenging terrain without prior preparation, they often get stuck due to the specific features of the AFV chassis and the terrain. The process of getting stuck goes like this: as the vehicle moves through the swamp, the height of the track increases from the first road wheel to the last, causing the vehicle to tilt significantly. As the road wheels move sequentially over the ground, the track under the last road wheel becomes almost five times deeper than the track under the first road wheel.

When the aft tip of the hull touches the surface of the swamp or another weak bearing base, the weight of the vehicle is redistributed from the chassis to the bottom. This increases the resistance to movement and makes it more difficult to gain traction, increasing the slipping of the tracks, which further escalates the ground and causes even more redistribution of loads under the road wheels. This cycle continues until the vehicle is completely bogged down and unable to move. The depth of the track increases until the vehicle is no longer able to move.

In summary, the redistribution of the weight of the vehicle from the chassis to the bottom leads to an increase in resistance to movement, requiring more traction force for movement. This causes an increase in the slipping of the track, the escalation of the ground by it, the redistribution of loads under the support rollers to an even greater extent, an increase in the track and the area of interaction of the bottom with the ground (swamp) until complete jamming and loss of the ability to move.

When the resistance to movement increases, the traction force decreases due to the decrease in the coupling weight and the destruction of the swamp crust. The tracks sink into less durable and more moist soil layers, which further decreases the traction force. The resistance to movement increases with the raking of the soil at the bottom. This causes the friction forces of the hull against the ground to exceed the forces of internal adhesion in the ground, creating sliding zones under the upper layer. As a result, a compacted core is formed under the bottom, which makes it difficult to evacuate the tank from its bogged-down position. In swamps, the AFV gets stuck mainly after the top layer of the swamp is torn off by the stern and stuffed under the hull. A stop occurs after the formation of a 0.3–0.8m high shaft in front of the nose. Therefore, the primary reason for getting stuck in a deep swamp and snow is the interaction of the hull with a non-existent base.

Image 9
Process of an AFV bogging down in a swamp

In addition, the size of the hull trim on the stern and the presence of corrugations, stamps, protrusions, and depressions of the hull at the stern are essential factors of interaction. Based on the above, to increase cross-country ability, it is necessary to reduce the resistance to movement by eliminating the interaction of the hull with the supporting base or by reducing the trim of the vehicle's hull.

To overcome difficult terrain, one method is to rest the back of the hull on a temporary support placed on the surface of the area to be crossed. The support can be a single beam placed in the middle of the intended path of movement, a tree trunk, or a log. When the tracks sink deeply into the ground, the back of the hull can rest on the log and slide along it. This helps prevent further sinking of the hull, tracks and soil, which can be a problem in swamp areas. It's important to note that although the weight distribution changes, the upper layer of the swamp can compensate for some of the weight reduction. This method reduces the complexity of preparatory work compared to other methods such as constructing temporary coverings like track, transverse decking, fascines or log packages. These methods require more materials and equipment and are more difficult to install.

Image 10
Method application to reduce the force of resistance by eliminating points of interaction between the hull of the tank and the terrain
1 - bracket: 2 - log 

It is rather unclear, how the above-mentioned devices, the cable drum and ground crawler blades would work on much heavier vehicles like T-72/T-80 MBTs. Considering the large weight difference and overall track formation which occurs in softer terrain it would be difficult to find a suitable tree which can resist the force double or triple that of a BMP-2 along with having to create larger size ground crawler blades which can somewhat assist a 46-ton vehicle in swamp terrain. 

Source 

• А.Л. Каменский, С.В. Рождественский, А.С. Смирнов, А.В. Доброхотов СПОСОБЫ ОБЕСПЕЧЕНИЯ ПЕРЕДВИЖЕНИЯ ВГМ В УСЛОВИЯХ ОГРАНИЧЕНИЯ ПРОХОДИМОСТИ ПО ТЯГОВО-СЦЕПНЫМ ХАРАКТЕРИСТИКАМ ДВИГАТЕЛЯ (ОАО «ВНИИТрансмаш», ОАО «УКБТМ»)