A tank is a tracked and armoured combat vehicle (armoured fighting vehicle), designed primarily to destroy enemy ground forces by direct fire. A modern main battle tank (MBT), designed predominantly for combat, is the most powerful direct-fire land-based weapon. It is distinguished from other armourd fighting vehicles primarily by its heavy armour and armament. It can cross rough terrain and move relatively quickly in short bursts, but is power-, maintenance-, and ammunition-hungry and is not designed for sustained operations.

Tanks were first used in World War I and have undergone many generations of design evolution since then. Tanks are now a fairly mature technology, but significant improvements continue to be made in tank subsystems. There is an ongoing arms race between tank armour and anti-tank munitions.

Full article: Tank history

There are a number of different accounts of the development of the first tanks. According to one popular story, Winston Churchill was a keen advocate of the idea of an armoured vehicle to defeat enemy trenches during World War I and as First Lord of the Admiralty he created a Landship Committee to develop the concept.

Whatever the degree of Churchill's involvement, the first prototype tank was tested for the British Army for the first time on September 6, 1915. Tanks were first used in combat at the Battle of the Somme on September 15 1916. The results were poor, with the tanks proving to be unreliable, underpowered, underarmoured, and incapable of navigating battlefield terrain. Nevertheless, the concept was seen to be sound, and allied tank designs improved during the war. Later tanks were more reliable, had more powerful engines, were capable of withstanding German armour-piercing bullets, and due to a rhomboid shape could navigate large obstacles. Germany also built a few tanks, but did not pursue the concept very far.

Between the two world wars, with the tank concept now established, several nations designed and built tanks. Many sizes of tank were considered, and a lot of development effort went into light tanks that would be useful primarily against infantry. However, with tank-versus-tank engagements now being a major consideration, it became clear that future tanks would need to be heavily armoured and carry large guns. Tank shape, previously guided purely by considerations of obstacle clearance, now became a trade-off, with a low profile desirable for stealth and stability. Turrets, which had always been considered but were not previously a universal feature on tanks, were recognised as the way forward.

World War II saw the greatest rate of advances in tank design. Germany initially fielded lightly armoured and lightly armed tanks, such as the Panzer I which was originally intended for training use only. These fast-moving tanks and other armoured vehicles were a critical element of the German Blitzkrieg. Experimentation with tank sizes reached its logical conclusion during the war, with the rather impractical German 188-ton Maus, of which only two were ever built and never saw combat, and the 1000-ton Ratte, of which none were ever completed.

It was during this war that tanks were first equipped with radios, vastly improving their command and control. Germany led the way in tank tactics and organisation, achieving some amazing successes due to their innovations despite having tanks generally inferior in armour and firepower to allied opponents. Tanks were also adapted to a wide range of military jobs, including mine clearance and engineering tasks. Some of these tank variants live on as other classes of armoured fighting vehicle, no longer called "tanks".

The role of tanks has evolved into a specific niche indicated by the modern term "main battle tank", and wide variations in size are no longer seen. Since World War II, tank design has gone through a series of refinements, rather than any radical changes. Guns have become larger, and targeting has improved to the point where a modern tank essentially never misses. Armour has evolved from steel plate into composite armour, with refinements such as reactive armour, as anti-tank weaponry has evolved. Crew comfort is improved, and the use of video cameras has removed the need for viewing ports through the armour.

The main battle tank (MBT) is the most powerful direct-fire land based weapon. It is used mainly to combat other MBTs, but its weapons are effective against almost all targets. Although some MBTs can carry infantry, this is not its primary task. The term MBT is used to distinguish the most powerful and most modern type of tank in any army from lighter, less costly tanks which are sometimes used for airborne or amphibious operations, or from older tanks which are used in secondary roles.

The main battle tank (MBT) is the most heavily armoured vehicle in modern armies. Its armour is designed to protect the vehicle and crew against all known threats, including kinetic energy penetrators fired from other tanks, anti-tank guided missiles (ATGMs) fired from infantry or aircraft, and mines. The amount of armour needed to protect against all these threats from all angles would be far too heavy to be practical, so when designing an MBT much effort goes into finding the right balance between protection and weight.

There are several different kinds of armour. The most common is called passive armour which comprise of layers of battle steel, alloys and ceramics. One of the best types of passive armour is the British-developed Chobham armour, which is comprised of spaced ceramic blocks contained by a resin fabric matrix between layers of conventional armour. A form of Chobham armour is encased in depleted uranium on the massively produced M1A1 Abrams MBT. Another type is reactive armour, which involves the armour "exploding" out towards the incoming round and diverting it. Reactive armour is most effective against shaped charge weapons while passive armour tends to be better against kinetic energy penetrators. Reactive armour tends to be attached as to the outside of an MBT in small, replacable units rather than being permanently incorporated into its main body. Secondary passive armour may also be attached to a tank, but this is not common.

Traditionally the thickness of the armour is very unevenly distributed. The thickest sections are usually on the front glacis plate and the front of the turret. The sides have much lighter armor and armour on the top of the turret is lighter still. The back of the tank, and the sections directly above the engine, in the rear, have the lightest protection of all. The tracks are only partly protected by steel skirts.

As a result of these design decisions, a tank group is relatively vulnerable to air attack and needs constant escort by anti-aircraft vehicles, when the enemy is at least partly in control of airspace. For the same reasons immobilized tanks are also very vulnerable to enemy artillery fire from anything from medium sized mortar to a large cannon. Unescorted tanks are also vulnerable to ambush by light infantry and guerrillas with hand held anti-tank weapons.

Paradoxically, a tank is usually in its safest state when the commander is in a personally unsafe position, riding in the open, head out of the turret, with no personal protection save his helmet and a flack jacket. In this rather high position the commander can see around the vehicle with no restrictions, and has the greatest chance of spotting enemy anti-tank operations or natural and unnatural obstacles which might incapacitate or slow down the tank. Tank periscopes and other viewing devices give a sharply inferior field of vision and sense of the countryside, despite constant advances in optics and electronics. Thus, when tanks advance in hostile territory with hatches closed, the commander and others might be personally safer but the tanks as a whole are more at risk, given the extremely reduced vision.

The main weapon of any modern tank is its gun, the size of which is exceeded by only the largest howitzers. It is usually 120mm calibre for western-built tanks and 125mm for eastern-built. The gun fires kinetic energy (KE) penetrator rounds as well as high explosive (HE) ones. Some tanks have the ability to fire missiles through the main gun barrel, which gives it longer range and makes it useful against airborne targets. Usually, the vehicle has a machine gun coaxially mounted with the main gun. This machine gun is of relatively small calibre (7.62mm - 12.7mm) and used against soft targets such as infantry. However, a couple of French tanks such as the AMX-30 and AMX-40 carry a coaxial 20 mm cannon, which has a high rate of fire and can destroy lightly armored vehicles. Additionally, many tanks carry a roof-mounted machine gun for anti-aircraft fire.

Historically, tanks weapons were unstabilised and aimed by eye and were consequently rather inaccurate, particuarly if the tank was moving when the weapon was fired. Modern tanks have a variety of sophisticated systems to make them more accurate. Gyroscopes are used to stablise the main weapon; laser range finders are used to calculate the range to the target; computers calculate the appropriate elevation and aim-point, taking into account wind speed, air temperature and other important factors. Night and infrared vision equipment is also commonly included. Laser target designators may also be used to pick out targets for guided munitions.

Many, if not most, MBTs carry smoke grenade launchers, which can rapidly deploy a smoke screen to visually shield a withdrawal from an enemy ambush or attack. The smoke screen is very rarely used offensively, since attacking through it blocks the attacker's vision and will give the enemy an early indication of impending attack. Modern smoke grenades work in the infrared as well as visual spectrum of light.

Some smoke grenades are designed to make a very dense cloud capable of blocking the laser beams of enemy target designators or range finders. In many MBTs, such as the Leclerc, the smoke grenade launchers are also meant to launch tear gas grenades and anti-personnel fragmentation grenades.

There are several types of ammunition designed to defeat armour, including HESH (High Explosive Squash Head), HEAT (High Explosive Anti Tank), APDS/APFSDS (Armour Piercing Fin Stabilized Discarding Sabot, a type of kinetic energy penetrator).

HESH rounds require a rifled gun while HEAT and other rounds can use a smooth bore gun as well as a rifled one. The British army and the Indian army, convinced of the superiority of HESH rounds, are now the only ones to field main battle tanks with rifled guns.

HESH rounds contain a charge of plastic explosive that splatters on a small area of the enemy tank. The force of the impact and power of the charge exploding causes part of the interior armor to spall off and ricochete around inside, damaging equipment and killing the crew.

A main battle tank is designed to be very mobile and able to tackle most types of terrain. Its wide tracks disperse the heavy weight of the vehicle over a large area, resulting in a specific ground pressure that might be lower than that of a man's foot. The types of terrain that do pose a problem are usually extremely soft ground such as swamps, or rocky terrain scattered with large boulders. In "normal" terrain, a tank can be expected to travel at about 30-50 km/h. The road speed may be up to 70 km/h.

The logistics of getting from point A to point B are not as simple as they appear. On paper, or during any test drive of a few hours, a single tank offers better off-road performance than any wheeled fighting vehicle. On a road the fastest tank design is not much slower than the average wheeled fighting vehicle design.

In practice, the huge weight of the tank combined with the relative weakness of the track assembly ensure that the maximum road speed of a tank is really a burst speed, which can be kept up for only a short time before there is a mechanical breakdown. The maximum off-road speed is much lower, but in general it cannot be kept up continuously for a day, given the variety of off-road terrains and their unpredictable nature, with the possible exception of plains and sandy deserts.

Since an immobilized tank is an easy target for mortars, artillery, and the specialized tank hunting units of the enemy forces, speed is normally kept to a minimum, and every opportunity is seized upon to move tanks on wheeled tank transporters and on railways instead of under their own power. Tanks invariably end up on railcars in any country with a rail infrastructure, because no army has enough wheeled transporters to carry all its tanks. Planning for railcar loading and unloading is crucial staff work, and rail bridges and railyards are prime targets for enemy forces wishing to slow a tank advance.

When moving in a country or region with no rail infrastructure and few good roads, or a place with good roads but mines or frequent ambushes, the average speed of advance of a tank unit in a day is comparable to that of a man on a horse or bicycle. Frequent halts must be planned for preventive maintenance and verifications in order to avoid breakdowns when the shooting starts. This is in addition to the tactical halts needed so that the infantry or the air units can scout ahead for the presence of enemy anti-tank groups.

Another mobility issue is getting the tank to the theater of operations. Tanks, especially main battle tanks, are extremely heavy, making it very difficult to airlift them. Using sea and ground transportation is slow, making tanks problematic for rapid reaction forces.

Most tanks continue to use a diesel engine because diesel fuel is very nonflammable even when exposed to a direct heat source. In many designs, such as the Israeli Merkava, diesel fuel tanks are placed around the crew area, effectively becoming the second layer of armor. A diesel engine is also very economical and offers better combat range than other engines. Drawbacks are that a diesel engine is hard to start because its fuel is stable and it lacks an instant burst of power. The dark plume and smell of exhaust are also problems and they make it harder for a stealthy approach. Some Russian tanks use this as an advantage and intentionally inject extra fuels to create a thick dark cloud of smoke for cover.

Some of the more recent tanks, like the latest iterations of the German Leopard MBT design, have multifuel internal combustion engines, which can operate on diesel, gasoline or other fuels. Certain designs, like the M1 Abrams from the United States, are powered by gasoline powered turbines, whose high pitched sound can be heard at a good distance. Using the gasoline powered engine has reverse problems to a diesel powered engine. Gasoline is highly flammable and the gasoline tank must be placed away from the tank's occupants. It also reduces the combat range. However, the engine starts faster and offers a burst of power when needed. It is also relatively quieter than a diesel engine, and much quieter than a turbine.

Turbines have proved popular in some recent tank designs. They offer "burst" power" immediately and their efficiency is greater than that of other engines in many conditions. However, in their lowest speed of operation they still continue to consume fuel at a much higher rate than other types of engines. During the first Gulf War this was a severe handicap for the M1s since they needed to keep their turbines running, burning huge amounts of fuel, in order to power the infrared imaging devices and other electronics, while diesel powered fighting vehicles around them could power their electronics at a much lower cost in fuel by having the engine run at low idling speed.

Most tanks are powered by a diesel engine of a power comparable to a diesel locomotive. From the outside a tank smells, sounds, and feels quite like a diesel locomotive. The deep rumble of even a single tank can be heard a great distance on a quiet day, and the sharp diesel smell can be carried far downwind. When a tank stands still with engine running the land trembles around it. When moving on most grounds the vibrations are greater. The acoustic and seismic signatures of multifuel engines are comparable. The acoustic signature of a turbine engine is much greater: its high pitch whine can be much more easily distinguished from other sounds, near or far.

The very large size, typically in excess of 1000 hp (750 kW), of a tank's engine ensures that it will always leave a distinct thermal signature. The unusually compact mass of metal of the tank hull dissipates heat in a fashion which marks it off sharply from other objects in the countryside. A tank is thus relatively easy to spot by good land based or aerial infrared scanners. One of the reasons for a one sided match during the Gulf Wars was that tanks like M1 Abrams had almost 4 times the nighttime infrared scanning range of T-72s used by the Iraqi army.

• Ogorkiewicz, Richard M. Technology of tanks Coulsdon, Surrey : Jane's Information Group, 1991.

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