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A tank is a tracked armoured combat vehicle designed to engage enemies head-on, using direct fire from a large-calibre gun and supporting fire from machine guns. Heavy armour as well as a high degree of mobility give it survivability, while the tracks allow it to cross even rough terrain at high speeds.

Tanks were invented and first manufactured for World War I in Lincoln, Lincolnshire, by the British Army, to break the deadlock of trench warfare; enabling troops to cross the "swept zone" (area under hostile fire) more rapidly than infantry. The breakthrough was then ideally to be followed up by deep penetration into enemy rear areas, supported by high mobility. Tanks seldom operate alone, being organised into armoured units, usually in combined arms forces. Without such support, tanks are vulnerable to special anti-tank artillery, other tanks, anti-tank mines, and (at short ranges) infantry, as well as specialised anti-tank aircraft.

While tanks are expensive to operate and support, they remain among the most formidable and versatile weapons of the modern battlefield, both for their ability to engage other ground targets (including fortifications) and their shock value against infantry. Tanks and armour tactics have undergone many generations of evolution over nearly a century. Although weapons systems and armour continue to be developed, often at very high cost, many nations have reconsidered the need for such heavy weaponry in a period characterised by unconventional warfare.

Etymology
There are at least three possible explanations of the origin of the name "tank". One is it first arose in British factories making the hulls of the first battle tanks: workmen and possible spies were to be given the impression they were constructing tracked water containers or tanks for the British Army, hence keeping the production of a fighting vehicle secret. Another is the term was first used in a secret report on the new motorized weapon presented to Winston Churchill, then First Lord of the Admiralty, by British Army Lt.-Col. Ernest Swinton. From this report, three possible terms emerged: "cistern", "motor-war car", and "tank". Apparently "tank" was chosen due to its linguistic simplicity. But perhaps the most compelling story comes from Winston Churchill's authoritative biography. To disguise the device, drawings were marked "water carriers for Russia." When it was pointed out that this might be shortened to "WCs for Russia," the drawings were changed to "water tanks for Russia." Eventually the weapon was just called a tank.

History
History of the tank

World War I: the first tanks
Tanks of WWI

Video clip of WWI tanks helping the Allies with an advance in Langres, France (1918).

British World War I Mark IV tank with experimental "Tadpole Tail"A tracked fighting vehicle was proposed in 1912 by Australian engineer L. E. de Mole. It was one of the earliest practical designs offered to the British War Office. He was notified in June 1913 that his idea had been rejected, though only some of his drawings were returned. He resisted urging from friends to sell the design to the German consul in Perth.

The stalemate on the Western Front later prompted the British Army to begin research into a self-propelled vehicle which could cross trenches, crush barbed wire, and be impervious to machine-gun fire.

Originally, tanks were part of the British Navy, which would train and provide all tank personnel, and were considered "landships". The First Lord of the Admiralty, Winston Churchill, sponsored the Landships Committee, which created the first successful prototype tank, 'Little Willie', in September 1915. The vehicles were colloquially referred to as water carriers, later shortened to tanks, to preserve secrecy; the name became official in December 1915.

The first tank to engage in battle was D1, a British Mark I, during the Battle of Flers-Courcellette (part of the Battle of the Somme), on 15 September 1916, one of two to breach German lines and reach Flers, but was knocked out by friendly fire. One of these two also mistakenly machinegunned the 9th Norfolks, who were preparing to attack. The French developed the Schneider CA1 working from Holt caterpillar tractors, and first used it on 16 April 1917. The first successful use of massed tanks in combat occurred at Cambrai on 20 November 1917. Tanks were also used to great effect in the Battle of Amiens, when Allied forces were able to break through entrenched German position due to armoured support.

Germany fielded very few tanks during World War I, with the A7V being the only type produced in Germany before the end of the war, and of which only 15 were built. The first tank versus tank action took place on 24 April 1918 at Villers-Bretonneux, France, when three British Mark IVs met three German A7Vs. German forces initially lacked countermeasures, though they did (accidentally) discover solid anti-tank shot, and introduced wider trenches to limit the British tanks' mobility. However, changing battlefield conditions and continued unreliability forced Allied tanks to evolve throughout the war, producing models such as the very long Mark V*, which could navigate large obstacles, especially wide trenches, more easily than their predecessors.

Initial results with tanks were mixed; significant reliability problems caused considerable attrition in combat, with up to one third breaking down due to mechanical problems unrelated to enemy fire, and their speeds were very slow, with the 13.4 km/h attained by the Whippet considered fast. Deployment in 'penny packets' also lessened their nonetheless formidable tactical value and impact. The spear-thrust type blitzkrieg-tactics were only to be developed fully in WWII, and while the tank would eventually make trench warfare obsolete, World War I came to an end before this fully came to pass.

During World War I two major types of tanks had evolved: the 'male', with large calibre guns, and the 'female', which only had machine guns. The 'female' was mainly designed as an anti-infantry platform to defend the 'male'. After World War I ended, the 'female' was largely replaced by machine gun carriers (such as the Bren carrier), and later by armoured personnel carriers.

Interwar years: advances in design and tactics

Polish Vickers E.With the tank concept now established, several nations designed and built tanks between the two world wars. The British designs were the most advanced, due largely to their interest in an armoured force during the 1930s. France and Germany did not engage in much development during the early interwar years due to the state of their economy, and the Versailles Treaty respectively (all German tanks had been destroyed as a condition of surrender ). The U.S. did little development during this period because the Cavalry branch was senior to the Armored branch and managed to absorb most of the (limited) funding earmarked for tank development. Even George S. Patton, with tank experience during WWI, transferred from Armor to Cavalry during this period (because the US Army decided not to fund a tank corps).

Throughout this period several classes of tanks were common, most of this development taking place in the United Kingdom. Light tanks, typically weighing ten tons or less, were used primarily for scouting and generally mounted a small-calibre gun useful only against other light tanks. The medium tanks (cruiser tanks, in the United Kingdom) were somewhat heavier and focused on long-range high-speed movement. Finally, infantry tanks were heavily armoured and generally very slow. The overall idea was to use infantry tanks in close concert with infantry to effect a breakthrough, their heavy armour allowing them to survive enemy anti-tank weapons. Once this combined force broke the enemy lines, groups of cruiser tanks would be sent through the gap, operating far behind the lines to attack supply lines and command units. This one-two punch was the basic combat philosophy of the British tank formations, and was adopted by the Germans as a major component of blitzkrieg. J.F.C. Fuller's doctrine of WWI was the fount for work by all the main pioneers: Hobart in Britain, Guderian in Germany, Chaffee in the U.S., de Gaulle in France, and Tukhachevsky in the USSR. All came to roughly the same conclusions, Tukhachevsky's integration of airborne pathfinders arguably the most sophisticated; only Germany would actually put the theory into practice, and it was their superior tactics, not superior weapons, that would make blitzkrieg so formidable.

There was thought put into tank-against-tank combat, but the focus was on powerful anti-tank guns and similar weapons, including dedicated anti-tank vehicles. This achieved its fullest expression in the United States, where tanks were expected to avoid enemy armour, and let dedicated tank destroyer units deal with them. Britain took the same path, and both produced light tanks in the hope speed would enable them to avoid being hit, comparing tanks to ducks. In practice these concepts proved dangerous. As the numbers of tanks on the battlefield increased, the chance of meetings grew to the point where all tanks had to be effective anti-tank vehicles as well. However, tanks designed to cope only with other tanks were relatively helpless against other threats, and were not well suited for the infantry support role. Vulnerability to tank and anti-tank fire led to a rapid up-armouring and up-gunning of almost all tank designs. Tank shape, previously guided purely by considerations of obstacle clearance, now became a trade-off, with a low profile desirable for stealth and stability.

World War II: Blitzkrieg and combined arms

Polish 7TP tanks during manoeuvres.World War II saw a series of advances in tank design. Germany, for example, initially fielded lightly armoured and armed tanks, such as the Panzer I, which had been intended for training use only, and were inferior to, for example, French tanks in service at the time. The German army had four models in operation at the start of the War. The first two models, (Panzerkampfwagen I and II, commonly Panzer) were designed for training. The lack of advanced models led to their use during the early stages of the war. The Panzer I light tank had a crew of 2 with two 7.92 mm machine guns. The Panzer II was an interim model with a crew of 3, a 20 mm main gun and one 7.92 mm coaxial machine gun. The Panzer III was the first definitive German battle tank. It initially fielded a 37 mm weapon, but was later upgraded to a 50 mm, had a crew of 5 and was the primary tank used during the German invasion of Russia in 1941. The Panzer IV was the main German battle tank of World War 2. It was initially designed as a support weapon and fielded a short-barrelled 75 mm gun. This was later improved with a long-barrelled version (and new ammunition), which greatly increased the weapon's anti-armour effectiveness. The Panzer IV has the distinction of being the only German tank produced throughout the war. Early in the war they fared poorly in direct combat with British tanks and suffered severely against Soviet T-34s, which were superior in armour, weaponry, and cross-country performance while being equal in speed. However, these fast-moving tanks and other armoured vehicles, competently used, nonetheless proved a critical element of the blitzkrieg.

By this time most tanks were equipped with radios (all U.S. and German, some Soviet; British radios were common, but often of indifferent quality), vastly improving the direction of units. Where previously, tanks had been seen as infantry support weapons, and were forced to move at the pace of the infantry, new doctrines and command structures allowed them to be used on their own, or in cooperation with infantry, instead of in a 'moving artillery' role. Closely associated requirements were to give infantry and logistics the speed to keep up with a rapid advance, and thus bringing into being mechanized infantry.

By the end of the war all forces had dramatically increased their tanks' firepower and armour; for instance, the ten ton Panzer I had only two machine guns; war's end, the standard German medium tank, the Panther, mounted a powerful, high-velocity 75 mm gun and weighed forty-five tonnes.

Another major wartime advance was the introduction of radically improved suspension systems. The quality of the suspension is the primary determinant of a tank's cross-country performance, and tanks with limited suspension travel subject their crew to massive shaking; this not only limits the speed at which the tank can travel, but also prevents firing while moving. Newer systems like the Christie or torsion bar suspension dramatically improved performance, allowing the late-war Panther to travel cross country at speeds that would have been difficult for earlier designs to reach on pavement.

Tank chassis were adapted to a wide range of military jobs, including mine-clearing and combat engineering tasks. All major combatant powers also developed specialised self-propelled guns: artillery, tank destroyers, and assault guns (armoured vehicles carrying large-calibre guns). German and Soviet assault guns, simpler and cheaper than tanks, had the heaviest guns in any vehicles of the war, while American and British tank destroyers were scarcely distinguishable (except in doctrine) from tanks.

Turrets, which were not previously a universal feature on tanks, were recognised as the most efficient siting of the main gun. In order to engage armoured targets the tank needed a single, powerful gun, unlike some prewar designs (like the Soviet T-35), which were often equipped with multiple turrets featuring low-calibre armament, or else mounted one larger gun in a fixed position. Most tanks retained at least one hull machine gun.

The Cold War and beyond
See also: Cold War

A Polish tank company equipped with T-55 tanks.
Serbian M-84AB1 tank, modernization of Yugoslav M-84 tank in Kuwait.After WWII, tank development proceeded largely as it had before, with improvement to both the medium and heavy classes. Light tanks were now limited to the reconnaissance role, and in U.S. use, airborne support as well. However, the weight limitations of air transport made a practical light tank almost impossible to build, and this class gradually disappeared over time.

The seeds for a true transformation had already been working their way into existing designs. A combination of better suspensions and greatly improved engines allowed late-war medium tanks to outperform early-war heavies. With only slightly more armour and somewhat larger engines to compensate, mediums were suddenly protected against almost all anti-tank weapons, even those mounted on heavy tanks, while at the same time having the mobility of a medium tank. Many consider the turning point to be the Panther, which became the inspiration for almost every Western post-war tank design - although the Panther was not quite up to the gun power and armour protection standards of the early Cold War.

Different tanks used by the German Armed Forces during the Cold War (M48A2C, Leopard 1A2, Leopard 2A4)A highly successful post-war tank was the Soviet T-54, which started production in 1947. This successor to the T-34 represented a direct evolution of Russian tank design principles, improving on its low profile, good armour, high mobility, and adding a 100 mm gun.

Another new tank was the British Centurion. Centurion marks built in the late 1940s were able to resist hits from the infamous German 88 mm gun, were ultimately armed with the deadly 105 mm Royal Ordnance L7 gun, and could reach 56 km/h due to the excellent 650 hp (485 kW) Rolls-Royce Meteor engine. The Centurion replaced all British cruiser tanks and finally led to the demise of the infantry tank entirely, becoming what the British referred to as the Universal Tank, soon to be known as the main battle tank (or MBT) in most forces.

In response to the threat of antitank guided missiles (ATGMs), the focus in development shifted from armour thickness to armour technology. Gun technology remained remarkably similar even to WWI-era gun technology, with most tanks in service still being manually loaded, but with big advances in shell effectiveness.

Although the basic roles and traits of tanks were almost all developed by the end of WWI, the performance of twenty-first-century counterparts had increased by an order of magnitude. They had been refined dramatically in response to continually changing threats and requirements, especially the threat of other tanks. The advancing capabilities of tanks have been balanced by developments of other tanks and by continuous development of anti-tank weapons.

In the Iraq War
As of 2005, there were 1,100 M1 Abrams tanks used by the United States army in the course of the Iraq War, and they have proven to have an unexpectedly high level of vulnerability to roadside bombs. However, with upgrades to their armour in the rear, they have proven invaluable in fighting insurgents in urban combat (a role that tactics otherwise proscribe), particularly at the Battle of Fallujah, where the Marines brought in two extra brigades.

Design

A Russian T-90, in service with India.The three traditional factors determining a tank's effectiveness are its firepower, protection and mobility. Also significant is shock action, the psychological effect of a tank's imposing battlefield presence on enemy soldiers.

Firepower is the ability of a tank to identify, engage, and destroy a target. Protection is the tank's ability to resist being detected, engaged, and disabled or destroyed by enemy fire. Mobility includes tactical mobility over diverse terrain on the battlefield, as well as strategic mobility, the ability of the tank to be transported by road, rail, sea, and perhaps by air, to the battlefield.

Tank design is traditionally held to be a compromise between these three factors--it is not considered possible to maximize all three. For example, increasing protection by adding armour will result in an increase in weight and therefore decrease mobility; increasing firepower by using a larger gun will decrease both mobility and protection (due to decreased armour at the front of the turret).

Further information: Tank classification

Firepower
Tank gun

A US Medium Tank M4A3E8 tank fires from a prepared position during the Korean War.A tank crew must be able to quickly identify, engage, and destroy many types of targets on the battlefield, while maintaining high mobility. To this end, they are equipped with sophisticated detection and fire-control equipment, a large gun capable of firing armour-piercing and high-explosive ammunition, and machine guns for defence against infantry, light vehicles, and aircraft.

The main weapon of any modern tank is a single large gun. Tank guns are among the largest-calibre weapons in use on land, with only a few artillery pieces being larger. Although the calibre has not changed substantially since the end of the Second World War, modern guns are technologically superior. The current common sizes are 120 mm calibre for Western tanks and 125 mm for Eastern (Soviet and Chinese legacy) tanks. Tank guns have been able to fire many types of rounds, but their current use is commonly limited to kinetic energy (KE) penetrators and high explosive (HE) rounds. Recent Russian and Chinese designed tanks can fire missiles through the gun. Smoothbore (rather than rifled) guns are the dominant type of gun today. The British and the Indian Armys are now the only ones to field main battle tanks carrying rifled guns. Bore evacuator/fume extractors have become a common feature.

Modern tank guns are generally fitted with thermal jackets which reduce the effect of uneven temperature. For instance, if it were to rain on a tank barrel, the top would cool faster than the bottom, or a breeze on the left might cause the left side to cool faster than the right. This uneven cooling will cause the barrel to bend slightly and will affect long range accuracy.

Close-up of a hull-mounted machinegun.Usually, tanks carry other armament for short range defence against infantry or targets where the use of the main weapon would be ineffective or wasteful. Typically, this is a 7.62 mm to 12.7 mm machine gun, mounted coaxially with the main gun. However, a couple of French tanks (such as the AMX-30 and AMX-40) carry a coaxial 20 mm with a high rate of fire, able to destroy lightly armoured vehicles. Additionally, many tanks carry a roof-mounted or commander's cupola machine gun for close-in ground or limited air defence. The .50cal/12.7 mm and 14.5 mm machine guns commonly carried on U.S. and Russian tanks and the French Leclerc are also capable of destroying lightly-armoured vehicles at close range.

Some tanks have been adapted to specialised roles and have had unusual main armament such as flame-throwers. These specialised weapons are now usually mounted on the chassis of an armoured personnel carrier.

Fire control
Historically, the tank's main gun is generally fired by 'direct fire', in which its weapons were aimed through simple optical sights and laid onto target by hand, with windage estimated or assisted with a reticle (markings in the gun sight which are aligned to frame an object of known size, in this case a tank) to estimate the range to the target. Consequently, accuracy was limited at long range and concurrent movement and accurate shooting were largely impossible. Over time these sights were replaced with stereoscopic rangefinders, and later by laser range-finders. Direct fire contrasts with the 'indirect fire' mostly used by the artillery, in which artillerymen seldom, if ever, see their target.

German Bundeswehr Leopard 2A5.Most modern main battle tanks in the armies of industrialised countries still utilize "direct fire", but deploy laser range-finders. However, some optical and reticule range-finders are still in use in older and less sophisticated vehicles. Modern tanks have a variety of sophisticated systems to make them more accurate. Gyroscopes are used to stabilise the main weapon; computers calculate the appropriate elevation and aim-point, taking input from sensors for wind speed, air temperature, humidity, the gun-barrel temperature, warping and wear, the speed of the target (calculated by taking at least two sightings of the target with the range-finder), and the movement of the tank. Infrared, light-amplification, or thermal night vision equipment is also commonly incorporated. Laser target designators may also be used to illuminate targets for guided munitions. As a result modern tanks can fire with reasonable accuracy while on the move.

Ammunition
Tanks traditionally fire fixed ammunition, contrasting with artillery's three piece ammunition consisting of projectile, powder charge and primer. Whereas artillery fires general service ammunition such as high explosive for general support, tanks, although capable of firing the same type of ammunition, generally fire armor-defeating projectiles such as High explosive squash head (HESH, also called high explosive plastic, HEP), High explosive anti-tank (HEAT), and kinetic energy penetrators (KEP, or armour-piercing discarding sabot APDS). For accuracy, projectiles are spun by gun-barrel rifling, or fin-stabilised (APFSDS, HEAT-FS, etc.).

Some tanks, including the M551 Sheridan, T-72, T-64, T-80, T-84, T-90, T-96, and PT-91 can fire ATGMs (anti-tank guided missile) through their gun barrel or from externally mounted launchers. This functionality can extend the effective combat range of the tank beyond the range afforded by conventional shells, depending on the capabilities of the ATGM system. It also provides the tank with a useful weapon against slow, low-flying airborne targets like helicopters. The United States has abandoned this concept, phasing out the M551 and M60A2, but CIS countries continue to employ gun-missile systems in their main battle tanks.

Protection

Sections of the side-skirt are swung aside on this M1 Abrams tank to expose the track so that a road wheel can be replaced.
An M1 Abrams tank on lookout. Heat haze from the turbine engine can be seen to the rear.A tank's protection is the combination of its ability to avoid detection, to avoid being hit by enemy fire, the ability of its armour to resist the effects of enemy fire, and its ability to sustain damage and complete its mission, or at least protect its crew.

Avoiding detection
Stationary tanks can be well-camouflaged in woodland and forested areas where there is natural cover, making detection and attack from the air more difficult. By contrast, in the open it is very hard to conceal a tank. In both cases, once a tank starts its engine or begins to move it can be detected much more easily due to the thermal signature and noise generated by its engine. The tank tracks across lands can be spotted from the air, and in the desert movement can stir up dust clouds several times the size of the tanks.

A recently-stopped stationary tank has a considerable heat signature. Indeed even if the tank itself is hidden, for example behind a hill, it is still possible for a skilled operator to detect the tank from the column of warmer air above the tank. This risk can be reduced somewhat by the use of thermal blankets which reduce the radiation of heat while the engine and tracks cool. Some camouflage nets are manufactured from unevenly distributed mix of materials with differing thermal properties, which are designed to "randomise" or at least reduce the regularity of the thermal signature of a tank.

Tanks are powered by a diesel or turbine engine of a power comparable to a diesel locomotive. From the outside a diesel powered 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, the vibrations are greater. The acoustic and seismic signatures of multi-fuel engines are comparable. The acoustic signature of a turbine engine is much greater: its high-pitched whine can be much more easily distinguished from other sounds, near or far.

The very large power output of modern tank engines (typically in excess of 750 kW or 1,000 hp) causes 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 moving tank is thus relatively easy to spot by good land-based or aerial thermal infrared scanners. One reason for the one-sided fighting during Operation Desert Storm was that tanks like the US M1 Abrams and the British Challenger had almost four times the night-time infrared scanning range of the older T-72s used by the Iraqi army. Another factor in Desert Storm was that, even when camouflaged and not moving, Iraqi tanks at night would cool at a different rate from their surroundings, making thermal detection easier.

Getting a tank to move proved to be important in the Kosovo conflict in 1999. During the initial few weeks of the conflict NATO air sorties were rather ineffective in destroying Serbian tanks. This changed in the final week of the conflict, when the Kosovo Liberation Army began to engage tanks. Although the KLA had little chance of destroying the tanks, their purpose was to get the tanks to move whereupon they could be more easily identified and destroyed by NATO air power. But even this proved ineffective, as the Serbian army had few tank losses on its side, while the KLA suffered heavy infantry losses.

Armour
Vehicle armour

Abandoning a disabled M-3 tank in training.The main battle tank is the most heavily armoured vehicle in modern land armed corps. Its armour is designed to protect the vehicle and crew against a wide variety of threats. Commonly, protection against kinetic energy penetrators fired by other tanks is considered the most important. Tanks are also vulnerable to anti-tank missiles; anti-tank mines, larger bombs, direct artillery hits, and NBC threats, which can disable or destroy them. Tanks are especially vulnerable to overhead attack. Most modern MBTs do offer near complete protection from artillery fragmentation and lighter anti-tank weapons such as rocket propelled grenades. The amount of armour needed to protect against all conceivable 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.

Most armoured fighting vehicles are manufactured of hardened steel plate, or in some cases aluminium. The relative effectiveness of armour is expressed by comparison to rolled homogeneous armour.

Most armoured vehicles are best-protected at the front, and their crews always try to keep them pointed toward the likeliest direction of the enemy. The thickest and best-sloped armour is on the glacis plate and the turret front. The sides have less armour, while the rear, belly and roof are least protected.

Before the Second World War, several tank designers tried sloped armour on new tank designs. The most famous and successful example of this approach at the time was the T-34. Angling armour greatly increases its effectiveness against projectiles, by increasing the effective perpendicular thickness of the armour (which however offers no weight advantage), and by increasing the chance of deflection. German tank crews were said to be horrified to find that shots fired at T-34s would sometimes simply ricochet.

During World War II, aircraft rockets earned a formidable reputation, especially in France after the Normandy landings (Operation NEPTUNE); post-war analysis, however, revealed many reported kills were near-misses. Aircraft cannon firing armour-piercing ammunition, such as the Hurribomber's 40 mm or Stuka's 37 mm, could be effective, also.

Today, tanks are vulnerable to specialised top-attack missile weapons and air attack, as well as specialised mines. Even light infantry anti-tank weapons however can immobilise a tank by damaging its suspension or track. Many tracked military vehicles have side skirts, intended to protect the suspension.

High explosive anti-tank (HEAT) warheads, such as those of the bazooka or Panzerfaust, were a new threat in the Second World War. These used a shaped charge, which focuses the force of an explosion into a narrow penetrating stream. Thin plates of spaced armour, steel mesh "RPG screens", or rubber skirts, caused HEAT rounds to detonate further from the main armour, greatly reducing their penetrating power (technically, it increased the "standoff distance").

Some anti-tank ammunition (HESH or HEP) uses flexible explosive material, which squashes against a vehicle's armour, and causes dangerous spalling of material inside the tank when the charge explodes. This may kill the crew without penetrating the armour, still neutralising the tank. As a defence, some vehicles have a layer of anti-spall material lining their insides.

Since the 1970s, some tanks have been protected by more complex composite armour, a sandwich of various 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. The Israeli Merkava takes tank design for crew protection to an extreme, using the engine and fuel tanks as secondary protection.

When the armour is defeated then the ability of the surviving crew to escape becomes an issue. The provision of escape hatches in for instance the bottom of the hull, as in the T-34, or the side, as in the Churchill, are necessary potential weaknesses in the armour.

Passive defenses
Most armoured vehicles 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 gives the enemy an early indication of impending attack. Modern smoke grenades work in the thermal infrared as well as visible spectrum.

Some smoke grenades are designed to make a very dense cloud capable of interfering with enemy laser target designators or rangefinders as well as obscuring vision, reducing probability of a hit from visually-aimed weapons, especially low velocity weapons, such as anti-tank missiles which require the operator to keep the tank in sight for a relatively long period of time. In many MBTs, such as the French Leclerc, smoke grenade launchers are also meant to launch tear gas grenades and anti-personnel fragmentation grenades. Many Israeli tanks contain small vertical mortar tubes which can be operated from within the tank, enhancing the anti-personnel capabilities and allowing it to engage targets which are behind obstacles. This idea first appeared in German tanks during WWII and there have been proposals to equip other tanks with dual-purpose smoke/fragmentation grenade launchers that can be reloaded from the interior.

Prior to the widespread introduction of thermal imaging, the most common smoke grenade in AFV launchers was white phosphorus which created a very rapid smoke screen as well as having a very useful incendiary effect against any infantry in the burst area (e.g., infantry attempting to close with hand-placed charges or mines).

Since the advent of thermal imagers, most tanks carry a smoke grenade that contains a plastic or rubber compound whose tiny burning fragments provide better obscurant qualities against thermal imagers.

Some tanks also have smoke generators which can generate smoke continuously, rather than the instantaneous, but short duration of smoke grenades. Generally smoke generators work by injecting fuel into the exhaust, which partially burns the fuel, but leaves sufficient unburned or partially burned particles to create a dense smoke screen.

Modern tanks are increasingly being fitted with passive defensive systems such as laser warning devices, which activate an alarm if the tank is "painted" by a laser range-finder or designator.

Other passive defences include radio warning devices, which provide warning if the tank is targeted by radar systems that are commonly used to guide antitank weapons such as millimetre and other very short wave radar.

Countermeasures
Passive countermeasures, like the Russian Shtora system, attempt to jam the guidance systems of incoming guided missiles.

Explosive reactive armour, or ERA, is another major type of protection against high explosive antitank weapons, in which sections of armour explode to dissipate the focused explosive force of a shaped charge warhead. Reactive armour is attached to the outside of an MBT in small, replaceable bricks.

Active protection systems go one step further than reactive armour. An APS uses radar or other sensing technology to automatically react to incoming projectiles. When the system detects hostile fire, it calculates a firing resolution and directs an explosive-launched counter-projectile to intercept or disrupt the incoming fire a few metres from the target.

Both types of armour are potentially dangerous for friendly infantry operating in close support.

Exposed crew

An Australian Sentinel tank during trials in 1942. Note the commander's lack of protection.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. In this rather high position (often called 'unbuttoned'), with no personal protection save maybe a helmet and a flak jacket, the commander can see around the vehicle with no restrictions, and has the greatest chance of spotting enemy antitank operations or natural and artificial obstacles which might immobilise or slow down the tank. Also, the tank itself is less visible as it can stay lower behind obstacles.

Using periscopes and other viewing devices gives a commander much inferior field of vision and sense of the countryside. Thus, when a tank advances in hostile territory with hatches closed, the commander and the crew might be personally safer, but the tank as a whole is more at risk given the extremely reduced vision. In order to overcome this problem improvements in onboard optical systems are ongoing.

Due to the limitations of the 'closed hatch', many World War II tank commanders of all sides fought their tanks with open hatches. Sometimes this was even standard operating procedure.

Mobility
Mobility of a tank is categorised as Battlefield Mobility, Tactical Mobility, or Strategic Mobility. The first is a function of its engine performance and capability of its running gear and is determined by aspects such as acceleration, speed, vertical obstacle capability and so on. This is what tankers and tank designers call 'agility'. The second is the ability of the tank to be readily transported within a theatre of operation. It depends from its operational range, what bridges it can cross, and what transport vehicles can move it. The third is its ability to be transported from one theatre of operation to other, dependent on its weight, air portability and so on.

A Leclerc crossing a gap.
T-72 Ajeya of the Indian Army fitted with reactive armour during an exercise.A main battle tank is designed to be highly mobile and able to tackle most types of terrain. Its tracks disperse the heavy weight of the vehicle over a large area, resulting in a specific ground pressure that might be lower than 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 to 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 the 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 makes the maximum road speed of a tank really a burst speed, which can be kept up for only a short time before there is a mechanical breakdown. Although the maximum off-road speed is lower, it cannot be kept up continuously all day long, given the variety and unpredictability of off-road terrain (with the possible exception of plains and sandy deserts).

Since an immobilised tank is an easy target for mortars, artillery, and the specialised tank hunting units of the enemy forces, speed is normally kept to a minimum, and every opportunity is used to move tanks on wheeled tank transporters and by railway 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 railway bridges and yards 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 roads riddled by mines or frequent ambushes, the average speed of advance of a tank unit in a day is comparable to a man on a horse or bicycle. Frequent halts must be planned for preventive maintenance and verifications in order to avoid breakdowns during combat. This is in addition to the tactical halts needed so the infantry or the air units can scout ahead for the presence of enemy antitank groups.

Another mobility issue is getting the tank to the theatre 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.

Some tank-like vehicles use wheels instead of tracks in order to increase road speed and decrease maintenance needs. These vehicles lack the superior off-road mobility of tracked vehicles, but are considered by United States planners as more suited for rapid reaction forces due to increased strategic mobility .

Water operations
For most tanks, water operations are limited to fording. The fording depth is usually limited to the height of the air intake of the engine, and to a lesser extent the driver's position. The typical fording depth for MBTs is 90-120 cm. (3-4 ft).

Deep fording

A T-90, engine snorkel erected.However, with preparation some tanks are able to ford considerably deeper waters. The German Leopard I and Leopard II tanks can ford to a depth of several meters, when properly prepared and equipped with a snorkel. The Leopard snorkel is in fact a series of rings which can be stacked to create a long tube. This tube is then fitted to the crew commander's hatch and provides air and a possible escape route for the crew. The height of the tube is limited to around three meters.

All modern Soviet/Russian tanks are also able to perform deep fording operations, however unlike the Leopard, the Russian snorkel is only a few inches round and does not provide a crew escape path, although it is more practical and can be stored on the tank.

This type of fording requires careful preparation of the tank and the ingress and egress sites on the banks of the water obstacle. Tank crews usually have a negative reaction towards deep fording. This has influenced tactics in those countries where the psychological health of the crews or their capacity for rebellion is taken into account. However, if properly planned and executed this type of operation adds considerable scope for surprise and flexibility in water crossing operations.

Amphibious tanks

Sherman DD (Duplex Drive) amphibious tank with waterproof float screens, in 1944. The float screen was raised in the water and rear propellers provided forward thrust.Some light tanks such as the PT-76 are amphibious, typically being propelled in the water by hydrojets or by their tracks. In 1969, the U.S. Army rushed the new M551 Sheridan to Vietnam. This 17 ton light tank was built with an aluminium hull, steel turret and gun (although the 152 mm gun was called a "launcher" at the time), and could swim across bodies of water. Because the U.S. Army had done away with the old heavy, medium, and light tank classifications prior to the Vietnam War, and had adopted the Main Battle Tank (MBT) system, the M551 was officially classified as an Airborne Reconnaissance Assault Vehicle. The M551 upon arrival in Vietnam began replacing the M48A3 Patton in all cavalry squadrons, leaving only the M48A3 in the U.S. Army's three armored battalions in Vietnam, the 1/77th, 1/69th, and the 2/34th Armor. However, the 11th Armored Cavalry Regiment did retain some M48s, as they were the only full regiment in country. Armor Crewmen Trainees at the U.S. Army's Armor School at Fort Knox Kentucky, at the time of the Sheridan entering service, were specifically instructed to refer to the Sheridan by its designated nomenclature. However, for nearly everyone today, civilian and military alike, the Sheridan is a "light tank." The Sheridan needed no modifications for river crossings, crewmen simply raised the cloth sides that were tucked inside rubber tubes along the hull's upper edges, raised the driver's front shield which had a acrylic glass window, the driver turned on his bilge pumps, shifted his transmission lever to water operations and the Sheridan entered the water. For newly arrived Sheridans, this might work as engineered. For "war weary" M551s, the driver's window was often "yellowed" and/or cracked as to obscure his vision, and the rubber tubes that contained the rolled up side sleeves were often cracked and/or frozen into place. The Sheridan could still cross a body of water, but like its swimming cousin, the M113 APC (Armoured Personnel Carrier, also built of aluminium) the river had to be narrow, less than 100 yards (100 m). In all cases the bilge pumps had to be working properly, and even then by the time the Sheridan or the APC reached the other side, water would often fill the insides up to their armoured roofs, spilling through the hatches' cracks and emptying onto the earth once safely ashore. Often a fold down trim vane is erected to stop water washing over the bow of the tank and thus reducing the risk of the vehicle being swamped via the driver's hatch.

In World War II the M4 Medium tank (named Sherman by the British) was made amphibious with the addition of a rubberized canvas screen to provide additional buoyancy. It was propelled by propellers driven by the main engine. This was referred to as the Sherman DD (Duplex Drive) and was used on D-Day to provide close fire support on the beaches during the initial landings. The Sherman DD could not fire when afloat as the buoyancy screen was higher than the gun. A number swamped and sank in the operation, due to rough weather in the English Channel (with some tanks having been launched too far out), and to turning in the current to converge on a specific point on the battlefield, which allowed waves to breach over the screens. Those making it ashore, however, provided essential fire support in the first critical hours.

Power plants

An M1 Abrams engine undergoing maintenance, with the turret turned sideways to expose the engine deck.The tank's power-plant supplies power for moving the tank and for other tank systems, such as rotating the turret or electrical power for a radio. Tanks fielded in WWI mostly used petrol (gasoline) engines as power-plants, unlike the American Holt Gas-Electric tank which was powered by a petrol (gasoline) engine and an electric engine. In the Second World War there was a mix of power-plant types used; a lot of tank engines were adapted aircraft engines. As the Cold War started, tanks had almost all switched to diesel, improved multi-fuel versions of which are still common. Starting in the late 1970s, gas turbines began to appear.

The weight and type of power-plant (influenced by its transmission and drive train) largely determines how fast and mobile the tank is, but the terrain effectively limits the maximum speed of all tanks through the stress it puts on the suspension and the crew.

Multi-fuel diesels
All modern non-turbine tanks use a diesel engine because diesel fuel is less flammable and more economical than gasoline. Some Soviet tanks used the smoke of burning diesel as an advantage and could intentionally burn fuel in the exhaust to create smoke for cover. Fuel tanks are commonly placed at the rear of the tank, though in some designs, such as the Israeli Merkava, the diesel fuel tanks are placed around the crew area to provide an additional layer of protection. Fuel has often been stored in auxiliary tanks externally, or by other means such as in a small trailer towed behind the tank, able to be detached during combat.

Modern tank engines are in some cases multi-fuel engines, which can operate on diesel, petrol or similar fuels.

Gas turbines
Gas turbine engines have been used as auxiliary power units (APUs) in some tanks, and as main powerplants in Soviet/Russian T-80s and U.S. M1 Abrams. They are comparatively lighter and smaller than diesels at the same sustained power output. However they are much less fuel efficient, especially at low RPMs, requiring more fuel to achieve the same combat range. Different models of M1 have addressed this problem with battery packs or secondary generators to power the tank's systems while stationary, saving fuel by reducing the need to idle the main turbine. T-80s can mount three large external fuel drums to extend their range. Russia has stopped production of the T-80 in favour of the diesel-powered T-90 (based on the T-72), while Ukraine has developed the diesel-powered T-80UD and T-84 with nearly the power of the gas-turbine tank.

Because of their lower efficiency, the thermal signature of a gas turbine is higher than a diesel engine at the same level of power output. On the other hand the acoustic signature of a tank with a muffled gas turbine can be quieter than a piston engine–powered one. The M1A2 was nicknamed 'Whispering Death' for its quiet operation.

A turbine is theoretically more reliable and easier to maintain than a piston-based engine, since it has a simpler construction with fewer moving parts. In practice, however, those parts experience a higher wear due to their higher working speeds. The turbine blades are also very sensitive to dust and fine sand, so that in desert operations special filters have to be carefully fitted and changed several times daily. An improperly fitted filter, or a single bullet or piece of shell fragment can render the filter useless, potentially damaging the engine. Piston engines also need well-maintained filters, but they are more resilient if the filter does fail.

Like most modern diesel engines used in tanks, gas turbines are usually multi-fuel engines.

Command, control and communications
Commanding and coordinating a tank organisation in the field has always been subject to particular problems. Because of the isolation of small units, individual vehicles, and even the crewmen of a tank, special arrangements have had to be made. Armoured bulkheads, engine noise, intervening terrain, dust, and smoke, and the need to operate "hatches down" (or "buttoned up") comprise severe detriments to communications.

Internal communications
Every action of a tank's crew, movement and fire, is ordered by its commander. In some early tanks, the tank commander's task was severely hampered by having to load or fire the main armament, or both. In many small armoured fighting vehicles, even into the late twentieth century, the tank commander would relay movement orders to the driver by kicks to his shoulders and back. Most modern AFVs are equipped with an intercom, allowing all crewmen to talk to each other, and to operate the radio equipment. Some tanks have even been equipped with an external intercom on the rear, to allow co-operating infantry to talk to the crew.

German Leopard 2A6M (2007)
Tactical communications
In the earliest tank operations, communications between the members of an armoured company were accomplished using hand signals or handheld semaphore flags, and in some situations, by crewmen dismounting and walking to another tank. In World War One, situation reports were sent back to headquarters by releasing carrier pigeons through vision slits. Signal flares, smoke, movement, and weapon fire are all used by experienced crews to coordinate their tactics.

From the 1930s to the '50s, most nations' armoured forces became equipped with radios, but visual signals are still used to reduce radio chatter. A modern tank is usually equipped with radio equipment allowing its crew to communicate on a company or battalion radio network, and possibly to monitor a higher-level network, to coordinate with other arms of service. Company or battalion commanders' tanks usually have an additional radio. Communications on a busy network are subject to a set of formalised language rules called radio voice procedure.

Most armoured forces operate with the tank commander, and possibly other crewmen, "hatches up", for best possible situational awareness. When taking fire, or in potential NBC conditions, tank crews "button up" and only view the battlefield through vision slits or periscopes, severely reducing their ability to acquire targets and perceive hazards. Since the 1960s, a tank's commander has had progressively more sophisticated equipment for target acquisition. In a main battle tank, the commander has his own panoramic sights (with night-vision equipment), allowing him to designate one or more new targets, while the gunner engages another. More advanced systems allow the commander to take control of the turret and fire the main armament in an emergency.

Computerised advances
A recent development in AFV equipment is the increased integration of fire control, the laser range-finder, GPS data, and digital communications. U.S. tanks are fitted with digital computers which are connected into battlefield networks. These integrate known information on enemy targets and friendly units to greatly improve the tank commander's situational awareness. In addition to easing the reporting burden, these systems also allow for orders to be given complete with graphics and overlays, via the network.

See also: Military communications, Command, control, and communications, and C4ISTAR

Vulnerability
Anti-tank warfare
Despite being a powerful weapon and an impressive sight on the battlefield, the tank is vulnerable. In fact, the tank's basic effectiveness has led to massive development of anti-tank weapons and tactics. Some critics have noted that due to these developments, and the rising importance of asymmetrical warfare, tanks have become too vulnerable to be worth the considerable cost they represent.

Attacks
Infantry
Unsupported tanks are vulnerable to attacks by foot soldiers who manage to reach blind or weak spots (such as very close under the tank, or behind it). Tanks may be immobilised by explosives used against the tracks, or attacks through open hatches (see above)). To protect themselves, tanks generally operate with closely coordinated infantry support to protect them from enemy infantry.

Artillery
While tanks are mostly invulnerable to shell fragments, specialised anti-tank ammunition can make a well-positioned artillery piece deadly against opposing tanks (assuming the artillery can react quickly enough and depress low enough). Modern anti-tank artillery shells also include guided projectiles.

See also: Anti-tank guns
Mines
Anti-tank minefields and landmines in general have a capability to pierce a tank's (relatively) thin bottom armour. In rare cases, even IEDs (though of comparatively massive size) have been capable of actually destroying a modern tank.

Aircraft
Ground attack aircraft may use heavy machine guns or cannons as well as rockets or guided missiles against tanks, often aiming for the top armour, which again is (relatively) weak. Attack helicopters, exploiting high mobility and the use of terrain for protection, and carrying guided missiles, have also become a mainstay of anti-tank tactics for many militaries.

Logistics
Tanks suffer a major drawback compared to wheeled vehicles like armoured cars, being mechanically more complicated and requiring far more maintenance. This places strains on an army's logistic system which may inhibit tank operations. In addition, design features may cause problems; the WW2 Panther, for instance, had interleaved road wheels, which tended to clog in mud or snow, and required five wheels to be removed to change a single inner wheel, while other German tanks of the period had driven front sprockets, which added complexity compared to British ones which drove the rear sprockets.

Climate
Tanks can also be disabled by the weather: starter batteries, lubricants, and even engines may fail in extreme cold (during World War II campaigns in Russian winters, tanks were often kept running to prevent restart problems with frozen-solid engines). Engines and crew-members can also suffer from overheating during hot weather (partly combated in newer tanks by air-conditioning systems), or dust clogging important ducts.

Terrain
Tanks are also at a disadvantage in wooded terrain and urban combat environments, which cancel the advantages of the tank's long-range firepower, limit the crew's ability to detect potential threats, and can even limit the turret's ability to traverse. Some of these disadvantages have now been taken into account by special modifications for urban combat, and it should be noted that urban operations create additional hazards for almost all unit types, with tanks often retaining a high survivability (especially against improvised and most man-portable weapons) by virtue of their armour.

Research and development
Tank research and development
Current research involves making the tank invisible to radar by adapting stealth technologies originally designed for aircraft and a variety of luminosity and colour shaping technologies. Research is also ongoing in armour systems and new propulsion units.

One clear trend is the increasing number of electrical and communication systems on a tank, such as thermal scopes and higher powered radios.

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