Since the beginning of warfare men have sought ways to protect themselves from their enemies weapons. Be it a wooden shield or the modern composite armor of a main battle tank, armors main purpose is to defeat the destructive effects of the oppositions weapon systems. However, armor is a trade off, as armor is added weight increases and this degrades mobility, which in itself can be a substantial defense. A tank with enough armor to counter every threat would be to heavy to move. No bridge or road could support it, and it would be nearly impossible to transport, especially by air. Also, heavier vehicles must have very powerful engines requiring large amounts of fuel to operate, this places a additional strain on the logistics system. Modern tanks have their heaviest armor in the front with thinner armor in the sides and the thinnest in the rear, top, and belly. This helps with the weight problem but leaves the vehicle with vulnerable spots. Many modern tanks use multiple types of armor. The M1 series, for example, uses a combination of composite armor, spacing, and sloping for protection.
Steel is the basic armor type and has been in use for hundreds of years. Some modern tanks such as the T-72 and the M60 use a one piece cast steel turret and hull which are produced by giant molds, others use welded together plates. Since steel armor relies on it's thickness, it's the least effective. Steel thick enough to protect against modern HEAT and KE warheads would be to massive to be realistic. However it's effectiveness can be increased by sloping or rounding the plates. Inclining a 4 inch armor plate to a 60 degree angle increases it's effective thickness to about 8 inches.
Reactive Armor
Reactive armor consists of boxes attached to the exterior of the tank. Each box contains a explosive charge sandwiched between steel plates. Impervious to small arms fire and artillery fragments the charge detonates on contact with a shaped charge warhead. The explosion blows the plates apart disrupting the warheads plasma jet, rendering the round ineffective. Reactive armor can increase the effectiveness of conventional armor up to 5 times but it has it's drawbacks. Once a panel blows it leaves that spot vulnerable to future attacks and it is not effective against KE rounds. Some modern warheads, such as the newer TOW missiles, are designed to defeat this type of armor. They use dual tandem warheads one detonating a split second after the other. The first detonates the reactive armor and the second attacks the conventional armor left naked underneath.
Spaced armor consists of armor containing hollow areas. Upon being hit by a HEAT warhead the plasma jet burns through the first layer and splashes inside the hollow space without penetrating the inner layer. Spacing armor does not add protection against KE rounds.
Although the exact composition of most composite armor is classified, it is basically a sandwich of steel and depleated uranium plates, ceramics, and plastic honeycomb. The British developed Chobham armor used in the M1 series, the German Leopard 2, and the British Challenger, is the most advanced composite in production.
Although anti-spall liners are not armor in the traditional sense, they provide protection for the crew and deserve mention here. When a round impacts the outside of the vehicle, wether or not it fails to penetrate, it can fracture the inner armor causing flakes to break off and bounce around the interior at high speed. This is known as spalling and can have a detrimental effect on the crew and delicate components inside, to say the least. Anti-spall liners are basically a kevlar or ballistic nylon liner inside the turret to prevent this.