Many armies are replacing some or all of their traditional heavy vehicles with lighter airmobile versions, often with wheels instead of tracks. Design Armour The level of armour protection between AFVs varies greatly – a main battle tank will normally be designed to take hits from other tank guns and anti-tank missiles, whilst light reconnaissance vehicles are often only armoured "just in case". Whilst heavier armour provides better protection, it makes vehicles less mobile (for a given engine power), limits its air-transportability, increases cost, uses more fuel and may limit the places it can go – for example, many bridges may be unable to support the weight of a main battle tank. A trend toward composite armour is taking the place of steel – composites are stronger for a given weight, allowing the tank to be lighter for the same protection as steel armour, or better protected for the same weight. Armour is being supplemented with active protection systems on a number of vehicles, allowing the AFV to protect itself from incoming projectiles. The level of protection also usually varies considerably throughout the individual vehicle too, depending on the role of the vehicle and the likely direction of attack. For example, a main battle tank will usually have the heaviest armour on the hull front and the turret, lighter armour on the sides of the hull and the thinnest armour on the top and bottom of the tank. Other vehicles – such as the MRAP family – may be primarily armoured against the threat from IEDs and so will have heavy, sloped armour on the bottom of the hull. Weaponry Weaponry varies by a very wide degree between AFVs – lighter vehicles for infantry carrying, reconnaissance or specialist roles may have only a autocannon or machine gun (or no armament at all), whereas heavy self-propelled artillery will carry howitzers, mortars or rocket launchers. These weapons may be mounted on a pintle, affixed directly to the vehicle or placed in a turret or cupola. The greater the recoil of the weapon on an AFV, the larger the turret ring needs to be. A larger turret ring necessitates a larger vehicle. To avoid listing to the side, turrets on amphibious vehicles are usually located at the centre of the vehicle. Grenade launchers provide a versatile launch platform for a plethora of munitions including, smoke, phosphorus, tear gas, illumination, anti-personnel, infrared and radar-jamming