Spaced armour

Tank spaced armour has been fielded since the First World War, when it was fitted to the French Schneider CA1 and Saint-Chamond tanks. The late variants of Panzer III had frontal spaced armour: a thick face-hardened steel layer in front of the thick main armour. Impacted projectiles were physically damaged by the 20 mm plate, so the main armour could withstand much greater hits. Due to lack of materials, German industry eventually switched to Rolled Homogeneous Armour (RHA), which is less effective and due to the slower production process, the technique was not widespread on German tanks. Many World War II–era German tanks used armoured skirts (Schürzen) to make their thinner side-armour more resistant to anti-tank rifles, contrary to popular belief that the German Schürzen were designed against shaped charge projectiles. The common Russian PTRS rifles could penetrate of armor at common combat ranges, whereas many German tanks only had of armor on their sides. The skirts thus added of additional thickness to make up the difference, and could theoretically cause the round to tumble, improving protection against those weapons. Nevertheless these rifles continued to be useful throughout the war. Postwar analysis of spaced armour at the US Aberdeen Proving Grounds found spaced armour to be ineffective if the layers are of roughly equal thickness. Numerous trials invariably showed that combinations of multiple plates provided "considerably less protection than a single solid plate of the same total thickness". This is because the midsection of plates provides more resistance to penetration than the front and rear surfaces, and thus having a thicker plate offers better performance. Instead additional layers of armour should be the thinnest required to obtain the possibility of fracturing the projectile, which has the best results in improving protection, though this effect was not consistent and could be mitigated by improved projectile design. Projectiles impacting against sloped spaced armour at greater standoff ranges could also result in the projectile turning to impact the second plate at a more perpendicular angle, making the added armour worse than nothing. This is because a projectile penetrating a plate is deflected towards the normal, an effect that could ruin an armour scheme. Though spaced armour appeared in some tanks like the Leopard 1 and the Merkava, the armour scheme was not considered to offer sufficiently better protection against armour-piercing projectiles to justify the increased complication they posed, and thus their use on post-war tanks was limited and eventually superseded by more effective composite armour. ==Against high explosive anti-tank rounds==

Most of the Cold War spaced armour was designed against medium-to-low caliber kinetic munitions, (e.g. autocannon and HESH rounds), especially vehicle side skirts. Most of them were made of RHA plates (Centurion), or thick reinforced rubbers (T-72), and worked in the same way as did WWII-era ones. Some WWII armoured vehicles used nets of wooden logs at a certain distance from the hull as makeshift spaced armour to protect the vehicle from magnetic mines, thrown shaped charges and grenades, and occasionally suicidal methods (e.g. the Japanese lunge mine). This method occurred on US M4 Sherman and Soviet T-34 medium tanks among others. The idea is that this thin layer of armour detonates explosive warheads prematurely. Such techniques were effective in warships against armour piercing shells with short fuzes. In T-64 and early T-72 (up to T-72M1) and T-80 (to mid T-80A) used stekloplastik (a special military-grade dense glass-fiber reinforced pressured plastic) as filling in the frontal upper glacis spaced armour. This plastic was effective in lowering the concentration of the jet of shaped charges and in destabilizing kinetic penetrators. Hardened steel plates have become commonplace for the outer part of spaced armour from the 1980s, not only on tanks but also on APCs and IFVs. With this add-on armour, even the APC's thin armour is sufficient against kinetic bullets of 12.7 mm (Stryker and BTR-80 upgrades) and 14.5 mm (Bradley, BMP-3) and also provides some protection against IEDs. The increase in the number of layers in spaced armour increases the physical damage and destabilization of jets and kinetic penetrators, so it is common in more modern armour to use successive layers alternating between softer (air, aluminium or plastic) and harder (RHA, SHS) layers. With multiple layers the likelihood of a bounce in case of kinetic projectiles is also increased. Thus, later T-72B and T-90 armour used seven-layered spaced armour (with hardened steel plates) to achieve much stronger protection at a cost of minimal weight increases. The more advanced late Cold War tanks were given multi-layer skirts (Leopard 2), in which passive (or reactive) effects significantly reduced the effectiveness of HEAT ammunition. At the same time, these elements are already heavy and have considerable thickness, which increases the size and weight of the vehicle and make maintenance difficult. Russian and some Western tanks carry explosive-reactive armour blocks to increase the effectiveness of spaced armour (particularly in the case of side skirts, e.g. TUSK and T-90), and main frontal armour. Almost all modern Western and Japanese and most Soviet tanks used some kind of spaced armour on the fronts and sides. Side panels of superstructures usually contain fuel, batteries and other less vital elements or munition of secondary weapons, because they also reduce the effectiveness of penetrating projectiles. In the most important areas (frontal armour and sides of turret) the cavity of spaced armour contains composite panels. From the 1980s, most Western tanks have composite armour blocks on the frontal part of the skirts, made of hardened steel or NERA armour (non-explosive-reactive armour, known as "Burlinghton armour" ==Spacecraft==

The Whipple shield uses the principle of spaced armour to protect spacecraft from the impacts of very fast micrometeoroids. The impact with the first wall melts or breaks up the incoming particle, causing fragments to be spread over a wider area when striking the subsequent walls. ==See also==