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Mortars had existed as a form of heavy artillery for centuries, but in 1914 the German Army introduced a limited number of small, cheap, portable minenwerfers, which were breech-loading, low-trajectory mortars. Other armies quickly copied the minenwerfer, and in March 1915, the English engineer Wilfred Stokes developed the grandfather of all current infantry mortars, the 3-inch muzzle-loading Stokes mortar.19 This weapon was much simpler to manufacture than artillery and therefore was employed extensively in all armies during the war.

All maneuver units require indirect fire to win. Mortars provide unique indirect fires that are organizationally responsive to the ground maneuver commander. Military history has repeatedly demonstrated the effectiveness of mortars. Their rapid, high-angle, plunging fires are invaluable against dug-in enemy troops and targets in defilade, which are not vulnerable to attack by direct fires. Although they are part of the total fire support system, mortar sections and platoons are not simply small artillery batteries. They play a unique and vital role on the AirLand Battlefield.

Mortars allow the maneuver commander to quickly place killing indirect fires on the enemy, independent of whether he has been allocated supporting artillery. Heavy forces use carrier-mounted mortars to allow the mortar platoon to move cross-country at speeds compatible with the battalion task force. Light forces use wheeled vehicles or hand carry mortars into firing positions. Some companies have light mortars that can be manpacked across all terrain. All mortar sections and platoons exist to provide immediate, organizationally responsive fires that can be used to meet the rapid changes in the tactical situation on the AirLand Battlefield.

The urban environment greatly restricts low-angle indirect fires because of overhead masking. While all indirect fire weapons are subject to overhead masking, mortars are less affected than field artillery weapons due to the mortar's higher trajectory. For low-angle artillery fire, dead space is about five times the height of the building behind which the target sits. For mortar fire, dead space is only about one-half the height of the building. Because of these advantages, mortars are even more important to the infantry during urban combat.

Not only can mortars fire into the deep defilade created by tall buildings, but they can also fire out of it. Mortars emplaced behind buildings are difficult for the enemy to locate accurately and even harder for him to hit with counterfire. Because of their lightweight, even heavy mortars can be hand carried to firing positions that may not be accessible to vehicles.

Mortars can be fired through the roof of a ruined building if the ground-level flooring is solid enough to withstand the recoil. If there is only concrete in the mortar platoon's area, mortars can be fired using sandbags as a buffer under the baseplate and curbs as anchors and braces. (This is recommended only when time is not available to prepare better firing area.) Aiming posts can be placed in dirt-filled cans.

The 60-mm and 81-mm mortars of the US Army have limited effect on structural targets. Even with delay fuzes they seldom penetrate more than the upper stories of light buildings. However, their wide area coverage and multioption fuzes make them useful against an enemy force advancing through streets, through other open areas, or over rubble. The 120-mm mortar is moderately effective against structural targets. With a delay fuze setting, it can penetrate deep into a building and create great destruction.

Mortar platoons often operate as separate firing sections during urban combat. The lack of large open areas can preclude establishing a platoon firing position. Two mortar sections, which are separated by only one street, can be effective in massing fires and be protected from countermortar fire by employing defilade and dispersion. All three of the standard mortar projectiles are useful during combat in urban areas. High-explosive fragmentation is the most commonly used round. WP is effective in starting fires in buildings and forcing the enemy out of cellars and light-frame buildings, and is the most effective mortar round against dug-in enemy tanks. Even near misses blind and suppress the tank crew, forcing them to button up.

The artificial relief of urban terrain reduces wind speed and increases atmosphere mixing, so mortar smoke tends to persist longer and give greater coverage in urban areas than in open terrain.

Urban masking impacts the use of illumination. In urban areas, it is often necessary to plan illumination behind friendly positions placing friendly troops in shadows and enemy troops in the light. Illumination rounds are difficult to adjust and are often of limited use because of the deep canyon nature of the urban area. Rapidly shifting wind currents in urban areas also affect mortar illumination, making it less effective.

The multioption fuze on newer US mortar rounds makes them effective weapons on urban terrain. Delay settings can increase penetration slightly, while proximity bursts can increase the lethal area covered by fragments. Tall buildings can cause proximity fuzed mortar rounds to detonate prematurely if they pass too closely.

The US currently has five models of mortars.

Light mortar
The 60-mm mortar, M224, provides air assault, airborne, ranger, and light infantry rifle companies with an effective, efficient, and flexible weapon. The inherent limitations of a light mortar (short-range and small-explosive charge) can be minimized by careful planning and a thorough knowledge of its capabilities. The M224 can be employed in several different configurations. The lightest weighs about 18 pounds; the heaviest weighs about 45 pounds. Each round weighs about 4 pounds.

The 60-mm mortar round cannot penetrate most rooftops, even with a delay setting. Small explosive rounds are effective, however, in suppressing snipers on rooftops and preventing roofs from being used by enemy observers. The 60-mm WP round is not normally a good screening round due to its small area of coverage. In urban combat, however, the tendency of smoke to linger and the small areas to be screened make it more effective. During the battle for Hue in South Vietnam, 60-mm WP rounds were used to create small, short-term, smoke screens to conceal movement across open areas such as parks, plazas, and bridges. Fragments from 60-mm HE rounds landing as close as 10 feet away cannot penetrate a single sandbag layer or a single-layer brick wall. The effect of a 60-mm mortar HE round that achieves a direct hit on a bunker or fighting position is equivalent to 1 or 2 pounds of TNT. Normally, the blast will not collapse a properly constructed bunker but can cause structural damage. The 60-mm mortar will not normally crater a hard-surfaced road.

Medium mortars
The 81-mm mortars, M29A1 and M252, are the current US medium mortars. The M252 is replacing the M29A1, but both will remain in the Army inventory for several years. Medium mortars offer a compromise between the light and heavy mortars. Their range and explosive power is greater than the M224, yet they are still light enough to be man-packed over long distances. The M29A1 weighs about 98 pounds. The M252 is slightly lighter, about 93 pounds. Both can be broken down into several smaller loads for easier carrying. Rounds for these mortars weigh about 15 pounds each.

The 81-mm mortar has much the same effect against urban targets as the 60-mm mortar. It has a slightly greater lethal area and its smoke rounds (WP and RP) are more effective. A direct hit is equivalent to about 2 pounds of TNT. The 81-mm round cannot significantly crater a hard-surfaced road. With a delay setting, the 81-mm round can penetrate the roofs of light buildings.

Heavy mortars
The 107-mm mortar, M30, and the 120-mm mortar, M120, are the current US heavy mortars. The M120 is replacing the M30, but both will remain in the US inventory for several years. The M30 is a rifled mortar, stabilizing its projectile by spinning it rapidly. The M120, like all other US mortars, fires fin-stabilized ammunition from a smooth bore. Although heavy mortars require trucks or tracked mortar carriers to move them, they are still much lighter than field artillery pieces. They outrange light and medium mortars, and their explosive power is much greater. The M30 weighs about 675 pounds. The M120 is much lighter at about 320 pounds. Rounds for the 107-mm mortar weigh about 28 pounds. Those for the 120-mm mortar weigh almost 33 pounds each.

The 120-mm mortar is large enough to have a major effect on common urban targets. It can penetrate deep into a building, causing extensive damage because of its explosive power. A minimum of 18 inches of packed earth or sand is needed to stop the fragments from a 120-mm HE round impacting 10 feet away. The effect of a direct hit from a 120-mm round is equivalent to almost 10 pounds of TNT, which can crush fortifications built with commonly available materials. The 120-mm mortar round can create a large but shallow crater in a road surface, but it is not deep or steep-sided enough to block vehicular movement. However, craters could be deep enough to damage or destroy storm drain systems, water and gas pipes, and electrical or phone cables.

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