arrangement of metal plates, ceramic blocks and open space; and saw a proposed design for a new British vehicle utilizing it. HEAT and sabot rounds enter the beginning layers of armor but are unable to penetrate the crew compartment. Ceramics have the ability to absorb a great deal of heat, and can blunt physical blows by cracking and deflecting the force. The remaining hot gasses and metal shrapnel spread out or settle in empty air pockets. Both contractors reevaluated their proposed armor configurations based upon the newly obtained data. This led to major changes in the General Motors XM1, the most prominent of which is the turret front changing from vertical to sloped armor. The Chrysler XM1 on the other hand retained its basic shape although a number of changes were made. The Ballistic Research Laboratory had to develop new armor combinations in order to accommodate the changes made by the contractors. Similar to most other main battle tanks, the M1 Abrams feature composite armor only on the frontal aspect of the hull. However, the Abrams' turret features composite armoring across both the front and the sides. In addition, the side skirts of the frontal half of the hull are also made of composite, providing superior ballistic protection against chemical energy munitions such as HEAT rounds. The composition of the Abrams' composite armor consists of sandwiched plates of non-explosive reactive armor (NERA) between conventional steel plates. The NERA plates feature elasticity, allowing them to flex and distort upon perforation, disrupting the penetrating jets of shaped charges and providing more material and space for a kinetic round to pass through, thus providing increased protection compared to conventional steel armor of similar weight. For the M1 Abrams base model, Steven Zaloga estimates the frontal armor at 350 mm vs armor-piercing fin-stabilized discarding sabot (APFSDS) and 700 mm vs high-explosive anti-tank (HEAT) warhead in the book, M1 Abrams Main Battle Tank 1982–1992 (1993). In M1 Abrams vs T-72 Ural (2009), he uses Soviet estimates of 470 mm vs APFSDS and 650 mm vs HEAT for the base model Abrams. He also gives the Soviet estimates for the M1A1, 600 mm vs APFSDS, and 700 mm vs HEAT. Armor protection was improved by implementing a new special armor incorporating depleted uranium and other undisclosed materials and layouts. This was introduced into the M1A1 production starting October 1988. This new armor increased effective armor particularly