M113 series of full-track armoured vehicles

M113

TRACKED ARMOURED VEHICLE SERIES

In January 1956 ATAC initiated a programme to design and build a lightweight armoured personnel carrier for the US Army to replace the M59 and M75 tracked apc's. It had to be amphibious and capable of air-drop operations with a good cross-country performance, and also be adaptable to other purposes. Within 90 days after the preliminary drawings had been completed, ATAC had a test bed built and running. It was powered by an Ordnance air-cooled engine, with rolled aluminium plate armour for the hull. Being a lot lighter than steel, for aluminium to gain comparable ballistic qualities it had to be produced 3 times thicker than it's steel equivalent. This however, was a small price to pay as aluminium is much stiffer than steel and structural supports could be eliminated, thus increasing the internal volume. This in turn reduced the overall weight and height of the vehicle, and also significantly cut down on the construction and welding times during manufacture. After extensive tests by ATAC and a number of problems had been solved, the project was put out to tender in June 1956. In September 1956, FMC Corp was announced as the winner and contracted to build the new APC. Five prototypes, designated T113, were built using aluminium armour and powered by Continental air-cooled engines. A further five prototypes, designated T117, were also built using steel armour and driven by a Packard water-cooled engine. By 1957 ATAC had received one of each of both types of hull. During testing of the T117 the Packard engine proved to be unsatisfactory and was replaced by the Ford 368UC engine. However even this engine was found to overheat and required an increase in engine compartment space to help it cool. As well as the problems faced with the Packard and Ford engines on the T117, the Continental air-cooled engine was also giving problems on the T113. By late 1957 FMC had developed two new prototypes, these were designated T113E1 and T113E2. Both types were termed "austerity" vehicles as they were designed to keep costs down to a bare minimum. With this line of thought in mind it was decided to adopt cheaper commercially available powerplants and transmissions. This led to the Chrysler A710B petrol engine being chosen because of it's performance, good cooling characteristics, and the fact that it would be in production for a lot more years. The transmission adopted was the Allison TX-100, again, because it was in widescale production, and it had an excellent track record in the truck industry. Two different hulls were developed which looked identical, but differed only in their armour thickness and weight so that ATAC could determine, through ballistic and durability tests, which of the hulls to adopt. At the same time the tracks were widened from 12" to 15" to improve on the vehicles reliability and performance. After successful testing the T113E2, with the thicker hull, was chosen as the basis for the M113 APC. FMC received an initial contract to build 900 examples and the first production M113 rolled of the FMC assembly lines at San Jose, California in June. 1960. These first M113s were powered by a 209HP Chrysler 75M petrol engine but attempts were made to develop a gas-turbine version under the designation M113E1. In June 1959 FMC were awarded a contract to study the feasability of fitting a diesel engine in place of the petrol one. The test vehicle was known as the M113E2 and was standardised as the M113A1 in May 1963. By 1964 the M113A1 was in production fitted with a GM 215HP Diesel engine which gave it an improved performance over the earlier M113. The M113 had a crew of two, a commander and a driver, and could carry eleven passengers and their equipment. The driver sat on the front lhs of the vehicle and had a single piece hatch cover that opens to the rear with four M17 pericopes around the hatch and an M19 periscope in the hatch roof. The commander was situated centrally in the vehicle with a fully rotating cupola containing five M17 periscopes. The commander also acted as the gunner and operated the .50 calibre machine gun, this had an 360° field of fire with an elevation of +53° and a depression of -21°. The ammunition for the .50 calibre was stored in boxes behind the infantrymen's seats. While a hatch is provided in the roof of the troop compartment the normal means of entry and exit is the large hydraulically-operated ramp in the hull rear, which is provided with an integral door on the left side in case the ramp should fail to open. The cold rolled alloy hull of the M113 was made from Aluminium, Manganese and Magnesium. It's armour thickness varied from ¾" to a maximum thickness of 1 ¼". This thickness provided protection from small arms fire and shell fragments, but would not stop a round from a LAW nor withstand the blast from a land mine. The M113 was constructed as a watertight unit, with all hatches and doors fitted with rubber seals to make them watertight. The front of the vehicle was fitted with a hinged splash plate which helped to stabilize the vehicle during operations through water. In the amphibious role it was propelled by it's tracks and aided by the rubber track shroud on each side of the hull which gave the M113 a top speed of 3mph while swimming.