World War, the economics of tank production governed by the complexity of manufacture and cost, and the impact of a given tank design on logistics and field maintenance capabilities, have also been accepted as important in determining how many tanks a nation can afford to field in its force structure. Some tank designs that were fielded in significant numbers, such as Tiger I and M60A2 proved to be too complex or expensive to manufacture, and made unsustainable demands on the logistics services support of the armed forces. The affordability of the design therefore takes precedence over the combat capability requirements. Nowhere was this principle illustrated better than during the Second World War when two Allied designs, the T-34 and the M4 Sherman, although both simple designs which accepted engineering compromises, were used successfully against more sophisticated designs by Germany that were more complex and expensive to produce, and more demanding on overstretched logistics of the Wehrmacht. Given that a tank crew will spend most of its time occupied with maintenance of the vehicle, engineering simplicity has become the primary constraint on tank design since the Second World War despite advances in mechanical, electrical and electronics technologies. Since the Second World War, tank development has incorporated experimenting with significant mechanical changes to the tank design while focusing on technological advances in the tank's many subsystems to improve its performance. However, a number of novel designs have appeared throughout this period with mixed success, including the Soviet IT-1 and T-64 in firepower, and the Israeli Merkava and Swedish S-tank in protection, while for decades the US's M551 remained the only light tank deployable by parachute. Command, control, and communications Commanding and coordinating tanks in the field has always been subject to particular problems, particularly in the area of communications, but in modern armies these problems have been partially alleviated by networked, integrated systems that enable communications and contribute to enhanced situational awareness. 20th century World War I and Interwar period armoured bulkheads, engine noise, intervening terrain, dust and smoke, and the need to operate "buttoned up" (with hatches closed) are severe detriments to communication and lead to a sense of isolation for small tank units, individual vehicles, and tank crew. Radios were not then portable or robust enough to be mounted in a tank, although Morse code transmitters were installed in some Mark IVs at Cambrai as messaging vehicles. Attaching a field