The W48 was an American nuclear artillery shell, capable of being fired from any standard 155 mmhowitzer, e.g. the M114, M198 or M109. It was manufactured starting in 1963, and all units were retired in 1992. It was known as the M45 AFAP in US service. The W48 was 6.1 inches in diameter and long. It was built in two models, Mod 0 and Mod 1, which are reported to have weighed and respectively. It had an explosive yield equivalent to 72 tons of TNT, which is very small for a nuclear weapon.
The W48 was a small diameter linear implosion nuclear fission weapon. An implosion weapon needs less nuclear material than is required to form a critical mass, at normal pressure and configurations, compared to a gun-type assembly. It uses precise explosive assemblies to collapse the material to many times normal density in order to attain critical mass. A bare critical mass of plutonium at normal density and without additional neutron reflector material is roughly. This amount of alpha-phase plutonium. To achieve a large explosive yield, a linear implosion weapon needs somewhat more material, about. The mass, known as the "pit", is configured in a lower density non-spherical configuration. A tamper or reflector could be used to reduce the mass, but the overall diameter of the fissile material plus tamper/reflector increases compared to the volume required for an untamped, unreflected pit. To fit weapons into small artillery shells bare pits are required. On firing the weapon, small to moderate amounts of high explosive collapse and slightly reshape the nuclear material into a supercritical mass which then begins a chain reaction, goes critical, and explodes in a small nuclear blast. Three methods are known to compress and reshape the nuclear material: collapsing hollow spaces inside the nuclear material; using plutonium-gallium alloy which is stabilized in the low density delta phase at a density of ; and shaping an explosive around the nuclear material so that the explosive pressure causes a stretched-out elliptical or rugby ball shape to collapse towards a more spherical end shape, which decreases the surface area of the pit. Linear implosion weapons have much lower efficiency due to low pressure, and require 2-3 times more nuclear material than conventional implosion weapons. They are also considerably heavier, and much smaller than conventional implosion weapons. The W54nuclear warhead used in the Davy Crockett nuclear artillery unit was about diameter and weighs. The W48 is in diameter and weighs over twice as much, and probably requires twice as much plutonium. Independent researchers have determined that one model of US conventional implosion fission weapon cost $1.25 million per unit produced, of which $250,000 was the total cost for all non-nuclear components and $1 million the cost of the plutonium. Linear implosion weapons requiring 2-3 times more plutonium are proportionally more expensive.
Replacement
The W82 was the most successful, but not complete, replacement for the W48.
