Rodman Gun

The Rodman gun is any of a series of American Civil War–era columbiads designed by Union artilleryman Thomas Jackson Rodman (1815–1871). The guns were designed to fire both shot and shell. These heavy guns were intended to be mounted in seacoast fortifications. They were built in 8-inch, 10-inch, 13-inch, 15-inch, and 20-inch bore. Other than size, the guns were all nearly identical in design, with a curving bottle shape, large flat cascabels with ratchets or sockets for the elevating mechanism. Rodman guns were true guns that did not have a howitzer-like powder chamber, as did many earlier columbiads. Rodman guns differed from all previous artillery because they were hollow cast, a new technology that Rodman developed that resulted in cast iron guns that were much stronger than their predecessors.

Guns had been traditionally cast solid and the bore was bored out of the solid metal. With this traditional method, the gun cooled from the outside inward. Castings shrink as they cool. As each succeeding layer cooled it contracted, pulling away from the still molten metal in the center, creating voids and tension cracks. Drilling out the bore removed the voids, but the tensions in the metal were still toward the outside. Rodman devised a method of hollow casting where the gun cooled from the inside out, so that as cooling occurred, it created compression rather than tension. This resulted in a much stronger gun.

With Rodman's method of casting, a cooling core was placed in the mold before casting. This core consisted of a watertight cast iron tube, closed at the lower end. A second, smaller tube, open at the bottom was inserted into the first. As the molten iron was poured into the mold, water was pumped through the smaller tube to the bottom of the larger tube. The water rose through the space between the two tubes and flowed out at the top. The water continued flowing as the metal cooled. To further ensure that the gun cooled from the inside out, a fire was built around the iron flask containing the gun mold, keeping the gun mold nearly red-hot. For an 8-inch Rodman columbiad, the core was removed 25 hours after casting and the flow of water continued through the space left by the core for another 40 hours. Over 50,000 gallons of water was used in the process. For larger guns, the cooling periods were longer and more water was used.

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