From "Rodman's Great Guns" (1962)

In 1844, Lieutenant Thomas J. Rodman, a young Ordnance officer only three years out of the military academy, began a long series of experiments aimed at overcoming the principal difficulty in casting extremely large iron cannon, a difficulty that set a maximum size limit for iron artillery pieces. At that time cannon, cast around solid cores, could be cooled only from the outside. His experiments resulted in the development of what is now known as the Rodman Gun.

History

The Rodman series of gun was developed by then- Lieutenant Thomas J. Rodman, who designed them to be used for seacoast fortification. There were five different sizes of bore that were developed for use, measuring 8, 10, 13, 15, and 20 inches. Rodman's design allowed the gun and cast to cool from the inside out, allowing the gun to maintain its structural integrity. Guns up to this point were cooled from the outside in, resulting in tension that would compromise the gun as it was fired repeatedly.

When tested against other weapons, the Rodman Gun was fired 1,500 times. The similar caliber gun that used the older method of cooling was fired 299 times before it was rendered useless. These results continued to show themselves when using inferior materials. Rodman's gun was able to sustain 250 shots, while the counterpart was rendered useless after only 19 rounds. With the success of these tests, the War Department authorized Rodman to cast larger guns with a similar design in 1860.

The 15-inch Rodman Gun was 15.83 feet long, with a bore length of 13.8 feet. It had a bottle-shaped appearance, which made the diameter at its widest point about four feet. It was designed to fire two types of ammunition: Solid shot (which was 450 pounds.) and exploding rounds (330 pounds, with a 17-pound explosive charge).

Almost 2,000 Rodman Guns were produced between 1861 and 1871. Nearly 200 remain today, including three (two 10-inch guns, and one 15-inch gun) that flank General Rodman's grave on Rock Island Arsenal.

Rodman Gun Facts

Produced: 1861-65 (8-inch); 1862-67 (10-inch); 1864 (13-inch); 1861-71 (15-inch); 1864-69 (20-inch)
Number: 213 (8-inch); 1,301 (8-inch), 1 (13-inch); 323 (15-inch), 2 (20-inch)
Surviving: 182
Purpose: Coastal Defense