The Rodman gun, developed in the mid-19th century, was the technological apex of smoothbore, muzzle-loading artillery. Cannons using chemical explosives to propel a projectile had made their first appearance on the battlefield in the 14th century, and for the next 500 years the technology changed very little. Almost all artillery pieces, cast of either iron or bronze, were smoothbore, muzzle-loaded, and used propellant charges based on black powder. Then, near the end of the 19th century, artillery technology made a radical leap forward with the introduction of rifled steel barrels, breech-loading systems, recoil mechanisms, and improvements in propellants. M During the 40 years between 1830 and 1870 the older types of artillery also underwent drastic scientific improvements, culminating in the Rodman gun. Although in the long run the gun was a technological dead end, the advances introduced by its designer, Brigadier General Thomas Jackson Rodman, revolutionized gun-barrel production and propellant design. M Rodman graduated from West Point in 1841, seventh in a class of 52. A career Ordnance Corps officer, his first assignment was to the U. S. Army Allegheny Arsenal in Pittsburgh. In 1844 he started working on the problems of cannon-barrel design after a 12-inch naval gun exploded on the USS Princeton, killing Secretary of State Abel P. Upshur and Secretary of the Navy Thomas Gilmer and almost killing President John Tyler. By the early 19th century most field artillery tubes were made from bronze,a material lighter than iron and less prone to casting defects. Bronze,however, was not strong enough for the chamber pressures generated by the larger-caliber fortress, siege,and naval guns, like the one that exploded on the Princeton. Those larger guns still had to be made from iron.
At the time, most iron gun tubes were cast solid and then bored out.The alternate process was to cast the barrel hollow around a sand core,and then smooth out the bore by machining. Either way, as the casting cooled and hardened from the outside in, the hot interior of the casting continued to contract, because it cooled more slowly. With the cooled external surface remaining rigid, shrinkage-induced cracks and cavities developed in the center of the tube that caused structural weaknesses. Theoretically, those flaws would have been removed when the tube was bored out—but they often weren’t. Rodman realized that even if the boring did remove the casting flaws, the barrel’s overall strength would still be compromised: its outer metal was in a state of compression (being squeezed inward) and its inner metal, around the bore, was in a state of tension (being pulled outward). A propellant charge exploding inside the chamber might at anytime increase the tension sufficiently to rupture the barrel—and it all too often did. The newer, rapidly burning black powders developed in recent years only increased that probability.
Rodman initially experimented with wrapping wire around the outside of the tube as a way to reinforce the compression.But maintaining constant and uniform tension on the wire proved difficult, so Rodman abandoned his wire-wrapping experiments when he hypothesized that gun barrels could be constructed using a variation on the principle employed by blacksmiths to shrink iron rims onto wooden wagon wheels.The iron-cooling process had to proceed from the inside out.
Rodman’s modified system of hollow casting replaced the sand core with an insulated iron pipe through which water circulated rapidly. As the water started the cooling process from the inside, hot coals packed around the casting mold kept the outside of the barrel hot: The coals were removed slowly as the internal cooling progressed. Rodman’s process caused each successive outer layer of metal to shrink upon the cooler inner layers. When the cooling finally finished, the entire barrel was in a uniform state of compression throughout its entire thickness, resulting in a far stronger tube capable of withstanding higher firing pressures. The entire process required some 65 hours for an 8-inch barrel to cool properly and used up to 50,000 gallons of water. Once the casting was completed, of course, the bore still required final machining to ensure exact diameter and smoothness.
Rodman’s gun barrels had a distinctive shape that made them instantly identifiable, even at a distance. The increased strength of the castings allowed him to design his tubes to correspond directly to the pressure curve generated by the propellant charge inside the barrel, as the projectile moved down the bore. Rodman invented his own bore pressure gauge so he could measure and plot that curve with precision. Following the curve exactly, Rodman’s guns assumed a streamlined bottle shape, tapering toward the bore. Rodman also replaced the standard spherical cascabel knob at the breech end of the barrel with a smooth disk, into which was cast a vertical line of ratchet grooves for an improved elevating mechanism.The new system greatly improved precision aiming for range.
Rodman had trouble convincing the U.S. Army’s Ordnance Department to adopt his casting process. The belief was widespread that circulating water through a molten casting was too risky, because the resulting steam from the water coming into contact with the heat would produce an explosion. Rodman traveled to Washington three times before he finally got permission from the chief of ordnance, General George Talcott,to patent the process and develop it privately. In 1845 Rodman entered into a partnership with the Pittsburgh foundry, Knap and Totten, which agreed to cover all the development and manufacturing costs in return for a half interest in the patent.
Rodman conducted the first test-firings in 1849, using a pairof 8-inch guns firing 64-pound spherical shot and 10 pounds of powder. One gun was cast conventionally, the other using Rodman’s system. The guns were identical in all other respects—size, weight, metal composition. The solid-cast gun burst on the 85th round. By the time the test was terminated after the 251st round, the hollow-cast barrel was still in firing order. During follow-up test firings in 1851, the conventional tube burst on the 73rd round, the hollow-cast gun survived1,500 rounds. By the time Rodman concluded his series of tests, six solid-cast guns fired a combined total of 772 rounds before all the tubes failed; six guns cast on Rodman’s principles fired a total of 5,515 rounds, and none failed.
The U.S. government finally approved Rodman’s casting process in 1859. He was then ordered to design and build a prototype for a 15-inch coastal defense gun. The resulting15-inch Rodman gun weighed 14,099 pounds, was 15 feet 10inches long, and had an external diameter of 48.1 inches at the barrel’s breech end. Using a 25-pound propellant charge,the gun fired both 330- and 450-pound solid spherical projectiles. The gun was test fired at Fortress Monroe, Virginia,in May 1860. The board of ordnance observed 49 test firings,using both projectile weights. On the first round it took the12-man gun crew 1 minute and 53 seconds to complete the firing cycle of sponging, loading, priming, and running into battery. (This was more than 35 years before the introduction of modern recoil systems.) By the sixth round the gun crew had the cycle down to 1 minute 3 seconds. The maximum range achieved during the tests was 5,730 yards—slightly more than 3.25 miles.
The board recommended acceptance of the design, and 15-inch Rodmans became standard as American coastal artillery pieces during the Civil War. Smaller 8- and 10-inch Rodmans were adopted as siege and naval guns. The 8-inch Rodman fired a 50-pound projectile to a range of 3,870 yards; and the10-inch Rodman fired a 128-pound projectile to a range of4,835 yards. All three calibers also fired explosive shells that were from 12 to 23 percent lighter than the equivalent size solid shot. Rodman produced two experimental 20-inch models,the largest guns ever cast in the United States, but they were only fired eight times in testing. During the course of the Civil War the government purchased 1,840 Rodmans. More significantly, all guns from then on, whether they externally resembled his guns or not, were cast using Rodman’s process,including the U.S. Navy’s Dahlgren 15-inch shell gun.
Rodman perfected his smoothbore guns just about the time rifled artillery appeared. Rifling increased both the range and the accuracy of guns. Rodman never opposed rifling, but he continued to believe that smoothbores still had a significant role to play. Testifying on February 6, 1864, before the Congressional Joint Committee on the Conduct of the War, he told the legislators that America’s coastal fortifications should be armed with a combination of rifled and smoothbore pieces. The rifled guns would engage approaching enemy ships at greater distances until the targets came within range of the smoothbores. When a member of the committee asked him directly if he thought that within their range arcs the smoothbores were superior to rifled guns, Rodman replied, “Yes, sir.
Rodman was on the wrong side of history on that point. But during the 1850s he had also conducted extensive experiments with artillery propellants that led to his greatest and most lasting contribution to ordnance engineering. Rodman was the first to understand that the physical configuration of the propellant, the exact shape of the powder granules, had to be purpose designed for the type of gun firing them. During his many test firings he had observed that the pressure generated inside the chamber was directly proportional to the rate at which the powder burned, and that the powder burned at a rate directly proportional to its external surface area.
All conventional powder at the time was compressed into small, solid granules of uniform size. Rodman at first thought that the size of the powder granules was the key to the problem. The powder’s greatest surface area, however, was at the moment of ignition. As the powder burned, the granule’s surface area decreased, the rate of burning slowed, and the generated pressure decreased with it. But as the projectile moved down the bore, the air volume in the chamber behind the round increased. That further compounded the drop in pressure behind the moving projectile. As the volume in the chamber increased, more rather than less pressure was needed. Rodman concluded that the optimum propellant “would be that which burnt so as to evolve its gas proportionally as the space increased behind the projectile while in the bore.”
Rodman’s solution was to press the powder into hexagonal-shaped cakes, which were then perforated with as many as seven longitudinal holes. With that configuration, the powder cakes burned simultaneously from the outside in and from the inside out. As the material around the holes burned, the holes got bigger. Thus, the powder’s exposed surface area increased, and with it the rate of burning. All of this happened in less than a second, of course, but it made a significant difference. The key point is that what Rodman called “prismatic powder” did not really increase the overall chamber pressure during firing. What it did was maintain pressure behind the projectile at a constant level as it moved down the bore. The result was greater muzzle-velocity without increasing the pressure on the tube.
During the Civil War Rodman commanded the Watertown Arsenal near Boston, which produced ammunition, artillery carriages, and other ordnance items. At the end of the war he was investigated by a U.S. congressional committee on charges of mismanagement of the arsenal and the clearly trumped-up charge of “disloyalty.” Among the elements of that latter charge was his failure to order the firing of a salute when Lee’s surrender was announced, and his alleged lack of appropriate sorrow at the death of Lincoln.
Cleared of all charges, Rodman was promoted to brevet brigadier general and assigned to establish and command what would become the Rock Island Arsenal in Illinois. On an island in the Mississippi River, the installation had been a camp for Confederate prisoners of war from 1863 to 1865.When Rodman assumed command, his mission was to convert the post into a modern ordnance manufacturing facility.He laid out the arsenal’s road system and designed and built10 large shop buildings, many of which remain in operation today, manufacturing gun mounts, recoil mechanisms, small arms, aircraft weapons subsystems, grenade launchers, and other ordnance components. Rodman was still in command of the arsenal when he died in 1871, at the age of 54. He was buried at the Rock Island Arsenal National Cemetery.
Ironically, relatively few of Rodman’s guns were actually fired in anger during the Civil War. Rodman’s process of casting iron gun tubes became obsolete when wrought-iron and forged barrels became more common at the end of the 1860sand then when steel was adopted as the main material for all artillery components. During the 1870s and 1880s various attempts were made to convert existing Rodman smoothbores to rifled guns of smaller calibers by inserting rifled, wrought-iron or steel liners into the bore. All those experiments failed,and smoothbore Rodmans remained in service well into the final years of the 19th century.
There are 182 known surviving Rodman guns, including several 8-inchers at Fort McHenry, which were installed during the Civil War when the fort was still active. Modern artillery propellants are chemically far more sophisticated than the simple black powder mixtures of the 19th century, but they still burn in accordance with the principles that Rodman discovered. His innovations in powder design continue to determine how artillery propellants are made to this day.
Major General David T. Zabecki (U.S. Army, ret.) is chief military historian of the Weider History Group and editor of the recently published encyclopedia Germany at War: 400 Years of Military History.
Originally published in the April 2015 issue of Military History Quarterly. To subscribe, click here.
