Rodman's Great Guns

Rodman's Great Guns is referenced from the web site http://www.einpgh.org/exhibit/neighborhoods/strip/strip_n41.html

From Ordnance, July-August 1962. Written by Donald B. Webster, Jr.

Due to the skill of a young Ordnance officer, a new type cannon was developed which was more effective than any constructed until then.

The five years of the Civil War are quite rightly considered a period of ordnance and artillery experimentation, development, and transition. The work of one man led, in fact, to the casting of one of the biggest guns ever built, even to the present day--a monstrous 20-inch muzzle-loader that fired a 1,000-pound solid shot.

In 1844 Lieut. Thomas Jackson 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 actually set a maximum size limit for iron artillery pieces. At that time cannon, cast around solid cores, could be cooled only from the outside.
This practice caused the cooling metal to contract toward the outer surface of a cannon barrel and in large castings created internal strains and structural irregularities in the metal, as well as "pipes" or "blowholes"-- actual cavities within the casting. In short, large guns all too often had a habit of cracking in cooling, breaking in transport, or finally bursting when fired.

Over a period of years, Rodman devised a theory to account for both internal strains and imperfections and for variations in the density, hardness, and tensile strength of the metal in cast-iron cannon. He outlined a plan to cast cannon around hollow cores, to be cooled from the inside, rather than externally, by a stream of running water.

This, Rodman felt, would cause the cooling metal to contract toward the bore and increase the density of the metal where it was most needed. The bore, of course, would later be reamed out and polished, eliminating any surface imperfections. The rate of cooling could be controlled by regulating the temperature and rate of flow of the water.

By following his procedures, Rodman claimed he could cast cannon of any practical size. Working at Knapp, Rudd & Company's Fort Pitt Cannon Foundry at Pittsburgh, casters of cannon for the government since 1803 and probably the largest foundry in the world, Rodman began a series of experiments and trials which lasted nearly ten years. Experimental cannon were carefully cast in pairs, one on the old solid core, the other around variations of Rodman's hollow core.

Of one pair, the gun cast by Rodman's principle was fired 1,500 times; its counterpart, cast on a solid core and cooled externally, burst on the 299th shot. In another test of guns purposely made of poor material, Rodman's internally cooled gun fired 250 times and held together; the other piece burst on the 19th round.

Completely satisfied by Rodman's results, in 1860 the War Department authorized the casting of a 15-inch smoothbore Columbiad, even at that time a gun bigger than anything the world had ever seen. The first 15-inch gun, "The Lincoln Gun", made under Rodman's personal supervision at the Fort Pitt Foundry, was sent to Fortress Monroe, Va., where it was tested in March 1861 and became a model for the many Rodman guns which followed. The new gun proved a great success, although its huge size and weight, 49,000 pounds for the barrel alone, made it practical only for fixed positions in forts or permanent batteries.

Specifications were impressive. The 15-inch Rodman gun was 15 feet, 10 inches long, with a bore length of 13 feet, 9 inches, or 11 times caliber, a good deal shorter than the general rule. Most black-powder artillery, other than howitzers and mortars, had a bore length of fifteen to twenty times caliber. With an odd bottle-shaped appearance, and the absence of reinforcing rings, something new to artillery, the gun had a maximum outside diameter of four feet.
Two types of ammunition were provided--a 450-pound solid shot, and a 330-pound explosive shell carrying a 17-pound bursting charge.

Perhaps even more important than his casting procedure was Rodman's development of progressive-burning powder. When any gun fires, of course, the volume of the bore behind the projectile increases as the projectile travels toward the muzzle. The normal black powder grain, however, irregular in shape, burns from the outside, so that its burning surface area continually decreases. Thus, in a normal black-powder piece, initial breech pressure is the highest obtained; the forward traveling projectile increases bore volume as the powder burns at a decreasing rate. Both occurrences reduce interior bore pressure.

Rodman proposed powder pressed into hexagonal grains perforated with several longitudinal holes so that as individual grains burned both inside and out, albeit almost instantaneously, the burning surface of each grain actually would increase. Rodman's powder didn't increase pressures--it simply maintained a higher bore pressure than normal powder could, as the projectile traveled forward. The result, logically, was an increased muzzle velocity of the projectile.
With charges of his hexagonal powder, Rodman's 15-inch gun, with its abnormally low bore, length-diameter ratio, fired its 330-pound shell at a muzzle velocity of 1,735 feet a second, much faster than the velocity achieved with any other gun, including many with bore length-diameter ratios as high as 20 to 1. With a 50-pound charge of hexagonal powder (two-fifths of the later standard 125-pound charge) the 15-inch gun at 25 degrees elevation had a maximum range of 4,680 yards.

Adopted as a standard heavy gun for coast artillery and in lighter versions for fortress, siege, and shipboard use, during the Civil War the Federal Government purchased 286 fifteen-inch, 1 thirteen-inch, 15 ten-inch, and 240 eight-inch Rodman guns from both the Fort Pitt Foundry and another established at West Point, N. Y.

Like the famed Gun Club of Jules Verne's "Journey from the Earth to the Moon and Around It," Rodman wanted an even bigger gun to test, and proposed building one as soon as the first 15-incher had been accepted. In his report of April 17, 1861, to the War Department, he expressed no doubt that a reliable gun of almost any size could be made with complete success.

He felt, or at least said, however, since he seems to have limited his ambitions rather reluctantly, that a 20-inch gun firing a half-ton shot would be quite big enough. Anything larger would require massive machinery for loading, and "it is not deemed probable that any naval structure, proof against that caliber, will soon if ever be built...."

Rodman's newest monster--one of the largest iron castings to say nothing of the largest gun ever attempted--was three years in the making. Expected to weigh over 100,000 pounds finished, the gun was much heavier than the 40-ton capacity of Knapp, Rudd's largest furnace. The foundry, however, had a total pouring capacity of 185 tons, and expected to cast the new gun from six furnaces at once. New plans had to be drawn, molds had to be made, new casting procedures were essential, and new finishing machinery had to be designed and built.

The great day finally came on February 11, 1864. With Major Rodman, then superintendent of Watertown Arsenal, Mass., supervising the operation, the huge gun was poured. Filled in sequence from different furnaces, the 4-piece mold took 160,000 pounds of molten iron. Cooling, by both running water and streams of air, took nearly a week, after which the gun was finished on a specially built lathe. The finished barrel weighed 116,497 pounds, and the muzzle of the gun was inscribed: "20 inch, No. 1, Fort Pitt, 116,497 lbs."

Destined for Fort Hamilton in New York harbor, the gun was placed on a double railway truck, also specially built, at the foundry to await shipment. As the Pittsburgh Gazettte reported on July 23, 1864, "Juveniles, aged from ten to fifteen years, were amusing themselves today in crawling into the bore on their hands and knees. A good sized family including ma and pa, could find shelter in the gun and it would be a capital place to hide in case of a bombardment....''
Rodman supervised the building of a special carriage for the 20-inch gun at Watertown Arsenal, for the cannon was far too big for any standard mount. The finished product, an iron front pintle barbette carriage weighing 36,000 pounds was shipped off to New York and assembled at Fort Hamilton.

The 20-inch gun was a sizable piece of artillery. Total length was 20 feet, 3 inches, with the bore length 17 feet, 6 inches; thus the bore length-diameter ratio of 10.5 was even lower than that for the 15-inch Rodman gun. Both the shot and the shell for the 20-incher were more than twice the weight of the same projectiles for the 15-inch model, the solid shot weighing 1,080 pounds, slightly over half a ton, and the explosive shell 725 pounds empty of the bursting charge.
The first test, not for range but simply to see if and how the gun would shoot, was held on October 25th, almost as soon as the gun was mounted. A huge crowd turned out, including Rodman, of course, and even Secretary of War Edwin Stanton. A 100-pound blank charge loaded for the first shot wouldn't fire with a standard friction primer, and at first it appeared that the gun had a blocked vent. After the charge was pulled, a man was sent down the bore, which Harper's Weekly reports he did very easily, to check for obstructions from the inside.
The trouble was finally found. The 20-inch gun, over 5 feet in diameter, had a small vent hole almost 23 inches long, and a standard friction primer simply hadn't the power to carry its flame that far to the charge. When the vent was filled up with fine powder before the primer was inserted, the blank charge fired perfectly.
The next shot was fired with a 50-pound powder charge and the 1,080-pound solid shot, at zero elevation. The Scientific American's on-the-spot correspondent wrote that "the shot struck the water throwing up showers of spray as large as a ship." The third and final shot of the day used 100 pounds of powder behind a solid shot, with the gun at an elevation of 25 degrees.

"At the report the ponderous globe rushed up through the air with a hoarse roar, and sweeping its long ellipse, fell a great distance, estimation 3 1/2 miles, away into the sea...." The shot's clearly visible flight was timed at 24 seconds.
The tests were continued on October 27th, again with a huge crowd present. Only two shots were fired, both with round shot and the gun at zero elevation. On the first shot, with a 100-pound [powder] charge, the ball hit about [...] yards away and ricocheted eight times on the water. Recoil drove the gun and carriage back 6 feet, 10 inches, on the base. The second shot, with a 125-pound charge, drove the gun back 7 feet, 5 inches, but the ball, hitting rough water, skipped only five times.

While the Ordnance Department announced that another test would be held as soon as a hulk or ship could be found for a target, the gun was never fired again during the Civil War. The huge cannon was simply included with a battery of 15-inch guns as a part of the permanent defenses of New York. Another test, held in March 1867, included four shots fired with 125-, 150-, 175- and 200-pound charges, all with the gun at an elevation of 25 degrees. The maximum range attained was 8,000 yards, or a little under 5 miles.

A second and slightly lighter 20-inch gun also may have been cast for the Navy in February 1864, and another was later cast in 1866. For obvious reasons, however, the guns were never much more than experimental pieces.
Rodman's heaviest cannons were fantastic weapons for their time, but from a practical point of view their usefulness was extremely limited.

Aiming time depended on the extent of adjustment, but it took an additional 2 minutes and 20 seconds to traverse the gun and barbette carriage 90 degrees. The 20-inch gun certainly would have required twice the loading and aiming time of the 15-incher.

Hitting a fast-moving ship at any reasonable range with the one shot that could be gotten off in time would have depended largely on luck.

Rodman's guns proved his theories, and particularly the advantages of progressively burning powder, but the 20-inch gun was still too big to be a really effective weapon.

The guns still exist. Old "No. 1" still sits at Fort Hamilton, now a public park, mounted on a concrete base, and another looks out over New York Harbor from Sandy Hook.

While both the British and Germans also experimented with progressive-burning powder charges by varying powder-grain sizes, no heavy muzzle loading black-powder artillery ever again approached the muzzle velocities that Rodman achieved with his 15-inch gun in 1861.