 |
THE CIVIL WAR ARTILLERY PAGE |
|---|
|
|
THE CIVIL WAR ARTILLERY PAGE |
|---|
This article was written by Donald B. Webster and was first published in Ordnance in the issue of July-August, 1962. It has been edited and corrected by Wayne Stark, co-author of The Big Guns: Civil War Siege, Seacoast and Naval Cannon.
Due to the skill of a young ordnance officer, a new type cannon was developed that was more effective than any previously constructed. The years of the Civil War, and the five years preceding it, are quite rightly considered a period of ordnance and artillery experimentation, development, and transition. The work of one man led to the casting of some of the largest cannon ever built, even to the present day: monstrous 20-inch muzzleloaders that fired 1,080-pound solid shot.
In 1844 Lieut. Thomas Jackson Rodman, a young ordnance officer only three years out of West Point Military Academy, began a long series of experiments aimed at overcoming the principal difficulty in casting large iron cannon, a difficulty that effectively set a maximum size limit for iron cannon. At that time cannon were cast solid and were 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 cannon 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 developed and patented a process for casting cannon around hollow cores and cooled from the inside, via a jacketed stream of running water, rather than external cooling.
Rodman was confident his process would cause the cooling metal to contract toward the bore and increase the density of the metal where it was most needed. The bore 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 bore size, 6.4 inches or larger. Working at Knap, Rudd & Company's Fort Pitt Foundry at Pittsburgh, founded in 1804 and perhaps one of the largest foundries in the world, Rodman conducted a series of experiments and trials that lasted nearly ten years.
Experimental cannon were carefully cast in pairs, one on the old solid core system, the other around variations of Rodman's hollow core process. Of one pair, the gun cast on 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 1859 the War Department authorized the casting of a 15-inch smoothbore columbiad. This gun, cast in 1860 under Rodman's personal supervision at 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 that followed. The new gun proved a great success, although its huge size and weight, 49,099 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 is 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 cannon, other than howitzers, seacoast howitzers and mortars, had bore lengths fifteen or sixteen times caliber. With an odd bottle-shaped appearance and the absence of decorative rings, something new to cannon design, the gun had a maximum outside diameter of four feet. Two types of projectiles 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 gunpowder. When a cannon is fired, the volume of the bore behind the projectile increases as the projectile travels toward the muzzle. The normal black powder grain, being irregular in shape, burns from the outside so that its burning surface area continually decreases. Thus, in a normal black powder cannon, initial breech pressure is the highest obtained; the forward traveling projectile increases bore volume as the gunpowder burns at a decreasing rate. Both occurrences reduce interior bore pressure.
Rodman proposed gunpowder 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 did not increase pressures, it simply maintained a higher bore pressure than normal powder could maintain as the projectile traveled forward. The logical result was an increased muzzle velocity of the projectile. With charges of his hexagonal powder, Rodman's 15-inch gun, even with its relatively low bore length to diameter ratio, fired its 330-pound shell at a muzzle velocity of 1,735 feet per second, much faster than the velocity achieved with any other gun, including many with bore length to 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.
The Rodman gun was adopted as the standard heavy gun for coastal artillery and in lighter versions for fortress and siege use. During the Civil War the Federal Government purchased about 130 fifteen-inch, 445 ten-inch, and 213 eight-inch Rodman guns from Cyrus Alger & Co. (Boston MA), Fort Pitt Foundry (Pittsburgh PA), Seyfert, McManus & Co. (Reading PA) and West Point Foundry (Cold Spring NY). Many more were cast post-war. 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 he proposed building one as soon as the first 15-inch had been accepted. In his report to the War Department dated 17 April 1861, he expressed no doubt that a reliable cannon of almost any size could be made with complete success.
20-inch Rodman gun, Model of 1861 Rodman felt, or at least claimed (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 cannon) 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 Knap, 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 arrived on 11 February 1864. With Major Rodman, then superintendent of Watertown Arsenal MA, supervising the operation, the huge gun was cast. Filled in sequence from different furnaces, the four-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 muzzle of the gun was inscribed: "No.1 116,497lbs / FORT PITT PA 1864." Destined for Fort Hamilton in New York harbor, the gun was placed on a double-trucked railroad flatcar, also specially built, at the foundry to await shipment. As the Pittsburgh Gazette reported on 23 July 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. The finished product, an iron front pintle barbette carriage weighing 36,000 pounds, was shipped off to New York and assembled at Fort Hamilton.
