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Menu.From Ordnance, July-August 1962. 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 ordinance 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 Jefferson 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, 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 reenforcing 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 blackpowder 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 frontpintle
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 richocheted 8 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 5 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 fifteen-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.