15-inch Rodman Prototype: The Result of All Those Experiments

Having discussed the early casting experiments by Thomas J. Rodman to refine his casting technique and subsequent gunpowder tests, it is time I turned to the prototypes of the Rodman Guns.  Instead of constraining the next tests to just 10-inch weapons, the first prototype of the improved pattern increased to 15-inch.  The result was this gun – with registry number 1 – the prototype of a long line of Rodman guns.

The Lincoln Gun - Fort Pitt Foundry #1

Concurrent with the gunpowder experiments, Rodman “ran the numbers” to determine the optimum thickness of metal for a 15-inch gun.  I’m reluctant to walk through the reasoning here, due to space and my “physics for poets” background.  The short version, for those of us willing to neglect the particulars of square roots and other higher forms of math, is the gun needed a thickness of around 16 inches around the seat of the charge.  The thickness increased to 25 inches from the bore bottom to the exterior breech face.

Rodman also took the time to determine, based on his complied data, the best profile for the gun’s chamber (or bore bottom as he called it).  The preceding experiments indicated the traditional flat-bottomed bore tended to crack at the corners.  In the course of experiments Rodman found the sub-caliber chambers of the old Columbiads detrimental to performance.  He wrote, “There should be no angles, either salient or re-entrant, in the termination of the bore, but he surfaces of the bore and of its termination should be tangent along their lines of junction…. the semi-ellipsoid is believed to be the best and true termination.”  However, the casting plan diagram showed that of a hemisphere.

Casting Diagram and Plan of 15-inch Prototype

The diagram indicates the use of the Rodman neck vice the traditional cascabels.  The thin lines around the finished gun’s form indicate Rodman planned for a large amount of excess metal.  On surface examination, this resembles the casting technique developed by John Dahlgren for his naval guns.

Another point of similarity, which Dahlgren would later cite, is the exterior form.  Yes, the exteriors of Rodman and Dahlgren guns feature blended curves, lacking external fixtures.  But Dahlgren’s pre-war shell guns show the use of a pure cylinder over the chamber, traditionally the location of the first reinforce on the gun.  Rodman guns, as seen on the prototype plan, featured a near continuous curve expanding from the breech face to a maximum thickness over the seat of the charge, thence gradually tapering down to the muzzle.

The exterior form matched that called the “ordnance shape” and offered few right angles where stress would accumulate.  Rodman guns offered no “flats” save that of the muzzle face, rimbases, and trunnions.  If this form borrowed from Dahlgren, the paper trail has yet to be established.  On the other hand, simple examination of Army ordnance from 1841 through 1861, considering the columbiad trials and even the shape of the 1857 “Napoleon,” demonstrate the evolution to such blended curves.

In the fall of 1859, Rodman tested and selected the best iron from supplies available at Fort Pitt Foundry.   The foundry lit furnaces on December 23 to start the casting process.  As with previous hollow core castings, after pouring the molten iron, the foundrymen  poured water into the insert.  According to Rodman’s notes, water entered the insert at 36° and exited at 58°.  Rather low temperatures considering the last 10-inch casting initially had water entering at 80° and exiting at 102°, but in the warm month of August.  Notice the difference between entry and exit temperatures remained 22°, summer or winter.

After twenty-one hours, the water temperature exiting the insert dropped to 47°, and the workers removed the insert.  As with the earlier castings, water then poured directly into the interior bore.  Exit temperatures jumped to 86°.  Cooling continued for over 140 more hours.  All told the gun sat in the pit for 168 hours, far less than the time taken to cool a solid cast gun of half the size.

The finished gun weighed 49,099 pounds, with a 1200 pound preponderance at the breech.  It measured 190 inches long with a maximum diameter of 48.1 inches.  The gun measured 25 inches in diameter at the muzzle.  The 15 inch bore ran 156 inches deep.  Regardless of the metric, this was a large gun – indeed the largest produced up to that time in the United States.

In May 1860, the gun went to Fort Monroe for trials.  And how was a 25 ton cannon moved from Pittsburgh to Old Point Comfort?

For this purpose two strong trussed beams, 50 feet long, were prepared.  These beams were placed parallel to each other, and about 36 inches apart, their ends resting upon two bolsters placed transversely across the middle points of two 8-wheeled platform cars.  The gun was suspended under the two trussed beams, and between the cars; so that its weight was equally distributed over the 16 wheels of the two cars.

Thus packed, the gun moved on the Pennsylvania Central Railroad, Northern Central, and finally the Washington Branch.  In Washington, D.C. the gun, still on the cars rolled onto a heavy cargo vessel, which took the setup to Fort Monroe.

Once at Fort Monroe, Army personnel prepared the gun for proofing.  It was time to fire some large projectiles from one of the world’s largest guns.  I’ll discuss the testing and proofing of the 15-inch prototype next.

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Thomas J. Rodman’s report of the 1859 experiments appears in Reports of Experiments on the Properties of Metals for Cannon, and the Qualities of Cannon Powder; with an Account of the Fabrication and Trial of a 15-inch Gun (Boston: Charles H. Crosby, 1861), pages 192-274, 281-282.

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