Origins of the Rodman – Early Experiments in the Rodman Method, Part 3

As seen in earlier posts, William Wade of the Fort Pitt Foundry conducted experiments in 1849 and 1851 to proof the system proposed by Lieutenant Thomas J. Rodman.  While providing promising results, the tests did not provide conclusive proof that hollow-core, water-cooled casting produced the quality guns needed by the Army (and the Navy).

Indeed after the 1851 experiments, Rodman felt the need to elaborate on the test results in correspondence directly to the Chief of Ordnance, Colonel H. K. Craig. In November 1851, Rodman provided a detailed explanation for the difference in performance between the 8-inch and 10-inch hollow-core Columbiads tested earlier that year.  After extensive examination of cooling and force dynamics, Rodman explained this difference of performance by the rate of cooling.  This of course due to the sand around the 10-inch gun.  Boldly, Rodman predicted, “I have the utmost confidence that with iron of the same quality as that used in these guns, and a proper application of the new mode, a 10 inch gun may be made to endure 1000 to 1250 fires.”1

Rodman tested his theories again in 1852 using two 32-pdr guns (I overlooked this test in my earlier post), one hollow and one solid cast (foundry numbers 160 and 161 respectively).  The solid cast gun fired 1000 rounds of standard service charge of 8 pounds of powder, then burst after six shots with extreme charge of 10 ⅔ pounds.  The hollow gun likewise fired 1000 rounds using the standard charge, then twenty with the extreme charge.  The gun finally burst on the 1021st fire with two solid shot and 16 pounds of powder.  These tests offered no additional conclusions.2

Compilation of Rodman's Tests 1849-1858

Not until August 1856 did Rodman again have permission to test the heavy Columbiads.  Using carefully selected iron, Rodman directed the casting another matched pair, one hollow and one solid, 10-inch Columbiads, foundry numbers 331 and 332 respectively.  Fracture diagrams made after testing show the pattern followed that of Model 1844.

Casting started on August 23.  The hollow core gun followed the same cooling technique as the 1851 casting, but with the central core removed after 17 hours of cooling.  Rodman noted the use of a separate wrought iron tube sunk into the gunhead to allow the flow of water out of the empty bore at that point, preventing water from touching the hot exterior.

All proceeded fine until the 38th hour of cooling.  The pit attendants, acting without instructions, piled hot coals around the casting.  This kept the casting at a constant temperature for the next twenty hours.  Upon noticing this issue, the foundrymen cleared the coals, but the gun required over a hundred more hours to cool.  This allowed the gun to anneal, lowering the tenacity of metal.  Hollow cast number 331 came out of its casket on September 1.  The  solid cast number 332 came out on September 5 (providing some comparison of cooling times).

The hollow cast gun ran into more problems.  In cooling it contracted significantly more than the solid cast gun.  Worse, when taken to the lathe, the workers found the bore to be off axis, requiring careful machining to correct.  Not a good start for the gun.

On the test range, both guns went through proofing with a first shot with 20 pounds of powder and a solid shot.  Second fire was a shell with 24 pounds of powder.  All subsequent fires used solid shot and 18 pounds of powder.  The solid cast gun burst on the the 26th fire.  The hollow cast gun lasted a bit longer to 315 fires.  This was not the endurance Rodman hoped for.3

Examination of the metal after bursting indicated the hollow cast gun exhibited slightly better density and tenacity over the solid cast.  Aside from detailed examinations of bore and vent enlargements, Rodman focused on the fracture behavior. Taking into account the strain and likely expansion of the metal, Rodman provided formulas that not only predicted the area of failure but explained in part why internal cooling (hollow core) worked better.4

Predicted Bursting Patterns

Rodman also focused on the performance of the powder in the firings.  Complaining of the selection, Rodman felt it burned too quickly, stressing the gun without any performance gain. With this in mind, Rodman also pursued tests to determine the best grain and size of powder for Columbiads.5  While a tangent, this would bear fruit later in the development of the large guns.

In his conclusions, Rodman placed blame on the failed hollow core gun on the improper cooling rate.  This was a “throw away” test in that regard.  But remarkably, Rodman appears to have taken advantage of this to gather data for the next round of tests.  Closing his report, he listed a full page of unknown factors, including iron quality, casting technique, chamber dimensions, exterior form, and powder charge. And Rodman set a course to resolve those unknowns through further experiments, which I will continue with shortly.  But let me offer up for now Rodman’s closing justifying further experiments:

And it is believed that the true interests of the country would be promoted, in a military point of view, by entering, at as early a period as practicable, upon a series of experiments which would supply positive knowledge in place of probability in some, and positive ignorance in many other points of the utmost importance to the national defense; for it is better that millions should be expended in time of peace, and from an overflowing treasury, than that a single gun should burst in action.6

———————————————

Notes:

  1. Thomas J. Rodman,  “Report on the Causes of Difference in the Endurance of Cannon, Cast Solid and Cast Hollow and Cooled from the Exterior and Interior” dated November 30, 1851,  Reports of experiments on the strength and other properties of metals for Cannon (Philadelphia: Henry Carey Baird, 1856), pages 209-13.
  2. Details from chart found in Thomas J. Rodman, 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), page 133.
  3. Ibid, pages 4-12.
  4. Ibid, pages 35-48.
  5. Ibid, page 15.
  6. Ibid, page 55.

Published by Craig Swain

"Historical marker hunter" and Civil War enthusiast.

2 thoughts on “Origins of the Rodman – Early Experiments in the Rodman Method, Part 3

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