Origins of the Rodman – Early Experiments in the Rodman Method, Part 2

In part one, I discussed two 8-inch Columbiads cast in 1849 and used to test the Rodman method of casting large guns.  The conclusion of that experiment indicated a slight improvement to the gun, but not great enough to prove the hollow-core, water-cooled method was greatly superior.  Gunfounder William Wade suggested more tests with better quality metal.

Lieutenant Thomas J. Rodman, working closely with Wade, waited until 1851 for the next full-scale tests.  Fort Pitt Foundry cast another pair of 8-inch Columbiads – one solid and one hollow-core – on July 30, 1851.   Then on August 21 of that year, Fort Pitt cast a similar pair of 10-inch Columbiads.  Wade selected what was considered high quality iron from Greenwood Furnace of New York.

Rodman revised the cooling technique after the first experiment.  For this 8-inch hollow-core columbiad, the gunmakers left the insert in the gun bore for the first twenty-five hours of cooling.  The insert (just as with the earlier experiment) consisted of a tube, closed at the bottom.  Inside that tube, a smaller diameter tube, open at the bottom, allowed water to flow down into the bore.  The water then ran off from the top of the casting mold into a trough.  All the while, a fire burned under the casting mold in the open pit to slow the overall cooling of the gun.

Diagram of Rodman Casting Arrangements

After that initial cooling phase, the foundrymen removed the core and let water flow directly into the now hardened gun bore.  Excess water continued to run out from the top of the casting mold into the trough.  This continued for forty hours.   About halfway through that period, the gunmakers extinguished the fire in the pit, allowing the gun to cool completely in the last twenty hours.  Wade indicated that for the 8-inch Columbiad 10,000 cubic feet (300 tons) of water circulated through the gun during the entire sixty-five hours of cooling.

As seen on the fracture diagrams supplied by Wade, the guns conformed to the Model 1844 pattern.

Fracture diagram of 8-inch solid cast Columbiad No. 3

Casting of the 10-inch hollow core Columbiad differed in both measures and technique.  Foundrymen left the core insert in the bore of the 10-inch Columbiad for ninety-four hours.  At that time extraction failed, as the bore had contracted around the core.  With the interior tube removed, water continued to flow at a reduced rate into the 10-inch Columbiad for an additional forty-eight hours.  Although Wade does not state such, presumably the core insert was drilled or wedged out after the final cooling cycle.

There was one other critical difference in the casting of the 10-inch hollow-core Columbiad.  Because the casting mold (both solid and hollow castings) for the 10-inch Columbiad were not large enough to allow what Wade considered sufficient clay, the foundrymen placed both 10-inch Columbiads in a pit filled with “green sand” to ensure the hot metal did not melt the casket of the casting mold.  This, as we shall see, changed the cooling rates and performance of the gun.

Fracture diagram of 10-inch solid cast Columbiad No. 5

With the guns cooled, all four went to the range.  The 8-inch Columbiads went through test firings between August 28 and October 2 of that year.  The 10-inch Columbiads followed in October 7 to October 18.  Just as with the 1849 tests, all four guns fired from a test mounting.  Gunners supplied proofing charges for the first two fires.  The 8-inchers fired a 12 pound charge for the first shot then a 15 pound charge for the second, after which the guns fired a 10 pound standard service charge.  Likewise the 10-inch Columbiads first fired a 20 pound charge, followed by a 24 pound, then settling on the 18 pound service charge.  The 8-inch Columbiads shots alternated between a 63 ½ pound solid shot or a 49 pound shell.  The 10-inch Columbiads fired 124 pound solid shot and 91 pound shells.

Wade reported a mixed bag of results:

  • 8-inch Columbiad, No. 3, cast solid – burst after 73 fires
  • 8-inch Columbiad, No. 4, cast hollow – survived 1500 fires
  • 10-inch Columbiad, No. 5, cast solid – burst after 20 fires
  • 10-inch Columbiad, No. 6, cast hollow – burst after 249 fires

The description of the bursting of 10-inch No.5 speaks to the power of black powder.  Wade wrote, “…the upper part, weighing 4400 pounds, was thrown upward, and fell in the rear, about 80 feet distant.  In its flight, it broke a limb of a tree, on a hill side, sixty feet above the level of the gun when fired.”

While the 8-inch No. 4 proved an excellent argument in favor of Rodman’s method, the 10-inch No. 6 dampened that success.  Upon examination of the fragments, Wade noted several cavities and fissures in the metal even describing some sections as “sponge-like”.  Wade attributed these flaws to the use of sand for exterior cooling, estimating that seven-tenths of the heat dissipated through the exterior as opposed to the water in the interior.  Most of these fissures appeared on the front part of the gun.  One large fissure was noted on the fracture diagram, along the chase of the Columbiad.

Fracture diagram of 10-inch hollow core Columbiad No.6

However, Wade insisted the fissures did not cause the gun’s failure.  As he noted, the crack in the gun started over the chamber and proceeded forward along the barrel, crossing over the fissure.  Instead, Wade attributed the failure of the gun to the brittle nature of the metal overall, a property caused by the rate of cooling of the gun.

Examination of the gun metal at the conclusion of these tests indicated, just as with the 1849 tests, no significant difference in the tensile strength of the metal of the different guns.  After assuring the paired castings took place under identical circumstances with the same batch of raw metal, Wade proclaimed, “The great difference of endurance must therefore be ascribed, to the different methods by which the castings were cooled; and to them alone.” The method, as Wade and Rodman would explain in detail, allowed the gun to contract upon itself while cooling, thus changing the way the metal reacted to the stress of firing.

In his report Wade made the conclusion that casting large caliber guns required even more fastidious selection of metal for gunmaking, calling for softer and weaker iron that underwent a managed cooling process.  He further added,

The method devised by Lieutenant Rodman, for accelerating the cooling of the interior of the guns, by passing a stream of water through them, and for retarding, at the same time, the cooling of the exterior, by surrounding it with heated air, appears to lead in the right direction, even if it does not fully accomplish the purpose designed.

Wade recommended further tests, with 32-pdr caliber guns in addition to the large caliber columbiads, with the aim to refine the process further.  (To my knowledge, the 32-pdr experiments were never conducted.)  CORRECTION: I overlooked the mention of tests in 1852.  Details in the next post in this series.

Rodman’s technique required even more refinement and some concurrent experiments with metal composition before it would bear fruit at the end of the 1850s.  As with many successful weapon systems throughout military history, the Rodman gun was not so much “revolutionary” but rather “evolutionary.”

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References:

William Wade.  “Report on the Manufacture and the Extreme Proof of two 8 inch Columbiads and of two 10 inch Columbiads….” dated January 24, 1852.  Reports of experiments on the strength and other properties of metals for Cannon. Philadelphia: Henry Carey Baird, 1856,  Pages 181-204.

Olmstead, Edwin, Wayne E. Stark, and Spencer C. Tucker. The Big Guns: Civil War Siege, Seacoast and Naval Cannon. Alexandria Bay, NY: Museum Restoration Service, 1997, Appendicies C115 and C142, pages 237 and 253.

Origins of the Rodman – Early Experiments in the Rodman Method, Part 1

I’ve mentioned the Rodman technique for manufacture of heavy cannons on a number of occasions.  That process, simplified to a thumbnail, involved casting the gun in a hollow mold then cooling the metal from the inside out using a flow of water through a pipe in what would become the bore.  This allowed the interior metal to cool faster than the exterior.  In the cooling process, the gun compressed itself around the bore, providing the highest strength at the very part of the gun most needed.

Before we get too far though, let’s set the record straight.  Thomas J. Rodman did not invent hollow casting.  In fact, many of the earliest cast cannon were hollow core.  Not until the early 1700s did solid casting become the dominant means of production.  By the American Revolution, gunmakers cast weapons as a complete block, then cut out the bore after cooling.   But as gun calibers and powder charges grew larger, some began to ponder other ways to construct stronger guns.

In addition to testing different metal composition, construction techniques, and exterior forms (recall the experimental 6-pdrs), gunmakers revisited hollow casting in the late 1840s.  In 1849, Fort Pitt Foundry received an order to produce two 8-inch Columbiads for tests – one cast solid and the other cast “… according to the plan invented by Lieutenant Rodman, on a hollow core, through which a stream of water passed while the metal was cooling.”

Civilian ordnance inspector (and later foundry owner), William Wade reported these two guns shared the same metal composition, furnace, and other production details – save of course the casting technique.  The hollow Columbiad cooled for forty hours with water flowing through an insert.  Then the gunmakers removed the insert and cooled for another twenty hours with water flowing through the empty, recently cooled, bore.  All told the process used 6000 cubic feet of water.

Both guns went to a test range (presumably near Pittsburgh).  The Columbiads first fired reduced charges for initial tests of both the guns and the support apparatus.  Satisfied the arrangements were set, the crews then started firing full service charges – 10 pounds of powder, one sabot, and either a 63 ½ pound solid shot or a 49 pound shell. The guns fired alternatively through the proof firings. The ordnance inspectors made meticulous observations after each firing, particularly noting the level of vent erosion.

The solid cast Columbiad burst on the eighty-fifth shot (firing a shell).  Firing the hollow core Columbiad continued until it burst on the 251st shot.  Both guns split through the reinforce to the breech, but at different planes of fracture.  Wade recorded the general shape of the breech fragment in his report:

Fort Pitt used the Model 1844 Pattern then in production, which featured base ring along with first and second reinforces.    Wade reported that metal samples were removed from sections of these fragments for further testing.  From those samples, observers concluded the hollow core Columbiad exhibited slightly higher density and tensile strength.  However this slight increase in strength was, in the words of Wade, “… not sufficient to account satisfactorily for the strongly marked difference in the endurance of the two guns.”

Concluding his report, Wade submitted:

No other cause for this unequaled endurance can be perceived, but that of the different methods by which the castings were cooled. The precautions taken, to ensure an equality of the material composing the two guns, and to preserve an exact uniformity in their respective proofs, were such, that the different endurance cannot be ascribed to inequities in either of these respects.  Neither of the guns, however, endured a sufficient number of fires to be satisfactory.

While promising, clearly the hollow-core, water-cooled process required more refinement.  Wade recommended another round of experiments, this time with 10-inch Columbiads cast from higher quality metal.  I’ll look at those tests next.

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References:

William Wade.  “Report on the Manufacture and the Extreme Proof of two 8 inch Columbiads….” dated October 26, 1849.  Reports of experiments on the strength and other properties of metals for Cannon. Philadelphia: Henry Carey Baird, 1856,  Page 169.

Olmstead, Edwin, Wayne E. Stark, and Spencer C. Tucker. The Big Guns: Civil War Siege, Seacoast and Naval Cannon. Alexandria Bay, NY: Museum Restoration Service, 1997, Appendix C115, page 237.

Balloons and Artillery at Dr. Gaines Farm

One of Civil War Trust’s latest preservation efforts is a large parcel of property, covering 285 acres, at Gaines Mill, Virginia.  The targeted ground is that over which General James Longstreet’s men attacked on June 27, 1862.

But like many locations in Virginia, the site was not a “one event” spot.  A month earlier, Federals occupied positions around Dr. Gaines farm.  At that time the balloonist made an appearance, as the Federals worked across Chickahominy  Creek at New Bridge.  An article on Civil War Trust’s site briefly discusses this skirmish with regard to the aeronaut activities.  Thaddeus Lowe established the “Balloon Corps” station on Gaines Farm in late May.  On May 23, batteries of the 2nd US Artillery, from the Army’s horse artillery, deployed in the same area.   Company A, 2nd US Artillery, under Captain John C. Tidball, shelled Confederate positions and received some direction from the balloonists.

In his official report on the action, Tidball wrote:

I have the honor to report that about 12 m. on the 23d instant my battery was ordered from its camp near New Bridge, over the Chickahominy River, for the purpose of shelling the ground occupied by the enemy in the vicinity of that bridge.

The pieces were placed in battery near the mansion of Dr. Gaines, and from there opened a steady and well-directed fire on the point indicated. The enemy made no reply, but, from the report of those in the balloon, fled from their position. After firing 93 rounds the battery was withdrawn, and a few minutes afterward started on its march toward Mechanicsville. A few rods after the head of the column, of which the left section of my battery constituted an advanced portion, had passed the bridge over Bell’s Creek, several cannon-shots were fired by the enemy from pieces on the eminence immediately in our front…. (OR Series I, Volume II, Part I, Serial 12, page 656).

Sort of a one-sided action, almost a routine action in many regards.  But Tidball’s mention of the balloon is worth analysis.

There are many mentions of “indirect” fire employed during the Civil War, or more specifically – “spotted” fire.  Contrary to our American pride, this style of fire control was not developed during our mid-19th century conflict, but rather dates back practically to the concept of siege weapons (in other words ancient stuff).  However the use of balloons, signal flags, telegraph, and other technologies came into play during the Civil War.

The question does arise why the combatants did not make more use of spotted fires.  Well there were some problems with technology.  The oft cited issue involved the lag in communications.  However one can imagine Lowe, a few hundred feet above Tidball’s guns, shouting down, “… a little to the left…” or “… give it one more turn of elevation….”  But at best Lowe could only offer a description of where the shot fell.  He could not aid the direction of the next shot.

You see, the real problem was not with communications, but with the guns themselves.  In order to “spot” a round, the observer and gunners must have some reasonable way to have the projectile fall in a predictable manner – in other words, consistent shot pattern.  Civil War artillery lacked recoil dampening or compensating systems.  The projectiles used rudimentary time fuses which were prone to failure.  The projectiles themselves, both rifled and smoothbore, were apt to take erratic ballistic courses.  And none of this took into account the effects of wind, weather and temperature.  Putting two successive rounds in the same spot required luck in addition to the skill of the gun crew.

Predicted fire would have to wait a few more decades until recoil systems, better projectiles and guns, advanced firing tables, and proper weather reports would enable gunners to place rounds on a target with some degree of regularity.  At that point, “spotting” was not only possible, but required!