Last month I focused on the early test of Thomas J. Rodman’s casting technique. But Rodman didn’t just offer a new casting technique. As we have seen looking at his notes and experiments, he sought to refine almost every aspect of the gun to produce the ultimate weapon of its class. Concurrently with testing the 10-inch columbiads, he also experimented with the behavior of the gunpowder.
Following the last tests with 10-inch columbiads in 1858, Rodman continued experiments with gunpowder and iron under stress. In mid-1859 Fort Pitt received a contract for one 42-pdr seacoast gun for experimental purposes (and this would be the foundry’s last of this type). Along with this 7-inch caliber gun, Rodman acquired two Dahlgren pattern guns – one 9-inch and one 11-inch – modified with Army style chambers. Standard Dahlgren guns used a “gomer” chamber resembling a thimble in profile, while the Army’s guns of the time had flat bottomed bores.
For the tests Rodman drilled a series of 0.4-inch holes from the bottom of the bore forward towards the muzzle at 14 inch intervals. These holes allowed measurement of pressure, at those intervals, during firing. Rodman used a device in which a pin, under pressure from the exploding powder, would press against a soft metal sample. By examining the indention of that metal, observers could determine the pressure exerted. Using a modified version of Captain Navez’s device for measuring velocity, Rodman could determine the type of powder giving the best performance. The tests used a cylindrical projectile weighing nearly double that of a service shot.
Rodman summarized the data obtained in his report:
Remarkably the tests demonstrated that during firing pressures rose and spiked in the case part of the barrel. Rodman attributed this behavior to the thinness of the metal at that point in the gun and the performance of the projectile. If I may put it in “modern” terms, the oblong projectile oscillated as it moved. In the chase portion of the bore, with less metal surrounding the bore, the gun could not counteract the vibrations and thus the pressures rose.
The tests also demonstrated the powder tended to burn out without imparting full effect against the projectile. Continuing with these tests, Rodman fired samples of powder with different sized grains in the 42-pdr. These tests used a standard service round with an 8 pound charge (thus making the tests more practical and applicable to field service!)
Notice the variation in velocity here. Although the smallest grain offered the highest velocity, the largest grain was only 80 feet-per-second slower. However, the larger grain powder created far less pressure. The behavior is due to progressive burning of the larger grains. The smaller grains are consumed faster, burning out before the projectile moved far down the bore. Larger grains took longer to burn, and the energy from that burning is imparted for a longer distance down the bore.
Rodman thus concluded that the same performance might be attained without increasing bore pressure simply by selecting the right size grain of powder. Based on these results, the Army adopted a new “mammoth” grain for use with the larger guns ranging between 0.7 and 0.9 inches in size. By comparison, the Army’s musket powder used 0.03 to 0.06 inch grains while standard cannon powder grains were between 0.25 and 0.35 inches.
Rodman’s tests also demonstrated how erroneous results from eprouvettes were. As indicated in the fifth column from the left on the table, the eprouvette test ranges of the powder dropped off significantly with grain size, even though actual performance tests demonstrated contrary results. So much for the old way of testing powder!
Rodman’s tests continued with observations from burning powder within a shell. These expanded the understanding of the behavior of the explosive force of gunpowder. Again, Rodman was not simply focused on better casting techniques, but better firing guns.
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 191-216.