Aside from gradual improvement in the formula, no great change in
powder making came until 1860, when Gen. Thomas J. Rodman of the U. S.
Ordnance Department began to tailor the powder to the caliber of the
gun. The action of ordinary cannon powder was too sudden. The whole
charge was consumed before the projectile had fairly started on its
way, and the strain on the gun was terrific. Rodman compressed powder
into disks that fitted the bore of the gun. The disks were an inch or
two thick, and pierced with holes. With this arrangement, a minimum of
powder surface was exposed at the beginning of combustion, but as the
fire ate the holes larger (compare fig. 20f), the burning area
actually increased, producing a greater volume of gas as the
projectile moved forward. Rodman thus laid the foundation for the
"progressive burning" pellets of modern powders (fig. 20).
[Illustration: Figure 20--MODERN GANNON POWDER. A powder grain has the
characteristics of an explosive only when it is confined. Modern
_propellants_ are low explosives (that is, relatively slow burning),
but _projectiles_ may be loaded with high explosive, a--Flake,
b--Strip, c--Pellet, d--Single perforation, e--Standard,
7-perforation, f--Burning grain of 7-perforation type. Ideally, the
powder grain should burn progressively, with continuously increasing
surface, the grain being completely consumed by the time the
projectile leaves the bore, g--Walsh grain.]
For a number of reasons General Rodman did not take his "perforated
cake cartridge" beyond the experimental stage, and his "Mammoth"
powder, such a familiar item in the powder magazines of the latter
1800's, was a compromise. As a block of wood burns steadier and longer
than a quick-blazing pile of twigs, so the 3/4-inch grains of mammoth
powder gave a "softer" explosion, but one with more "push" and more
uniform pressure along the bore of the gun.
It was in the second year of the Civil War that Alfred Nobel started
the manufacture of nitroglycerin explosives in Europe. Smokeless
powders came into use, the explosive properties of picric acid were
discovered, and melanite, ballistite, and cordite appeared in the last
quarter of the century, so that by 1890 nitrocellulose and
nitroglycerin-base powders had generally replaced black powder as a
propellant.
Still, black powder had many important uses. Its sensitivity to flame,
high rate of combustion, and high temperature of explosion made it a
very suitable igniter or "booster," to insure the complete ignition of
the propellant. Further, it was the main element in such modern
projectile fuzes as the ring fuze of the U. S. Field Artillery, which
was long standard for bursts shorter than 25 seconds. This fuze was in
the nose of the shell and consisted essentially of a plunger, primer,
and rings grooved to hold a 9-inch train of compressed black powder.
To set the fuze, the fuze man merely turned a movable ring to the
proper time mark. Turning the zero mark toward the channel leading to
the shell's bursting charge shortened the burning distance of the
train, while turning zero away from the channel, of course, did the
opposite. When the projectile left the gun, the shock made the plunger
ignite the primer (compare fig. 42e) and fire the powder train, which
then burned for the set time before reaching the shell charge. It was
a technical improvement over the tubular sheet-iron fuze of the
Venetians, but the principle was about the same.
[Illustration: Figure 21--MODERN POWDER TRAIN FUZE.]