Well, the multicharge gun was 25 feet long (50 calibers). It therefore was quite heavy (25 tons), whereas the Krupp gun weighed only 3.8. The Ordinance Board was of the opinion that the multicharge gun's performance should be compared, not to guns of equal caliber, but to those of equal weight. While the 10-inch gun, weighing 18 tons, had a lower muzzle velocity (1400 psi), its 400 pound projectile carried greater kinetic energy and, in the Board's opinion, was likely to have more penetrating power. (Walker; Haskell).
Propellants (Gunpowder, etc.)
Gunpowder is a mixture of saltpeter, charcoal and sulfur. The saltpeter (potassium nitrate) is an oxidant, and it burns the charcoal (carbon), forming carbon dioxide gas. The sulfur combines with the potassium ion of the saltpeter, forming potassium sulfide, and in the process generates a lot of heat. Since the heated gas is confined by the gun, that results in an increase in pressure. And that's what pushes the projectile out. It also stresses the barrel, so you can't use too much powder and how much can be used depends on its burn rate.
Among the down-timers, there's no consensus as to the proper formula for gunpowder (black powder). Just one master gunner, Peter Whitehorne (1560), presented 20 different recipes, with saltpeter content of 16–84 %, charcoal of 8-64 %, and sulfur of 8-28 %. (Walton 123). EB11/Gunpowder says that the following formula was used in Britain in 1647: 66.6 % saltpeter, 16.6 % charcoal, 16.6 % sulfur. By 1781, the proportions were 75-15-10, the ones given in H. Beam Piper's Lord Kalvan of Otherwhen. Other formulae of possible interest included 52.2-26.1-21.7 (Germany 1596), 68.3-23.2–8.5 (Denmark 1608). 75.6-13.6-10.8 (France 1650), and 73-17-10 (Sweden 1697). Even in the nineteenth century, different countries had different preferences, with saltpeter 70–80 %, charcoal 11–18.5 %, and sulfur 9.5-13 %. (Beauchant 149). The proportions given in the modern EB (2002CD) is 75-14-11. The Medieval Gunpowder Research Group, using a replica of the Loshult Gun, found that muzzle velocity peaked at a saltpeter content of about 72 %. (MGRG2).
The burn rate is proportional to the burning surface. It thus is dependent in part on initial particle ("grain") size; the smaller the particles, the greater the total surface area for a given weight of powder, and the faster the "deflagration" reaction. However, the reaction shouldn't be too fast; you want it to continue until the projectile reaches the muzzle. Thus, the grain size must be matched to the barrel length; muskets used finer powder than did "great guns." The term "powder" became a bit misleading; the "grains" can be several inches in diameter-please look at Fig. 1 in EB11/Gunpowder.
The quality of gunpowder has improved over time. In 1587, gunners used "serpentine," which was floury. Because of the small particle size, it was necessary to leave part of the powder chamber empty, to provide oxygen. The powder also absorbed moisture readily. The charge for a culverin was equal to the shot weight, and for a cannon, half that weight.
By 1625, "corned" powder, which was granulated, was common. (Lavery 135). The size of the grains could be controlled by sieving. In 1673, a culverin used a two-thirds shot weight charge, and a cannon, one-half.
Improvements were also made in the preparation of the components of gunpowder. By 1740, the charges ranged from 40 % for a 42-pounder to 66 % for a 9-pounder. (Id.) In 1783, "cylinder powder" was introduced, although it didn't come into common use until 1803 (Rodger 421). It incorporated a better grade of charcoal. The wood was placed in cast iron cylinders, and heated over a stove, rather than charred in a kiln. (Id.; Douglas 201). This permitted reducing the standard charge to one-third the weight of the ball for ordinary guns, and a mere 8 % for carronades (Lavery). The method is described by EB11/Gunpowder but without discussion of its advantages over former practice.
You could use less if you were trying to conserve powder, or were hoping to produce more splinters if the shot didn't hole the target. A one-sixth charge is sufficient to "drive a ball from any large gun through the side of a ship at 1100 yards" but for a 24-pounder would require twice the elevation as a one-third charge, thus reducing accuracy. (Douglas 54).
In the mid-nineteenth century, the increase in gun size led to incompatibility with the ordinary black powder; it burned too quickly, creating conditions that strained the gun. A slower-burning "brown powder," described in EB11/Gunpowder, was introduced.
The shape of the grains is also significant. Normally, as deflagration continues, the particles are consumed inward, reducing the total burning surface and thus reducing the burn (regressive burn). This is experienced with all solid grains, whether they be spheres, cylinders or plates.
In 1860, what EB11 calls "shaped powders" were introduced. The grains had one or more perforations so they were consumed both inward and outward, resulting in a constant (neutral burn) or even increasing (progressive burn) burn rate. EB11 describes how they were made.
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In the late-nineteenth century, gunpowder was largely replaced by nitrocellulose-based propellants (the so-called "smokeless powders"). These produced less smoke and flash, burned progressively, and caused less erosion to the barrels. They are classified as being single-base (nitrocellulose) or double-base (nitrocellulose combined with nitroglycerin or some other liquid organic nitrate).
Ballistite (1887) was 40 % nitrocellulose and 60 % nitroglycerin (EB2002CD/explosive). Cordite was similar; 37 % nitrocellulose, 58 % nitroglycerin, 5 % Vaseline (Rinker 34) or later 65-30-5 (EB11/Cordite). EB11 doesn't say anything about stabilizers, but EB2002CD suggests diphenylamine.
A member of a Civil War reenactment group would probably be familiar with Pyrodex, which was developed in the 1970s. It's essentially black powder with various additives so it burns more cleanly-less fouling of the bore, less smoke. However, the formula of Pyrodex is proprietary, and the person who developed it (Powlak) lost his life in the process.
The decomposition products of black powder are 43 % gaseous and 57 % solid, the latter being responsible for the smoke of the proverbial "smoking gun." In contrast, modern smokeless powder is more than 99 % gaseous. Gases can be accelerated to higher velocities than solids, for a given internal pressure. Consequently, black powder has a low "specific impulse" (pounds thrust produced per pound propellant burned per second)-~50–70 seconds-whereas double base powders provide ~180–210 seconds. (Guilmartin 300).
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Average muzzle velocities increased over the nineteenth century, from 1575 fps for ordinary black powder, to 2133 fps for prismatic powder and 2225 fps for early (1885) smokeless powder. (Breyer 38).
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With black powder, the principal manufacturing considerations were "strength", freedom from fouling, and proneness to deterioration. These were affected by composition, density, moisture content, and grain size, shape, hardness and "glazing."
Until 1868, powder density was measured by "cubing"-weighing it in a box of standardized volume. This was improved upon by the mercury densimeter. (Farrow 313).
There was no quantitative test for hardness; the grain would be broken between finger and thumb. (Smith).
Powder strength will vary from manufacturer to manufacturer, from lot to lot, and even from barrel to barrel. (Dahlgren 180). Even at the end of the black powder era, powder manufacture was an art, not a science. In 1881, 150,000 pounds of Westphalian Company prismatic powder was rejected because it didn't meet the standards; the representative blamed it on manufacturing "during very cold weather." (Buchanan 325).