The case is then dried and charged by placing on a “spindle,” which is a strong gun-metal base with a nipple fitting into the vent of the rocket and having a tapering spindle which fits tightly in the choke and projects up into the bore of the rocket. The composition is poured in in small quantities measured in a scoop, each scoopful being consolidated by blows with a wooden mallet or a wooden “drift” hollowed to take the spindle. Before the first scoop of composition is introduced, the rocket is “set down,” that is, several blows are given on the drift to consolidate the paper at the choke and give it accurate shape. Next, a scoop of ground dry clay is poured in and charged firm as a protection to the paper of the choke. The charging is then proceeded with as detailed above. Varying drifts are used in order that the hole may approximately correspond with the diameter of the tapering spindle as the composition rises in the case.
A short portion of the case above the spindle is charged solid; this is referred to as the “heading,” and is usually about one and a half times the bore in depth.
Large rockets are charged in a mould which fits tightly round the outside of the case and prevents the case being split under the pressure of the blows whilst being charged.
Rocket Manufacture, from Frézier’s “Feu d’Artifice,” 1747.
In early times these moulds were used for all sizes and were of cast metal, and it is from them that the classification of the sizes is derived. Rockets are designated by the weight of a ball of lead which fits the bore of the corresponding mould. Thus we have rockets varying in size from ½ oz. to 6 lbs. and over, war rockets being made up to 9 and 24 lbs., but their use is now almost extinct.
This classification, although it serves its purpose well enough, is somewhat misleading, as the thickness of the case varies in practice, at any rate under modern conditions. In the seventeenth and eighteenth centuries pyrotechnists seem to have had a standard proportion between the case and bore, i.e., the thickness of the case was one-quarter the internal diameter of the rocket.
In modern commercial practice a rocket—say for example of 1 lb.—is a rocket rolled on a former whose diameter is that of the bore of a 1 lb. rocket of standard thickness, but whose outside diameter is governed by the strength of the paper employed in the case.
Several writers on pyrotechny, one Frézier writing in 1747 in particular, have endeavoured to supersede this classification of rockets by replacing it with a series of internal diameter measurements, so far without success. It is hard to supersede the traditions of centuries on a plea of mere rationalism.
Rocket compositions, although containing the same ingredients, namely, saltpetre, sulphur, and charcoal, have them in differing proportions. Broadly speaking, the larger the rocket the greater the proportion of charcoal and sulphur, the variations in proportion being considerable, from the half-ounce rocket mixing of 13 saltpetre, 2 sulphur, and 5 charcoal to the 9 lb. and 24 lb. war rocket, with 13 saltpetre, 3 sulphur, and 4 charcoal approximately, and even higher proportions of the second and third ingredients for special purposes. A larger proportion of charcoal gives a larger tail—a desirable feature in display and signal rockets. Some compositions have a proportion of mealed gunpowder to produce fiercer burning.