Thus we have rockets varying in size from 1/2 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.
