The French official experiments in the manufacture of rockets were carried on with great care, and the results of the work have been preserved in records of the ordnance department, from which the following specific accounts were gathered concerning all essential details.
The mold serves to sustain the case in position firmly while it is being loaded. Its exterior is so fashioned usually as to give it the appearance of a miniature cannon. It is hollow throughout its length, and this cavity, into which the case fits, should be perfectly round and very smooth. The mold is commonly made of hard wood. (PL II, fig. 1.)
The height of the mold should diminish in proportion as the interior diameter of the mold increases. The reason for this diminution is that, since the force of the burning material in a rocket does not increase in the same ratio as the diameter of the rocket, the gases developed by combustion would be unable to raise a rocket of large size, with its length proportioned to the diameter in the ratio employed for small rockets.
The mold is supported by a cylindrical base of the same material, which is called the culot. (PL II, fig. 2.)
The height of the culot, or base, is equal to the interior diameter of the mold, and its width is one diameter and one-quarter.
The mold carries a piercer, or spindle, of iron running upward from the middle. This spindle, although in a single piece, has four distinct parts: The first part is called the tail of the piercer. This is wholly below the surface of the cylindrical base, which it enters sufficiently to permit of being solidly fastened in position. The second part of the 102 piercer has a cylindrical form, with a diameter that of the interior of the mold and a height equal to its diameter. The third part of the spindle is the half-ball. This is immediately below the cylinder. It has a diameter two-thirds the interior diameter of the mold and a height one-half the same diameter. The half-ball fits snugly in the throat of the case during the loading, thus serving to maintain the pasteboard in shape. The fourth part is the piercer proper. The purpose of this is to maintain an empty space within the interior of the rocket. The hole thus contrived is called the soul of the rocket, because it is essential to that display of energy which causes the upward flight. The vacant space offers to the fire a larger surface of inflammable material. The gases thus formed exert their force on the surrounding walls and also against the air in the direction of the orifice. The resistance of air is such that the reaction of the gases within the chamber drives the rocket forward.
The difference between the height of the mold and the length of the piercer, when the mold is in position on the culot, gives the height to which the charge reaches. Experience has taught that it becomes necessary to diminish the height of the mold, while increasing the length of the piercer proportionately, as the rockets are given a larger size.
If this proportion were not modified for the various sizes of rockets, the results would be found wholly unsatisfactory. If, for example, the height of the charge should be measured in each case by one and one-quarter of the diameter of the mold, the result would be that in small rockets the charge would be consumed too quickly, so that the garniture would be thrown out long before the completion of the flight, while in the case of large rockets the charge would be lighter in effect, so that the garniture would ouly be thrown out some time after the fall back to earth was begun.
Little rockets, having an exterior diameter of 5 lines or less, require no piercing in order to ascend. These should be loaded within a mold resting on a base to which no spindle is attached. Such rockets, with a stick duly fastened in place, ascend properly. If pierced, the flight upward is so rapid that it is almost impossiple to perceive the effect.