ROCKET is a cylindrical vessel or case, of pasteboard or metal, attached to one end of a light rod of wood, and containing a compo sition which, being fired, the vessel and rod arc projected through the air by a force arising from the combustion.
Rockets have long been used as a means of making signals for the purpose of communication when the parties have been invisible distance or darkness, or otherwise inaccessible to each other ; and they have occasionally served the important purpose of determining the difference of longitude between two places. In the latter case the rocket is fired at some convenient spot between the stations, from both of which the explosion must be visible ; and the latter being nearly instantaneous, the difference between the times at which it is observed, as iudicated by chronometers regulated so as to show the mean times at the places, is the required difference of longitude. Rockets have also been constructed for the purpose of being used in warfare, and such missiles were so employed for the first time at the siege of Copen hagen hi 1807.
In signal rockets the part of the case which contains the composition, by whose combustion the projectile force is produced, is joined, at the upper extremity, to a conical case containing the composition for pro ducing the explosions or stars of light which constitute the signal, and the length of this part is always rather greater than the diameter of the cylindrical part of the case. Such rockets are made to weigh half a pound, one pound, or occasionally even two pounds. The exterior diameter of the one-pound rocket is inch; the length of the cylindrical case is 121 inches, and the length of the conical head is 31 inches. The rod is generally attached near the base and on one side of the rocket ; its length is about 8 feet, or 60 diameters of the rocket, and its thickness is about half a diameter of the latter. The composition with which the cylinder is filled consists generally of saltpetre, sulphur, and charcoal or gunpowder; the whole is reduced to a mealed state, and well mixed together in the following proportions :—saltpetre, 4 lbs.; sulphur, I lb. ; and charcoal, 1 lb. 8 oz. The composition which produces what are called the stars consists of saltpetre, 8 lbs.; sulphur, 2 lbs.; sulphide of antimony, 2 lbs.; mealed powder, 8 oz.; and isinglass, 3 oz. The latter is dissolved in one quart of vinegar, after which one pint of spirit of wine is added, and then the mcaled composition is mixed with the liquid till the whole becomes of the same consistency as a stiff paste. It is then moulded into short cylinders, and when dry, these cylinders are packed into the head with a small bursting charge. When the composition of the rocket is burnt out, the burster blows open the head and frees the lighted stars.
The rocket case having been placed on a conical spindle, which passes a certain distance up the centre, the composition for burning is rammed or driven round it, so that in the interior about the axis a void space of a conical form is left in order that a considerable surface of the com position may be at once in a state of combustion. The neck of the
rocket (the pact to which the rod is attached), is then choked., or reduced in size, leaving a small bole or vent through which the fire is communicated to the composition. The combustion of the latter immediately takes place on all the concave conical surface about the void space just mentioned.
In order to understand the cause of the rocket's motion, let it be observed, that if the composition were to be fired within a vessel or case closed on all sides, the pressure of the gas generated would be equal in every direction, and the case would either burst in pieces, or, if sufficiently strong, would remain at rest while all the composition was being consumed. But, the case having an aperture or apertures at the choke or lower extremity of the cylinder, the pressure which would have taken place against that extremity is reduced by the escape of the gas into the atmosphere; and there is an excess of pressure out the bead, which is not counteracted by a pressure on the other ex tremity, equal in amount to the pressure due to a surface of the size of the orifice, that is, disregarding the resistance of the atmosphere to the escape of gas ; and this pressure impels the rocket forwards or upwards. This force acts in a manner similar to that by which a gun recoils when the charge is fired ; but, in the latter case, the fluid escaping almost instantly from the bore, the force is one of impulse on the bottom of the chamber which ceases nearly as soon as it is generated'; whereas, in a rocket, the composition continuing to burn during several seconds, the force of impulse becomes a force of pressure, which con tinues to act till the material is consumed. Hence it follows that a rocket ascends, or moves forward, with an accelerated motion till the resistance of the air becomes equal to the accelerative force : and when the composition is burnt out, the rocket falls to the ground. We see then that veteris paribus, the larger the vent or aperture, the greater will be the pressure on the head, driving the rocket forward. But this is modified by the consideration that the smaller the aperture the greater will be the condensation, and, therefore, pressure of gas generated ; or, in fact, that the gas should not be allowed to escape as fast as it is generated. Again, the more rapidly the gas is generated, or the stronger the composition is, the greater will be the pressure ; but at the same time, the sooner will it be burnt out, and the stronger and heavier must be the case, These considerations are most import.