Rocket, a projectile which is set in motion by a force residing within itself, thus performing the twofold functions of piece and projectile. Rockets are used as night signals, missiles of war, and in pyrotechnic displays. They are said to have been invented about the close of the 9th century, and to have been used for war purposes in India and China even before the invention of gunpowder. Their inferior force and accuracy limited their use by Europeans to incendiary and signal purposes till 1804, when Sir William Congreve turned his attention to their improvement. He substituted sheet iron for paper cases, made the guide stick shorter, and attached it to the centre instead of the side of the base. He prepared and used them successfully at the siege of Boulogne and the battle of Leipsic, and is said to have increased the range of the six-pounder rocket from 600 to 2,000 yards. Rockets are now constructed of cylindrical cases composed of paper or wrought iron, according to the use for which they are intended, and are filled with a composition of nitre, charcoal, and sulphur, like gunpowder, except that the ingredients are compounded for a slower rate of combustion.

If penetration and range are required, as in the case of war rockets, the head is surmounted by a solid shot; if explosion or incendiary effect, by a shell or a case shot with a fuse attached, which is ignited by the flame of the composition. The base is perforated by one or more vents for the escape of the gas generated within, and sometimes with a screw hole into which a guide stick is fastened. A rocket is set in motion by the reaction of a rapid stream of gas escaping through the vents and impinging against particles of the air, although the reaction of the escaping gas would cause it to move also in vacuo. The velocity of the flight depends upon the size and shape of the vents and the velocity with which the gas escapes, and it has been found in practice that the best results are obtained by conical vents. As the composition burns in parallel layers of uniform thickness, the amount of gas generated in a given time or the velocity of its escape from the case depends on the extent of the inflamed surface; and experience shows that to obtain the required surface of inflammation it is necessary to form a long cavity in the mass of the composition.

This cavity is called the bore, and in small rockets it is obtained by driving the composition around a spindle, which is afterward withdrawn, while in large rockets the composition is driven in solid, and afterward bored out; the bore is made concentric with the case, and slightly conical. The propelling force of a rocket changes its direction with the axis along which it acts, so that without means of giving stability to this axis the trajectory will be very irregular. Instances have been known where these projectiles have returned to the point whence they started; the "serpent," a species of small rocket, owes its peculiarity to this fact. The two means used to give steadiness to the flight of rockets are rotation, as in the case of a rifle ball, and the resistance of the air, as with an arrow. The first is exemplified in Hale's rockets, where rotation is produced by the escape of the gas through vents situated obliquely with reference to the axis. The common signal rocket is guided by a long stick projecting from its base in the continuation of its axis, or by four shorter ones attached to the side of the case at its base, and each making a small angle with the axis.

Congreve's war rocket is guided by a long wooden stick attached to its base. Rockets are generally fired from wooden tubes or gutters, but where they have four sticks, these, forming a pyramid, permit them to be fired from the ground, by standing them on end. The advantages claimed for war rockets over cannon are unlimited size of projectile, portability, freedom from recoil, rapidity of discharge, and the terror which the noise and fiery trail produce upon troops, especially cavalry or mounted infantry. The numerous conditions to be fulfilled in the construction in order to obtain accuracy of flight, and the difficulty of preserving the composition uninjured, have restricted their usefulness for military purposes. They were not used at all during the civil war in America, though two sizes are prescribed for service, namely, the 2-inch (interior space) weighing 6 lbs., and the 3-inch, weighing 16 lbs., and having with an elevation of from 4° to 5° a range of 500 or 600 yards, and with an elevation of 47° ranges of 1,760 and 2,200 yards respectively.

They have also fallen into disuse in Europe, and hereafter, except in peculiar cases, they will probably be confined to the uses of signalling and pyrotechny. - See Benton's "Ordnance and Gunnery" (3d ed., New York, 1867).