Rocket Batteries would be a siege engine, the third to be implemented beside the catapult and the ballista. Unlike the former which can attack enemies from multiple z-levels(a feature that should be implemented) and the latter which plies through single rows of enemies with brute power, the rocket battery would be more of an Area of Effect weapon, which damages multiple enemies in a radius.
The Rocket Battery would look similar to 15th century Ming Dynasty rocket batteries(see: “wasp nest” rocket launchers), basically a 3by1 box in the middle with the stick and warhead of the missiles poking out. Unlike other siege engines, the Rocket Launcher can hold up to 3 rockets and fire them all in rapid succession. The rockets themselves would explode after contacting an enemy or after traveling a certain distance(provably around 60-80 tiles). The explosion would look similar to a deadly dust cloud, except with fire, smoke and no dust. Creature may be flung several tiles horizontally and vertically by the explosion, while blast damage itself has a high tendency to detach limbs which have no armour(naked, civilian clothes) and seriously damaging the body through tearing/killing through exploding into gore. Unarmoured enemies are very susceptible to explosions, while armoured enemies usually deflect the blow or get minor bruising, only rarely having limbs torn off or rendered otherwise useless. Combat actions taken by explosions usually reads in the combat log as “The blast hits the (creature) in the (part), (action)!”. Blasts may also set enemies on fire.
Rockets themselves are usually made from 1 metal bar, 1 wood and 1 piece of gunpowder. Gunpowder is made by reacting 1 Brimstone, 1 Charcoal/Ash and 1 Saltpeter. Brimstone would be changed to spawn in both igneous extrusive and intrusive layers to facilitate the feasible creation of gunpowder. Saltpeter may also be made from bat guano, a rare “mineral” that is white in colour and spawns on cavern floors(natural ones, not artificial ones). Giant bats can also produce guano, meaning tamed ones can be farmed for their guano to make gunpowder.
Drawbacks: The missiles are indiscriminatory, being able to blow apart your recruits as easily as invading goblins. Furthermore, they cannot be moved, unlike other siege engines, preventing them from being used for raids/sieges. Finally, if a siege operator at Competent skill or under operates the battery, there is a chance that the missile will “backfire” and explode in the battery, usually killing the poor operator, deconstructing the battery and exploding all missiles within the blast radius, which may prove !!FUN!! especially if they start a fire.