Rockets are an artillery unit available to the British and United States. They replace Heavy Cannon for the former, while the latter can train them from the Artillery Foundry as with any regular artillery piece. In the case of the British, they are one of the two unique units produced from the Factory, the other being the Great Bombard. They have a faster rate of fire than both the Heavy Cannon and the Great Bombard. They are as effective against infantry units as they are against buildings due to their high rate of fire and high attack. They are also more effective against cavalry due to a lack of damage penalty against them.
Producing Rockets at a Factory requires 98 seconds (93 with Chinese TEAM Engineering School card). The average resource gather rate of a Factory is 5.5 every second (before the building upgrade (+20%)). This means that the Rocket, while appearing to be free, will have a cost dependent upon how effective the Factory gathers a needed resource. In many cases, this cost can exceed the base value of the unit.
The first rockets used in warfare were developed by the Chinese and Indians. The principle is not dissimilar to a bottle rocket, where explosive propellant (gunpowder) propels a missile (the rocket) and is stabilized by a stick that extends well beyond the body of the rocket. It was William Congreve, a British officer, who developed their use for warfare in the Napoleonic era. His rockets were tubes capped by cones and filled with gunpowder and either shrapnel or shot. They were fired from structures that consisted of a ladder with supports for the body of the rocket and a frame to prop up both ladder and rocket. These were tall affairs, and needed to be, when the stick of a large rocket could be as long as 15 feet! The "rockets' red glare" in the Star-Spangled Banner, the national anthem of the US, refers to the rockets used by the British navy in their assault on Fort McHenry in 1814.