A merculite rocket was a projectile weapon used by the Talarian Republic and the U'lanna. It was considered obsolete by Starfleet standards of the 24th century. (TNG: "Heart of Glory")
U'lanna galleons used five heavy merculite rocket launchers to penetrate hulls. (Star Trek: The Stoneship Files: "For a New World")
Merculite rocket article at Memory Alpha, the canon Star Trek wiki.