From 2367 to 2369, the Starfleet Corps of Engineers enlisted and destroyed 21 separate Phoenix-named starships in an effort to perfect transwarp engine technology, partially starting with the USS Phoenix and fully starting with the NCC-65420-A to the NCC-65420-U. The final and remaining vessel was Phoenix-X with the registry NCC-65420-X. (Star Trek: Phoenix-X: "Secret Occurrences")