Star Trek: 10 Things You Didn't Know About Warp Drive
2. The Great Experiment
In the 2280s, Starfleet had begun a series of experiments that began with the USS Excelsior NX-2000. This ship, said to have transwarp capabilities, was captained by Styles - a deeply arrogant commander who stepped over Sulu for promotion. He received a nasty shock when the Excelsior suffered a total system shutdown after Scotty sabotaged their propulsion system.
Though little was said of the transwarp experiment again, it was said that these trials were deemed a failure. The Excelsior was retrofitted with a conventional warp drive, entering service later that decade. Starfleet would not manage to achieve transwarp until the 2370s.
However, perhaps failure is too strong a word, as the discoveries made while testing the Excelsior led directly to the upgrading of their engines and the advanced propulsion systems onboard the Ambassador class ships. Both classes of vessels remained in service well into the 24th century, thanks primarily to their durability in action.
In fact, both were seen as strong to bear the name Enterprise. A refit of the Excelsior-class became the Enterprise-B, while an Ambassador class ship became the Enterprise-C.