The DY-100 class received extensive design and performance reviews to become the DY-500 class. Its almost pure fission engine plant produced a clean gas exhaust who powered a MHD, providing electricity to the ship. At the moment of its commission it was one of the most popular transports. The loss of the DY-500, UNSS Courageous, was the worst tragedy in the 65 first of space exploration. (TOS reference: Star Trek Space Flight Chronology)