S.S. Woden just before being destroyed by the M-5 computer on stardate 4729.4 during a wargame exercise
Image scanned from Star Trek Spaceflight Chronology

Soon after the first major advances in fission rockets occured, the aging DY-100 fleet of ships was upgraded to the DY-500 class, some of which saw use up through the early 24th century. In addition to adding a most efficient fission drive to this class of ship, the other major advancement over the original DY-100 class was the addition of magnetohydrodynamic power taps, through which the fission exhaust passed, creating enough energy to power the onboard systems of the ship. This advancement was one of the technological breakthroughs which serves on board Starfleet ships even today. The physical design of the DY-500 did not change much from the DY-500, except for some ships which were lengthened to accomodate more coargo modules.

The year 2022 marked the first and most tragic accident since the space shuttle Challenger exploded after launch 36 years before. The UNSS Courageous, a DY-500 class ship carrying 62 people was ireperably crippled in deep space when a solar flare penetrated a shielding flaw in the reaction chamber. The fission drives' throttle was locked at 100 percent thrust, causing the ship to fly off uncontrollably without the hope of rescue before it got out of range. Because of this misfortune, the Courageous had the dubious distinction of becoming the fastest ship at the time, attaining a speed of 0.86c before its fuel was expended.