The final half-decade of launches in the Thor/Delta family consisted of one Delta IV Heavy each year from 2020 through 2024.
This followed the retirement of Delta II in 2018 and single-stick (non-Heavy) variants of the Delta IV in 2019.
With the start of 2024, Vulcan Centaur took its first flight in time to maintain continuity of ULA's service in the heavy-lift launch vehicle market before the Delta IV Heavy program came to an end three months later on April 9, 2024.