The Thorad-Agena was an American expendable launch system, derived from the Thor and Delta rockets. The first stage of the rocket was a stretched Thor variant named "Long Tank Thrust Augmented Thor". The Long Tank Thor first stage was later adopted by NASA's Delta program for its "Thrust Augmented Improved Delta", which first flew in 1968. The second stage was the Agena-D, which had already been used in conjunction with the standard configuration Thor, as the Thor-Agena.