 The Atlas-Agena was a two-and-a-half-stage rocket, with a stage-and-a-half Atlas missile as the first stage, and an RM-81 Agena second stage.Initially, Atlas D missiles, redesignated as the LV-3, were used as the first stage.These were later replaced by the standardized Atlas SLV-3, and its derivatives, the SLV-3A and B.