Delta D

The three-stage core vehicle was essentially the same as the Delta C. The first stage was a Thor missile in the DSV-2A configuration, and the second stage was the Delta D, which was derived from the earlier Delta. An Altair-2 SRM was used as a third stage. The main difference between the Delta C and Delta D was the presence of three Castor-1 solid rocket boosters, clustered around the first stage.