The first stage was a Thor missile in the DSV-2C configuration, and the second stage was the Delta E, which was derived from the earlier Delta. Three Castor-1solid rocket boosters were clustered around the first stage. Two different solid-fuel upper stages were available; an Altair-2 was used on the baseline version, however this could be replaced with an FW-4D to increase performance. A Delta E with the FW-4D upper stage was designated Delta E1. Six flights used the Delta E configuration and seventeen used the Delta E1.