The Delta E, or Thor-Delta E was an American expendable launch system used for twenty-three orbital launches between 1965 and 1971. It was a member of the Delta family of rockets. 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-A. Three Castor-1 solid 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. Delta E rockets were launched from Cape Canaveral Air Force Station Launch Complex 17 and Vandenberg Air Force Base Space Launch Complex 2E. All 23 flights were successful. On December 16 1965, a Delta E launched the Pioneer 6 space probe.