[2][3] The basic design of the original Vanguard upper stages, featuring a pressure-fed nitric acid/UDMH, regeneratively cooled engine, was kept in place, but with an improved AJ10-118 engine.

More significantly, the Delta stage featured cold gas attitude control jets allowing it to be stabilized in orbit for restart and more precise burns.

The Thor-Delta was the first rocket to use the combination of a Thor missile and a Delta upper stage.

This configuration was reused for many later rockets, and a derivative, the Delta II, remained in service until 2018.

All launches were successful except the maiden flight, which failed to place Echo 1 into orbit due to a problem with the second stage.