Delta 2313 was a three-stage Delta 2000-series configuration with the RS-27-powered extended long-tank first stage, three Castor II strap-on motors, the TR-201-powered second stage, and a TE-364-3 solid third stage. Three vehicles flew from 1974 through 1977. Skynet 2A was stranded in an unusable low orbit after a second-stage electronics failure; Skynet 2B and Sirio 1 launched successfully.