The Delta N consisted of two stages. The first stage was the Long Tank Thor, a stretched version of the Thor missile, previously flown on the Delta L and Delta M. Three Castor-2solid rocket boosters were attached to increase thrust at lift-off. A Delta E was used as the second stage. If an increased payload capacity was required, six boosters were flown instead of three, in a configuration known as the Delta N6, or "Super Six". Three launches used the N6 configuration.