Several variants existed, which were differentiated by a four digit numerical code. The first stage was the RS-27 powered Extended Long Tank Thor, first flown on the 2000-series. Three or nine Castor-4 solid rocket boosters were attached to increase thrust at lift-off, replacing the less powerful Castor-2 boosters used on earlier models. Two second stages were available; the Delta-P, which had been flown on the Delta 1000 and 2000 series, or the Delta-K, an uprated version with the Aerojet engine. Some launches used a three-stage configuration in order to reach higher orbits. A Star-37D, Star-37E, or Star-48B PAM-D could be used as an upper stage. Launches with PAM-D upper stages were designated Delta 3XX0 PAM-D, rather than assigning a code to the upper stage for use in the four-digit sequence. From the 4000-series onwards, the PAM-D received the upper stage code "5", however this was not applied retrospectively to 3000-series rockets.