The Delta M or Thor-Delta M was an American expendable launch system used for thirteen orbital launches between 1968 and 1971. It was a member of the Delta family of rockets.The Delta M had a three-stage configuration. The first stage was the Long Tank Thor, a stretched version of the Thor missile, previously flown on the Delta L. Three Castor-2 solid rocket boosters were attached to the first stage to increase thrust at lift-off.