The Saturn I and its successor the Saturn 1B were built quickly using available propulsion systems to permitt orbital testing of Apollo components before the Saturn V was ready. This was necessary to acheive the goal of landing a man on the moon within the 60's decade. The first stage of this vehicle was humorously refered to as "Cluster's last stand" - Eight Redstone tanks around Jupiter tank core, eight Jupiter engines. The 2nd Stage of the Saturn 1, known as the S-4 was powered by six RL-10 hydrogen fueled engines. This launch vehicle was soon superceded by the Saturn 1B using a larger 2nd stage called the S-4B with a single J2 engine.