Today is the 45th anniversary of the first Saturn V test flight. The naming history is complicated, so a quote from NASA’s history page:
Evolution of nomenclature for the Saturn family of launch vehicles was one of the most complex of all NASA-associated names. On 15 August 1958 the Department of Defense’s Advanced Research Projects Agency (ARPA) approved initial work on a multistage launch vehicle with clustered engines in a 6.7-million-newton-thrust (1.5-million-pound-thrust) first stage. Conceived by designers at the Army Ballistic Missile Agency (ABMA), the vehicle was unofficially known as “Juno V.” (Juno III and Juno IV were concepts for space vehicles to follow Juno II but were not built.) 1
In October 1958 Dr. Wernher von Braun, the Director of ABMA’s Development Operations Division, proposed the Juno V be renamed Saturn and on 3 February 1959 ARPA officially approved the name change. The name Saturn was significant for three reasons: the planet Saturn appeared brighter than a first-magnitude star, so the association of this name with such a powerful new booster seemed appropriate; Saturn was the next planet after Jupiter, so the progression was analogous to ABMA’s progression from missile and space systems called “Jupiter”; and Saturn was the name of an ancient Roman god, so the name was in keeping with the U.S. military’s custom of naming missiles after mythological gods and heroes.
Image of all Saturn V launches courtesy of NASA, image in the public domain.