how did he get known as a rocket man?

He got known as "Rocket Man" primarily due to the famous song "Rocket Man" by Elton John, which was inspired by a short story by science fiction writer Ray Bradbury called "The Rocket Man." The song, released in 1972, tells the story of a Mars-bound astronaut experiencing mixed feelings about leaving Earth. Elton John's association with this iconic song helped him become colloquially known as "Rocket Man".