Catch you on the flip side

In the early stages of spaceflight, NASA developed a technique that saved much needed fuel to return space craft back to Earth by using the moon’s orbit like a slingshot. Astronauts in space and ground control would lose radio signal once the space craft makes its way to the “back-side” of the moon and away from line of sight with Earth. They would of course regain radio signal when they reach the other side of the moon (the “flip side”). It was customary for Ground Control to say “Catch you on the flip side” before they would lose radio signal. The none-space working-class community adapted the phrase as a “good-bye” or “see you later.”