I kept getting asked about a sequence from the Apollo 11 documentary showing the ignition of a Saturn V upper Stage, known as the S-IVB, in space. After asking around I tracked it down to Apollo 9, which was needed the full Saturn V for launch but didn't need the upper stage to go to the Moon. Skylab Launch video was assembled by Retro Space https://www.youtube.com/channel/UCZJgMAoS30yYoLOYUe_lAXQ