Research into the origins of the serialized feature The Space Explorers, by folks who grew up obsessed with it from TV viewings in the late 50s/early 60s, never ends. Years ago I posted all the information I had about this Fred Ladd pastiche on my Cartoon Research FAQ. Ladd apparently combined live action shots from a German sci-fi film Weltraumschiff 1 Startet (Spaceship 1 Launches) with scenes from some random foreign outer space cartoon. Ladd has never been able to recall the name of the animated film from which the cartoon segments were culled.
You can all sleep easier tonight. The mystery has been solved. All character animation, from the interior of the spaceship to the scenes of the planet exploration, were extracted from a Russian short called Polet na lunu, (Flight to the Moon), produced in 1953. More information (with frame grabs) is posted on a French website located HERE.