X Minus One

Long before Rod Serling created The Twilight Zone, probably the best science fiction and fantasy program made, there was X Minus One, the radio drama which ran during 1955-1958 on the NBC radio network. Using material supplied by the two major science fiction magazines of the day, Astounding Science Fiction and later Galaxy, the show produced a weekly radio drama involving science fiction, aimed at adults. (One of the program's sponsors was Pabst Blue Ribbon beer.)

As noted on The Other Wiki, the show opened with the same teaser:

Countdown for blastoff... X minus five, four, three, two, X minus one... Fire! [Rocket launch SFX] From the far horizons of the unknown come transcribed tales of new dimensions in time and space. These are stories of the future; adventures in which you'll live in a million could-be years on a thousand may-be worlds. The National Broadcasting Company, in cooperation with Street and Smith, publishers of Astounding Science Fiction presents... X Minus One.

Later they would use Galaxy in place of Astounding. Some openings did not include the word 'transcribed'.

Each week, a new story, taken from either Astounding or Galaxy, was performed as a radio drama (or comedy) in which the material was played straight for an adult audience, as opposed to typical 1950s science fiction (especially science fiction movies) which either trivialized this type of story (as in the case of most "bug eyed monster" style low-budget "earth is being invaded by monsters (or) martians" films) or targeted it for juveniles.

For its time, the show was somewhat groundbreaking in that it covered science fiction as serious fiction directed to adults. Story lines included characters with gambling and addiction problems, criminal tendencies, and even a story about forcible relocation of the American Indians, told with Martians.

MP3s of the show can be downloaded for free from the Internet Archive https://archive.org/details/OTRR_X_Minus_One_Singles , although the copyright status of old time radio is somewhat dubious (they are not subject to US federal copyright, but may be subject to state copyright until 2067).

Tropes used in X Minus One include:

Double "Don't Know": One episode had the character using this trope
Downer Ending: Many of the shows ended up with a bad result for the protagonist