The first book in the "Rick Brant" series by John Blaine. "John Blaine" is a pen-name for Harold Leland (Hal) Goodwin. The first 3 books were written in collaboration with Peter Hawkins, starting in 1947. After Hawkins moved to Arkansas, Goodwin continued on his own, writing 21 more over the next 20 years (although the last didn't see publication until 1990.) When I read these as a teen, I placed them midway between the Hardy Boys and Tom Swift, with the science gimmick for each book believable and attainable.

The Rocket's Shadow - (the flap-copy introduction): "Rick Brant and his pal, Scotty, have the kind of adventures all boys would like to have. They live on an island called Spindrift, where Rick's father heads a group of scientists working in the field of electronics. Here and abroad, the boys encounter many thrilling adventures and solve many baffling mysteries." Here is the beginning, where Rick flys errands in his Piper Cub for the scientists on the island. He finds clues leading him to suspect someone is trying to sabotage the main project - a rocket to the moon. Rick is rescued by ex-marine Scotty when trailing some baddies who were buying up all of a necessary component needed for the project. They become fast friends, and Scotty is hired as security for the project... For 1947, this is definitely SF. But the moon-shot is believable - they're not trying to land, but just show that a rocket can be controlled well enough to get there, and make a big enough explosion to be seen as proof.