Omnibus of: ROCKET SHIP GALILEO (1947) + SPACE CADET (1948) + RED PLANET (1949) + FARMER IN THE SKY (1950.) These are his first 4 "juveniles" for Scribners.

I have trouble remembering the plot because I read it so long ago. This one was clearly aimed at teenage boys, and works quite well at that - it just doesn't quite have the universal appeal that his later juveniles did. The basic idea is about 3 boys who accompany a scientist on the first piloted rocket ship flight to the moon. Science and engineering details are worked out nicely (from a 1947 perspective.) Heinlein adapted some of the ideas from this book when he wrote the screenplay for the 1950 movie DESTINATION MOON.

Farmer in the Sky (1950.) A shorter version was serialized in Boys' Life in 1950 as "Satellite Scout."

Bill Lermer is a Scout, who emigrates to Ganymede with his engineer-father. Of course, to emigrate, you had to be a family, so he suddenly acquires a new mother and stepsister. Bill has trouble adjusting, and his dad has to explain that he didn't get married to emigrate, but because he loved his new wife, and wanted room to raise a family. Travel to the colony in the torch-ship Mayflower takes months, and the details of shipboard life are shown with great inventiveness. When they arrive on Ganymede, all is not as rosy as indicated when they signed up. An artificial heat trap is what gives the moon a livable climate. Ganymede is nothing but rock, and has to be turned into a farming world by lots of hard work.

Heinlein has worked out the details of what has to be done, and what it will be like when doing it. His characters are survivors, who learn to adapt and do what is needed to survive.