There’s a bit of a progression in the Heinlein juveniles; several progressions, in fact. The physical distance of each book’s journey was to steadily increase, with Heinlein essentially finishing off the Solar System with The Rolling Stones and kicking off his faster-than-light travel and interstellar adventure phase with Starman Jones.
There’s also somewhat of an emotional progression. All of Heinlein’s boys’ books star the sort of teenage characters he would have every young man strive to be. The age of his heroes was usually intentionally vague, but he seems to have mostly had in mind a young adult on the cusp of his majority, respectful of his parents but entitled to make some of his own decisions.
But the earlier adventures usually involve some kind of adult supervision or at least parental permission, whereas the later books are more likely to have a young man taking charge of his own destiny (again, see Starman Jones). Bildungsromans, in other words. Between Planets is somewhat of a bridge, as Heinlein has contrived for his protagonist to make adult decisions and take an early adult role in society by that most classical of circumstances: war.
Don Harvey was born in space, to an Earth-born father and Venus-born mother, both of whom consider themselves, like Don himself, citizens of the Solar System rather than any individual planet. Don is attending a private school on Earth and gets, like everybody else, maybe a day’s warning that an interplanetary war is about to break out. Suddenly, citizenship and loyalty are a big deal.
His parents send for him to meet on Mars, which should be neutral. Unfortunately, the summoning comes too late. Don manages to get off Earth but only to end up on Venus, where his mixed heritage and refusal to declare a side is only slightly more tolerated than on Earth.
Unable to communicate with his parents and with limited funds, Don is pretty much carried away on the rising tide until the fateful moment he takes a decision and picks a side (the one he thinks has the moral right), becoming a soldier and a man all at the same time. There’s also a plot with a secret message for his genus scientist parents, cloak and dagger stuff, which you just know is going to end up being important at some point.
Heinlein knew what he wanted to do here. A former Navy man himself, he wanted to take something of his own feelings on military service and coming of age and how well the two things seem to go together. The violence is mostly either implied or off-stage, though one gruesome exception comes to mind. The reasons behind the war are not overly dwelt on as Heinlein just needed the kid to get caught in a military conflict in order to provide the emotional journey he was interested in writing about.
The writing is competent, as always: straightforward and readable. The plot and characterization were a little thin for my taste. Heinlein’s moralizing has been known to get the better of him. One of the things he moralizes about is patriotism and integrity and sometimes he does a really good job at getting those ideas in there at no expense to the story, but not this time. There’s no balance here, no lightness or humour or romance to cut the duty and grim determination.
I guess that’s the problem, though. It’s very serious subject matter that can’t be made light of without undercutting his message (though if the message is “join the army and be a grown-up if you don’t die”, maybe undercutting the message just a bit would have been a good thing). On the other hand, more serious treatments of the subject of war and a young man’s experience of it (All Quiet on the Western Front; General’s Die in Bed) have become classics not by translating true events into the form of a cleaned-up juvenile adventure story (further removed from history by the science fictional setting), but by going whole hog and embracing all the grit and darkness and nihilism of it all.
Between Planets ends up in somewhat of a no man’s land of being too cleaned up to be a serious war story, but still too real for an escapist adventure story. Oh, it’s serviceable, but why read serviceable Heinlein? Stick with the good stuff lest your palate be ruined.
Right after this Heinlein wrote The Rollings Stones which is basically just a fun romp, written as a series of adventures by a man with no idea where it’s going, with a sort of fade-to-black ending that should probably feel like a cop-out but manages to work, and an after-the-fact theme to lend cohesion where there was none. And it stands up to repeated readings. With Between Planets, by contrast, the man knew exactly what story he wanted to tell before he even typed a word. He just didn’t know how to tell it.