Tunnel in the Sky is an underappreciated gem of a Heinlein juvenile. In the future, an overpopulated Earth expands into space through wormhole gates. Any job in the Outlands, as the other worlds are called, requires passing Outland Survival. The final exam is being dumped on an unknown planet, and surviving a few days until recall. Rod Walker is a high school student facing such an exam.

We meet his family, and get some cool hints at the expanded setting. A new religion called Monism has joined the big three Semitic faiths. China has conquered Australia and irrigated the outback, but population continues to rise. Rod's sister Helen is an officer in the all-female Amazon space marines. We see pioneers going through the a gate with horses and Conestoga wagons, because 'grass-burners' make their own replacements and resupply will be rare until the colony can export food or Uranium back to Earth. Rod's parents are facing a hard choice as well, a 20 year wait in cryogenic stasis while his father's rare metabolic disease is cured.

We get some useful advice on survival, "don't carry a gun, your job is to be a rabbit and live", and then we're off. Rod does fine the first few days, but then someone knocks him on the head and steals everything but his back-up knife. Worse, as days pass and the gate home fails to appear, it becomes apparent something has gone very wrong. The survivors of the 100-odd high school and college students have to settle down and figure out some kind of long term solution for survival. Rod's is pushed aside in favor of a smooth talking college kid, who's early attempt at democracy becomes mired in committees and social niceties, like building houses for newlyweds rather than a defensive wall. The colony is well on its way to becoming a stone age society, when a seasonal migration of 'dopey joes' turns a previously harmless species into a vicious killer. Rod is vindicated, and becomes mayor of their colony for a year, when the gate reopens and suddenly he is no longer an independent leader on a frontier world, but a kid again, with all that that entails.

There are lots of hints of ideas that Heinlein would develop in later works. The themes of power and responsibility in Starship Troopers, the frontier space colonies of Time Enough for Love, and the survivalism of Farnham's Freehold. Heinlein has lighter touch on these topics, focusing more on the coming of age of his protagonists. There are some missteps, the characterization is a little thin, and who stole Rod's survival gear is a dropped thread. Heinlein's attitudes on gender and race are progressive for their time, but they haven't aged very well. His female characters (Caroline, Jack), are the equals of the men, but it takes Rod some time to get over his prejudices against women. Even so, men hunt and women cook. The 'yellow hordes' bit in the beginning is not great, but according to a letter from Heinlein Rod is canonically African-American, which is solid for a book published a year after Brown v. Board of Education.