Apollo 4 sent an unmanned Command and Service Module on a high-speed reentry test atop the towering Saturn V, the first all-up test of the rocket's three stages together. The launch validated the heat shield design needed to survive lunar-return speeds. Walter Cronkite, watching from miles away, famously had his broadcast booth's windows rattled by the rocket's roar.
NASA engineers under Wernher von Braun took an enormous gamble with Apollo 4, testing all three stages of the new Saturn V rocket simultaneously in a single flight rather than incrementally, a strategy known as 'all-up testing' that program managers had adopted to save time in the race to the Moon.
The rocket, standing 363 feet tall, roared off its pad at Kennedy Space Center with a force that stunned observers miles away. CBS anchor Walter Cronkite, broadcasting from a booth he had believed was a safe distance away, watched in real astonishment as the shockwave rattled ceiling tiles and shook his broadcast window, exclaiming that the building itself seemed to be trying to shake apart.
The uncrewed Command Module completed a high-speed reentry test, plunging back into the atmosphere at velocities simulating a return from the Moon to prove its heat shield could handle the thermal extremes. The mission's success gave NASA the confidence to proceed toward crewed lunar missions, setting the stage for Apollo 8's crewed lunar orbit just over a year later and Apollo 11's landing in 1969.