If you look up a list of all Apollo missions NASA flew in the 1960s and 70s, you’d see Apollo 1, then Apollo 4 through 17. So what exactly happened to the missing Apollos 2 and 3?
When NASA started testing Apollo and Saturn hardware in the early 1960s — the hardware that would eventually fly to the Moon — it established a pretty standard nomenclature. Every rocket was given a letter designation for the rocket and payload type followed by a number indicating the launch.
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