Project Apollo followed Projects Mercury and Gemini as the final phase in meeting President John F. Kennedy's ambitious aim, which was stated in a speech on May 25, 1961: "I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the moon and returning him safely to the Earth." This was at the height of the Cold War, and the United States was behind in the space race with the Soviet Union. Forty-three days before the speech, the Soviet Union had put the first person in space, Yuri Gagarin, who made one orbit of Earth in a 108-minute trip.

Flight Mode

One of the key technological decisions of the early Apollo program was the flight mode used to travel to the Moon and back. Early plans focused on direct ascent (DA) and Earth-orbit rendezvous (EOR). In DA a single...