I remember the Gemini program with great fondness. How many kids today even KNOW what that was?
That was the program that a lot of the early astronauts liked the best, as the craft was designed to the requirements of the men who were going to fly it. I’ve read that Grissom in particular had a lot to say about it, and those who flew it could tell it was a pilot’s machine.
That superb design philosophy survived in some fashion into the Apollo design, y’know, other than that little mishap where the shitty Apollo 1 ignited inside and burned those three astronauts to death.
Von Braun was a fanatic about safety; the Mercury missions were a gamble because the boosters were pretty iffy. But Gemini was used to provide practical experience in rendezvous and docking — both needed for the Apollo missions still on the drawing board — and reentry (ditto).
The first three Apollo missions were more of the same; Apollo 8 took a similar weighted chunk of ballast to simulate the lunar module; 9 went only to Earth orbit, but practiced with the actual lunar module; 10 took the module to the Moon, and practiced everything including descent but then just dropped the legs and went back up to the Command Module. They practiced the maneuvers needed for the later missions, to test both the hardware and the execution.
Space walks were practiced by the Soviet cosmonauts because their lunar missions involved nearly simultaneous launches of two vehicles — one would have taken two cosmonauts into lunar orbit, where they’d rendezvous with the unmanned lander vehicle sent by the other launch. Since they didn’t have room in their mass budget for a docking module, the lucky solo cosmonaut would leave the craft, make a space walk over to the lander, get in, turn on, drop down, get out, plant the flag, make a broadcast, get back in the craft, take off from the surface, rendezvous, make another space walk, get back into the return vehicle, and return to Earth with the other cosmonaut.
The N1 boosters to be used for this never had a successful flight test, and actually never had their engine clusters systematically tested either — the clusters were built and shipped, then installed. The first N1 test destroyed the entire launchpad, one of the two needed for the lunar mission, and due to budget shifts (part of Korolev’s budget was shifted to Gluschko) it was never rebuilt.