I was in high school, a freshman. Trig and physics and chemistry were in the future. I went to see it with some friends, who were a lot less interested in astronomy and space travel than I was.
Kubrick knew he wasn't shooting a technical film, even though he was aware of its technical aspects. Style and cinematography would have been a concern. To shoot on a set that reflected an efficiently-designed spacecraft would have produced a claustrophobic image. A lot of space -- empty space -- inside the space vehicles would help to communicate the loneliness, the isolation, the vast emptiness of space on the outside of the vehicles.
We're also talking about a period in American history when buildings, vehicles, living spaces, and movies were physically expansive. To imagine a future in space that was tiny and cramped like the Apollo capsules? Though that would have appealed to the technically aware, the rest of us wouldn't have thought in those terms at all. Certainly we would, by that time, have moved into luxury spacecraft!
Sure, the image was unrealistic, but it reflected the dream. Mechanics could come later.
Space/Science » in reply to re: 2001 A Space Odyssey question
Not all the viewers were technically sophisticated.