In "Of Other Worlds", in a conversation with Brian Aldiss and Kinsley Amis (or Martin Amis? Don't remember), he says that he had a rocket take Ransom to Mars, but he knew better by the time he wrote the second book, and had angels take Ransom to Venus. That is, he wasn't trying to write hard science fiction; he was trying to write stuff where you were confronted with "the unknown". Fantasy and science fiction were both aimed at that, but fantasy was better, because you didn't have to worry about the rules of actual science.
He also said (quoting from memory): "If I were briefed to attack my own books, I would say that though the scientist has to be a physicist for the plot, his concern seems to be almost exclusively biological. I would also ask whether it was credible that such a gas-bag could invent a mousetrap, let alone a spaceship. But then, I wanted comedy as well as adventure."
He also said (quoting from memory): "If I were briefed to attack my own books, I would say that though the scientist has to be a physicist for the plot, his concern seems to be almost exclusively biological. I would also ask whether it was credible that such a gas-bag could invent a mousetrap, let alone a spaceship. But then, I wanted comedy as well as adventure."