Oh, that - what I've known as the Interplanetary Trilogy or Space Trilogy. I'm quite familiar with that - even named one of my kids after Ransom (middle name).
This is always how the difference between the two genres has been explained to me, and why Star Wars (until the thing with midichlorians) is often classified as fantasy rather than sci-fi. The (admittedly) little I know about the Cosmic Trilogy makes it seem more like science fantasy than true science fiction, but that's neither here nor there.Science fiction differs from fantasy in that, within the context of the story, its imaginary elements are largely possible within scientifically established or scientifically postulated laws of nature (though some elements in a story might still be pure imaginative speculation).
This is always how the difference between the two genres has been explained to me, and why Star Wars (until the thing with midichlorians) is often classified as fantasy rather than sci-fi. The (admittedly) little I know about the Cosmic Trilogy makes it seem more like science fantasy than true science fiction, but that's neither here nor there.
If I know Lewis, he was much more concerned with the underlying allegory than following the conventions of a particular genre. It seems a lot like what Madeline L'Engle did with her Time quartet/quintet, combining science, fantasy, and religious mythology.