Science fiction based on outdated science.
One such I read, Celestial Matters, is set in a world where Hellenistic civilization survived, and Hellenistic science was right about pretty much everything (just followed out to its logical conclusions). The world is geocentric and ships follow Aristotelian physics, that is to say they revolve in circles with thrusters pushing them further from or closer to the world center. Medicine is based on humoral theory, so people use syringes of biles and blood as stimulants and depressives; a blood transfusion makes the recipient giddy, a yellow bile transfusion makes them domineering and nasty. The central theme of the book was that both Hellenistic and Taoist science are correct, but the two sides of their Warhammer 40K style permawar have to cooperate to understand them.
Planetary romances (like John Carter of Mars) are more fun than space operas. Old stuff from an era where it seemed plausible that Mars and Venus could have borne life and civilizations.
I miss old gray/green aliens and flying saucers.