There's actually two things to think about when coming up with a sci-fi RPG.
Longevity - If it's intended for short campaigns or games, science can be safely ignored.
Liberty - How free are the characters to choose their own course of action? If the PC can be successfully constrained, then you never need to think about science.
For example:
Star Wars - successfully constrained by source fiction.
Star Frontiers - never played a game in which players didn't drop asteroids on planets, or otherwise become the most feared criminals in the game world.
Just because you ignore science, doesn't mean players will. If you feel like you need to ignore science, for the story you want to tell, then include compelling setting fiction, or rules that reward maintaining genre conventions or penalize violating genre conventions.