Hiya!
I'm still very partial to
Star Frontiers (Digitally Remastered). The "Remastered" version is the improved, expanded and errata'ed version that the SF community worked on and put out over the decades....with WotC's nod of approval, mind you. Then, when WotC decided it was going to sell PDF's of the original game books, they "rescinded that nod". That said, I'm sure you can find them somewhere on the net.
But if you want it easier...just go to DriveThru and pick up the WotC original PDF's. They are very well done, and you can even get them in PoD Softcover (I did; I even picked them up in Hardcover; although I still prefer, and use, my SF Digitally Remastered 'community improved' version...it's just better).
No matter which route you go, picking up the fan-made "Frontiersman" e-Mag's is almost a must-have. They offer soooo much good, creative-fueling ideas, examples, adventures, new equipment, new 'subsystem skills' to tweak the game system itself, etc. that you'd be a fool not to grab at least a couple of them to check it out.

HIGHLY recommended!
A recent newcomer, if you don't mind "setting specific", is
The Expanse RPG. It uses the AGE system by Green Ronin. I did the kickstarter on this one and have my hardbacks, screen, etc, as well as the PDF's. I've given it a skim-over, but haven't read it...I have the series on DVD and have watched it twice+, and I also have the DragonAge three-box set, the AGE System books (all of them), and have played it and read it....so, yeah. Anyway, if it sticks mostly to the lore of The Expanse, and the AGE System with some changes to reflect the setting (which it seems to do), then I could see it being a great sci-fi game system! (EDIT: Just thought...'unless you want FTL travel to strange, alien planets' and all that....then it's NOT the expanse)/
Other than those two specifics, the generic RPG systems might be worth looking into. Masterbook and Silohuette CORE are my go-to's for "not too crunchy, but very flexible"; Masterbook is the lesser-crunch of the two.
^_^
Paul L. Ming