It's definitely the closed cycle. I've narrowed the issue down to something about my save file - a fresh install of KSPI-E and fuel switcher from curse and the closed cycle can use any fuel in a sandbox game. But if I try to use it in my career save, it ends up only being able to use Nitrogen, LqdArgon, Argon, Hydrogen, Liquid Fuel and Lithium as propellants. I'd like to not have to restart my career - is there anywhere in the persistent.sfs that might be affecting the propellant types?