The highest-performing rocket engine ever tested burned molten lithium with cyrogenic fluorine, plus added hydrogen for some light reaction mass.
This has never been developed for actual flight, in significant part because it's terrifying.
And if you haven't heard of Things I Won't Work With, it's an (infrequently updated) blog by a research chemist on substances frightening enough that he doesn't want to be in the same lab as them. A common thread is compounds that would *really* prefer to be nitrogen gas and will energetically turn into such if given the slightest opportunity.
http://blogs.sciencemag.org/pipeline/archives/category/things-i-wont-work-with
@starkatt And in small part because it's far more expensive and only slightly more powerful than an LH₂/LO₂ rocket.
This is actually a trivium I like to bring up when discussions about rocketry happen! (which is surprisingly often, probably due to the company I keep. that's a good thing!)
@starkatt ..you know that's not as complex as a lot of rocket fuel attempts but it gets major "oh hell no" points anyway.
@starkatt Nope nope nope nope
BTW if you enjoy rocketry history or the Things I Won't Work With blog, I highly recommend the book /Ignition!/
It's an informal history of the development of rocketry propellants. Contains explosions and a whole lot of frightening chemistry.
https://library.sciencemadness.org/library/books/ignition.pdf