The highest-specific-impulse chemistry ever test-fired in a rocket engine was lithium and fluorine, with hydrogen added to improve the exhaust thermodynamics (all propellants had to be kept in their own tanks, making this a tripropellant). The combination delivered 542 s specific impulse in vacuum, equivalent to an exhaust velocity of 5320 m/s.
The impracticality of this chemistry highlights why exotic propellants are not actually used: to make all three components liquids, the hydrogen must be kept below −252 °C (just 21 K), and the lithium must be kept above 180 °C (453 K).
Lithium and fluorine are both extremely corrosive. Lithium ignites on contact with air, and fluorine ignites most fuels on contact, including hydrogen. Fluorine and the hydrogen fluoride (HF) in the exhaust are very toxic, which makes working around the launch pad difficult, damages the environment, and makes getting a launch license more difficult.
Both lithium and fluorine are expensive compared to most rocket propellants. This combination has therefore never flown.
i love how utterly expectable and yet humorous this whole article reads.
“oh so you think you finally found the best possible fuel, that has the highest specific impulse? think again: it’s toxic, corrosive, expensive, difficult to work with, etc.”
If you enjoy this topic, I can highly recommend Ignition! An informal history of liquid rocket propellants by John D. Clark. He an excellent writer, and makes the whole subject wryly humourus as well as understandable to the lay person.
If the first couple images and captions weren’t enough to convince me to read it, the foreword by Isaac Asimov settled it. Sucks that Asimov turned out to be a shithead but the late fees I paid on his short story collections as a kid (okay, fine, my parents paid in lieu of an allowance for chores) probably helped the local library afford a new wing or two.
I can also highly recommend Derek Lowe’s series of things he won’t work with in which he describes various fluorine compounds as things he definitely won’t work with.
That’s a brilliant series that had me chuckling along to chemical formulas, he has a fantastic way with words.
Another excellent source that’s based/inspired from the book: Alexander the ok! His vid on the Li-F engine and as a bonus, his video on a Soviet tripropellant engine