Editing Liquid rocket propellant (section)

==Lithium and fluorine==
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 rocket|tripropellant]]). The combination delivered 542&nbsp;s specific impulse in vacuum, equivalent to an exhaust velocity of 5320&nbsp;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&nbsp;°C (just 21&nbsp;K), and the lithium must be kept above 180&nbsp;°C (453&nbsp;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.<ref>{{cite web |title=Current Evaluation of the Tripropellant Concept |first=Robert |last=Zurawski |date=June 1986 |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19860018652.pdf }}</ref>

During the 1950s, the Department of Defense proposed lithium/fluorine as ballistic-missile propellants. A 1954 accident at a chemical works that released a cloud of fluorine into the atmosphere convinced them to use LOX/RP-1 instead.{{cn|date=September 2024}}