From the same "Ignition" book
«It happened at their Shreveport, Louisiana, installation, while they were preparing to ship out, for the first time, a one-ton steel cylinder of CTF (Chlorine trifluoride,). The cylinder had been cooled with dry ice to make it easier to load the material into it, and the cold had apparently embrittled the steel. For as they were maneuvering the cylinder onto a dolly, it split and dumped one ton of chlorine trifluoride onto the floor. It chewed its way through twelve inches of concrete and dug a threefoot hole in the gravel underneath, filled the place with fumes which corroded everything in sight, and, in general, made one hell of a mess. Civil Defense turned out, and started to evacuate the neighborhood, and to put it mildly, there was quite a brouhaha before things quieted down. Miraculously, nobody was killed, but there was one casualty — the man who had been steadying the cylinder when it split. He was found some five hundred feet away, where he had reached Mach 2 and was still picking up speed when he was stopped by a heart attack.»
You must imagine how will the launch pad look like after the launch since F2 + H2 → HF
= Hydrogen fluoride, in other words, hydrofluoric acid. It is highly corrosive and dissolves almost everything, even glass (watch "Breaking Bad")
Now imagine a rocket containing 1000 tons of fluorine crashing in Florida. After that event, even enriched Uranium will look like eco-friendly fuel.
Fluorine is toxic starting from 0.1 part of fluorine per million parts
of air (ppm).
Xenon Ion thruster engine has even higher specific impulse - about 3000 seconds.
Specific impulse is important, but there are also other aspects to consider, such as:
- thrust (if the engine is used at the ground level)
- fuel cost
- cost of engine R&D
- cost of handling the fuel on earth
- environmental impact
Fluorine was R&D heavily in the 60s. RD-301, RD-350 and RRC studies It was canceled for obvious reasons.
The highest specific impulse chemistry ever test-fired in a rocket engine was lithium and fluorine, with hydrogen added to improve the exhaust thermodynamics (making this a tripropellant) [ARBIT, H. A., CLAPP, S. D., DICKERSON, R. A., NAGAI, C. K., Combustion characteristics of the fluorine-lithium/hydrogen tripropellant combination. AMERICAN INST OF AERONAUTICS AND ASTRONAUTICS, PROPULSION JOINT SPECIALIST CONFERENCE, 4TH, CLEVELAND, OHIO, Jun 10-14, 1968. ] . The combination delivered 542 seconds (5.32 kN·s/kg, 5320 m/s) specific impulse in a vacuum. 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, fluorine ignites on contact with most fuels, and hydrogen, while not hypergolic, is an explosive hazard. Fluorine and the hydrogen fluoride (HF) in the exhaust are very toxic, which damages the environment, makes work around the launch pad difficult, and makes getting a launch license that much more difficult. The rocket exhaust is also ionized, which would interfere with radio communication with the rocket.