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One of notes near the end of "Ignition!" included:

Using lithium and fluorine alone (no hydrogen) their maximum specific impulse was 458 seconds. But when they proportioned the lithium and fluorine to burn stoichiometrically to LiF, and injected hydrogen to make up 30 percent of the mass flow, they measured 542 seconds —probably the highest measured specific impulse ever attained by anything except a nuclear motor. And the chamber temperature was only 2200 K! Performance like that is worth fighting for.

(emphasis mine).

That was about 60 years ago. And it really leaves LH2/LOX performance in the dirt. Nuclear rockets are a political impossibility, and electric propulsion makes only sense for very light spacecraft.

Is this tripropellant combination entirely forgotten, or does it find any use - in actual plans, or just projects - or was there some terrible fault found that made it not viable at all even out of atmosphere? What is its current status?

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    $\begingroup$ Flourine is nasty stuff. And lithium is a solid at room temperature. $\endgroup$ – Organic Marble Jan 19 '17 at 13:07
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    $\begingroup$ Re nuclear rockets are a political impossibility -- so is anything that uses fluorine. $\endgroup$ – David Hammen Jan 19 '17 at 18:56
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    $\begingroup$ "but while mishandling consequences may be worse" Um, yeah. $\endgroup$ – Organic Marble Jan 19 '17 at 20:57
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    $\begingroup$ Henry Spencer: "Unfortunately, it has all the problems of liquid fluorine, all the problems of liquid lithium (which is not only hot but fiercely corrosive), and all the problems of liquid hydrogen. Pity." $\endgroup$ – Russell Borogove Jan 21 '17 at 13:12
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    $\begingroup$ @RussellBorogove, +1 for quoting Henry. It's been a long time since I've been to SAS. $\endgroup$ – ShadoCat Oct 16 '18 at 23:52
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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.

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    $\begingroup$ 21K versus 453K? Wow, that's insane. What happens when you mix liquid lithium at 21K with hydrogen at 453K, and how do you keep such a close proximity with such an extreme temperature difference? $\endgroup$ – Magic Octopus Urn Oct 17 '18 at 1:22
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    $\begingroup$ Not only flourine is toxic, hydrogen flouride is also toxic. Even skin contact is dangerous. $\endgroup$ – Uwe Oct 17 '18 at 8:39
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So for the moment ignoring the large-scale ecological damage issues that a large quantity of fluorine/hydrofluoric acid would represent even in the event of a successful launch (let alone an explosion on the launch pad), the other difficulty is an engineering one.

As mentioned in Ignition! and in ilyakharlamov's answer, you have to keep three wildly different substances in liquid form. One at -252 °C, one at 180 °C and one at -219 °C.

This is not an impossible engineering challenge. However, a greater quantity of insulation is needed, in addition to some form of electrical heating to keep the lithium liquid in-situ (on the pad at least, and in the air at most). Rocket design suffers (or benefits, depending on who you talk to) from a square-cube law. The mass of the propellant tank, including insulation, increases as the square of the scale of the rocket, while the volume of the propellant increases as the cube. So for complicated rocket designs involving three tanks with lots of insulation, only the largest designs of Li/H/F rockets become more efficient compared to hydrolox/kerolox rockets.

When not ignoring the ecocidal nature of such a rocket, only having a really big one be feasible creates... issues.

Also, this is ignoring the engineering nightmare of designing a liquid rocket engine that can handle all three propellants and inject them into the same chamber and not completely fail when the lithium solidifies in the pipes. Getting bipropellant engines functioning well took a good chunk of the 20th century (I'd say they were the limiting aspect of progress on rocketry), so I can't imagine the difficulties involved with tripropellant rocketry, even if the propellants themselves were relatively benign.

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    $\begingroup$ Great point on the 'only feasible when big'. $\endgroup$ – SF. Oct 17 '18 at 4:27
  • $\begingroup$ Devil's advocate here: couldn't you get around the problems with handling liquid lithium by using a hybrid-rocket design, with liquid fluorine flowing across a solid-lithium fuel grain, and hydrogen then injected in downstream? $\endgroup$ – Sean Jun 20 at 0:11
  • $\begingroup$ I don't know much about liquid-solid engines, but what you describe sounds frightening to me. $\endgroup$ – Ingolifs Jun 20 at 1:23
  • $\begingroup$ @Ingolifs: How so, and why would that be any scarier than using liquid fluorine with molten lithium? $\endgroup$ – Sean Jul 14 at 0:43

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