The use of liquid fluorine as an oxidizer diluted with liquid oxygen has been tested, but wasn't flown, by Bell and Rocketdyne, seperately.


It was determined that its toxic and explosive nature made it unsuited to use as a launch vehicle propellant.

Unmanned tugs or landers for lunar orbit and landing, if we aim to make them reusable to save on launch costs, would be most useful with high energy propellants.

Given the advantages of this propellant mix, and its lack of danger when on an unmanned lunar vehicle of any type, what has prevented this from being utilised already?

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    $\begingroup$ Pardon me a moment. AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA! Okay sorry, go ahead. $\endgroup$ Feb 21 at 20:25
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    $\begingroup$ In a small combustion research company in 1960s, we were exploring fluorine oxidizer ignition and combustion properties with various fuels. At an initial test that we did in the parking lot after hours, we cracked the valve on the fluorine bottle to release fluorine to the test apparatus and the regulator just burned up virtually instantaneously. The regulator was not properly passivated. A whole bottle of fluorine was released over Monrovia California. $\endgroup$
    – tckosvic
    Feb 21 at 23:38
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    $\begingroup$ New rule, anyone suggesting propellant combinations must read Ignition! first $\endgroup$ Feb 22 at 9:53
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    $\begingroup$ Have you by chance ever read the Things I Won't Work With blog? If not, I recommend it for his descriptions of anything involving fluorine. $\endgroup$
    – Graham
    Feb 22 at 13:13
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    $\begingroup$ In addition to these other good points, there wouldn't be much point to it because if you were willing to brave the hazards of fluorine chemistry, you'd skip the pure stuff and go with chlorine trifluoride (or perhaps pentafluoride) which offers non-cryogenic liquid storage and better specific impulse. $\endgroup$
    – Cadence
    Feb 22 at 14:45

2 Answers 2


The fact that you're thinking about using it in an unmanned vehicle doesn't really help with the whole "toxic and explosive" situation. (And that phrase is rather underselling it, if you ask me.)

That is to say, the crew of a space craft is not the only party vulnerable to the nasty effects of incredibly dangerous fuels. Unless you're somehow intending to get the liquid fluorine (shudder) from a lunar source, it would have to come from earth, which means somebody somewhere has to produce and bottle this stuff for transport, and somebody somewhere has to load up a rocket with it and fire it off into space.

There are ways to stabilize fluorine for transport (possibly by making potassium fluoride and then electrolyzing it later), but if you're going to all that trouble, it's probably cheaper to simply transport a larger volume of a lower energy fuel in the first place.

And even if you could completely isolate the stuff from humans, the fact that no human lives are endangered won't make dangerous fuels a good plan. Cold fluorine is extremely reactive with practically anything that can possibly give up an electron, and violently hypergolic with practically anything that contains hydrogen. I'm not sure diluting it with pure oxygen actually helps that much. The risk of blowing up a very expensive lunar lander because of a drop of hydraulic fluid in the wrong place is probably too big to be acceptable.

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    $\begingroup$ Instead of diluting it with oxygen, react it instead to make FOOF for extra fun! $\endgroup$ Feb 21 at 20:50
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    $\begingroup$ @fyrepenguin I'm not sure I can handle that much fun, you can do it. $\endgroup$ Feb 21 at 21:21
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    $\begingroup$ Actually that's an interesting idea. Launch it as potassium fluoride, electrolyze it in space with solar panels, and use when you want the thrust. So many engineering difficulties. $\endgroup$
    – Joshua
    Feb 22 at 5:04
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    $\begingroup$ @Joshua That's kinda what I was thinking about, but like... at that point just load up with plain old kerolox,, honestly? Or any of the old standby hypergolics like MMH/N2O4? I don't see the benefit of playing with fluorine just to get a relatively tiny Isp advantage. $\endgroup$ Feb 22 at 15:58
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    $\begingroup$ The one Ignition! refers to as being violently hypergolic with everything including test engineers? Everything in my answer that I said about LF2 applies to ClF3, possibly even more so. Leave the Nightmare Juice alone! $\endgroup$ Feb 23 at 17:22

Fluorine is an engineering nightmare.

Fluorine is a safety nightmare.

Fluorine is a logistical nightmare.

It is also a regulatory nightmare. First because all of the above and second because Fluorine is also an importani ingredient and precursor of sarin, as well as some other relatively low-tech militarized nerve agents.

  • $\begingroup$ All of this is true, but pretty much exactly the same could also be said about the plutonium in RTGs. Yet these nightmares can be worth it if it solves a problem sufficiently better than all available alternatives. $\endgroup$ Feb 22 at 12:39
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    $\begingroup$ @leftaroundabout Except the quantity of plutonium in them is only enough to keep the RTG going. That's not even just orders of magnitude different quantities - it's orders of magnitude of orders of magnitude. If you want to compare like for like, compare it to Project Pluto or Project Rover, neither of which ever put a rocket in the air because they were too damn dangerous. $\endgroup$
    – Graham
    Feb 22 at 13:20
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    $\begingroup$ @leftaroundabout RTG plutonium is different isotope (238) and pretty much unwanted in weapon-grade Pu where 239 is the essence. $\endgroup$
    – fraxinus
    Feb 22 at 13:31
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    $\begingroup$ @Graham The usual RTG used in space has as much as 1/10 of the bomb core Pu mass $\endgroup$
    – fraxinus
    Feb 22 at 13:32
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    $\begingroup$ The whole reason they had PU238 hanging around for use in space is because it was a waste product of nuclear weapon production. $\endgroup$ Feb 22 at 16:00

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