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Fluorine is a bit more efficient chemically than oxygen. It has never been used as a launcher oxidizer because it and its exhaust products are so toxic. That's as much I've heard about it anyway. But, would it be useful for engines used only in space where toxicity doesn't matter, as in a third stage or for satellite station-keeping in small engines? Or is it too corrosive even for that?

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  • $\begingroup$ It's awfully difficult to handle, and at some point you are going to be filling tanks of it on the ground prior to launching them into space. $\endgroup$ – Organic Marble Sep 12 '15 at 21:22
  • $\begingroup$ @OrganicMarble Is it really that bad? Hypergolics and hydrogen peroxygen aren't nice either. $\endgroup$ – LocalFluff Sep 12 '15 at 21:25
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    $\begingroup$ See Why are hydrogen-fluorine fuels not used for rockets more frequently? $\endgroup$ – TildalWave Sep 12 '15 at 21:31
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    $\begingroup$ Check out this book: it's a fun read about early attempts to use exotic fuels. I remember reading that there was no way to extinguish a metal/flourine fire because it would burn all firefighting agents(!) web.gccaz.edu/~wkehowsk/ignition.pdf $\endgroup$ – Organic Marble Sep 12 '15 at 21:46
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    $\begingroup$ Incidentally, "in space where toxicity doesn't matter", exhaust from hydrazine thrusters on ISS does actually pose a toxicity concern to crews returning from EVA. Obviously this isn't a general problem for unmanned satellites, but something to keep in mind. $\endgroup$ – Russell Borogove Sep 12 '15 at 23:23
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The safety-related costs of developing, testing, and fueling fluorine-oxidized rockets outweigh the costs of building slightly bigger and less efficient rockets using hypergolics or LOX.

In launcher stages, hydrolox is only 5% less efficient by Isp than hydrogen-fluorine; the higher density of fluorine makes for more compact tankage, though, which means less structural mass overall. Your rocket will wind up bigger using lox, but it will also be cheaper, safer, and less toxic.

Hydrogen-fluorine also burns about 900 degrees hotter than hydrolox; keeping the chamber and nozzle robust against hot corrosive gases is going to add weight one way or another.

Fluorine is cryogenic, liquifying at close to the same temperature as oxygen, so it's impractical to store for long-term missions. For long-term satellite stationkeeping, the go-to solution nowadays is an electric propulsion system such as ion thrusters; way more efficient than H/F and again, much safer.

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  • $\begingroup$ But what about tanks with a few tens of kilograms of fluorine prepared and sealed in lab environment, for satellites' station keeping on the top of the rocket equation, so to speak, where those 5% or so in extra performance count the most? Is there no reasonable way to contain it for a few years in space? $\endgroup$ – LocalFluff Sep 12 '15 at 22:17
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    $\begingroup$ You're trying to solve a non-problem. For long-term station-keeping operations, electric propulsion systems (such as ion thrusters) are in wide use, and are vastly more efficient and safer. americaspace.com/wp-content/uploads/2012/08/… space.stackexchange.com/questions/8105/… $\endgroup$ – Russell Borogove Sep 12 '15 at 23:06

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