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Nitrogen trifluoride (NF3) is a liquid between 66K and 144K and has an incredible density of 1.885 g/cm2. It has a pretty great specific impulse, although combustion temp is a bit high, and AFAIK is stable, although very poisonous. NF3/N2H4 with O/F 2.95, 7.4 Ae/At, and chamber pressure and temp. 6MPA and 4260K, respectively, have a vacuum-specific impulse of 345s and a density impulse of 535s! Why is it not a commonly used propellant, or at least received attention for its potential?

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    $\begingroup$ Safety concerns, sensibly enough, have generally eliminated fluorine-based oxidizers from serious consideration. $\endgroup$ Commented Sep 20, 2022 at 20:47
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    $\begingroup$ I could see it as a reasonable second/third stage oxidizer. Keeping the upper stages compact and as result light, high Isp, spraying poison where no-one minds it. Of course there's still the whole ground handling and tests side to it, never mind abort scenarios. so I bet the gap of technology readiness becomes the ultimate roadblock: if we had rockets on it running reliably, they'd be great, but getting them requires the stage of rockets running unreliably and we can't really afford that. $\endgroup$
    – SF.
    Commented Sep 21, 2022 at 8:42
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    $\begingroup$ "although very poisonous". That's your answer in the US and Europe. Russia and China probably think it's a great idea. $\endgroup$
    – RonJohn
    Commented Sep 21, 2022 at 13:42

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It is possibly a jack of all trades and master of none Ignition by J Clark only talks about NF3 as a precursor to making other more exciting fluorine compounds, NF3 itself being mentioned as hard to make:

making NF3 is tricky enough, It's done by electrolyzing molten ammonium biflouride, using graphite electrodes. They have to be graphite - if you use nickel you do not get any NF3 and the yield depends on who made the graphite. Don't ask me why.

and that it is 'rather inert', presumably for a fluorine compound.

It is still cryogenic, so no particular advantage over pure fluorine there. Most of the work on fluorine compounds seems to have been in pursuit of something liquid at room temperature.

It will still produce all the exciting compounds that burning fluorine gets, making a mess of your plumbing and launch pad.

So it is complex to work with, still a fluorine compound and still must be keep cold and is inferior in pure performance to boring hydrogen/Oxygen.

If you want to do all of those things, why not go the whole hog with lithium/Hydrogen/fluorine at 542 seconds ISP?

The major needs in space flight are: First and second stages, where simplicity/safety/cost matter over peak performance

Transfer/injection stages where in almost all cases you need to have something that can sit in the tank being boring till you need it, and high performance.

So it is not used because while it excels at one thing(density) space launches do not actually need density over all else (see use of liquid hydrogen). If there is a situation where drag or total volume does matter than it might see use - maybe in some type of low altitude very high speed weapon where keeping drag and radar cross section low matters over all else (and toxic exhaust becomes a feature)?

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    $\begingroup$ Great answer. I was expecting more along the lines of 'because it will melt your face', which is the typical answer for questions of this type. $\endgroup$
    – GdD
    Commented Sep 20, 2022 at 16:17
  • $\begingroup$ You're right about its uses at low altitude, although for military purposes I can't imagine a cryogenic would be appreciated. Thankyou very much for your answer, it was very detailed $\endgroup$
    – R. Hall
    Commented Sep 21, 2022 at 3:05
  • $\begingroup$ Thank you for an interesting article on a kerolox/hydrolox engine that I had never heard of before. $\endgroup$ Commented Sep 21, 2022 at 3:58
  • $\begingroup$ According to Ignition, cryogens were tolerated in strategic ICBMs, at least as long as they were about the only option. $\endgroup$
    – ikrase
    Commented Sep 21, 2022 at 5:28
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    $\begingroup$ Too bad nobody got acetylene/ozone working. $\endgroup$
    – Joshua
    Commented Sep 21, 2022 at 18:59

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