The bond energy of $O_2$ is 498kJ/mol. The bond energy of $O_3$ is 364kJ/mol.

That means the energy to break up the particles into 3 moles of monoatomic oxygen (which then binds with fuel to release energy) is 747kJ for $O_2$ and only 364kJ for $O_3$. You're saving 127kJ/mol of $O$ using ozone instead of diatomic oxygen.

241.8 kJ of energy surplus is produced burning a mole of hydrogen using (half a mole of) $O_2$. It looks like using ozone could bump this by over a half! 50% better specific energy of LFO fuel?

...why aren't we doing this? Why isn't ozone at least a considerable part of the oxidizer composition? Or did I make a mistake somewhere? Or is ozone so hard to obtain or otherwise ungrateful that it's not worth the headache?

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    $\begingroup$ You're vastly overestimating the effect of bond energy on the specific impulse. Look up the specific impulses in literature and you find about less exciting numbers - its not that it hasn't been tried, just the gains do not outweight the complications. $\endgroup$
    – Durandal
    Commented Dec 20, 2016 at 12:00
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    $\begingroup$ @Durandal: I don't think I'm underestimating it. Sure it scales with square root of specific energy, but then launch mass scales with exponent of ISp, meaning a small gain in ISp is a big gain in launch mass. And this is a unique situation where we can considerably improve ISp of high thrust chemical engines, whose fuel currently is the lion share of launch mass. Of course the explosive nature of ozone is kinda bummer... $\endgroup$
    – SF.
    Commented Dec 20, 2016 at 16:05

2 Answers 2


The book 'Ignition!' tells the history of propellant research and has this to say about ozone from page 112 available here

For it has its drawbacks. The least of these is that it's at least as toxic as fluorine. (People who speak of the invigorating odor of ozone have never met a real concentration of it!) Much more important is the fact that it's unstable — murderously so. At the slightest provocation and sometimes for no apparent reason, it may revert explosively to oxygen. And this reversion is catalyzed by water, chlorine, metal oxides, alkalis —and by, apparently, certain substances which have not been identified. Compared to ozone, hydrogen peroxide has the sensitivity of a heavyweight wrestler. Since pure ozone was so lethal, work was concentrated on solutions of ozone in oxygen, which could be expected to be less dangerous.

Further reading of that chapter will explain more the issues with dealing with ozone. One of the major issues was that when mixed with O2 to increase stability and reduce toxicity at 75% O2 and 25% O3, it tended to separate from O2 which has a lower boiling point (at 90K compared to ozone at 161K) in effect distilling into higher and higher concentrations of ozone, so we are back to the more volatile, more toxic product. (see page 113 of Ignition for this info)

In conclusion, from page 114

For ozone still explodes. Some investigators believe that the explosions are initiated by traces of organic peroxides in the stuff, which come from traces, say, of oil in the oxygen it was made of. Other workers are convinced that it's just the nature of ozone to explode, and still others are sure that original sin has something to do with it. So although ozone research has been continuing in a desultory fashion, there are very few true believers left, who are still convinced that ozone will somehow, someday, come into its own. Im not one of them.

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    $\begingroup$ That book is well worth a read, even if it is a little heavy on the chemistry $\endgroup$
    – Chris H
    Commented Dec 19, 2016 at 16:45
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    $\begingroup$ It's a fantastic book, and even though he talks about all the chemistry behind the research, it reads more like a person telling you a story, rather than a dry textbook. I actually had to stop myself from laughing repeatedly while reading. $\endgroup$ Commented Dec 19, 2016 at 20:57
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    $\begingroup$ Ah, 'Ignition!', I am always happy when that lovely book shows up. $\endgroup$ Commented Dec 19, 2016 at 22:38
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    $\begingroup$ "I have heard of no motor runs with the mixture." note that this reffers specifically to ozone/flourine mixtures, not to ozone in general. $\endgroup$ Commented Dec 20, 2016 at 17:02
  • $\begingroup$ @PeterGreen I'll edit that portion of the quote out (this was an attempt to make the paragraph more readable, but i seem to have changed the meaning) $\endgroup$ Commented Dec 20, 2016 at 17:31

Ozone is not stable, storing in liquid form as a solution in LOX requires stabilizers. It will oxidise most metals. A gaseous mixture of ozone and oxygen may decompose explosively. And it is poisonous in low concentrations. Nobody wants such boiloff gases at the launch pad. See https://en.wikipedia.org/wiki/Ozone

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    $\begingroup$ Ozone's reputation as a "poisonous" gas is perhaps overly simplistic when stated like that. It has many beneficial uses, such as destroying bacteria, fungus, tumors, viruses and helping athletes perform better when directly injected into the body. $\endgroup$
    – CPHPython
    Commented Dec 20, 2016 at 16:39
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    $\begingroup$ @CPHPython That link emphasizes that ozone is responsible for a number of deaths in the medical setting, and nearly every use of ozone is better served by other substances, especially around people $\endgroup$ Commented Oct 14, 2018 at 10:26

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