Presumably, it would be expensive to use ozone ($\ce{O3}$) as an oxidizer instead of $\ce{O2}$, but would the extra oomph be worth it? Does $\ce{O2}$ provide as much thrust/energy/heat as can be provided, given the liquid hydrogen propellant?

I figured Chemistry was better than Physics, Engineering or Astronomy for this question.

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    $\begingroup$ All that would happen is a massive explosion, before even getting to the rocket. Ozone is not as temperamental as FOOF, but it cannot be tamed. $\endgroup$
    – Ed V
    Commented Dec 19, 2020 at 3:29
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    $\begingroup$ It is way too reactive (like aggressive) if stored at in high concentrations. Maybe the conditions performing an ozonolysis are not familiar to you. However, descriptions about ozone itself and reactions against e.g., metals (think about tubings, fittings, framework of a rocket) quickly outweigh the little «extra umpf» to reach a higher temperature of combustion. Because rockets' propulsion is based on momentum ($p = m \cdot v$), overall, it is cheaper to generate more gaseous volume per time. $\endgroup$
    – Buttonwood
    Commented Dec 19, 2020 at 8:04
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    $\begingroup$ Liquid ozone has tendency for spontaneous explosion. especially at contact with anything ozone can oxidize. As an astronaut or a satellite, I would not like to sit on a basin of liquid ozone. Additionally, it is serious environmental pollutant even at as low concentration as 100 ppb v/v. $\endgroup$
    – Poutnik
    Commented Dec 19, 2020 at 10:58
  • $\begingroup$ Ozone's boiling point is −112 °C ("normal" oxygen's is −183 °C). $\endgroup$ Commented Dec 19, 2020 at 13:38
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    $\begingroup$ Read 'Ignition', linked in Matt's answer. Recommend any technical friends read Ignition too, even if they're not into rockets. It's a great read, you don't need to know anything about rockets to appreciate the humour, of which there is a foretaste in the quotes Matt has chosen to include in his answer. $\endgroup$
    – Neil_UK
    Commented Dec 20, 2020 at 11:47

2 Answers 2


As is usual with rocket fuels, the problems of ozone are practicality not performance

Almost every answer for why a specific rocket fuel component is used or not will end up referring to John D Clarke's magnificent and sparklingly written book: Ignition: An informal history of liquid rocket propellants (a rare technical book worth reading for the brilliant and humorous style in addition to the technical content).

His summary of why ozone, despite its apparent functional advantages, is not more widely used is fairly simple: the practical problems outweigh the apparent advantages.

He points out the advantages:

What makes it attractive as a propellant is that (1) its liquid density is considerably higher than that of liquid oxygen, and (2) when a mole of it decomposes to oxygen during combustion it gives off 34 kilocalories of energy, which will boost your performance correspondingly. Sänger was interested in it in the 30’s, and the interest has endured to the present. In the face of considerable disillusionment.

But every available way of creating a liquid with a high proportion of ozone is dangerous. Ozone is extremely toxic and unstable:

For it has its drawbacks. The least of these is that it’s at least as toxic as fluorine. ... 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.

Work was done on solutions of ozone in liquid oxygen which is somehow more stable. But has the disadvantage that ozone/oxygen mixtures separate into two phases the ozone rich version of which is difficult to prevent in feed tubes after firing and is extremely unstable. Another mixture considered to make handling easier was with liquid fluorine (!!!).

Ultimately he concludes the known work on ozone mixtures of any sort:

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. I’m not one of them.

Maybe there are theoretical advantages, but they are outweighed by the practical and safety problems. In a profession used to testing things like FOOF (fluorine dioxide) and chlorine trifluoride, this is some admission to make.

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    $\begingroup$ Ignition! and Things I Won't Work With have some of the greatest chemistry quotes of all time. $\endgroup$ Commented Dec 20, 2020 at 0:38
  • $\begingroup$ In this context, does "mixture considered to make handling easier" mean "mixture which was evaluated for the purpose of making handling easier" or "mixture which, in people's opinions, made handling easier?" (If it's the latter, then it's awfully interesting that people tried it and it appeared to work.) $\endgroup$ Commented Dec 20, 2020 at 12:45
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    $\begingroup$ I think it should be apparent by now that "we" use the most powerful fuels and oxidizers which are consistent with practicality. If you -- an amateur -- has thought of it, experts have thought of it; if they aren't using it, there's a good reason why they aren't using it. $\endgroup$
    – RonJohn
    Commented Dec 20, 2020 at 16:43
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    $\begingroup$ @RonJohn: That's more or less the difference between science and engineering. I never understood why "rocket science" is used as a placeholder for "insanely difficult", because it is essentially high school level maths and newtonian mechanics, and undergrad organic chemistry and fluid mechanics. Rocket engineering is where it gets hard and rocket economics is where it get really hard. (Actually, I consider economics part of engineering: "an engineer is someone who can do with a dollar what any fool can do with two". [Source unknown]) $\endgroup$ Commented Dec 20, 2020 at 20:57
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    $\begingroup$ @JörgWMittag orbital mechanics is only "high school level math" after "egghead" mathematicians figure out all the complicated maneuvers and orbits. $\endgroup$
    – RonJohn
    Commented Dec 20, 2020 at 21:14

Liquid ozone is rather stable (as in "can be stored") as an up to 70% solution in LOx, if you add some stabilisers. That would give quite a bit of additional boost in a rocket, but it's surely going to explode already in the rocket's fuel pumps and tubing, instead of burning in the engine's nozzle.

Rocket engines are tricky enough to build, you don't want to add the requirement that all material is of a kind that does not catalyse a premature decomposition of ozone. It's usually a runaway process: Decomposition increases temperature, temperature increases decomposition rate, .... boom.

  • $\begingroup$ Apart from practical considerations, what are the actual numbers with regard to the difference in (potential) energy? Is there only a 1% difference between using oxygen and ozone? $\endgroup$ Commented Dec 19, 2020 at 13:34
  • $\begingroup$ @PeterMortensen That's a very simple piece of math. Just look up the formation enthalpies of water and ozone. $\endgroup$
    – Karl
    Commented Dec 19, 2020 at 21:19
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    $\begingroup$ Back-of-envelope math says +25%. $\endgroup$
    – Karl
    Commented Dec 19, 2020 at 21:33
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    $\begingroup$ @MSalters: Oxygen radicals usually result in an engine-rich combustion. $\endgroup$
    – Joshua
    Commented Dec 21, 2020 at 20:13
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    $\begingroup$ @MSalters "Does it matter if the ozone decomposes just before use?"... not really. as long as your plumbing is up to the job of having its contents jump in temperature by about 730K, with very little notice. (this may occur in your main oxidizer tank, if the Ozone is in a bad mood) $\endgroup$
    – PcMan
    Commented Dec 22, 2020 at 9:43

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