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I was recently doing a chemistry assignment about bond energy when I noticed the incredibly low bond energy of nitric acid. Given the high bond energy of a nitrogen triple-bond, this set me wondering about its utility as a rocket fuel, and I quickly wrote out a few equations:

Methane-Oxygen Combination

CH4 + 2O2 --> CO2 + 2H2O

2652 kJ/mol --> 3462 kJ/mol

yields 810 kJ/mol

Nitric Acid-Hydrogen Combination

6HNO3 + 15H2 --> 3N2 + 18H2O

12492 kJ/mol --> 18596 kJ/mol

yields 6104 kJ/mol

*Bond energies from Chemistry LibreTexts

Nitric acid is famously hazardous. But surely a reaction output an order of magnitude higher than current chemical fuels combined with a completely clean exhaust should make this the leading liquid rocket fuel in the aerospace industry. Why is this particular combination not used? Further research from Ignition (as well as SF's comment below) indicate that nitric acid has been used as an oxidizer in the past, so it's not completely unfeasible. Is my chemistry just wrong? Thank you for taking the time to answer this.

*Fixed error in the second equation, 2N3 was changed to 3N2. 20/3/2024

Robert Goddard-Wright
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    RFNA (Red Fuming Nitric Acid) and/or nitrous oxide (these two seem to co-exist in precarious balance) is used as oxidizer with unsymmetrical dimethylhydrazine + (plain) hydrazine bipropellant rockets, in particular in military; these two are hypergolic with each other, making the engine construction more simple, robust and cheaper. – SF. Mar 19 '24 at 11:15
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    It is interesting that Ignition completely skips Nitric Acid and Hydrogen despite talking about all sorts of other combinations with both suggesting the issue is obvious to those in the field. Possibly just that Nitric acid freezes at liquid hydrogen temps making tanks and injectors challanging? – GremlinWranger Mar 19 '24 at 12:45
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    For one reaction, you have 20 molecules sharing the energy, while for the other you have three. – John Doty Mar 19 '24 at 12:58
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    Seems like it combines all the inconvenience of cryogenic propellants with all the inconvenience of highly corrosive chemicals, and might be a bit of a pain to start. I'm not seeing an obvious upside. – Starfish Prime Mar 19 '24 at 13:01
  • @GremlinWranger There's a general sense - which is stated outright a few times - that propellant engineers thought of liquid oxygen as the ideal, energetically speaking, and only bothered with nitric acid and the rest because of its less-than-ideal handling characteristics. But I can't quite put my finger on why that should be so, or whether they were correct. – Cadence Mar 19 '24 at 15:17
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    The equations of motion refer to masses, not moles; what are the respective energies per mass unit for those reactions? – Russell Borogove Mar 19 '24 at 16:12
  • @SF. The red stuff is nitrogen dioxide. – PM 2Ring Mar 21 '24 at 21:36
  • one can write chemical reactions like the one for nitric acid and hydrogen, but that does not mean that those are the reactions that happen when the two compounds are mixed. It is fairly implausible–even ignoring the reaction conditions of cryogenic hydrogen and a very freezable liquid–that this reaction would yield molecular dinitrogen. The biggest problem might be the chemistry. – matt_black Mar 21 '24 at 23:20

2 Answers2

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"Kilojoules per mole" is not the most important measure of a rocket fuel's effectiveness. The most important is effective exhaust velocity, often expressed as "specific impulse".

The main contributors to effective exhaust velocity are the number of exhaust molecules the reaction energy is distributed over, and the mass of those molecules. Despite nitric acid/hydrogen producing about seven times as much energy as methalox, that energy is spread over about seven times as many molecules, and those molecules are only marginally lighter than those of methalox.

A nitric acid/hydrogen engine might be marginally higher-performance than a methalox one (particularly if you boost the hydrogen beyond stoichiometric levels to bring the exhaust mass down), but it's nowhere near as spectacular as your calculations make it look. And hydrogen has a density problem: once you add in the extra tankage mass, the performance of a nitric acid/hydrogen rocket, as opposed to a nitric acid/hydrogen engine, will probably be inferior to methalox.

Mark
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The "why not this combination" answer: in Ignition! it's mentioned that the main drive for using RFNA as a propellant is that it's an oxidizer that's liquid at outdoor temperatures (say, -40 °C to +50 °C) and can be stored "indefinitely". The US military wanted something suitable for missiles on permanent alert. That implies using it with a fuel that's also liquid at outdoor temperatures, such as hydrazine.

Once you switch one of fuel or oxidizer to cryogenic, you might as well switch the other one, because you now have the inconvenience of managing boil-off. Cryogenic rockets are usually fuelled a short time before launch. Whereas the ICBM use case was "store for ten years and maybe launch with a few minutes' notice". Hence the use of liquid fuels by basically all ICBMs.

(See Why can’t cryogenic propellants be storable, at least on the ground, via refrigeration?.)

Peter Mortensen
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pjc50
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    Most ICBMs actually use solid fuel, once they figured out how to do it reliably. Liquid fueled missiles tend to have toxicity and corrosion issues to work around, which may make them harder to store for long periods of time. – costrom Mar 20 '24 at 17:31
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    I think you mean "Hence the use of non-cryogenic fuels by basically all liquid-fuelled ICBMs." The earliest ones (R-7, Atlas, Titan I) used liquid oxygen, but everyone gave that up as soon as possible. – John Dallman Mar 20 '24 at 19:11
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    @costrom Worth noting that Ignition cuts off right at about Apollo and also does not cover solid fuels at all. – ikrase Mar 21 '24 at 08:44
  • "Once you switch one of fuel or oxidizer to cryogenic, you might as well switch the other one" - that's an exaggeration: the combination of cryogenic oxidizer with stable fuel is common, usually Kerolox. This way around it makes sense because the oxidiser can be vented without too much risk, and oxygen has huge advantages over liquid or solid oxidizers. With fuels, not so much. – leftaroundabout Mar 21 '24 at 11:32
  • The last US ICBM to use a hypergolic combination (or liquids of any kind) was Titan II, which we got rid of in the 1990s partly as a trade in a strategic arms limitation treaty with the Soviets. – Chris Ison Mar 22 '24 at 20:45
  • @leftaroundabout That's also because as far as cryogenics go,LOX is a relatively easy one. Compared to that,liquid hydrogen is a real beast to handle (actually, it's a real beast to handle compared to basically everything): it seeps right through lots of ordinary materials, including most seals for example,and hydrogen embrittlement of steels is also no fun. If you can avoid LH2 without suffering an unacceptable performance penalty,you certainly will. – TooTea Mar 23 '24 at 21:34
  • @TooTea not really, oxygen has its own problems. Most cryogenic fuels, like methane, are arguably safer and generally "easy" if you disregard the cooling/venting issue... which is a big if! Hydrogen is a very special special case. – leftaroundabout Mar 23 '24 at 22:21