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Jet engines can run on almost any fuel, and the operating temperatures of modern jet engines' hottest sections are anywhere between 3000 and 3150 degrees F (1648 and 1732 degrees Celsius). Does that mean that a hydrogen on-demand system could work on modern jets?

Water is pumped and heated first by the exhaust section, then directed towards the hotter sections of the engine (when hot enough to not cause cooling and lower engine efficiency) where it's broken down into hydrogen and oxygen at a heat above 1472 degrees F (800 degrees Celsius), then those gases are pumped into the engine for combustion.

The advantages are that firstly, water is abundant and therefore cheap. Even sea water could be used because at those temperatures it's easy to design a system that would get rid of the impurities that would otherwise corrode critical engine parts.

Secondly, it would save on manufacturing costs given that non-heat critical parts in the exhaust section would not need to be made of sophisticated and expensive materials and alloys given the cooling effect of water.

Thirdly, the costs of the fuel weight would be reduced given that the energy density of hydrogen is twice that of fossil fuels, so less would need to be carried. And most importantly, the environment problem would be solved in aviation given that there would be little or no carbon dioxide emissions.

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    $\begingroup$ It won't work. You can't oxidize water exothermically. $\endgroup$
    – MaxW
    Sep 20, 2017 at 16:37
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    $\begingroup$ Where the energy is going to come from? $\endgroup$
    – Ivan Neretin
    Sep 20, 2017 at 17:44
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    $\begingroup$ What's the combustion product of burning hydrogen with oxygen? Water. So why have big fuel tanks at all? We can just condense the water coming out of the back of the engine, pipe it back to the fuel pump, and burn the same few liters of water forever. Now that's fuel weight savings. $\endgroup$
    – hobbs
    Sep 20, 2017 at 19:28
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    $\begingroup$ Dude, we already told you this is impossible over on Aviation SE. Why did you waste everybody's time by posting it again? (cc @Mithoron, who suggested the question should be elsewhere -- it is!) $\endgroup$
    – David Richerby
    Sep 21, 2017 at 9:55
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    $\begingroup$ I'm voting to close this question as off-topic because it's was already asked and answered on Aviation, where it belongs. $\endgroup$
    – Mithoron
    Sep 21, 2017 at 13:30

3 Answers 3

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It can't work because of the fundamental thermodynamics

What you are proposing is, basically, the plane carries water; the water is broken down into its components, hydrogen and oxygen; the components are recombined by burning them as fuel. Burning hydrogen and oxygen is a perfectly good way to create a lot of heat. But it doesn't much matter how you break the water apart into hydrogen and oxygen, the thermodynamics of the reaction won't work.

The problem is simple: you need to have a source of energy to split the water apart. In chemistry we know the energy levels of the reactants and the products and we can work out whether energy is released or stored in a reaction. Burning hydrogen and oxygen releases a lot of energy, but by the rule of thermodynamics, breaking water apart to its components requires the input of exactly the same amount of energy. You can't get round this. Worse, in the real world, there are losses at every conversion step so you can't even break even (ain't things unfair!)

In your plane you could, in principle, split water and burn it in the engine for propulsion. But you would need to have some other source of the vast amount of energy required to split the water. That implies both another fuel and another engine. In reality they would vastly outweigh any imagined savings in weight and cost.

Even if you could build some sort of SciFi engine that both splits water and then burns it again you would still be nowhere: the entire output of the burning would be required for the splitting with nothing left over to generate thrust (and in the real world there are losses so your plane would rapidly drop out of the sky).

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    $\begingroup$ Now, if you had a concentrated, compact power source (like, say, a miniaturized nuclear reactor) you could use it to heat water into steam and use that to generate thrust. You could even do it with just air. The latter option was experimented with back in the '50s, and the Russians even managed to make one fly, although the crews were not re-usable... $\endgroup$
    – Perkins
    Sep 20, 2017 at 19:18
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    $\begingroup$ @LordFarquaad 1) Yes it does apply to other fuels. The (improper) analogy would be that water and CO2 are "separated" into fuel and oxygen, then burned in the jet. Except, of course, this "separation" was done on the ground for a very long time and took a lot of sunlight (and other energy). If you tried to synthesize the fuel on the jet and then burn it again, you'd have the same problem. $\endgroup$
    – Ordous
    Sep 20, 2017 at 19:58
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    $\begingroup$ It doesn't work this way for other fuels because they release energy when they are burned in air (producing water and carbon dioxide, mostly). Fission is a nuclear process not a molecular one and can't be compared to chemical stuff (except that even in nuclear processes, energy is only released when the products have less than the starting materials: you don't release energy by splitting oxygen nuclei in two). $\endgroup$
    – matt_black
    Sep 20, 2017 at 20:00
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    $\begingroup$ @LordFarquaad 2) Fission is pretty much the same. You have some heavier elements, they fall apart (because they are unstable or because you bombard them with neutrons or both or other) and give off energy. We've found ways how we can trigger said fission using less energy than is released in the process. But that doesn't explain how we got those heavier atoms in the first place. They were (for the most part) created in a star, and used tons of energy. If we tried to "make" them, we'd use more energy than we can get by breaking them apart. Fission is not "free energy", it's concentrated solar. $\endgroup$
    – Ordous
    Sep 20, 2017 at 20:00
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    $\begingroup$ @LordFarquaad Pretty much! Energy is conserved, so you always have to get it from something or transform something (taking it's energy). $\endgroup$
    – Ordous
    Sep 20, 2017 at 20:13
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The steam engine has already been invented, but you need a source of heat outside the system. It is thermodynamically impossible to use the heat from the steam generated to produce more steam. It gets worse if the heat is converted to something else first, like the heat generated from friction. You are throwing (kinetic) energy and mass out the back of the turbine. Both need to be constantly replaced.

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This is possible when the jet engine has multiple stages.

The first low temperature stage (low temperature meaning 2500°C in this case), water is split into hydrogen and oxygen.

In a next stage, the hydrogen is separated from the oxygen (which is discarded) and compressed to a few bar of pressure.

The third stage is kept at a temperature of above 10.000.000°C. In this stage, the hydrogen is exothermally burned into helium, supplying the heat for stages one and three and also propulsion.

This engine also produces large amounts of radioactive radiation, which can be seen as a disadvantage in some situations.

Building a jet engine which can withstand a temperature of 10.000.000°C is left as an exercise for the reader.

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    $\begingroup$ Ya beat me to it. But you did forget Step 3 discover cold fusion. $\endgroup$
    – Carl Witthoft
    Sep 21, 2017 at 13:00

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