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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    $\begingroup$ Do you know how conservation of energy works? $\endgroup$ – Floris Sep 20 '17 at 15:55
  • $\begingroup$ Breaking water down into hydrogen and oxygen costs as much energy as the combustion of hydrogen and oxygen to water releases. $\endgroup$ – noah Sep 20 '17 at 15:56
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    $\begingroup$ You're trying to have your cake and eat it. Not unlike a perpetual motion machine. $\endgroup$ – StephenG Sep 20 '17 at 16:17
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    $\begingroup$ You have cross posted this on Aviation SE and Chemistry SE, which is not generally good behavior on a site like this. And the answers have been quite emphatically "no, you can't use water". $\endgroup$ – StephenG Sep 20 '17 at 20:54

Energy has to come from somewhere--for example, to keep the hot parts of the engine from cooling down. If you start with water, split it, then recombine it, then you're back where you started. You can't get energy that way. It's not analogous to real jet fuel, where you start with a higher-energy chemical configuration (hydrocarbons + oxygen in the air) and end up with lower-energy (carbon dioxide + water vapor). In the process, energy is released, in the process creating heat and performing mechanical work, yet without violating Conservation of Energy.

So in this context water is not a "fuel".

  • $\begingroup$ Water is the by-product. This is essentially asking "can I use the combustion products and waste heat to power a jet?" And the answer is "No" unless you become a wizard who can reverse irreversible processes. $\endgroup$ – JMac Sep 20 '17 at 16:00
  • $\begingroup$ @JMac sorry even a reversible process doesn't produce net power... $\endgroup$ – Floris Sep 20 '17 at 16:19
  • $\begingroup$ I don't see how this violates conservation of energy: the water would absorb heat energy from the engine components and carry it away in the exhaust. The efficiency increases because waste heat is reduced... unless that high temperature is critical for engine operation somehow, of course. $\endgroup$ – Asher Sep 20 '17 at 16:21
  • $\begingroup$ @Asher - if you are using "some of the exhaust heat" to convert water to hydrogen and oxygen so the over all efficiency of the engine goes up, then no laws are violated. But that's not really "using water as fuel" in the way most people read the question. $\endgroup$ – Floris Sep 20 '17 at 16:22
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    $\begingroup$ @Floris Yes, you're right. And I've just realised that this is the same thing: if you can prevent knocking you can increase the compression ratio & thus efficiency. I hadn't connected this before: thanks! $\endgroup$ – tfb Sep 21 '17 at 11:05

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