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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.

securitydude5
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    It won't work. You can't oxidize water exothermically. – MaxW Sep 20 '17 at 16:37
  • How do you heat your engine to 3000 degrees in the first place? – Waylander Sep 20 '17 at 16:51
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    You use stored hydrogen or excess stored hydrogen from previous flight hopefully – securitydude5 Sep 20 '17 at 17:14
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    Where the energy is going to come from? – Ivan Neretin Sep 20 '17 at 17:44
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    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. – hobbs Sep 20 '17 at 19:28
  • I feel it's not best site to ask - it's mostly technical problem about jets. In principle this could work, but it wouldn't be practical, but that's for jet engineers to tell not chemists. – Mithoron Sep 20 '17 at 21:59
  • @hobbs Not "forever" but partial steam exhaust condensing to reuse it, could be possible in theory, and better then tanking all the water in the beginning. – Mithoron Sep 20 '17 at 22:04
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    You're asking, why don't we unburn water and then burn it? The simple answer is that the energy you get from burning it has to be put back in to unburn it. – user253751 Sep 21 '17 at 00:23
  • This would work if you could crack apart the water molecule outside the plane and before the flight, and store the hydrogen and oxygen in tanks as fuel ready to use. – Criggie Sep 21 '17 at 04:41
  • The fluorine reacts with water. But don't try at home. – Martín-Blas Pérez Pinilla Sep 21 '17 at 07:41
  • @Criggie You don't take oxygen to burn jet fuel with you on a plane, why take it to burn hydrogen? - Still, the whole proposal is just a perpetuum mobile, can't understand why people seem to have such a hard time wrapping their heads around it. :) – I'm with Monica Sep 21 '17 at 08:13
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    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!) – David Richerby Sep 21 '17 at 09:55
  • @Waylander "How do you heat your engine to 3000 degrees in the first place?" By having a "start cart" at the airport. This isn't the problem with this idea... – David Richerby Sep 21 '17 at 09:57
  • Is this correct :https://i.stack.imgur.com/6oIFp.png I'd assume in the h2O mix 2 thirds is hydrogen therefore for 3.5 kgs of water you get 2.33kgs of hydrogen – securitydude5 Sep 21 '17 at 10:19
  • @AlexanderKosubek fair point, but you're missing my point that the cracking can happen sometime before the flight using ground-based energy. This might work as a way to time-shift solar energy, for example. But then again, a tank of Hydrogen might not be the safest fuel in the air. – Criggie Sep 21 '17 at 10:50
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    @Criggie Well, okay, you probably were focussing your comment on the implied question in the title, if hydrogen would be a suitable fuel for jet engines? The full text question is about a perpetuum mobile, though. Due to the specific assumptions layed[! :) ] out in the question, especially cracking up the water to make hydrogen in-flight, I strongly believe that OP should just do the math and realize that what he supposes will not, as we all know, work. – I'm with Monica Sep 21 '17 at 11:57
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    I'm voting to close this question as off-topic because it's was already asked and answered on Aviation, where it belongs. – Mithoron Sep 21 '17 at 13:30
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    @securitydude5 3.5 kg of water contains 0.4 kg of hydrogen not 2.33kg. Hydrogen atoms weigh a lot less than oxygen atoms and it's 2/3 of the atoms that are hydrogen not 2/3 of the mass. That's basic chemistry. – matt_black Sep 21 '17 at 15:40
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    @Mithoron Don't vote to close it for that reason. The chemical explanation why it can't work is a good explanation of chemistry and thermodynamics. Probably better explained here than on aviation. – matt_black Sep 21 '17 at 15:41
  • Iranians already have created a hydro engine.
    1. Using an efficient catalyst you could get hydrogen from water with a lower amount of electricity needed in hydrolysis process.
    2. After hydrolysis, you'll have oxygen in hand which could be involved in other energy provider processes.
     – Eftekhari Mar 15 '18 at 01:49
  • https://www.youtube.com/watch?v=qSHKt9kWRGg – Eftekhari Mar 15 '18 at 01:54

3 Answers3

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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).

Michael DM Dryden
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matt_black
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    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... – Perkins Sep 20 '17 at 19:18
  • Two questions: 1) Why does this barrier apply to water but not to other fuels? Is it that there's no step to breaking down a fossil fuel before it is burned? 2) I know that fission works because splitting atoms releases more energy than is put into it, so I assumed something similar was possibly for water, but now I'm guessing at that level we're looking at a completely different set of equations, right? If I misunderstand fission you can just say "you misunderstand fission", I'm not asking you to give me a nuclear crash course here. – Lord Farquaad Sep 20 '17 at 19:53
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    @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. – Ordous Sep 20 '17 at 19:58
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    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). – matt_black Sep 20 '17 at 20:00
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    @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. – Ordous Sep 20 '17 at 20:00
  • @Ordous and matt_black, ok, thanks. It sounds like for both those processes then we're taking advantage of energy that's already been introduced into the system, so OP's issue comes from trying to start from scratch? – Lord Farquaad Sep 20 '17 at 20:05
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    @LordFarquaad Pretty much! Energy is conserved, so you always have to get it from something or transform something (taking it's energy). – Ordous Sep 20 '17 at 20:13
  • Matt, I'm afraid you're answer is totally invalid: "How do you heat your engine to 3000 degrees in the first place? You use stored hydrogen or excess stored hydrogen from previous flight hopefully" OP isn't talking about any perpetuum mobile, but recycling waste energy from turbine. – Mithoron Sep 20 '17 at 21:35
  • @Mithoron But how do you generate excess hydrogen on the previous flight? You need it all to provide enough energy to spit the water (even if you assume a lossless system). The best you in the best possible system is to cycle between water and the oxygen/hydrogen mixture with no energy left over for flight. – matt_black Sep 20 '17 at 21:40
  • @LordFarquaad To avoid confusion on the issue with other fuels: the energy produced from combustion of a fuel has nothing to do with splitting it apart. It comes because the products of the combustion have a lower energy then the fuel and oxidant mixture. Energy release comes from the net difference in bond energy between the products and the reactants: some things split up and release energy; some things combine and release energy. – matt_black Sep 20 '17 at 21:53
  • Geez, you simple have "normal" engine using hydrogen tanked before flight, but also carrying some water, taking off some waste heat and partially braking down to O2/H2 which may be used later. – Mithoron Sep 20 '17 at 21:55
  • @Mithoron And how are you going to recover the hot gas to store the fuel?Jet engine thrust is dependent on expelling hot gas at high velocity. Even with out this practical barrier, how do you get more energy out than you put in? – matt_black Sep 20 '17 at 22:01
  • Stealth bombers have exhaust cooled to reduce thermal signature, so I don't see much problem with this. Main obstacle would be probably that it would be very difficult to get actual water dissociation, as probably it would not get to high enough temp. – Mithoron Sep 20 '17 at 22:14
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    @Mithoron Cooling the output form stealth engines comes at a big cost in efficiency of the engine. It costs to use the heat so doesn't provide a means to get "free" fuel however you organise the recovery. – matt_black Sep 20 '17 at 22:17
  • Let us continue this discussion in chat. – Mithoron Sep 20 '17 at 22:38
  • Iranians already have created a hydro engine. 1. Using an efficient catalyst you could get hydrogen from water with a lower amount of electricity needed in hydrolysis process. 2. After hydrolysis, you'll have oxygen in hand which could be involved in other energy provider processes.https://www.youtube.com/watch?v=qSHKt9kWRGg – Eftekhari Mar 15 '18 at 01:52
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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.

Scot Parker
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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.

Klaws
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