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Could a jet engine that uses water work?

The idea is that water is sucked in, compressed, and boiled instantly by a dozen mirror focused industrial grade lasers. The resulting super heated gases would be routed out of the engine via a pipe able to adjust where the gases go by up to 10 degrees in any direction. This engine would be mounted on a submarine. Some of the hydrogen and oxygen would be collected for energy generation and life support. Some sodium (from the salt in the water) could also be used in the food. Could this design work in a real ocean? Please tell me any flaws in the design.

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    – L.Dutch
    Commented Apr 28, 2021 at 18:21
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    $\begingroup$ Related, and topical IEEE : Plasma Jets May One Day Propel Aircraft. Lasers use electricity, so you're really proposing a variety of electric engine. There are more efficient, practical, and promising ways to turn electricity into thrust. $\endgroup$
    – J...
    Commented Apr 28, 2021 at 18:55
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    $\begingroup$ KISS, and use the electricity you want to power the lasers to instead drive an electric motor. Whether that motor turns a traditional propeller, or a pump jet impeller is up to you. $\endgroup$
    – RonJohn
    Commented Apr 29, 2021 at 4:48

8 Answers 8

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Let's analyze this step by step.

  • First, you wouldn't be using the water as fuel, but as reaction mass, like an ordinary jet engine uses air.
  • You can't really compress water, fortunately, that's not necessary, turning it to steam will provide the required expansion.
  • Using lasers to heat the water would be a colossal waste of energy and engineering, as lasers aren't very efficient at turning electricity to heat (at the business end; the energy will end up heating the laser itself instead). It would be much easier and more efficient to heat the water directly like an immersion heater does.
  • You seem to assume that heating water to steam will somehow separate it into elemental hydrogen and oxygen, which won't happen. You can electrolyze water, but the energy you'd get from burning that is at most the same you put in. If you can fusion the hydrogen, that's different, but it's not currently attainable technology.
  • the sodium (and other stuff in the raw sea water) is actually a huge problem as it will accumulate and gunk up your pipes, you'll need a way to continously get rid of it; also, sea water is notoriously corrosive. Collecting the salt for food would be kind of wasted effort, the small amount of salt you need is trivially carried in store.

That said, if you have a reliable source of high heat, using steam for propulsion might be possible. But if it would provide an advantage over propellers, nuclear subs would already do that probably. Since nuclear reactors primarily produce heat, it would be advantageous for them to not have to produce electrical power first.

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  • $\begingroup$ WRT the first point, that's exactly what your ordinary Jet Skis and similar do. Water is taken in, accelerate, and shot out the back at higher speed. $\endgroup$
    – jamesqf
    Commented Apr 28, 2021 at 16:29
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    $\begingroup$ @AlexeiLevenkov The first point in the answer is about using water as reaction mass. ALL water-jet engines (including those cute little water rockets) do this, by definition. It has nothing to do with heating, just that the water is used as the reaction mass for propulsion. In this context "reaction mass" doesn't mean that there is a chemical or nuclear reaction, rather it refers to the "equal and opposite reaction" of Newton's Third Law. $\endgroup$
    – Corey
    Commented Apr 29, 2021 at 3:46
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    $\begingroup$ "But if it would provide an advantage over propellers, nuclear subs would already do that probably." One huge disadvantage is that blasting a gas jet out into the water is extremely not covert. As a general rule, the whole point of nuclear submarines is to not be detected. One that use a gas jet propulsion system would be trivially detected by aircraft, satellites, any boat in visual range, any boat with sonar detection equipment, etc. At that point, you may as well just use a surface ship because you won't be significantly harder to spot. $\endgroup$
    – reirab
    Commented Apr 29, 2021 at 16:23
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You have updated the venerable pop-pop boat.

pop pop boat

Pop-pop boats are sweet. They boil water and use the steam for propulsion. The vacuum caused by the steam leaving the boat causes the water reservoir to refill.

Historically these were powered by a candle. The update here is the LASERS. A good reason to heat things with lasers is that the lasers are heavy but you can heat things at a distance so you dont need them on your boat. They can be on shore. There are proposals for space ships that fly because distant lasers are boiling water on their tail ends to propel them with steam. This is laser propulsion and you are here using it to make steam for a boat. That is fine!

I hope you have supersniper robots keeping the lasers aimed at the boiler or I might be nervous waterskiing behind this boat. Also some robots need to keep anyone else from coming out on the water, because there are lasers. That also helps make waterskiing better because I have the whole lake.

The lasers would look cool at night because you would be able to see them in the mist coming up off the water. It would be like a rock show, and you could play Pink Floyd from the boat. Maybe you could get the pop pop engine to go in time with the music but that might require another robot.

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    $\begingroup$ Interesting visual, but the question says that this is for a submarine, which means that a shore-mounted laser will need to penetrate (heat) all the water between itself and the sub. $\endgroup$ Commented Apr 27, 2021 at 23:04
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    $\begingroup$ I missed the submarine part. There goes the waterskiing scheme. $\endgroup$
    – Willk
    Commented Apr 27, 2021 at 23:34
  • $\begingroup$ Maybe have a towed energy receiver like the antenna packs that submarines (I think) use when at low depths. Although then there will be more energy losses in turning received beamed energy impacting the receiver into heat at the stern. And yes, even with that change I can't figure out waterskiing options, but you can still get the light show in the mist. $\endgroup$ Commented Apr 27, 2021 at 23:53
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    $\begingroup$ @Willk Not at all - you just need the lasers inside the submarine. And somehow you need them to transfer all their energy to the water, instead of into the opposite wall of the sub. (Of course this is "just engineering". :) The downside is that you lose the Pink Floyd light show, but this would not generally be considered a desirable feature anyway by the "silent service". On the upside, waterskiing is back on the agenda. $\endgroup$
    – Graham
    Commented Apr 28, 2021 at 11:59
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    $\begingroup$ @BobJarvis-ReinstateMonica I'm thinking more Snake Plissken myself. :) $\endgroup$
    – Graham
    Commented Apr 29, 2021 at 9:26
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Faulty physics on multiple levels.

Vaporizing water does not produce oxygen or hydrogen; It produces water vapour.

To produce hydrogen and oxygen from water you need electrolysis, not heating, and so cannot contribute to propulsion. And you still can't use the hydrogen and oxygen to fuel your ship because it takes more energy to crack the water than you can get back from it by burning the hydrogen with the oxygen. If it didn't you would have a perpetual motion machine.

You might as well hang onto your fuel and use it directly for propulsion and not waste it in a roundabout, energy-wasting process.

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    $\begingroup$ Actually it is possible to crack water into oxygen and hydrogen using heat, although not at a commercial scale just yet. See energy.gov/eere/fuelcells/downloads/… $\endgroup$ Commented Apr 28, 2021 at 8:12
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    $\begingroup$ @PaulJohnson the report you link does not appear to say anything about cracking water to form oxygen and hydrogen gas using heat alone, so your claim is a bit misleading. All of the processes considered in the report are thermochemical processes involving one catalyst or another. In principle, of course, the process imagined by the OP could do similar, but that's not what they seem to be describing. They seem to think that simply heating water is sufficient to crack it. $\endgroup$ Commented Apr 28, 2021 at 18:08
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    $\begingroup$ Honestly, I'm not surprised since everything dissociates at high enough temperatures eventually turning into plasma then subatomics past that. $\endgroup$
    – DKNguyen
    Commented Apr 29, 2021 at 3:38
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    $\begingroup$ The problem is that water will dissociate at temperatures which will also turn the rest of the jet engine into a plasma. $\endgroup$
    – MSalters
    Commented Apr 29, 2021 at 7:56
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    $\begingroup$ The other problem is that, regardless of how you go about splitting water into hydrogen and oxygen, you will always use more energy to do that than you will get back from subsequently recombining them into water (which is what burning hydrogen does.) It doesn't matter what process you use, it will never possibly be able to get more energy out than you put in. $\endgroup$
    – reirab
    Commented Apr 29, 2021 at 16:30
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No

You might be able to use water as a reactant to combine with the vessel's fuel (in effect giving you bonus fuel because you get reaction mass from the outside), but you are wanting to use water as the only fuel source.

Unfortunately, water is fairly stable. It is a product of the decay of energy-rich compounds. It is less energetic than elemental hydrogen and oxygen gas (which is why, when you burn hydrogen, energy and water are released).

This means you can't use any chemical process using only water for fuel to generate more energy than you put into the system. It doesn't matter how you try to break the water apart, the laws of thermodynamics require you to pay for lunch. There ain't no such thing as a free lunch.

The vessel would need to go nuclear

There is, in fact, a way that to extract more energy from pure water than you have to pump into it. Fusion. The first problem is we don't have working fusion reactors, so this requires sci-fi future tech.

The next problem is you want a jet engine. Even if you have futuristic fusion plant technology, this would be a plasma jet engine, and the problem with a jet of radioactive plasma exhaust hot enough to undergo nuclear fusion is it will be essentially impossible to contain in an engine. The plasma will destroy the engine.

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Here's my concern even if everything else worked, which is not certain at all.

This is for a submarine, it is supposed to be stealthy. Your sub would have a huge thermal signature which could be detected. Then send in a few heat seeking missiles (i.e., torpedoes) and they are going to go straight for the engines and blow you to smithereens.

If and only if stealth was a non-factor would this even be useful. If you were exploring the ocean floor for scientific reasons and not military.

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  • $\begingroup$ @AlexeiLevenkov Yes, but in this case the thermal plume would be more intense and localised to the jet outlet... making it a far more useful target for an enemy heat-seeking munition. $\endgroup$
    – Corey
    Commented Apr 29, 2021 at 3:41
  • $\begingroup$ Heat seeking missiles track infra-red emissions (thermal radiation), but water absorbs IR pretty well. This is not a concern. Submarines have always been hunted by sound, because sound does travel very well in water. And that would be a real concern with a jet engine. $\endgroup$
    – MSalters
    Commented Apr 29, 2021 at 7:58
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    $\begingroup$ I'm quite surprised this is not the main comment. No matter the engineering details of how you achieve such a thing, laser, nuclear, whatever, the noise level of a steam propelled submarine would be absolutely off the roof and that's a immediate no-no. $\endgroup$ Commented Apr 29, 2021 at 13:15
  • $\begingroup$ @ZeroCredibility Yup, now that you mention it noise would also be a huge problem. Also if the steam caused bubbles in the water at the surface that would also give it away. In a nuclear solution at least you could have heat pipes that distribute the heat across the whole ship so there is no one single hot spot. $\endgroup$
    – cybernard
    Commented Apr 29, 2021 at 14:46
  • $\begingroup$ @MSalters But, unless the depth were extremely deep - enough for all of the steam exhaust to turn back to liquid before reaching the surface - then you would have jets of gaseous water coming out of the ocean surface in your trail, which would be a pretty big giveaway thermally, acoustically, and visually. It would probably be the easiest-to-detect sub ever built. Even if you were deep enough for all of the steam to condense before reaching the surface, you'd still be leaving a trail of warm water that would probably be trivially detected by thermal imaging satellites or aircraft. $\endgroup$
    – reirab
    Commented Apr 29, 2021 at 16:35
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Yes! Sort of.

You have water. You have electricity (which you'd have needed for your lasers anyhow). You even have air since you're in a submarine, not a spaceship, just need to surface occasionally to resupply. Congratulations: You can make hydrazine. Electrolysis to split the water into hydrogen and oxygen. Mix the hydrogen with nitrogen from the air, and you've just made hydrazine, a powerful rocket fuel. (It's slightly more complicated than that, but easily explainable with tech-tech.)

Caveat: It will probably not be very stealthy, but you'll be one of the fastest subs in the ocean, and you might not need stealth all the time if you can outrun everything. You'll probably want to use a more conventional propulsion when you're not in a hurry and want to remain undetected.

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    $\begingroup$ You might be very fast, but your position will be quite easily predicted, making it very convenient for a surface ship or aircraft to place some very inconvenient depth charges in your path. Or just aim a torpedo at your path. You will still be much slower than any aircraft, due to the drag from water vs. from air. And, of course, you still have the problem that you're putting more energy into making the hydrazine than you'll get out from burning it. But, still, +1 for creativity. :) $\endgroup$
    – reirab
    Commented Apr 29, 2021 at 17:05
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    $\begingroup$ @reirab If instead of 1 rocket engine, you've got 3 or more, pointed at different angles for maneuverability, you might be slightly less predictable. And if you're really in a pinch, point that sucker straight up, and you've got a submarine that can fly into space, Sea Dragon style - they won't see that coming. (Probably too much mass to realistically reach orbit, but if you're gonna go bonkers, it could make for some entertaining sci-fi...) $\endgroup$ Commented Apr 29, 2021 at 20:43
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You're pointing the lasers in the wrong direction - there is a technology called supercavitating torpedoes, which point an infrared laser or other gas generating technology in the direction of flight so they can travel flying hypersonically in a bubble of steam rather than having to push water out the way.

  • Water absorbs IR quite well so using lasers for heating isn't an insurmountable problem, though I don't know if anyone has a torpedo deployed using them, though I have heard it being talked about as a possibility
  • it's not enclosed, so scaling up with precipitated matter is not such an issue as it would be vaporising the water in a tube
  • this occurs at depth so the bubble is compressed and turns back to water behind the torpedo before reaching the surface, so bubble tracks are not an issue
  • travelling at 200 mph the stealth tactics of conventional submarine warfare don't apply, but the noise will scare any whales you don't boil to death

So, most of the issues have mitigations for some applications, the only issue is to create thrust without scaling up with deposits. Something like an Aerospike might work, where the expansion is around a core and bounded by the fluid rather than within an enclosed tube like a ramjet would be (the VA-111 does use a ramjet with water as reaction mass, but it has to operate for a minute or so so it doesn't matter if it would be clogged after an hour). Or maybe just turn it off for a few minutes per hour so the water gets to it and most of the deposits can redissolve.

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Here's my (sci-fi) idea.

The engine rips the water into hydrogen and oxygen, cold-fusions the hydrogen for energy, and uses the energy proceeds to jet the oxygen and fusion by-products out the rear.

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