So I've recently read about NTR's and how they can be applied in space. Here is a link to the website that I've looked at: http://www.projectrho.com/public_html/rocket/enginelist2.php#ntrsolidcore

It says that water is a viable propellant. I am looking at creating a sci-fi story with near-future tech, but more focused on the planetary aspect instead of in space. Anyway, I thought up a submarine where water is collected from the ocean it's in then passed through the reactor and shot out at high speeds in the back. Assume stealth isn't an issue.

Could this work? Would it be efficient? Are there any unintended consequences such as the ocean being polluted?

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    $\begingroup$ Submarines are in water. There is no reason why they wouldn't use a very much simpler nuclear-electric power plant driving conventional propellers. (And of course a submarine propelled by expelling ginormous amount of very hot gas would be as non-stealthy as humanly possible.) $\endgroup$
    – AlexP
    Mar 29, 2022 at 5:34
  • $\begingroup$ @KerrAvon2055, the submarine would be for cargo/exploration. I just gave the example of stealth as I thought the idea of something like that trying to be stealthy was funny. $\endgroup$ Mar 29, 2022 at 6:02
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    $\begingroup$ @KerrAvon2055 Quite obviously, the purpose of the submarine is ocean ecocide. I mean, c'mon... learn to read between the lines a little. $\endgroup$
    – John O
    Mar 29, 2022 at 12:58

3 Answers 3


Could this work?

Sure! We already have rocket-propelled torpedoes that make use of sea water as part of their propulsive reaction mass:

Once accelerated, speed is maintained by an underwater ramjet fueled by hydroreactive metals using seawater as both reactant and the source of oxidizer; the torpedo travels at around 200 kn (370 km/h; 230 mph).

Clearly overpowered jet systems can work!

Would it be efficient?

That's very difficult to establish, because fluid dynamics is hard and I'm not about to calculate the drag force on a supercavitating submarine for you. Remember though that the drag force scales according to the square of the velocity, so if you wanted efficiency you'd be moving slowly rather than expending all your energy ramming seawater out of your way.

ocean being polluted?

The ocean will inevitably become polluted, because you're exhausting very hot nuclear coolant directly into the environment, and that coolant will contain at the very least all sorts of interesting neutron activation products from the dissolved materials in seawater, notably sodium-24 and chlorine-38 which will at least be thoroughly diluted and have relatively short half-lives.

Are there any unintended consequences

The biggest problem that solid-core nuclear thermal rockets are likely to face is buildup of neutron poisons in the reactor core which will reduce reactor power output to the point where it eventually becomes useless. Project rho has a few things to say on the subject, but doesn't get into the gory details.

A clever nuclear reactor design can work around this to some degree and be factory sealed and still operate for years... apparently some US navy reactors are like this, and the ships they're mounted in will be end-of-life before they run out of fuel. Your ridiculous nuclear rocket on the other hand is operating at a much higher power level, which means the fissiles in the reactor core will be used up more quickly and the neutron poisons will build up faster.

This means you'll have to refuel more often than a regular nuclear reactor, and you'll either need a good source of highly enriched fissiles, or a nuclear fuel reprocessing facility. The former is obviously a proliferation risk, and the latter will also be a source of plutonium as well as clean fuel rods.

Such facilities are hard to hide, even without you running a giant underwater rocket that you could hear from the other side of the world and leaves a surface-visible wake of warm radioactive bubbling seawater. This makes you a global nuclear threat, and inevitably there will be some kind of consequence unless you're a powerful nation-state.

  • $\begingroup$ I never thought of the build up of nuetron poisons. If the exhaust was pointed at a normal boat/submarine I wonder how much damage it could do. This is certainly an intresting topic, without any practicality at all! $\endgroup$ Apr 22, 2022 at 4:05
  • $\begingroup$ @sirOrange17 the kzinti lesson definitely applies, but it will be quite short ranged in water. Consider though that modern warships aren't massively solid heavily armored things, and a super high speed submarine necessarily needs to be quite toughly built. Instead of pointing your exhaust at your opponents, just turn around and sail straight through them... $\endgroup$ Apr 22, 2022 at 10:00

The pop-pop boat!

Pop-pop boats are toys powered by a steam engine with no moving parts.


A heat source boils water and the steam blasts out the back along with some water. The consequent vacuum then refills the boiler through the same pipes used for propulsion. I think that is nifty. In practice I think you would lurch along. Maybe that is why they called them pop-pop boats but the linked article suggests that there was a flexing piece of copper than made the sound.


pop pop boat

Throughout the operation, the boiler contained a little water, but at least 3/4 of the area was dry -- and presumably hotter than the boiling point of water. That showed, second, that the water expelled in the pulses at the rear of the boat is not supplied from the water put in for priming. It means that between the pulses out, water must be sucked into the pipes to replace that expelled.

To propel with air (as a ramjet plane) you can heat the air and it will expand. Water does not expand so much so you would need to boil some of it and use the phase change and pressure increase to push your vessel. Heat is heat whether from a candle or a battery or a nuclear power plant. Pop pop!

  1. Cavitation will be hard to calculate.
  2. Do not use sea water to pass trough reactor. Use metal or salt cooled reactor and then use hot, liquid metal(salt) to heat up water. That will reduce nuclear polution down to almost 0.
  3. You kill marine life by shockwave and heating up sea. Short range but with time and more ships all medium and big life will be killed. This do shortage in food for small. At some point You will be left with bacteria, protozoa, fungi and maybe some algae.
  4. Remember to do some sparks in exhaust. At this temperature and speed part of water will split to oxygen and hydrogen and You do not need that in atmosphere.

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