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Nuclear fission! How hard can that be? Nuclear reactors have been quietly humming under the earth's surface only a few billion years ago after all.

It's very convenient then, that my planet's quite a bit younger than earth, and has many of them deep below, containing naturally enriched uranium (about 3-4%, as in the wiki article), waiting to be discovered by eager inventors.

...The main issue however being, that my civilization has roughly the same tech-level as late 19th to early 20th-Century Europe.

Atoms are known about, but it would probably take some time for the scientists of this world to realize the potential of fission-energy. For this question, we'll assume that they cracked the code, at least enough to understand that radioactive elements can be used for fission and the main application of this question.

The setting is more orientated around steam-punk, so you won't find any digital computers or extremely modern materials to keep these reactors working. Instead, they'll have to settle for mechanical/analog computers, brass, steel, iron and all other materials and chemicals which were commonly used for construction in Europe during the late 19th and very early 20th century. (probably 1880-1915)

Though, i'm not just looking for a reactor...i'm looking for a specific application.

A thruster.

The thruster i linked to isn't the exact type i need. I just want to know how it could even be possible for a civilization of this tech-level to build a type of nuclear thruster with the materials and technology available. The type of thruster that i need, is essentially the one that's most plausible for them to build, though it's intended purpose is inter-orbital travel (and yes, that means no lift-off or re-entry, these thrusters, and the ship they're attached to, will never experience any planetary, atmospheric effects).

I'll try to clarify anything that might be important for the question.

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    $\begingroup$ A en.wikipedia.org/wiki/Nuclear_salt-water_rocket perhaps? Kinda dirty, but it doesn't have to land... and it has plenty of power. $\endgroup$
    – Monty Wild
    Sep 27, 2023 at 15:13
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    $\begingroup$ How are they even getting to orbit in the first place with 1915-level technology? $\endgroup$ Sep 27, 2023 at 15:14
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    $\begingroup$ @NuclearHoagie That'll be it's own question. Hence, why it's not asked for in this one. $\endgroup$
    – NimRad
    Sep 27, 2023 at 15:19
  • $\begingroup$ if you are handwaving technology that much, does it matter? You want them to build something before the the very concept the makes it work was even discovered. when the real thing took more than hundred years of the most drastic technological revolution in history. At that point you can just say they did and no one can really argue. $\endgroup$
    – John
    Sep 27, 2023 at 20:50
  • $\begingroup$ "a nuclear engine is similar to a liquid chemical engine. Both hold the working mass in a large tank and pump it to the reaction chamber using a turbopump. ... the reaction chamber is generally larger, the size of the reactor. ... a means had to be found of controlling reactor temperature and power output. ... a means had to be devised to hold the propellant." $\endgroup$
    – Mazura
    Sep 28, 2023 at 2:22

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Wondering if perhaps for your specific situation a more primitive approach but still using nuclear tech to achieve the result.

Decaying nuclear material is used with a Thermocouple to produce electricity. This is what powers Voyager and many deep space probes where solar is not feasible. This is an RTG. RTGs can heat up to 600C / 1112F. Well past the boiling point of water. Remove the thermocouple from the device and running water over the hot "RTG" flashing it to steam, then though a pressure system to build and release it in a controlled manner in a sort of monstrous put-put mode of propulsion you have a sufficiently scarry space rocket. Admittedly ham-fisted and primitive, but that is Steampunk almost by definition.

With some refinements and input from more learned folks than myself, the amount of handwavium could be minimized.

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  • $\begingroup$ Ham-fisted and primitive is perfect! As long as it could produce enough thrust, this would be a great option. $\endgroup$
    – NimRad
    Sep 27, 2023 at 16:09
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    $\begingroup$ @NimRad - it cannot produce enough thrust. You need a full-on nuclear reactor to produce sufficient energy. Decay energy from an RTG isn't enough to lift the RTG off the ground. $\endgroup$
    – jdunlop
    Sep 27, 2023 at 18:55
  • $\begingroup$ @jdunlop, the question is about inter-orbital travel, "no lift-off or re-entry" in those bold letters at the bottom. So it would be enough for the RTG to produce thrust which could propel the ship through the vacuum of space for the needed time and distance. $\endgroup$
    – NimRad
    Sep 27, 2023 at 18:59
  • $\begingroup$ The bigger issue is not so much thrust, it's specific impulse. A first ballpark upper bound for the maximum possible specific impulse of 600C steam is ~240 seconds, which is somewhat terrible - and again, that's an upper bound. (Specific Enthalpy of ~2730kJ/kg; calculate velocity assuming that energy was 100% converted into kinetic energy and divide by g.) $\endgroup$
    – TLW
    Sep 28, 2023 at 1:10
  • $\begingroup$ @TLW Not my wheelhouse, but out of curiosity I looked up the Specific impulse, of the Saturn 5 F1 engine: vacuum 304 s (2.98 km/s) Specific impulse, sea-level 263 s (2.58 km/s). Surprisingly (very!) not that far apart, I am probably missing something. Granted you calculate 100% efficiency, and in fairness if a Victorian era "RTG" performed anywhere near 50% it would be a miracle of science. But this is a steampunk universe and some leeway and suspension of belief is expected. It wont' move fast it will NOT be efficient, and it will probably self destruct, but you sail with the boat you have. $\endgroup$
    – Gillgamesh
    Sep 28, 2023 at 13:01
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It rather stretches belief that a late Victorian engineer would have a launch vehicle that could stick a nuclear rocket into space (without Cavorite, of course). Jules Verne's 'From Earth to the Moon' (1867) gives some project details for the Columbia giant cannon. A rocket is a much harder thing to build, as it needs an active process to keep it pointing up, as the centre of gravity is behind the centre of thrust.

However, let us suppose it is so. There is a good review of different nuclear propulsion systems here. Perhaps the easiest to achieve would be the Nuclear Salt Water Rocket. This would need a large amount of water to act as a neutron moderator and to provide the bulk of the reaction mass, but if you can get this water in space, and the tube to put it in, then the remainder to be launched from Earth might be relatively light.

PS: The original proposal for the Nuclear Salt Water Rocket sounds pretty dangerous. However, there is no need for the fissile material to be actually dissolved - as long as it is distributed finely enough for the neutrons to pass between the two phases, which may be millimetres, so you could combine jets of water and fissile material dust. This would be the nuclear equivalent of a hypergolic propellant.

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    $\begingroup$ NSWR might be the most difficult to achieve. There are good reasons to think it's impractical or even impossible. $\endgroup$
    – BMF
    Sep 27, 2023 at 16:04
  • $\begingroup$ "A rocket is a much harder thing to build, as it needs an active process to keep it pointing up, as the centre of gravity is behind the centre of thrust." - even if you put the center of thrust ahead of the center of gravity, you still need active stabilization. Tractor configurations aren't passively stable - thinking they would be is the pendulum rocket fallacy. $\endgroup$ Sep 28, 2023 at 5:12
  • $\begingroup$ What was stabilisation technology in the Nazi V2? Electronic (valves, pre-transistor), Electrical, or mechanical? $\endgroup$
    – nigel222
    Sep 28, 2023 at 9:34
  • $\begingroup$ @BMF Added safer NSWR option. Yes, the original proposal was pretty hairy, but it should be possible. Like to see it work with Thorium. $\endgroup$ Sep 28, 2023 at 9:40
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You're looking at throwing something behind you at extreme velocity. The Falcon rocket throws exhaust out at around 4 km/s in order to provide thrust. Here are a few options, plus or minus a little handwavium.

Record speed for an electric powered rail gun is around 11 km/s (destroying the rail), but your typical weapons grade weapon will launch them at 2.5 km/s.

Coil guns only shoot at around .2 km/s, but that could be improved with a longer coil.

Ion thrusters launch particles at 20-50 km/s. Our ion thrusters have microscopic thrust because the mass is low, but I'm sure your engineers are cleverer. er.

Water can be pressurized and ejected at around 1 km/s. Steam can be driven to much higher velocities, but I wasn't able to find a quote.

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