The idea is to use a fission reactor to turn water into steam pumping it outside the reaction chamber through heat pipes.

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    $\begingroup$ What you are describing is an NTR (en.wikipedia.org/wiki/Nuclear_thermal_rocket), although steam makes for a bad working fluid. LH2 is much more efficient. $\endgroup$
    – Polygnome
    Mar 24 at 7:27
  • $\begingroup$ It'd totally be possible to build an NTR engine using steam, however i'd imagine it'd struggle to launch anything at least from the ground as NTR's have a characteristically low thrust to weight. $\endgroup$ Mar 24 at 9:29

Could you make a nuclear thermal rocket using water as a reaction mass? Sure! People have looked at that, as it would be nice to be able to refuel rockets in space from things you find (see Nuclear thermal rockets using indigenous extraterrestrial propellants)

Could you launch from Earth with such a rocket?

Well, maybe.

Would it be worth it?

Maybe not.

Lets assume first that you can have a nuclear rocket with a sufficient thrust-to-weight ratio to get off the ground. The classic NERVA design might not have had this property, but things like Project Timberwind or DUMBO both suggested this might be possible... the latter even suggested a better thrust-to-weight than the Space Shuttle's main engines using water as reaction mass. Real world politics shelved those designs (at least in part due to ecological concerns about running an open cycle nuclear reactor in the biosphere we live in).

Anyway. I'll be pulling in some stuff from this related answer which may be of interest to you. An important figure of merit in a rocket engine is the "characteristic velocity":

$$c_* \propto \sqrt{\frac{T_t}{M_w}}$$

where $T_t$ can be taken to be the exhaust temperature, and $M_w$ is the molecular weight of the gas species in the exhaust.

Problem is that having your exhaust made up of big, heavy molecules like H2O limits its velocity. You can counter this by increasing your operating temperature, but now you're running into problems with your engine melting.

A solid-core nuclear thermal rocket has a chamber temperature pretty similar to that of a modern hydrogen-oxygen rocket, and if your NTR runs on water then its exhaust products will be the same too, which means you end up with the same rocket performance.

We don't have any single-stage-to-orbit LH/LOX rockets. Your water-NTR would end up needing something like the Space Shuttle's arrangement of disposable boosters and a massive reaction mass tank. That's a lot of hazardous nuclear material, radioactive exhaust, untried technology and great expense for not a lot of gain! it'd be better to use a regular rocket to fly a low-thrust, high-efficiency nuclear rocket into space instead.

If you were prepared to stretch the bounds of plausibility a little, you could imagine a Closed-Cycle Gas-Core NTR which might be able to offer an efficient SSTO when fueled with water. Maybe. The engineering required to build such a thing is quite beyond us right now, and quite frankly I'd expect to see working fusion reactors and electromagnetic launch systems before anyone built a nuclear lightbulb like this.

  • $\begingroup$ Another problem is that high temperature water vapor is very corrosive and will react with hot carbon (en.wikipedia.org/wiki/Water_gas), so you wouldn't be able to use materials like graphite in the reactor without protective coatings which might turn important parts of the reactor to hydrogen and carbon monoxide gas if they fail. $\endgroup$ Mar 24 at 11:37
  • $\begingroup$ @ChristopherJamesHuff yeah, there was some work done on solid-core NTRs with unfriendly propellants (such as CO, and I think CO2 is also bad for graphite under the right circumstances), though I've not come across any details. It might be one of those things that was handwaved away as a Simple Matter Of Engineering. $\endgroup$ Mar 24 at 11:44
  • $\begingroup$ CO2 will oxidize C to make CO, and CO will react with itself to make C and CO2 (en.wikipedia.org/wiki/Boudouard_reaction), which will effectively convert vital engine components into soot that gets deposited on other vital engine components or flung out the exhaust. $\endgroup$ Mar 24 at 12:14
  • $\begingroup$ There would almost certainly be a tremendous public outcry if it was proposed to start flying nuclear reactors around in the in the atmosphere on rockets. Even the Radio Thermal Generators used on probes such as Cassini caused a stir. $\endgroup$
    – Slarty
    Mar 24 at 16:11
  • $\begingroup$ @Slarty it was weirdly quieter around the Mars 2020 launch... it is possible that public attitudes are softening. It'd be a much easier argument to oppose big NTRs operating in an atmosphere, simply by pointing out that they're not really any better than decent conventional rocketry. That is, after all, why all the high-thrust NTR designs were all shelved, with the low-thrust deep-space alternatives limping on for a bit longer before getting the axe. $\endgroup$ Mar 24 at 16:22

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