In the setting I'm making, Orion drives are one of the primary forms of propulsion used in spacecraft. I have been thinking of adding Nuclear Salt Water Rockets (the lithium variety) alongside Orion in order to have a 'classical' rocket punk feel to the setting alongside the raw power of Orion, though why anyone would used a NSWR over Orion is still something I'm figuring out.

So the question is:

Why would a NSWR be used over Orion?

-Here are some links for more info for both drives-




Nuclear Salt Water:



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    $\begingroup$ Do you really have to ask why a rocket that uses constant nuclear bombs going off would be less popular than a rocket that uses a reactor to heat saltwater? Which would you rather have in orbit of your planet? A single nuclear reactor or a rocket with enough nuclear weapons on board to completely glass the surface of your planet several times over? $\endgroup$
    – stix
    Feb 10, 2021 at 17:53
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    $\begingroup$ @stix that's a bit misleading. Controlling a NSWR is like moderating a continuous nuclear reactor meltdown. It's extremely tricky and things can go south very quick. $\endgroup$
    – BMF
    Feb 10, 2021 at 20:30
  • $\begingroup$ @BMF The fuel is in a gaseous state. It is, by definition, impossible to melt down. The geometry of the reaction chamber is what allows it to maintain criticality. The reactor is self limiting in a problem since it requires a constant supply of fission fuel and if the reactor chamber is damaged it can't maintain a geometry conducive to criticality. It is still orders of magnitude safer than going around with thousands of nuclear weapons on board and far more politically palatable. $\endgroup$
    – stix
    Feb 11, 2021 at 17:49

4 Answers 4



Orion does not scale down very well, the minimum sized start at 10 000 tonnes and much heavier.

Whereas NSWR scales down quite well. The minimum occurs when a critical mass is barely achievable, and that is possible with a mass flow of a few tens of grams per second.

Maximum Thrust (Actually thrust to weight ratio, but allowed by higher maximum thrust)

Also, the potential maximum thrust from an Orion design is quite limited by vibration and thermal limitations. With Orion, more thrust = more heat in the vehicle. Plus God's jackhammer beating on your skull at a faster rate might overload the dampening systems.

With NSWR, all you need for extra thrust is a faster pump, and a mechanically stronger engine chamber. The heat passed to the vehicle remains constant (although neutron flux may get interesting!). Virtually all of the heat in a NSWR exits with the exhaust, thus not bothering the vehicle itself.

and the silly real reason

If you have a whole civilization employing Orion and Nuclear Salt Water Rocket all over the place, then obviously your species is not Human. Or even any form of carbon-water-protein life. Rather they would have to be some form of radiation-eating BEM.

The environmentalist lobby could be demanding the return of NSWR simply because these newfangled super-efficient Orion drives are simply not seeding enough nutrients into the spacelanes. Why, last quarter's neutron count is below 4e10 per cubic meter already, and there's virtually no sightings of free antimatter being reported at all! We must drastically increase the nutrient release before travelers start starving on the flight to Blito-P3. Never should have scrapped those rest-stops with their fast breeder drive-throughs, I tell you!

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    $\begingroup$ I'm not sure why you think an NSWR would produce less fallout than an Orion drive system. It's not even close that the Orion would be far more nasty. Not that it matters, since space is incredibly vast. $\endgroup$
    – stix
    Feb 10, 2021 at 18:22
  • $\begingroup$ @stix not sure why you would think i said that? Maybe try reading that again? $\endgroup$
    – PcMan
    Feb 10, 2021 at 20:23
  • $\begingroup$ because your comments imply that an NSWR creates more fallout? You explicitly state an Orion drive is "super-efficient" (untrue) and that your radiation eating aliens want NSWRs because they increase the neutron count (also untrue). $\endgroup$
    – stix
    Feb 10, 2021 at 21:53

Nowadays we have electric cars, supersonic jets and bullet trains, yet people use their bikes or feet to go places sometimes. Why?

Well, go visiting aunt Penny down the block with a bullet train would be quite wasteful, don't you think? Even the astronauts boarding the Apollo or the Space Shuttle took a bus and walked to reach it.

To be less extreme, for short distance travels the train is often more competitive than the airplane.

Simply said, for some use cases Orion is an overshooting and NSWR is better suited.


A lithium NSWR could have a much higher specific impulse than an Orion drive... if your nuclear engineering was up to the task, you could theoretically get an exhaust of pure fission products, which combined with a suitable magnetic nozzle give you the sort of engine you'll be wanting for going a really long way, like to the Oort cloud, or Alpha Centauri or somewhere.

"Classic" Orion drives can't have an exhaust velocity that high because of the need for a whole physics package and propellant disc and so on. You can modify them to make increasingly exotic things like ACMF or z-pinch compression of subcritical assemblies, etc etc, and in the limit you end up with ICF zapping pure fuel pellets in a magnetic nozzle but that's really stretching the definition of Orion a bit.

So, why would a lithium NSWR be used over an Orion? why, when you wanted to fly to Sedna instead of Jupiter.

That doesn't quite gel with your desire for classic atomic rocketry, alas.


Simple economics. Fissionable materials are scarce, difficult to refine, and thus expensive. Nuclear explosives are wasteful: they literally blow themselves apart before all of the uranium or plutonium can fission, and the remaining fragments (which could be useful in a higher tech) are also widely dispersed. (That is, fallout, though of course it wouldn't fall in space.)

With a reactor, everything is kept neatly in a package. Once your fuel elements are expended, you can pull them out and refine/recycle what's left. If you design your reactor properly, it can even produce more fuel than it started with - see "breeder reactor" for details.

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    $\begingroup$ Uranium isn't particularly rare, especially if the technology is developed to extract it from seawater. Thorium is even more abundant, and is currently a waste product of rare earth mining. Your scarcity argument doesn't hold. $\endgroup$
    – stix
    Feb 10, 2021 at 19:09
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    $\begingroup$ In what way does this answer relate to the question of "Why would Nuclear Salt Water Rockets be used when Orion drives are around"? Unless you thought the "R" in NSWR stood for "Reactor"? It does not. It is an open-exhaust Nuclear Rocket. $\endgroup$
    – PcMan
    Feb 10, 2021 at 21:08
  • $\begingroup$ @stix: Uranium may not be particularly rare, but the fissionable isotope U-235 is, and separating it from the non-fissionable U-238 is difficult. $\endgroup$
    – jamesqf
    Feb 11, 2021 at 4:32
  • $\begingroup$ @jamesqf That's irrelevant. You can run reactors on natural uranium with the appropriate moderator and don't need to enrich it. U-238 is not fissile but it is fertile. The U-238 in a natural uranium reactor will transmute to plutonium-239 which is itself fissile. $\endgroup$
    – stix
    Feb 11, 2021 at 17:46

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