The Nuclear Salt Water Rocket is a rocket engine concept that uses a rapid nuclear reaction in a Uranium salt dissolved in water to create a high thrust, high efficiency engine which eclipses the performance of any rocket engine ever designed. It's a concept originally presented by Robert Zubrin, which is appealing because it looks more scientifically plausable than many other futuristic propulsion concepts.
It's also scary on so many levels, using a propellent that has to be stabilized by specially designed tanks, and relies on managing a small nuclear explosion with power outputs of hundreds of gigawatts.
The original paper can be read here:
https://arc.aiaa.org/doi/abs/10.2514/6.1990-2371
And Atomic Rockets has a section on the device:
http://www.projectrho.com/public_html/rocket/enginelist2.php#nswr
The Kerbal mod version is available as part of Nertea's "Far Future Technologies":
https://forum.kerbalspaceprogram.com/index.php?/topic/199070-111x-far-future-technologies-jan-7/