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.
Orthodox chemical rockets use heat energy produced by chemical reactions in a reaction chamber to heat the gas products. The products are then expelled through a propulsion nozzle at a very high speed, creating thrust. In a nuclear thermal rocket (NTR), thrust is created by heating a fluid by using a nuclear fission reactor. The lower the molecular weight of the exhaust, hydrogen having the lowest possible, the more efficient the motor can be. However, in this engine the propellant can be anything with suitable properties as there will be no reaction on the part of the propellant. In a NSWR the nuclear salt-water would be made to flow through a reaction chamber and out of an exhaust nozzle in such a way and at such speeds that critical mass will begin once the chamber is filled to a certain point; however, the peak neutron flux of the fission reaction would occur outside the vehicle.
Advantages of the design
There are several advantages relative to conventional NTR designs. As the peak neutron flux and fission reaction rates would occur outside the vehicle, these activities could be much more vigorous than they could be if it was necessary to house them in a vessel (which would have temperature limits due to materials constraints). Additionally, a contained reactor can only allow a small percentage of its fuel to undergo fission at any given time, otherwise it would overheat and melt down (or explode in a runaway fission chain reaction). The fission reaction in an NSWR is dynamic and because the reaction products are exhausted into space it doesn’t have a limit on the proportion of fission fuel that reacts. In many ways NSWRs combine the advantages of fission reactors and fission bombs.
Because they can harness the power of what is essentially a continuous nuclear fission explosion, NSWRs would have both very high thrust and very high exhaust velocity, meaning that the rocket would be able to accelerate quickly as well as be extremely efficient in terms of propellant usage. The combination of high thrust and high specific impulse is a very rare trait in the rocket world. One design would generate 13 meganewtons of thrust at 66 km/s exhaust velocity (or exceeding 10,000 seconds ISP compared to ~4.5 km/s (450 s ISP) exhaust velocity for the best chemical rockets of today).
The design and calculations discussed above are using 20 percent enriched uranium salts, however, it would be plausible to use another design which would be capable of achieving much higher exhaust velocities (4,700 km/s) and use 2,700 tonnes of highly enriched uranium salts in water to propel a 300 tonne spacecraft up to 3.6% of the speed of light.
“NSWRs share many of the features of Orion propulsion systems, except that NSWRs would generate continuous rather than pulsed thrust and may be workable on much smaller scales than the smallest feasible Orion designs (which are generally large, due to the requirements of the shock-absorber system and the minimum size of efficient nuclear explosives).”