Robert Zubrin’s Nuclear Salt Water Rocket Design

Robert Zubrin is best known for his advocacy of the manned exploration of Mars.

Zubrin also had a design for interplanetary propulsion called the Nuclear Salt Water Rocket.

A nuclear salt-water rocket (NSWR) is a theoretical type of nuclear thermal rocket. A conservative design for the rocket would be fueled by salts of 20 percent enriched uranium or plutonium. The solution would be contained in a bundle of pipes coated in boron carbide (for its properties of neutron absorption). Through a combination of the coating and space between the pipes, the contents would not reach critical mass until the solution is pumped into a reaction chamber, thus reaching a critical mass, and being expelled through a nozzle to generate thrust.

The low 20% enrichment level is what is permitted for nuclear energy reactors. The system would achieve an ISP of about 67300. This would be about 20 times more efficient than most chemical rockets. Exhaust velocity would be 66 km/second.

The 300 ton design has a thrust of 2.9 Mlb, and a T/W of 40 is assumed. The specific impulse is 6730 s, and the tankage fraction is 0.04. The very high thrusts inherent in the NSWR causes this system to depart LEO with an acceleration of 3.4 Earth g’s. The total jet power output of the engine is 427,000 MW.

The NSWRs is like a hybrid between fission reactors and fission bombs.

In the mission described, the NSWR used 83.6 tonnes of
aqueous propellant (41.8 for each of TSI and EOI), 16.7 tonnes out which is uranium.
Of this uranium, 3.34 tonnes are actually fissile U235. This is rather a large amount of U235 to expend on a single mission but if NSWRs should come into use, large amounts of Pu239 or U233 could be bred (out of U238 or cheap Th232, respectively) in either fission breeder reactors or (much better) fusion/fission hybrid reactors. These fissiles could serve equally well in an NSWR as U235 and could be made cheap enough that propellant cost would not be a significant problem.

The exhaust of the NSWR is highly radioactive, as no attempt has been made to retain the fission products within the engine, however, with an exhaust velocity of 66 km/s, the radioactive products are emitted with a velocity far exceeding the escape velocity of the Earth and, providing the engine was directed to thrust perpendicular to the radial vector connecting the spacecraft in LEO to the Earth’s centre (i.e. tangent to the direction of circular orbital velocity), the amount of contaminant reaching the Earth could be insignificant It is thus appropriate to contemplate using the NSWR for LEO departure. Of course, if public concern prevented such an application, NSWRs could still be used on high energy missions by boosting the spacecraft first to a hyperbolic excess velocity of say 3 km/s with an NTR, and then firing the NSWR a 4 days later when the spacecraft was a million kilometres away from Earth

The potential ultimate performance is calculated with more optimistic assumptions.

Consider for example, an NSWR utilizing a 2% uranium bromide solution with 90% enriched U233, and obtaining a 90% fission yield. Assuming a nozzle efficiency of 0.9, the exhaust velocity of this system will be 4725 km/s, or about 1.575% of the speed of light (a specific impulse of 482,140 seconds). If the 300 tonne Titan mission spacecraft is endowed with 2700 tonnes of propellant (for a mass ratio of 10) a maximum velocity of 3.63% of speed of light could be obtained, allowing
the ship to reach Alpha Centauri in about 120 years.

Deceleration could be accomplished without the use of substantial amounts of rocket propellant by using a magnetic sail (or “magsail”) to create drag against the interstellar medium.

In a more ambitious approach, one could envisage a group of interstellar emigrants selecting a small ice asteroid with a mass of 30,000 tonnes and using it as propellant (together with 7,500 tonnes of uranium obtained elsewhere) for a 300 tonne spacecraft. In this case the ship could obtain a final velocity of about 7.62% light speed, and reach Alpha Centauri in about 60 years.