Nuclear salt-water rocket
A nuclear salt-water rocket (NSWR) is a theoretical type of nuclear thermal rocket which was designed by Robert Zubrin. In place of traditional chemical propellant, such as that in a chemical rocket, 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.
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 in the best case, 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 an NSWR the nuclear salt-water would be made to flow through a reaction chamber and out an exhaust nozzle in such a way and at such speeds that critical mass will begin once at the chamber is filled to a certain point, however, the peak neutron flux in 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 meltdown (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 this makes NSWRs like a hybrid between fission reactors and fission bombs.
Due to their ability to 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. Such an engine being able to produce both high thrust and high ISP is a very rare trait in the rocket world, owing to the possibility to use a low density propellant such as hydrogen. One design would generate 13 meganewtons of thrust at 66 km/s exhaust velocity (compared to ~4.5 km/s 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)."
The propellant used in the initial design would contain a rather large amount of the relatively expensive isotope 235U, which would not be very cost effective. However, if the use of NSWR began to rise, it would be possible to replace this with the cheaper isotopes 233U or 239Pu in either fission breeder reactors or (much better) fusion/fission hybrid reactors. These fissiles would have the right characteristics to serve nearly equally as well, while having the boon of coming at a relatively low cost.
The vessel's exhaust would contain radioactive isotopes, but in space these would be rapidly dispersed after travelling only a short distance; the exhaust would also be travelling at high speed (in Zubrin's scenario, faster than Solar escape velocity, allowing it to eventually leave the Solar System). This is however of little use on the surface of a planet, where a NSWR would eject massive quantities of superheated steam, still containing fissioning nuclear salts. Terrestrial testing might be subject to reasonable objections; as one physicist wrote, "Writing the environmental impact statement for such tests [...] might present an interesting problem ..." It is also not certain that fission in a NSWR could be controlled: "Whether fast criticality can be controlled in a rocket engine remains an open question".
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