Nuclear salt-water rocket
Encyclopedia
A nuclear salt-water rocket (or NSWR) is a proposed type of nuclear thermal rocket
designed by Robert Zubrin
that would be fueled by water
bearing dissolved salt
s of Plutonium
or U235
. These would be stored in tanks that would prevent a critical mass from forming by some combination of geometry or neutron
absorption (for example: long tubes made out of boron
in an array with considerable spacing between tubes). Thrust would be generated by nuclear fission
reactions from the nuclear salts heating the water and being expelled through a nozzle. The water would serve as both a neutron moderator
and propellant.
In a conventional chemical rocket, chemical reactions of the fuel
and oxidizer (e.g. Oxygen
and Kerosene
) heat the byproducts of the chemical reaction (e.g. CO2 and H2O
) to high temperatures as they are forced through a rocket nozzle. The fast moving molecules in the exhaust focused in one direction create thrust. In a nuclear thermal rocket
(or NTR) a nuclear fission reactor would serve as a source of heat which would be transferred to a propellant that is then exhausted through a rocket nozzle. The propellant in this case can be any material with suitable properties, it need not react during the operation of the rocket, it is simply a source of mass to be heated up and exhausted out of the rocket at high speeds. 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 the peak neutron
flux in the fission
reaction would occur outside of the vehicle. This has several advantages relative to conventional NTR designs. Because the peak neutron flux and fission reaction rates would occur outside of 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
). Because 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.
Because of 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, a rare combination of traits in the rocket world, meaning that the rocket would be able to accelerate quickly as well as be extremely efficient in terms of propellant usage. 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). Another design would achieve 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 vessel's exhaust would contain radioactive isotopes, but these would be rapidly dispersed after traveling 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, little use on the surface of a planet, where a NSWR would eject massive quantities of superheated steam, still containing fissioning nuclear salts. This is extremely dangerous from an ecological point of view, and such engines could not be used on the surface of a planet without terrible environmental damage occurring.
Nuclear thermal rocket
In a nuclear thermal rocket a working fluid, usually liquid hydrogen, is heated to a high temperature in a nuclear reactor, and then expands through a rocket nozzle to create thrust. In this kind of thermal rocket, the nuclear reactor's energy replaces the chemical energy of the propellant's...
designed by Robert Zubrin
Robert Zubrin
Robert Zubrin is an American aerospace engineer and author, best known for his advocacy of the manned exploration of Mars. He was the driving force behind Mars Direct—a proposal intended to produce significant reductions in the cost and complexity of such a mission...
that would be fueled by water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
bearing dissolved salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
s of Plutonium
Plutonium
Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...
or U235
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
. These would be stored in tanks that would prevent a critical mass from forming by some combination of geometry or neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
absorption (for example: long tubes made out of boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
in an array with considerable spacing between tubes). Thrust would be generated by nuclear fission
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...
reactions from the nuclear salts heating the water and being expelled through a nozzle. The water would serve as both a neutron moderator
Neutron moderator
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
and propellant.
In a conventional chemical rocket, chemical reactions of the fuel
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...
and oxidizer (e.g. Oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and Kerosene
Kerosene
Kerosene, sometimes spelled kerosine in scientific and industrial usage, also known as paraffin or paraffin oil in the United Kingdom, Hong Kong, Ireland and South Africa, is a combustible hydrocarbon liquid. The name is derived from Greek keros...
) heat the byproducts of the chemical reaction (e.g. CO2 and H2O
H2O
H2O is the chemical formula for water and is also used as an abbreviation for the word "water". H2O or H2O It may also refer to:* H2O , a punk band**H2O , their self-titled debut album...
) to high temperatures as they are forced through a rocket nozzle. The fast moving molecules in the exhaust focused in one direction create thrust. In a nuclear thermal rocket
Nuclear thermal rocket
In a nuclear thermal rocket a working fluid, usually liquid hydrogen, is heated to a high temperature in a nuclear reactor, and then expands through a rocket nozzle to create thrust. In this kind of thermal rocket, the nuclear reactor's energy replaces the chemical energy of the propellant's...
(or NTR) a nuclear fission reactor would serve as a source of heat which would be transferred to a propellant that is then exhausted through a rocket nozzle. The propellant in this case can be any material with suitable properties, it need not react during the operation of the rocket, it is simply a source of mass to be heated up and exhausted out of the rocket at high speeds. 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 the peak neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
flux in the fission
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...
reaction would occur outside of the vehicle. This has several advantages relative to conventional NTR designs. Because the peak neutron flux and fission reaction rates would occur outside of 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
Nuclear chain reaction
A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...
). Because 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.
Because of their ability to harness the power of what is essentially a continuous nuclear fission explosion, NSWRs would have both very high thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....
and very high exhaust velocity, a rare combination of traits in the rocket world, meaning that the rocket would be able to accelerate quickly as well as be extremely efficient in terms of propellant usage. 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). Another design would achieve 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
Project Orion (nuclear propulsion)
Project Orion was a study of a spacecraft intended to be directly propelled by a series of explosions of atomic bombs behind the craft...
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 vessel's exhaust would contain radioactive isotopes, but these would be rapidly dispersed after traveling only a short distance; the exhaust would also be travelling at high speed (in Zubrin's scenario, faster than Solar escape velocity
Escape velocity
In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...
, allowing it to eventually leave the Solar System). This is however, little use on the surface of a planet, where a NSWR would eject massive quantities of superheated steam, still containing fissioning nuclear salts. This is extremely dangerous from an ecological point of view, and such engines could not be used on the surface of a planet without terrible environmental damage occurring.