Fission-fragment rocket
Encyclopedia
The fission-fragment rocket is a rocket engine
design that directly harnesses hot nuclear fission product
s for thrust
, as opposed to using a separate fluid as working mass
. The design can, in theory, produce very high specific impulse
s while still being well within the abilities of current technologies.
and related designs, the nuclear energy is generated in some form of "reactor" and used to heat a working fluid to generate thrust. This limits the designs to temperatures that allow the reactor to remain "whole", although clever design can increase this critical temperature into the tens of thousands of degrees. A rocket engine's efficiency is strongly related to the temperature of the exhausted working fluid, and in the case of the most advanced gas-core engines, it corresponds to a specific impulse of about 7000 s (69 kN·s/kg).
The temperature of a conventional reactor design is actually the average temperature of the fuel, the vast majority of which is not actually reacting at any given instance. In fact the atoms undergoing fission are at a temperature of millions of degrees, which is then spread out into the surrounding fuel, resulting in an overall temperature of a few thousand. In the fission-fragment design, it is the individual atoms that actually undergo fission that are used to provide thrust, by extracting them from the rest of the fuel as quickly as possible before their energy is spread out into the surrounding fuel mass.
This is easier to achieve than it might sound. By physically arranging the fuel such that the outermost layers of a fuel bundle will be most likely to undergo fission, the high-temperature atoms, the fragments of a nuclear reaction, can "boil" off the surface. Since they will be ionized due to the high temperatures of the reaction, they can then be handled magnetically
and channeled to produce thrust. Numerous technological challenges still remain, however.
. In their design the fuel was placed into a number of very thin carbon
bundles, each one normally sub-critical. Bundles were collected and arranged like spokes on a wheel, and several such wheels were stacked on a common shaft to produce a single large cylinder. The entire cylinder was rotated so that some bundles were always in a reactor core where additional surrounding fuel made the bundles go critical. The fission fragments at the surface of the bundles would break free and be channeled for thrust, while the lower-temperature un-reacted fuel would eventually rotate out of the core to cool. The system thus automatically "selected" only the most energetic fuel to become the working mass.
The efficiency of the system is surprising; specific impulses of greater than 100,000 are possible using existing materials. This is high performance, although not that which the technically daunting antimatter rocket
could achieve, and the weight of the reactor core and other elements would make the overall performance of the fission-fragment system lower. Nonetheless, the system provides the sort of performance levels that would make an interstellar precursor mission possible.
A newer design proposal by Rodney A. Clark and Robert B. Sheldon theoretically increases efficiency and decreases complexity of a fission fragment rocket at the same time over the bundle proposal. In their design, nanoparticle
s of fissionable fuel (or even fuel that will naturally radioactively decay) are kept in a vacuum chamber subject to an axial
magnetic field
(acting as a magnetic mirror
) and an external electric field
. As the nanoparticles ionize
as fission occurs, the dust becomes suspended within the chamber. The incredibly high surface area of the particles makes radiative cooling simple. The axial magnetic field is too weak to affect the motions of the dust particles but strong enough to channel the fragments into a beam which can be decelerated for power, allowed to be emitted for thrust, or a combination of the two. With exhaust velocities of 3% - 5% the speed of light and efficiencies up to 90%, the rocket should be able to achieve over 1,000,000 sec Isp
.
Rocket engine
A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
design that directly harnesses hot nuclear fission product
Fission product
Nuclear fission products are the atomic fragments left after a large atomic nucleus fissions. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons and a large release of energy in the form of heat , gamma rays and neutrinos. The...
s for 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....
, as opposed to using a separate fluid as working mass
Working mass
Working mass is a mass against which a system operates in order to produce acceleration.In the case of a rocket, for example, the reaction mass is the fuel shot backwards to provide propulsion. All acceleration requires an exchange of momentum, which can be thought of as the "unit of movement"...
. The design can, in theory, produce very high specific impulse
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...
s while still being well within the abilities of current technologies.
Design considerations
In traditional nuclear thermal rocketNuclear 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...
and related designs, the nuclear energy is generated in some form of "reactor" and used to heat a working fluid to generate thrust. This limits the designs to temperatures that allow the reactor to remain "whole", although clever design can increase this critical temperature into the tens of thousands of degrees. A rocket engine's efficiency is strongly related to the temperature of the exhausted working fluid, and in the case of the most advanced gas-core engines, it corresponds to a specific impulse of about 7000 s (69 kN·s/kg).
The temperature of a conventional reactor design is actually the average temperature of the fuel, the vast majority of which is not actually reacting at any given instance. In fact the atoms undergoing fission are at a temperature of millions of degrees, which is then spread out into the surrounding fuel, resulting in an overall temperature of a few thousand. In the fission-fragment design, it is the individual atoms that actually undergo fission that are used to provide thrust, by extracting them from the rest of the fuel as quickly as possible before their energy is spread out into the surrounding fuel mass.
This is easier to achieve than it might sound. By physically arranging the fuel such that the outermost layers of a fuel bundle will be most likely to undergo fission, the high-temperature atoms, the fragments of a nuclear reaction, can "boil" off the surface. Since they will be ionized due to the high temperatures of the reaction, they can then be handled magnetically
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
and channeled to produce thrust. Numerous technological challenges still remain, however.
Research
One such design was worked on to some degree by the Idaho National Engineering Laboratory and Lawrence Livermore National LaboratoryLawrence Livermore National Laboratory
The Lawrence Livermore National Laboratory , just outside Livermore, California, is a Federally Funded Research and Development Center founded by the University of California in 1952...
. In their design the fuel was placed into a number of very thin carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
bundles, each one normally sub-critical. Bundles were collected and arranged like spokes on a wheel, and several such wheels were stacked on a common shaft to produce a single large cylinder. The entire cylinder was rotated so that some bundles were always in a reactor core where additional surrounding fuel made the bundles go critical. The fission fragments at the surface of the bundles would break free and be channeled for thrust, while the lower-temperature un-reacted fuel would eventually rotate out of the core to cool. The system thus automatically "selected" only the most energetic fuel to become the working mass.
The efficiency of the system is surprising; specific impulses of greater than 100,000 are possible using existing materials. This is high performance, although not that which the technically daunting antimatter rocket
Antimatter rocket
An antimatter rocket is a proposed class of rockets that use antimatter as their power source. There are several designs that attempt to accomplish this goal...
could achieve, and the weight of the reactor core and other elements would make the overall performance of the fission-fragment system lower. Nonetheless, the system provides the sort of performance levels that would make an interstellar precursor mission possible.
A newer design proposal by Rodney A. Clark and Robert B. Sheldon theoretically increases efficiency and decreases complexity of a fission fragment rocket at the same time over the bundle proposal. In their design, nanoparticle
Nanoparticle
In nanotechnology, a particle is defined as a small object that behaves as a whole unit in terms of its transport and properties. Particles are further classified according to size : in terms of diameter, coarse particles cover a range between 10,000 and 2,500 nanometers. Fine particles are sized...
s of fissionable fuel (or even fuel that will naturally radioactively decay) are kept in a vacuum chamber subject to an axial
Axial
Axial may mean:* Along the same line as an axis of rotation in geometry* A type of modal frame in music* One of several anatomical directions in an animal body* Axial age, the period from 800 to 200 BC in China, India and the western world...
magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
(acting as a magnetic mirror
Magnetic mirror
A magnetic mirror is a magnetic field configuration where the field strength changes when moving along a field line. The mirror effect results in a tendency for charged particles to bounce back from the high field region....
) and an external electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
. As the nanoparticles ionize
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
as fission occurs, the dust becomes suspended within the chamber. The incredibly high surface area of the particles makes radiative cooling simple. The axial magnetic field is too weak to affect the motions of the dust particles but strong enough to channel the fragments into a beam which can be decelerated for power, allowed to be emitted for thrust, or a combination of the two. With exhaust velocities of 3% - 5% the speed of light and efficiencies up to 90%, the rocket should be able to achieve over 1,000,000 sec Isp
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...
.