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The fission-fragment rocket is a rocket engine design that directly harnesses hot nuclear fission products for thrust, as opposed to using a separate fluid as working mass. The design can, in theory, produce very high specific impulse while still being well within the abilities of current technologies.
In traditional nuclear thermal rocket 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 Isp.
The temperature of a conventional reactor design is the average temperature of the fuel, the vast majority of which is not reacting at any given instant. 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.
By physically arranging the fuel into very thin layers or particles, the fragments of a nuclear reaction can escape from the surface. Since they will be ionized due to the high energy of the reaction, they can then be handled magnetically and channeled to produce thrust. Numerous technological challenges still remain, however.
A design by the Idaho National Engineering Laboratory and Lawrence Livermore National Laboratory [1] uses fuel placed on the surface of a number of very thin carbon fibres, arranged radially in wheels. The wheels are normally sub-critical. Several such wheels were stacked on a common shaft to produce a single large cylinder. The entire cylinder was rotated so that some fibres were always in a reactor core where surrounding moderator made fibres go critical. The fission fragments at the surface of the fibres would break free and be channeled for thrust. The fibre then rotates out of the reaction zone, to cool, to avoid melting.
The efficiency of the system is surprising; specific impulses of greater than 100,000s are possible using existing materials. This is high performance, although 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 L. Clark and Robert B. Sheldon theoretically increases efficiency and decreases complexity of a fission fragment rocket at the same time over the rotating fibre wheel proposal. [2] Their design uses nanoparticles of fissionable fuel (or even fuel that will naturally radioactively decay) of less than 100 nm diameter. The nanoparticle 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.
In 1987 Ronen & Leibson [3] [4] published a study on applications of 242mAm (one of the isotopes of americium) as nuclear fuel to space nuclear reactors, noting its extremely high thermal cross section and energy density. Nuclear systems powered by 242mAm require less fuel by a factor of 2 to 100 compared to conventional nuclear fuels.
Fission-fragment rocket using 242mAm was proposed by George Chapline [5] at LLNL in 1988, who suggested propulsion based on the direct heating of a propellant gas by fission fragments generated by a fissile material. Ronen et al. [6] demonstrate that 242mAm can maintain sustained nuclear fission as an extremely thin metallic film, less than 1/1000 of a millimeter thick. 242mAm requires only 1% of the mass of 235U or 239Pu to reach its critical state. Ronen's group at Ben-Gurion University of the Negev further showed that nuclear fuel based on 242mAm could speed space vehicles from Earth to Mars in as little as two weeks. [7]
242mAm's potential as a nuclear fuel derives from the fact that it has the highest thermal fission cross section (thousands of barns), about 10x the next highest cross section across all known isotopes. 242mAm is fissile and has a low critical mass, comparable to that of 239Pu. [8] [9] It has a very high cross section for fission, and is destroyed relatively quickly in a nuclear reactor. Another report claims that 242mAm can sustain a chain reaction even as a thin film, and could be used for a novel type of nuclear rocket. [6] [10] [11] [12]
Since the thermal absorption cross section of 242mAm is very high, the best way to obtain 242mAm is by the capture of fast or epithermal neutrons in Americium-241 irradiated in a fast reactor. However, fast spectrum reactors are not readily available. Detailed analysis of 242mAm production in existing PWRs was provided in. [13] Proliferation resistance of 242mAm was reported by Karlsruhe Institute of Technology 2008 study. [14]
In 2000 Carlo Rubbia at CERN further extended the work by Ronen [4] and Chapline [5] on fission-fragment rocket using 242mAm as a fuel. [15] Project 242 [16] based on Rubbia design studied a concept of 242mAm based Thin-Film Fission Fragment Heated NTR [17] by using direct conversion of the kinetic energy of fission fragments into increasing of enthalpy of a propellant gas. Project 242 studied the application of this propulsion system to a crewed mission to Mars. [18] Preliminary results were very satisfactory and it has been observed that a propulsion system with these characteristics could make the mission feasible. Another study focused on production of 242mAm in conventional thermal nuclear reactors. [19]
On 2023-01-23 NASA announced funding the study of "Aerogel Core Fission Fragment Rocket Engine", where fissile fuel particles will be embedded in an ultra-low density aerogel matrix to achieve a critical mass assembly. The aerogel matrix (and a strong magnetic field) would allow fission fragments to escape the core, while increasing conductive and radiative heat loss from the individual fuel particles.
Americium is a synthetic chemical element; it has symbol Am and atomic number 95. It is radioactive and a transuranic member of the actinide series in the periodic table, located under the lanthanide element europium and was thus named after the Americas by analogy.
A nuclear thermal rocket (NTR) is a type of thermal rocket where the heat from a nuclear reaction replaces the chemical energy of the propellants in a chemical rocket. In an NTR, 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. The external nuclear heat source theoretically allows a higher effective exhaust velocity and is expected to double or triple payload capacity compared to chemical propellants that store energy internally.
A nuclear electric rocket is a type of spacecraft propulsion system where thermal energy from a nuclear reactor is converted to electrical energy, which is used to drive an ion thruster or other electrical spacecraft propulsion technology. The nuclear electric rocket terminology is slightly inconsistent, as technically the "rocket" part of the propulsion system is non-nuclear and could also be driven by solar panels. This is in contrast with a nuclear thermal rocket, which directly uses reactor heat to add energy to a working fluid, which is then expelled out of a rocket nozzle.
A gas nuclear reactor is a proposed kind of nuclear reactor in which the nuclear fuel would be in a gaseous state rather than liquid or solid. In this type of reactor, the only temperature-limiting materials would be the reactor walls. Conventional reactors have stricter limitations because the core would melt if the fuel temperature were to rise too high. It may also be possible to confine gaseous fission fuel magnetically, electrostatically or electrodynamically so that it would not touch the reactor walls. A potential benefit of the gaseous reactor core concept is that instead of relying on the traditional Rankine or Brayton conversion cycles, it may be possible to extract electricity magnetohydrodynamically, or with simple direct electrostatic conversion of the charged particles.
The nuclear salt-water rocket (NSWR) is a theoretical type of nuclear thermal rocket 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 plutonium or 20 percent enriched uranium. The solution would be contained in a bundle of pipes coated in boron carbide. 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.
In a traditional nuclear photonic rocket, an onboard nuclear reactor would generate such high temperatures that the blackbody radiation from the reactor would provide significant thrust. The disadvantage is that it takes much power to generate a small amount of thrust this way, so acceleration is very low. The photon radiators would most likely be constructed using graphite or tungsten. Photonic rockets are technologically feasible, but rather impractical with current technology based on an onboard nuclear power source.
A fusion rocket is a theoretical design for a rocket driven by fusion propulsion that could provide efficient and sustained acceleration in space without the need to carry a large fuel supply. The design requires fusion power technology beyond current capabilities, and much larger and more complex rockets.
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. The advantage to this class of rocket is that a large fraction of the rest mass of a matter/antimatter mixture may be converted to energy, allowing antimatter rockets to have a far higher energy density and specific impulse than any other proposed class of rocket.
Nuclear pulse propulsion or external pulsed plasma propulsion is a hypothetical method of spacecraft propulsion that uses nuclear explosions for thrust. It originated as Project Orion with support from DARPA, after a suggestion by Stanislaw Ulam in 1947. Newer designs using inertial confinement fusion have been the baseline for most later designs, including Project Daedalus and Project Longshot.
In nuclear engineering, fissile material is material that can undergo nuclear fission when struck by a neutron of low energy. A self-sustaining thermal chain reaction can only be achieved with fissile material. The predominant neutron energy in a system may be typified by either slow neutrons or fast neutrons. Fissile material can be used to fuel thermal-neutron reactors, fast-neutron reactors and nuclear explosives.
Mixed oxide fuel, commonly referred to as MOX fuel, is nuclear fuel that contains more than one oxide of fissile material, usually consisting of plutonium blended with natural uranium, reprocessed uranium, or depleted uranium. MOX fuel is an alternative to the low-enriched uranium fuel used in the light-water reactors that predominate nuclear power generation.
Nuclear propulsion includes a wide variety of propulsion methods that use some form of nuclear reaction as their primary power source. The idea of using nuclear material for propulsion dates back to the beginning of the 20th century. In 1903 it was hypothesized that radioactive material, radium, might be a suitable fuel for engines to propel cars, planes, and boats. H. G. Wells picked up this idea in his 1914 fiction work The World Set Free. Many aircraft carriers and submarines currently use uranium fueled nuclear reactors that can provide propulsion for long periods without refueling. There are also applications in the space sector with nuclear thermal and nuclear electric engines which could be more efficient than conventional rocket engines.
A subcritical reactor is a nuclear fission reactor concept that produces fission without achieving criticality. Instead of sustaining a chain reaction, a subcritical reactor uses additional neutrons from an outside source. There are two general classes of such devices. One uses neutrons provided by a nuclear fusion machine, a concept known as a fusion–fission hybrid. The other uses neutrons created through spallation of heavy nuclei by charged particles such as protons accelerated by a particle accelerator, a concept known as an accelerator-driven system (ADS) or accelerator-driven sub-critical reactor.
Fertile material is a material that, although not fissile itself, can be converted into a fissile material by neutron absorption.
Americium (95Am) is an artificial element, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no known stable isotopes. The first isotope to be synthesized was 241Am in 1944. The artificial element decays by ejecting alpha particles. Americium has an atomic number of 95. Despite 243
Am being an order of magnitude longer lived than 241
Am, the former is harder to obtain than the latter as more of it is present in spent nuclear fuel.
A minor actinide is an actinide, other than uranium or plutonium, found in spent nuclear fuel. The minor actinides include neptunium, americium, curium, berkelium, californium, einsteinium, and fermium. The most important isotopes of these elements in spent nuclear fuel are neptunium-237, americium-241, americium-243, curium-242 through -248, and californium-249 through -252.
Similar to how the fission-fragment rocket produces thrust, a fission fragment reactor is a nuclear reactor that generates electricity by decelerating an ion beam of fission byproducts instead of using nuclear reactions to generate heat. By doing so, it bypasses the Carnot cycle and can achieve efficiencies of up to 90% instead of 40-45% attainable by efficient turbine-driven thermal reactors. The fission fragment ion beam would be passed through a magnetohydrodynamic generator to produce electricity.
Project Timberwind aimed to develop nuclear thermal rockets. Initial funding by the Strategic Defense Initiative from 1987 through 1991 totaled $139 million (then-year). The proposed rocket was later expanded into a larger design after the project was transferred to the Air Force Space Nuclear Thermal Propulsion (SNTP) program.
Nuclear gas-core-reactor rockets can provide much higher specific impulse than solid core nuclear rockets because their temperature limitations are in the nozzle and core wall structural temperatures, which are distanced from the hottest regions of the gas core. Consequently, nuclear gas core reactors can provide much higher temperatures to the propellant. Solid core nuclear thermal rockets can develop higher specific impulse than conventional chemical rockets due to the low molecular weight of a hydrogen propellant, but their operating temperatures are limited by the maximum temperature of the solid core because the reactor's temperatures cannot rise above its components' lowest melting temperature.
A pulsed nuclear thermal rocket is a type of nuclear thermal rocket (NTR) concept developed at the Polytechnic University of Catalonia, Spain, and presented at the 2016 AIAA/SAE/ASEE Propulsion Conference for thrust and specific impulse (Isp) amplification in a conventional nuclear thermal rocket.
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