Nuclear electric rocket
Encyclopedia
In a nuclear electric rocket, nuclear thermal energy
is changed into electrical energy that is used to power one of the electrical propulsion
technologies. Technically the powerplant is nuclear, not the propulsion system, but the terminology is standard. A number of heat-to-electricity schemes have been proposed.
One of the more practical schemes is a variant of a pebble bed reactor
. It would use a high mass-flow nitrogen
coolant near normal atmospheric pressures. This would take advantage of highly developed conventional gas turbine
technologies. The fuel for this reactor would be highly enriched, and encapsulated in low-boron
graphite
balls probably 5-10 cm in diameter. The graphite serves to slow, or moderate, the neutron
s.
This style of reactor can be designed to be inherently safe. As it heats, the graphite expands, separating the fuel and reducing the reactor's criticality. This property can simplify the operating controls to a single valve throttling the turbine. When closed, the reactor heats, but produces less power. When open, the reactor cools, but becomes more critical and produces more power.
The graphite encapsulation simplifies refueling and waste handling. Graphite is mechanically strong, and resists high temperatures. This reduces the risk of an unplanned release of radioactives.
Since this style of reactor produces high power without heavy castings to contain high pressures, it is well suited to spacecraft.
Research in nuclear propulsion began with studies for nuclear thermal propulsion, where the ejected mass was pumped right through the reactor thus getting heated and ejected. See KIWI, NERVA
. The reports at the time (and since) indicated that keeping the system light would require high temperature, densely packed designs, such as fast metal cooled reactors or hexagonal pin fueled, high temperature gas cooled reactors. In the past several decades the attention has turned to using the nuclear reactor to drive a turbine to produce electricity, which is used to create a plasma which is accelerated. See Project Prometheus
. The present best of tech is the SAFE-400, which uses a 400kW thermal reactor and a gas turbine (called a closed Brayton cycle
) to produce electric power. Heat rejection is kept low-mass using advanced heat pipe systems (such as are now used in some laptop computers for cooling as well). Safety comes from ruggedness, proper shielding, control pins and spoiler pins inside the reactor which arrest the reaction.
The key elements to NEP, as they are being pursued today are: 1. A compact reactor core 2. A gas turbine or stirling engine used as an electric generator 3. A compact heat rejection system such as heat pipes 4. A power conditioning and distribution system 5. A high power propulsion system based on plasma propellants
The SAFE-400 is the current best of tech for items 1-3. Item 4 is common to all spacecraft. Some examples of thrusters that might be suitable for this are VASIMR, DS4G and Pulsed inductive thruster
. PIT and VASIMR are unique in their ability to trade between power usage, specific impulse (a measure of efficiency, see specific impulse
) and thrust in-flight. PIT has the additional advantage of not needing the power conditioning system between itself and the electric generators.
Thermal energy
Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system's temperature....
is changed into electrical energy that is used to power one of the electrical propulsion
Spacecraft propulsion
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the...
technologies. Technically the powerplant is nuclear, not the propulsion system, but the terminology is standard. A number of heat-to-electricity schemes have been proposed.
One of the more practical schemes is a variant of a pebble bed reactor
Pebble bed reactor
The pebble bed reactor is a graphite-moderated, gas-cooled, nuclear reactor. It is a type of very high temperature reactor , one of the six classes of nuclear reactors in the Generation IV initiative...
. It would use a high mass-flow nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
coolant near normal atmospheric pressures. This would take advantage of highly developed conventional gas turbine
Gas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....
technologies. The fuel for this reactor would be highly enriched, and encapsulated in low-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...
graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
balls probably 5-10 cm in diameter. The graphite serves to slow, or moderate, the 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...
s.
This style of reactor can be designed to be inherently safe. As it heats, the graphite expands, separating the fuel and reducing the reactor's criticality. This property can simplify the operating controls to a single valve throttling the turbine. When closed, the reactor heats, but produces less power. When open, the reactor cools, but becomes more critical and produces more power.
The graphite encapsulation simplifies refueling and waste handling. Graphite is mechanically strong, and resists high temperatures. This reduces the risk of an unplanned release of radioactives.
Since this style of reactor produces high power without heavy castings to contain high pressures, it is well suited to spacecraft.
Research in nuclear propulsion began with studies for nuclear thermal propulsion, where the ejected mass was pumped right through the reactor thus getting heated and ejected. See KIWI, NERVA
NERVA
NERVA is an acronym for Nuclear Engine for Rocket Vehicle Application, a joint program of the U.S. Atomic Energy Commission and NASA managed by the Space Nuclear Propulsion Office until both the program and the office ended at the end of 1972....
. The reports at the time (and since) indicated that keeping the system light would require high temperature, densely packed designs, such as fast metal cooled reactors or hexagonal pin fueled, high temperature gas cooled reactors. In the past several decades the attention has turned to using the nuclear reactor to drive a turbine to produce electricity, which is used to create a plasma which is accelerated. See Project Prometheus
Project Prometheus
Project Prometheus was established in 2003 by NASA to develop nuclear-powered systems for long-duration space missions. This was NASA's first serious foray into nuclear spacecraft propulsion since the cancellation of the NERVA project in 1972...
. The present best of tech is the SAFE-400, which uses a 400kW thermal reactor and a gas turbine (called a closed Brayton cycle
Brayton cycle
The Brayton cycle is a thermodynamic cycle that describes the workings of the gas turbine engine, basis of the airbreathing jet engine and others. It is named after George Brayton , the American engineer who developed it, although it was originally proposed and patented by Englishman John Barber...
) to produce electric power. Heat rejection is kept low-mass using advanced heat pipe systems (such as are now used in some laptop computers for cooling as well). Safety comes from ruggedness, proper shielding, control pins and spoiler pins inside the reactor which arrest the reaction.
The key elements to NEP, as they are being pursued today are: 1. A compact reactor core 2. A gas turbine or stirling engine used as an electric generator 3. A compact heat rejection system such as heat pipes 4. A power conditioning and distribution system 5. A high power propulsion system based on plasma propellants
The SAFE-400 is the current best of tech for items 1-3. Item 4 is common to all spacecraft. Some examples of thrusters that might be suitable for this are VASIMR, DS4G and Pulsed inductive thruster
Pulsed inductive thruster
Pulsed inductive thrusters are a form of ion thruster, used in spacecraft propulsion. A PIT uses perpendicular electric and magnetic fields to accelerate a propellant. A nozzle releases a puff of gas which spreads across a flat induction coil of wire about 1 meter across...
. PIT and VASIMR are unique in their ability to trade between power usage, specific impulse (a measure of efficiency, see 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 ,...
) and thrust in-flight. PIT has the additional advantage of not needing the power conditioning system between itself and the electric generators.
See also
- Spacecraft propulsionSpacecraft propulsionSpacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the...
- Nuclear propulsionNuclear propulsionNuclear propulsion includes a wide variety of propulsion methods that fulfil the promise of the Atomic Age by using some form of nuclear reaction as their primary power source.- Surface ships and submarines :...
- PolywellPolywellA polywell device is a type of fusion reactor that was originated by Robert Bussard under a U.S. Navy research contract. It traps electrons in a magnetic confinement inside its hollow center. The negatively charged electrons then accelerate positively charged ions for the purpose of achieving...
- Nuclear reactorNuclear reactorA nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
- Radioisotope thermoelectric generatorRadioisotope thermoelectric generatorA radioisotope thermoelectric generator is an electrical generator that obtains its power from radioactive decay. In such a device, the heat released by the decay of a suitable radioactive material is converted into electricity by the Seebeck effect using an array of thermocouples.RTGs can be...
- Ion thrusterIon thrusterAn ion thruster is a form of electric propulsion used for spacecraft propulsion that creates thrust by accelerating ions. Ion thrusters are categorized by how they accelerate the ions, using either electrostatic or electromagnetic force. Electrostatic ion thrusters use the Coulomb force and...
- Magnetic sailMagnetic sailA magnetic sail or magsail is a proposed method of spacecraft propulsion which would use a static magnetic field to deflect charged particles radiated by the Sun as a plasma wind, and thus impart momentum to accelerate the spacecraft...