Nuclear electric rocket
Also known as nuclear electric propulsion and space nuclear fission electric power systems, 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: Rankine cycle, Brayton cycle, Stirling cycle, thermoelectric (including graphene-based thermal power conversion), pyroelectric, thermophotovoltaic, thermionic, magnetohydrodynamic type, or some as yet undiscovered technology or as-yet unknown thermoelectric materials.
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 neutrons.
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 power spacecraft.
Research in nuclear propulsion began with studies for nuclear thermal propulsion, where the reactor heated a propellant (usually hydrogen) that was allowed to expand through a nozzle. This was essentially an ordinary chemical rocket with the nuclear reaction replacing chemical combustion as the rocket's heat source. Because the reactor could supply more heat to the propellant than chemical combustion, higher exhaust velocities, i.e., higher specific impulses were possible. 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 400 kW 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[vague], proper shielding, control pins and spoiler pins[clarification needed] inside the reactor which arrest the reaction.
The key elements to NEP, as they are being pursued today are:
- A compact reactor core
- A gas turbine or Stirling engine used as an electric generator
- A compact heat rejection system such as heat pipes
- A electric power conditioning and distribution system
- A high electric 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.
Other types of nuclear power in space
Nuclear electric propulsion is a field that is distinct from other space nuclear power areas, such as radioisotope systems (including radioisotope thermoelectric generators, radioisotope heater units, radioisotope piezoelectric generators & the radioisotope rocket - all of which use the heat from a static radioactive source (usually Plutonium-238) for a low level of electric or direct propulsion power), a nuclear thermal rocket (energy is used to heat the liquid hydrogen propellant), direct nuclear (fission products from a nuclear reaction directly propel the rocket), nuclear pulse propulsion (nuclear explosions propel the rocket), or space based nuclear fusion systems, either a fusion rocket, or some as-yet theoretical or unproven experimental fusion technology.
- Spacecraft propulsion
- Electrically powered spacecraft propulsion
- Nuclear reactor
- Radioisotope thermoelectric generator
- Ion thruster
- Magnetic sail
- David Buden (2011), Space Nuclear Fission Electric Power System: Book 3: Space Nuclear Propulsion and Power
- Joseph A. Angelo & David Buden (1985), Space Nuclear Power
- NASA/JPL/MSFC/UAH 12th Annual Advanced Space Propulsion Workshop (2001), The Safe Affordable Fission Engine (SAFE) Test Series)
- NASA (2010), Small Fission Power System Feasibility Study Final Report
- Patrick McClure & David Poston (2013), Design and Testing of Small Nuclear Reactors for Defense and Space Applications
- Mohamed S. El-Genk & Jean-Michel P. Tournier (2011), Uses of Liquid-Metal and Water Heat Pipes in Space Reactor Power Systems
- U.S. Atomic Energy Comission (1969), SNAP Nuclear Space Reactors
- Space.com (May 17, 2013), How Electric Spacecraft Could Fly NASA to Mars
- Technolog Review, March 5, 2012: Graphene Battery Turns Ambient Heat Into Electric Current
- Scientific Reports, Aug. 22, 2012: Graphene-based photovoltaic cells for near-field thermal energy conversion
- MIT News, Oct. 7, 2011: Graphene shows unusual thermoelectric response to light