Image Credit: NASA JPL
Fission-fragment propulsion is a propulsion design whereby fission fragments from a fissioning reactor core are utilized to provide thrust, thus theoretically capable of achieving speeds of .03c (3% of the speed of light)*
While most fission propulsion designs heat up and expand a working fluid (propellant) like hydrogen to attain thrust, this ultimately limits the velocity of the spacecraft to 7000 specific impulse (70 km/s) due to the structural limits of available engine materials**. The fission fragment concept uses the energetic particles created directly from the fissioning of critical material to work as the propellant. Since the fission fragments are ionized a magnetic field can be used to direct the highly energized fragments to produce a thrust beyond 1 million Specific Implulse*.
The Idaho National Engineering Laboratory (INEL) and Lawrence Livermore National Laboratory (LLNL) developed the design above, which illustrates revolving wheels of thin graphite fibers coated with nuclear fuel. Spent nuclear fuels such as americium or curium would be likely candidates for the fissionable material due to their ability to provide high specific power and high specific impulse. The graphite wheels revolve through a reactor causing the nuclear fuel to fission inside the reactor. Heat and charged fissioned-fragment nuclei accelerate out of the reactor through a fragment extraction port and are then directed by a magnetic field for thrust*. Cont 1, 2