Image Credit: NASA JPL
Robert L. Forward also designed a fission fragment propulsion concept in which the fissionable material was coated on a sheet analogous to a solar sail, where by the fission fragments would be propelled off the sheet, as a result, pushing the spacecraft forward**. When near a star, however, the sail would rely on the propulsion of photons emitted from that star*.
Steven Howe recently devised a similar concept to Forward's in which a carbon sheet covered with uranium would fission as a result of antiproton collisions, thereby creating even more efficient fission conversions of mass into energy, thus greater specific impulse and power***. back 1, 2
*Frisbee, H Robert. Advanced Propulsion Concepts Fragment-Fission. http://www.islandone.org/APC/Nuclear/10.html
**Frisbee, H Robert. Advanced Space Propulsion for the 21st Century. Journal of Propulsion and Power. Vol. 19, No. 6, November-December 2003.
***Howe, D Steven, Jackson. P Gerald. Antimatter Driven Sail for Deep Space Propulsion. Interstellar Robotics Missions for the 21st Century Workshop. JPL/Cal Tech, 1998