The Fusion Driven Rocket

A very persuasive reason for investigating the applicability of nuclear power in rockets is the vast energy density gain of nuclear fuel when compared to chemical combustion energy. The Fusion Driven rocket (FDR) represents a new approach to fusion propulsion where the power source releases its energy directly into the propellant, not requiring conversion to electricity. It employs a solid lithium propellant that requires no significant tankage mass. The propellant is rapidly heated and accelerated to high exhaust velocity (> 30 km/s), while having no substantial physical interaction with the spacecraft thereby avoiding damage to the rocket as well as limiting the thermal heat load and therefor radiator mass. The FDR concept is based on research conducted at MSNW and the University of Washington on magnetically driven implosions of thin metal bands onto magnetized plasmoids to achieve nuclear fusion. Here, Lithium bands are inductively driven to converge radially and axially forming a thick blanket surrounding the plasmoid. Virtually all of the radiant, neutron and particle energy from the fusion plasma is absorbed by the encapsulating shell thus isolating the thruster and spacecraft from the intense energy release. This energy, in addition to the powerful Ohmic heating in the shell at peak magnetic field compression, is adequate to vaporize and ionize the Lithium. The expansion of this hot, ionized metal propellant is inhibited from blow-back onto the thruster as it is produced inside the large region filled with a divergent axial magnetic field. The propellant thermal energy is consequently converted into high thrust at an optimal Isp through this magnetic nozzle. The back emf from magnetic flux compression due to the expanding Lithium plasma can be utilized to resupply the electrical energy needed for subsequent pulses. This paper outlines these key elements of the FDR concept and the state of their current development. These include: 1) The results from laboratory testing of the metal band compression with sufficient imploded foil kinetic energy (~ 0.5 MJ) to reach conditions required for fusion breakeven conditions. (2) The results from both a 1D code for analyzing the magnetically driven metal band implosion dynamics as well as a full 3D ANSYS® calculation of the implosion dynamics. (3) Based on these results the FDR spacecraft design and mass budget were formulated. (4) A range of mission architectures for which FDR would be enabling, including a rapid, single launch manned Mars mission were developed and are described.