Impact of energy gain and subsystem characteristics on fusion propulsion performance

Rapid transport of large payloads and human crews throughout the solar syst em requires propulsion systems having very high specific impulse (1(sp) gre ater than or equal to 10(4) to 10(5) s). It also calls for systems with ext remely low mass-power ratios (alpha less than or equal to 10(-1) kg/kW). Su ch low alpha are beyond the reach of conventional power-limited propulsion, but may be attainable with fusion and other nuclear concepts that produce energy within the propellant. The magnitude of energy gain must be large en ough to sustain the nuclear process while still providing a high jet power relative to the massive energy-intensive subsystems associated with these c oncepts. This paper evaluates the impact of energy gain and subsystem chara cteristics on alpha. Central to the analysis are general parameters that em body the essential features of any "gain-limited" propulsion power balance. Results show that the gains required to achieve alpha similar to 10(-1) kg /kW with foreseeable technology range from similar to 100 to over 2000, whi ch is three to five orders of magnitude greater than current fusion state o f the art. Sensitivity analyses point to the parameters exerting the most i nfluence for either 1) lowering alpha and improving mission performance or 2) relaxing gain requirements and reducing demands on the fusion process. T he greatest impact comes from reducing mass and increasing efficiency of th e thruster and subsystems downstream of the fusion process. High relative g ain, through enhanced fusion processes or more efficient drivers and proces sors, is also desirable. There is a benefit in improving driver and subsyst em characteristics upstream of the fusion process, but it diminishes at rel ative gains greater than or equal to 100.