TheCollidingBeamFusionReactorSpacePropulsionSystem,CBFR-SPS,isananeutronic, magnetic-field-reversed configuration, fueled by an energetic-ion mixture of hydrogen and boron 11 (H-B 11 ). Particle confinement and transport in the CBFR-SPS are classical, hence the system is scaleable. Fusion products are helium ions, α-particles, expelled axially out of the system. α-particles flowing in one direction are decelerated and their energy recovered to "power" the system; particles expelled in the

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... te direction provide thrust. Since the fusion products are charged particles, the system does not require the use of a massive-radiation shield. This paper describes a 100 MW CBFR-SPS design, including estimates for the propulsion-system parameters and masses. Specific emphasis is placed on the design of a closed-cycle, Brayton-heat engine, consisting of heat-exchangers, turbo-alternator, compressor, and finned radiators. CONCEPT DESCRIPTION Exploration of the solar system (and beyond) requires propulsion capabilities that far exceed the best available chemical-or electric-propulsion systems (Stuhlinger, 1964) . For advanced-propulsion applications the Field-Reversed Configuration, FRC, (Tuszewski, 1988 and Steinhauer, 1996) is a promising concept, providing: design simplicity, high-thrust, high-specific impulse, and high specific-power-density. An improved version of the FRC is the Colliding Beam Fusion Reactor, CBFR, where the ion temperature is 100's of keV and the size of the ion gyro-orbits is comparable to the radial dimension of the system. The CBFR, first conceived by Rostoker, et al. (Rostoker, 1993 and , provides a pathway for the use of advanced fuels, i.e., fuels that produce little or no radioactivity. Moreover, in a CBFR plasma confinement and transport are expected to be classical, based on Tokamak experiments involving energetic ions (Heidbrink, 1994) . Analysis suggests that a CBFR could operate on a wide range of fusion fuels, i.e., D-D, D-T, D-He 3 , H-B 11 , H-Li 6 , etc., and could scale over a wide range of output-power levels, from MWs to GWs (Rostoker, 2002). A CBFR, fueled with H-B 11 , is shown schematically in Figure 1 . A super-conducting magnet provides the ambient-magnetic field. Energetic ion beams are injected tangentially (Wessel, 1990) into the fusion core to provide current drive and to re-fuel the reactor and the confined plasma has a large-angular velocity. The injected-ion velocities are the same, but their energies are different. A fusion reaction produces three, helium nuclei (i.e., α-particles), each with an average particle energy of 3.5 MeV. Since fusion products are produced isotropically in the plasma core, approximately half of the α-particles co-rotate with the fuel ions and the remaining half are counter-rotating. Surrounding the fusion-plasma core is a "magnetic separatrix" that defines the boundary between open and closed magnetic field lines. Plasma particles possessing nonresonant gyro-orbits diffuse rapidly to the separatrix and are expelled from the system, out both ends along magnetic-field lines.