The thermal-hydraulic behavior of rotating bed reactors is analyzed using the drift-flux model to predict fuel bed expansion. A new correlation for onset of fluidization is developed for this purpose. Parametric effects are discussed and first priority research areas are delineated. Reactor design curves are developed showing small cores between 25- and 50-cm radius should be capable of generating power up to 5000 MW. With exhaust temperatures approaching 3000 K using hydrogen as a propellant, these devices seem especially suited for use in space-tug concepts.