NASA seeks high-power conversion systems for space applications and their requirements emphasize high efficiency and low mass. This paper presents preliminary designs and specific power estimates of high efficiency, free-piston Stirling machines for 5 kWe , 10 kWe and 25 kWe outputs. In each layout, several engine configurations have been considered: single and dual opposed beta and three, four and six cylinder alphas. A critical factor in obtaining high specific power is related to the specific power optimization of the alternator. This is shown to be a strong function of the piston amplitude. Monocoque finned heads are found to be compromised by temperature gradients across the pressure-containing walls. These temperature gradients become significant at power levels above 2 kW. A stepped piston three-cylinder alpha arrangement is shown to have favorable dynamics for high efficiencies and good specific powers.