NASA SBIR-2015 Topic S3.01 seeks to evaluate and advance Stirling convertors as a potentially more efficient alternative to the radioisotope-heated thermoelectric power generators used on some U.S. space missions. Most free-piston Stirling (FPS) convertors are piston-displacer machines that use complex gas bearing/ flexure strategies to manage the wear of reciprocating and close-fitting internal components. The reliability of these mechanisms is critical to overall system performance and longevity. Converter Source, LLC has developed a double-acting free-piston Stirling arrangement that eliminates displacers and the problem of displacer seal wear while at the same time reducing the number of distinct parts required for the machine. The key design features of the new convertor most relevant to the space power application include: integrated alternator functionality within the piston assembly, eliminating connecting structures and buffer volumes with associated pressure walls, thereby reducing system mass compared to conventional technology, compatibility with simple lightweight hydrodynamic gas bearings to eliminate piston seal wear, dynamically-balanced radial layout that surrounds the heat source at the center, and heat rejection to ambient at the outer perimeter.