Flight performance of long-range spacecraft is limited by the need to lift large amounts of fuel into orbit; thus, a propulsion system which will not depend on mass-ejection will be a most welcome development. Various past proposals which attempted to achieve this were rejected because it was thought that they contradicted basic laws of physics. An experimental mechanical device which operates without mass-ejection, (or interaction with the environment), and which can serve as a spacecraft propulsion system, was designed, constructed and successfully tested. It is based on manipulating centrifugal forces, internally generated by rotating masses, and is not size or scale dependent. Full explanation, with photos and movies of its construction and operation, is presented. When fully developed, this technology will enable constant long-duration acceleration of spacecraft (decreasing flight times), permit novel physics research, or enable hovering with or without the presence of atmosphere.