I pointed earlier to the set of videos from the Tennessee Valley Interstellar Workshop (TVIW) held in Oak Ridge last November. One of the talks was given by Marc Millis, who was chief of NASA’s Breakthrough Propulsion program, which ran from 1996-2002. Schemes to provide propellantless propulsion without violating conservation of momentum require some type of non-standard view of inertial frames. In his TVIW talk, he gave an intro to this topic. He also has this online tutorial: Breakthrough Propulsion Physics – a tutorial by Marc Millis – h+ Media