A Reaction Drive Powered by External Dynamic Pressure as Second Stage for Interstellar Flight Author: Jeffrey Greason, B.S., Chairman, Tau Zero Foundation Abstract Background: The Plasma Magnet work sponsored by NIAC in 2004-2005 developed a means of producing drag against the interplanetary solar wind or interstellar medium with high drag-to-mass ratio. By itself that would be sufficient for interstellar precursor missions as summarized in a talk at TVIW 2017, but that potential has not been realized, in part because of a lack of methods for braking after such fast transits for planetary missions. Abstract Objectives: A new class of reaction drive is discussed, in which reaction mass is expelled from a vehicle using power extracted from the relative motion of the vehicle and the surrounding medium, such as the solar wind or interstellar plasma. Abstract Methods: The physics of this type of drive are reviewed analytically and shown to permit high velocity changes with modest mass ratio while conserving energy and momentum according to well-established physical principles. Abstract Results: For interplanetary missions, use of the plasma-magnet device as based on past NIAC research updated with modern superconductors offers acceleration ~0.05 m/s^2. For acceleration, the mass ratio needed is the square of the ratio of final to initial velocity. Departure velocities of ~7500-15000 km/s (advanced fusion rocket) then give 0.1-0.2c at mass ratio 16 for the second stage. TRL Assessment: The plasma magnet itself has been demonstrated in a realistic environment for TRL5. The new reaction principle for braking in the solar system and interstellar acceleration has now had the physical principles and governing equations worked out and submitted for publication, with some paths for implementation at conceptual design level, making it TRL2. Abstract Near-Term Technical Milestones: Next step is to extend the equations to include losses and inefficiencies and lateral thrust, to explore fast flight to Mars near conjunction. Then take design of the interstellar implementation to a preliminary stage, and test a subscale version in a laboratory.Next step is to extend the equations to include losses and inefficiencies and lateral thrust, to explore fast flight to Mars near conjunction. Then take design of the interstellar implementation to a preliminary stage, and test a subscale version in a laboratory. Abstract Conclusions: For interplanetary missions, combination of this principle with plasma magnet permits fast interplanetary transits (one year to accelerate, coast, and brake to a Neptune orbit). If used as a second stage for a fusion or other advanced rocket, 0.1-0.2c velocities appear achievable.