There's also solar electric propulsion, which when combined with water as propellant, leads to dramatic (10-100x) cost reductions due to increased propellant ef

There's also solar electric propulsion, which when combined with water as propellant, leads to dramatic (10-100x) cost reductions due to increased propellant efficiency plus the synergies created by using water as reaction mass (it can be used for everything from radiation shielding to crew consumables prior to being sent to the engines, which effectively turns most of your consumables mass into propellant). This approach also benefits from ongoing improvements in component technologies (thin film PV, electric propulsion, and inflatable structures), so a reusable spacecraft based on this design pattern could be upgraded as things improve. (Release early, release often, versus the 30 year Gantt chart methodology favored by space agencies). http://www.centauri-dreams.org/?p=33078 http://www.centauri-dreams.org/?p=33078