> <i><font color="yellow">There is another kind of re-usability: on-orbit, Cis-Lunar utilization of second stages. Propellant depots, space tugs</font>/i><br /><br />One concept of operation that intrigues me is two or three stage system for moderate amounts of mass (e.g., humans or basic supplies) in which all the components are reusable. The first stage is Earth-to-LEO, and these vehicles would be optimized for traveling through the atmosphere. The second stage is for travel between LEO and Lunar orbit, and these vehicles would be optimized for zero-G travel (e.g., no landing gear). The third stage is Lunar orbit to Lunar surface, and it would be optimized for short VTVL trips with no atmosphere.<br /><br />For time-sensitive cargo (e.g., humans), the second stage could use chemical rockets to limit transit time. Otherwise, ion propulsion could be used to deliver things like supplies.<br /><br />By standardizing the interfaces (e.g., docking rings, communications, procedures, etc.), multiple parties could participate. For example, there are clearly many groups looking seriously looking at delivering solutions to the first stage (moderate size mass to LEO).<br /><br />Such a system would only become useful once a large enough presense is established on the Moon which would require frequent, low-to-moderate mass transportation, so the Ares I & V are still needed as pathfinder systems.</i>