Propellant tanker

Feasibility of propellant depots

• Propellant tanker delivery. In this approach, a single tanker performs a rendezvous and docking with an on-orbit spacecraft. The tanker then transfers propellant and departs. This approach is "much like an airborne tanker refuels an aircraft".
• In-space depot. An alternative approach is for many tankers to rendezvous and transfer propellant to an orbital depot. Then, at a later time, a spacecraft may dock with the depot and receive a propellant load before departing Earth orbit.