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Propellant transfer is a flexible approach. Propellant might be delivered by a variety of providers using big rockets or small ones, and the depot could be placed in whatever orbital location makes the most sense. However, by embracing that flexibility, advocates have avoided specifying the details that determine whether a design closes—that is, whether it really works. At least one viable proof of concept example is needed before the idea can be evaluated. Each reference design should specify, among other things: how big the tankers are and how and when they are launched; how the various propellant spacecraft operate together; the orbits, trajectories, and maneuvers assumed; and whether the in-space propellant transfer assets can be reused for multiple missions or are deployed fresh for each mission.
In engineering, there are four fundamental steps to inventing the next big thing. First, come up with a new idea that looks promising. Second, figure out the details of how it would work. Third, compare it to known alternatives to figure out whether the new idea is actually any better. Propellant depots for exploration have gotten through step one. But until someone performs steps two and three to figure out whether depots really are an improvement, it’s premature to move to step four: making it happen.