Concepts for relieving the terrible mass-fraction penalties of launching out of Earth’s gravity well by re-fueling spacecraft on-orbit have been proposed for decades (Reference 1). Certainly it makes theoretical sense to avoid carrying all needed mission propellants (as well as other vital fluids and/or gases) from liftoff to the final orbit by ‘topping off’ at an in-space ‘filling station.’ which is clearly analogous to the re-fueling of cars on an Interstate highway on a long auto trip.

Indeed, analogies have been drawn to the Pony Express in the 1880s and to steam engine trains before that. In those cases, the ‘fuel’ (feed and water for the ponies and wood and water for the engines) was generally available in situ at the way station; later, for coal-driven and then diesel trains, and finally, automobiles, the fuel was produced elsewhere and delivered to re-fueling locations.

This analogy is usually applied to concepts for on-orbit depots, wherein cryogenic or storable propellants are produced on the ground, transported by rocket-tankers to an orbital storage depot, where it is then available for transfer into upper stages and/ or orbital maneuvering vehicles (OMV). Two other concepts, ‘in-situ production’ (ideally, also, ‘on- demand’) of cryogenics at the depot, and ‘in-space re-fueling from the ground’ (analogous to ‘mid-air re-fueling of aircraft’) have also been proposed.

This paper will address the considerable technological, operational and economic challenges for all of these concepts at a very high, qualitative level, and also assesses the prospects of commercialization of such depot/re-fueling systems. In addition, we will discuss potential technology demonstrations or study projects that might help to focus the logical and cost-effective way forward to such an ‘in-space refueling system’ as part of future space infrastructures.