The word `refuel' is actually somewhat inappropriate when the Black Horse is concerned. Current plans call for transfer of oxidant, rather than fuel. Hence, the official term for what the Black Horse is designed to do is `Aerial Propellant Transfer'. However, I like to use the term `refuel' in some circumstances to help people understand the Black Horse concept by relating it to something they already know.
The reason that the Black Horse is designed to transfer oxidant rather than fuel is strightforward: the oxidant wieghs much more than the fuel. Depending on the propellant choices, the oxidant to fuel mass ratio should be near 8:1. Thus, even if the vehicle caries all of its fuel from takeoff, most of the propellant mass is transferred from the tanker.
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