This propellant depot was proposed by United Launch Alliance, as a derivative of their Advanced Cryogenic Evolved Stage (ACES), later renamed to Centaur V. The lower part of the depot is composed of a standard Centaur V Mk3/ACES upper stage, with the LH2 tank vented and converted to store liquid oxygen. On top, an additional tank is added for liquid hydrogen storage, separated from the stage by an adapter ring and truss structure, and surrounded by a deployable multilayer insulation sunshade to allow multi-month storage without boiloff. The adapter ring also supports radiators, solar arrays, and the interfaces used by visiting tankers and client spacecraft for docking and propellant transfer
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