Drop Tank SEP Reuse (DTSR)

Drop Tank would be pressurized with Xenon or more likely  pressurized with xenon  after cryogenic propellent transfer.Drop Tank needs IVF and a air liquefaction/fractionation unit.

Drop tank needs a solar array and communications and carries a secondary payload such as cubesats

Drop tank itself could be the secondary payload of the primary mission and undock with the primary mission after the primary mission completes its injection.This would require a common berthing mechanism in addition to the robotic arm fuel transfer system.

Cryogenic propellent boil off could be used to ZBO cryogenic Xenon to save space and mass from its tank and the drop tank would need a in space air liquefaction/fractionation system to segregate gases Xenon fro gasses O2 and H2 in order for the IVF engine to work.

ZUEXS   as drop tank