gas turbine engine systems involving tip fans are provided . in this regard , a representative gas turbine engine system includes : a multi - stage fan having a first rotatable set of blades and a second counter - rotatable set of blades , the second rotatable set of blades defining an inner fan and a tip fan and being located downstream of the first set of rotatable blades ; and an epicyclic differential gear assembly operative to receive a torque input and differentially apply the torque input to the first set of blades and the second set of blades .