A gas turbine engine has, in at least a first mode of operation, a core engine, a fan, and an auxiliary turbine engine coupled as follows. The core engine has, along a core flowpath, at least one compressor section, a combustor, and at least one turbine section. The fan is upstream of the core flowpath and drives air downstream along the core flowpath and along a bypass flowpath. The auxiliary turbine engine has an axial compressor, a fan, a combustor, and an axial turbine. The fan has fan blades and a continuation of the bypass flowpath extends sequentially downstream through the axial compressor and radially outward through passageways in the blades of the fan and a continuation of the core flowpath passes axially through the fan. The combustor is downstream of the fan along the continuation of the bypass flowpath. The axial turbine is downstream of the combustor along the continuation of the bypass flowpath and comprises at least one stage of blades rotating with the fan.