A gas turbine engine defines an axial direction and a radial direction and comprises a turbomachine having an unducted primary fan, a core engine a combustor casing enclosing a combustor and defining an outer surface, a core cowl surrounding at least a portion of the core engine. The outer surface of the core cowl defines a peak cowl diameter (D) in the radial direction, and the outer surface of the combustor casing defines a maximum combustor casing diameter (d) along the radial direction. The core engine defines an overall core axial length (L) along the axial direction and an under-core cowl axial length (L1) along the axial direction. The gas turbine engine defines a core cowl diameter ratio (CDR) equal to the peak cowl diameter (D) divided by the maximum combustor casing diameter (d) and a core cowl length ratio (CLR) equal to the under-core cowl axial length (L1) divided by the overall core axial length (L). The CDR is between 2.7 and 3.5 and the CLR is between 0.25 and 0.50.