The turbofan engine is a branch of the jet engine. In terms of blood relationship, the turbofan engine should be regarded as a variant of the turbojet engine. Structurally, the turbofan engine is nothing more than adding a fan before (after) the turbojet engine. Yet it is this meager few pages of fans that strictly separate turbojets from turbofans. The turbofan engine is also blue and green, relying on a few pages of its own fans.