The diffusers used on model turbine compressors drive the air neatly round 90 deg (in side view). This provides a different efflux geometry, compared to the regular spiral arrangement used on turbochargers for example, which exhausts out of the side. If the pressure rise is better with a Centrifugal (single stage) in the context of a compressor can such an arrangement (with the outer diffuser ring) be used in place of a Ducted Fan in free air for propulsion? What are the comparative efficiencies if so? Secondly what about heat? I wondered if the exhast gases from an IC motor were added to the efflux, and re-ignited (in a stainless steel chamber) is there a capacity for 'boost' therein?