Compound propulsion engine

A compound propulsion engine comprises a primary stage and a secondary stage. The primary stage includes an axial compressor having compressor blades and a rotary shaft defining an axial direction such that ram air is compressed and exits the axial compressor in the axial direction, a sliding vane rotary vane turbine connected by an interconnecting shaft to rotationally drive the axial compressor, and a combustor that is disposed downstream of the axial compressor and that adds heat energy to the exiting compressed air to produce combustion products. The secondary stage includes an axial turbine having turbine blades and a rotary shaft extending in the axial direction and driven by the combustion products exiting the combustor, and a sliding vane rotary vane compressor connected by an interconnecting shaft to the axial turbine so as to be rotationally driven by the axial turbine, the sliding vane rotary vane compressor being connected to receive and further compress a portion of the ram air or a portion of the compressed air exiting the axial compressor to produce a secondary compressed air flow that is directed through one or more transfer passages to an intake of the sliding vane rotary vane turbine, said fluid transfer passages between sliding vane rotary vane compressor and sliding vane rotary vane turbine comprise at least one check valve and/or at least a cyclo valve.