A closed cycle turbine system is provided for the generation of power in a light, high altitude airborne vehicle. The closed cycle turbine system may operate according to either the Brayton or the Rankine cycles. The working fluid for the closed cycle may be alternately heated by the combination of an external burner and heat exchanger and cooled by expansion and radiation. The use of the external burner operating at near the atmospheric pressure may eliminate the requirement for a compressor to compress large amounts of low density, ambient air for use in the turbine. Additional ambient air may be provided to the burner by either using a fan to concentrate the ambient air or pressurizing the fuel stream so as to entrain the ambient air therein. The external burner may use a gaseous fuel such as hydrogen or liquid fuel such as jet fuel to provide heat for the closed cycle.