This paper documents the design and validation of a closed cycle propulsion system suitable for use on the Perseus A high altitude research aircraft. The atmospheric science community is expected to be the primary user of this aircraft with initial missions devoted to the study of ozone depletion and global warming. To date large amounts of funding are not available to the atmospheric science community, so to be useful, the aircraft must satisfy stringent cost and performance criteria. Among these, the aircraft has to be capable of carrying 50 kg of payload to altitudes of at least 25km, have a initial cost in the $1-2M range, be capable of launch from remote sites, and be available no later than 1994. These operational criteria set narrow boundaries for propulsion system cost, complexity, availability, reliability, and logistical support requirements.The propulsion system developed for this mission employs a spark ignited internal combustion engine operated in a closed cycle configuration. In this system oxygen and cooled exhaust gas are supplied to the engine intake to maintain a manifold pressure of one atmosphere regardless of the existing ambient conditions. A microprocessor based system is used to control loop pressure, oxygen admission, fuel injection and spark timing.Two years of research and development effort have produced a full scale demonstration system. This hardware has completed full power, full mission durations runs while operating independent of the local atmosphere. Demonstrated output and fuel and oxidizer specifics are consistent with the performance requirements of the Perseus A aircraft.