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For global environmental research it is essential to provide scientists with a tool to perform in-situ studies of our stratosphere; such a tool is a high-altitude subsonic aircraft capable of climbing into the layers of ozone at 25 kilometres and higher. This paper describes the design and development of ram air heat exchangers for the Perseus high-altitude unmanned scientific research aircraft, designed to carry a 50 kg payload to 25 km. The powerplant of the Perseus aircraft consists of a closed-cycle internal combustion engine, where the exhaust gas is cooled and fed back into the engine as the diluent. Thus, manifold pressure can be kept at a desired level, independent of atmospheric conditions. The aircraft carries its fuel and liquid oxygen as reactants. Cooling systems are required for the thermal balance of this powerplant. The recirculated exhaust gas requires a cooler for temperatures up to 1100 Kelvin.

An exhaust gas recirculating power plant has been developed using an existing lightweight spark ignition four-stroke engine. Application is a remotely piloted unmanned aircraft for global environmental research at very high altitudes. The engine uses gasoline and oxygen as reactants. Liquid oxygen is carried in a cryogenic tank on board of the aircraft. Mass balance is maintained through a waste gate in the loop. A specially tailored electronic engine management system had to be developed to control fuel injection, oxygen metering and ignition timing. Additional control parameters are the waste gate to control the loop pressure and the aerodynamic cooling flaps on the cooling ram air side of the exhaust gas heat exchanger to control the air charge temperature. The advantages of this control scheme are the manifold pressure and the ratio of reactant to diluent can be held at any desired level, independent of atmospheric conditions.