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. Also needed are heat exchangers for the glycol-water mix used for the cylinder head cooling and for the lubrication oil of the engine. Computational models have been developed and used to optimise the heat exchangers and to predict heat transfer, aerodynamic performance and weights. General results are shown for the design of optimum aircraft ram air coolers as a function of atmospheric conditions and flight Mach number. Examples of optimum duct designs are discussed and presented.