In fuel cell systems, the size of the radiator that rejects stack waste heat is of concern because the coolant temperature is limited to 80 ◦C. In order to address this concern, we have formulated and used simple heat transfer and fluid mechanics [8] models of ram air cooled radiators for a hybrid fuel cell propulsion system. In applying the model we assumed that the low-temperature radiator has the same frontal area (see Table 3 ) as the high-temperature radiator and is located directly in front of it since we prefer that the ambient air cool the low-temperature coolant first. We further assumed that a single-speed radiator fan supplies a pressure head of 380 Pa (2 of water), and the coolant flow in the hightemperature circuit is proportional to the stack power and in the low-temperature circuit is proportional to the heat duty of the condenser and traction inverter motor.