Hydrocarbon Fuels as Multifunctional Structure-Power for Unmanned Air Vehicles

A new multifunctional system concept has been developed at the Naval Research Laboratory (NRL). It combines structure and power functionalities for performance enhancement in unmanned air vehicles (UAVs). A large proportion of vehicle mass is devoted to the structural and fuel subsystems in all current classes of fixed-wing aircraft, hence combining fuel and structure in a single multifunctional system provides an opportunity for more efficient material utilization leading to extended flight endurance and/or increased payload capacity. We describe the development and testing of a prototype structure-power system, multifunctional design, fuel selection, critical system efficiencies, and technical challenges for UAV implementation. The NRL prototype structure-power system uses a two-phase (liquid/gas) hydrocarbon fuel to rigidize and strengthen a light- weight structural shell. The fuel vapor-pressure acts to keep the high-stiffness composite shell layers properly positioned to achieve maximal component bending-stiffness and to prevent component failure (i.e., loss of load-carrying capability). When the current prototype is filled with n-butane (117-kPa vapor pressure at room temperature), we observe a 2.5-fold increase in bending stiffness and a 2.4-fold increase in buckling strength. Hydrocarbon fuels, in general, have high energy-content (ca. 13 kWh/kg) and can be used directly to power internal combustion engines or can be transformed via combustion- thermoelectric-conversion. This latter option was explored using commercial bismuth- telluride-based thermoelectric modules. Whereas low thermoelectric conversion efficiency (<5%) limits the specific-power of structure-power systems utilizing this type of energy conversion process, the specific-energy and specific-power levels can be made to approach those of state-of-the-art Li-ion cells by increasing the fraction of fuel-storage volume.