Flight Endurance Enhancing Thermal Management Part II: Thermal Endurance Gauge and Mission Planning

Modern aircraft designs require thermal management using fuel flow to cool increasingly demanding on-board systems. The topology and control of such thermal management systems has been shown to substantially impact the potential flight endurance. Excess fuel flow is employed for temperature regulation. The flow is recirculated from the engine through a cooler. If the flow is insufficiently cooled and directed to a common reservoir, the reservoir temperature may rise excessively. The excessive rise results in the fuel pump saturating at its maximum output and onboard systems being insufficiently cooled. The result is a reduction in flight endurance. Leveraging previous works, a dual tank topology with a reservoir tank, containing the majority of the fuel reserves, and a smaller recirculation tank is used in conjunction with an online fuel flow control scheme to manage the on-board thermal loads. Previous work focused on studying operation with flight mission parameters that allowed for thermal constraints not to limit the flight endurance using proper temperature and flow control. Implicitly, there was no prediction of the flight endurance when the thermal constraints were limiting. The current design extends the control scheme to explicitly address operation when flight endurance is limited by thermal constraints. During overly taxing mission phases, thermal management is achieved by saturating the fuel flow rate and controlling the devision of the recirculation fuel flow between the two tanks. The complementary online control design is provided in a companion paper. Herein, the modeling, design, and analysis yield the ability to characterize and predict the flight endurance subject to thermal limitations using closed form expressions. The closed form expressions allow for efficient online computation of the flight endurance, or offline evaluation of a broad range of vehicle design and mission planning parameters. The main feature is the efficient computation of the Thermal Endurance Gauge ratio, namely the ratio between the flight endurance with and without thermal constraints. The capabilities of these expressions are showcased in a suite of four prototype software tools, simulating online flight endurance evaluation, and offline mission and vehicle evaluation. The utility of these tools is demonstrated in connection with the case study in the companion paper.