Staging energy sources to extend flight time of a multirotor UAV

Energy sources such as batteries do not decrease in mass after consumption, unlike combustion-based fuels. We present the concept of staging energy sources, i.e. consuming energy in stages and ejecting used stages, to progressively reduce the mass of aerial vehicles in-flight which reduces power consumption, and consequently increases flight time. A flight time vs. energy storage mass analysis is presented to show the endurance benefit for multirotors on staging. We consider two specific problems in discrete staging -- optimal order of staging given a certain number of energy sources, and optimal partitioning of a given energy storage mass budget into a given number of stages. We then derive results for two continuously staged cases -- an internal combustion engine driving propellers, and a rocket engine. A fundamental flight time limit is seen for the internal combustion engine case, but not for the rocket engine, suggesting that rocket engines could be a better choice in certain scenarios. Lastly, we conduct flight experiments on a custom two-stage battery-powered quadcopter. This quadcopter can eject a battery stage after consumption in-flight using a custom-designed mechanism, and continue hovering using the next stage. The experimental flight times are compared with those predicted from the analysis for our vehicle. The values match well which validates the presented staging analysis.