  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 of staging to
multirotors. 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. Notably, we show that
a multicopter powered by internal combustion has an upper limit on achievable
flight time independent of the available fuel mass, but no such limit exists
for rocket propulsion. Lastly, we validate the analysis with 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 match
well with those predicted from the analysis for our vehicle. We achieve a 19%
increase in flight time using the batteries in two stages as compared to a
single stage.
