We present the concept of staging energy sources to
increase the endurance of unmanned aerial vehicles. A single-stage multirotor usually takes-off, hovers
and then lands once its energy source is depleted. On the other hand, a multi-stage multirotor
consumes its energy sources in stages, and ejects the depleted energy sources. The energy contained in an energy source
is typically proportional to its mass. The power consumption of propeller driven multirotors
is proportional to the mass of the vehicle raised-to 3/2. The flight time for a single-stage multirotor is
the total energy divided by power consumption, which remains approximately constant. The flight time formula can be
extended to multi-stage multirotors by calculating the flight times of
individual stages, and then adding them up. For the same multirotor,
carrying the same total energy, a multi-stage multirotor will ideally always
fly longer than a single-stage multirotor. The derived flight time equations are applied to our
quadcopter to get the flight time plot as shown. With a total battery mass of 380 grams,
the analysis predicts flight times of about 20 and 24 minutes respectively
for the single-stage and two-stage case. To validate the analysis,
we conduct flight experiments, where the quadcopter starts
hovering using the first battery. In one of the experiments, we eject the first stage
once it is depleted by turning a detaching motor. The quadcopter then continues
hovering using the second battery. The experimental flight times match
well with the analysis predictions. The quadcopter flew for 19.7 minutes with a
single-stage and 23.6 minutes with two-stages, showing a 20% increase in flight time
using just one additional stage.

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