  We design and test an octopus-inspired flexible hull robot that demonstrates
outstanding fast-starting performance. The robot is hyper-inflated with water,
and then rapidly deflates to expel the fluid so as to power the escape
maneuver. Using this robot we verify for the first time in laboratory testing
that rapid size-change can substantially reduce separation in bluff bodies
traveling several body lengths, and recover fluid energy which can be employed
to improve the propulsive performance. The robot is found to experience speeds
over ten body lengths per second, exceeding that of a similarly propelled
optimally streamlined rigid rocket. The peak net thrust force on the robot is
more than 2.6 times that on an optimal rigid body performing the same maneuver,
experimentally demonstrating large energy recovery and enabling acceleration
greater than 14 body lengths per second squared. Finally, over 53% of the
available energy is converted into payload kinetic energy, a performance that
exceeds the estimated energy conversion efficiency of fast-starting fish. The
Reynolds number based on final speed and robot length is $Re \approx 700,000$.
We use the experimental data to establish a fundamental deflation scaling
parameter $\sigma^*$ which characterizes the mechanisms of flow control via
shape change. Based on this scaling parameter, we find that the fast-starting
performance improves with increasing size.
