Wedesign and test an octopus-inspired flexible hull robot that demonstrates outstanding fast-startingperformance. The robot is hyper-inflated with water, and then rapidly deflates to expel the fluid so asto power the escape maneuver. Using this robot we verify for the first time in laboratory testing thatrapid 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 isfound to experience speeds over ten body lengths per second, exceeding that of a similarly propelledoptimally streamlined rigid rocket. The peak net thrust force on the robot is more than 2.6 times thaton an optimal rigid body performing the same maneuver, experimentally demonstrating large energyrecovery 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 estimatedenergy conversion efficiency of fast-starting fish. The Reynolds number based on final speedand robot length isRe ≈ 700 000.We use the experimental data to establish a fundamental deflationscaling parameter ?^∗ which characterizes the mechanisms of flow control via shape change. Based onthis scaling parameter, we find that the fast-starting performance improves with increasing size.