A Squid-like Bio-Inspired Repeatable Explosive Water-Jet Thruster for Aquatic-Aerial Vehicles

Flying squids reconcile the competing density requirements of water-to-air transition: water retained in the mantle cavity helps keep their bulk density close to that of the surrounding water during submerged motion, whereas rapid mantle contraction expels this water as a high-momentum jet and lowers body mass and bulk density for the aerial phase. This strategy uses surrounding water as reaction mass and avoids large external propulsive appendages. Here, we implement this coupled impulse-generation and transient-unloading mechanism in a repeatable cavity-piston water-jet thruster driven by metered butane–oxygen deflagration. The design maps the mantle cavity, mantle contraction and funnel to a refillable water chamber, a rigid free piston and a converging tail nozzle, while closed-loop reactant metering and vacuum-assisted reset enable successive cycles. A corrected gas-supply model, a reduced-order structural model, an equivalence-ratio thrust-pulse model, a two-stage deflagration and fluid-structure interaction (FSI) framework, and prototype experiments are combined to guide the design and assess its operation. The analytical models relate commanded reactant volumes and nozzle geometry to gas-delivery requirements and thrust-pulse trends. At a common initial gas pressure obtained by scaling the deflagration-model output with an engineering pressure factor, the FSI simulations quantify how nozzle geometry affects estimated peak thrust, useful pulse duration and water-release rate. Prototype experiments establish gas-metering accuracy and the dependence of the measured thrust pulse on equivalence ratio, with the pulse histories represented by an empirical model. The prototype produces a peak thrust of approximately 80 N and completes refill and reset in approximately 20 s. Together, these results identify nozzle geometry and reactant metering as design and control variables for squid-inspired propulsion that combines transient thrust, water unloading and repeatable operation for aquatic-aerial applications.