Low-density gas cavity-mediated underwater shaped charge jet penetration enhancement: Fluid shielding and shock-induced target pre-weakening

To address the penetration performance degradation of underwater shaped charge jets due to severe kinetic energy attenuation and interfacial instability in water, this study proposes a coupled structure combining a linear shaped charge (LSC) and flexible detonating cord (FDC) and examines the interactions among the shock wave, gas cavity, and LSC jet. In this structure, the FDC is first detonated to create a gas cavity. At its maximum expansion, the LSC is initiated, allowing the jet to form and stretch within the low-resistance gas cavity. This reduces resistance and limits momentum exchange with the surrounding water. The study describes the “slow–fast–slow” three-stage stretching behavior of the jet as it travels across the water–gas interface, along with the interaction between the shock wave and the gas cavity. A criterion is proposed for selecting the coupling distance. The changes in jet velocity, effective stretch length, and penetration depth at the selected coupling distance are also discussed. Results show that, at a 5-cm coupling distance, jet length increased from 5.12 to 7.49 cm, compared with 4.76 to 6.31 cm for the single LSC; thus, net jet extension increased by 52.9%. Jet-tip velocity increased from 893 to 1160 m/s (+29.9%), while at 183 μs, penetration depth increased from 4.19 to 7.03 mm (+2.84 mm). The proposed criterion can be used to estimate the coupling distance for various water depths and charge parameters, providing a reference for structural design and parameter optimization in underwater shaped charge applications.