Effect of the synergistic mechanism between projectile inertial confinement and radial expansion confinement on the internal ballistic energy conversion efficiency of supercritical CO₂

Although research on SCO₂ launching technology has spanned several decades, a systematic understanding of the mechanisms governing interior ballistic energy conversion efficiency remains lacking. To address this gap, this study developed a ballistic numerical model that incorporates transient energy triggering and non-equilibrium phase change, introduced the dimensionless mass ratio (MR), revealed the influence mechanism of projectile inertial confinement on energy conversion efficiency, identified the optimal MR range, and analyzed the in-bore flow field evolution and phase transition path. The control variable method was further employed. By varying the barrel caliber, the variation patterns of interior ballistic energy conversion efficiency under the synergistic effect of projectile inertial confinement and radial expansion confinement were systematically investigated, and the influence mechanism of radial expansion confinement on energy conversion efficiency was thoroughly examined. The results show that projectile inertial confinement modulates the evolution of thermophysical properties by controlling post-projectile space expansion, shock superposition, and secondary compression from expansion and reflected waves, thereby affecting energy conversion efficiency. Radial expansion confinement improves energy conversion efficiency by coordinating post-projectile space expansion, phase transition sequence, and pressure wave propagation. Under the synergistic effect of the two confinements, the optimal radial expansion confinement increases with increasing inertial confinement, revealing their synergistic matching law. This study provides important theoretical foundations for in-depth investigation of energy conversion mechanisms and engineering design of SCO₂ launching systems. • MR is introduced to systematically reveal the three-stage thermodynamic conversion mechanism of SCO₂ interior ballistics under projectile inertial confinement. • The piecewise regulation law of inertial confinement on energy conversion efficiency is clarified: low efficiency occurs at MR < 5; the range of MR = 5–15 features a high peak pressure, a narrow conversion window, and a high energy density, making it suitable for high-efficiency energy transfer over a short travel distance; MR = 15–100 delivers uniform energy release and a stable overload, applicable to long-distance propulsion with low overload; and an inertial saturation state emerges when MR > 100. • The synergistic matching relationship between inertial confinement and radial expansion confinement has been uncovered, whereby the optimal barrel diameter increases with increasing MR. • A nonlinear effect of radial expansion confinement on energy conversion efficiency is identified. For a fixed inertial confinement condition, an optimal barrel diameter exists to achieve optimal temporal coupling among shock compression, secondary compression, and expansion attenuation, which substantially suppresses energy dissipation.