Theoretical Calculation of Combustion Performance of Capsule-type Gun Propellant

To address the critical requirement for energy release control under high-pressure conditions in gun bores, and to resolve the conflict between high loading density and combustion progressivity, a novel end-capped tubular combined charge structure containing spherical propellants based on the "capsule" structure concept is proposed. This design utilizes the tubular shell and end-capped discs to form a slow-burning "capsule shell", with high specific surface area discrete spherical propellants filled inside acting as fast-burning "capsule agents". Through physical isolation and geometric burn-through of the outer shell, a high-progressivity combustion mode characterized by "slow burning followed by fast burning" is established. A theoretical combustion model incorporating the form function and gas generation intensity (Γ) is developed to quantitatively analyze the influence of component geometric parameters on combustion performance. Calculation results indicate that the combined charge exhibits typical "step-wise" double-peak characteristics during the combustion process. The burning thickness ratio of the end-capped disc to the tubular shell (r_p-k) is critical for regulating the phase of high-pressure gas intervention into the internal combustion, thereby determining the onset timing of the Γ surge. The burning thickness ratio of the spherical propellant to the tubular shell (r_q-k) directly governs the magnitude of combustion progressivity. When r_q-k<0.5, the spherical propellants with high specific surface area burn out before the tubular shell within the high-pressure field, triggering a drastic surge in Γ; moreover, a smaller r_q-k corresponds to a larger initial burning surface area, leading to a more significant gain in the Γ peak. Conversely, when r_q-k>0.5, the tubular shell burns out before the spherical propellants, resulting in a step-wise drop in intensity at the late stage of combustion (Ψ>0.95). Furthermore, an increase in loading density further amplifies this surface area augmentation effect induced by geometric phase transition. Theoretically, this combined charge with "capsule" characteristics possesses high combustion progressivity, offering a novel approach for the development of combined gun charges.