Optimal depth of entrenched infantry positions as protection from FPV strikes: an engineering analysis of survivability under detonation of a 200 g rdx charge

This article develops an engineering-grounded framework for determining the optimal depth of entrenched infantry positions intended to defeat top-attack strikes delivered by firstperson view (FPV) quadcopter drones carrying approximately 200 g of RDX-based high explosive — a charge mass now canonical on the Russo-Ukrainian frontline. The analytical problem is framed around three coupled physics regimes that jointly determine survivability: the peak incident and reflected airblast overpressure field generated by a near-vertical detonation above an open trench, the fragmentation flux produced by the drone's steel or cast-iron casing and embedded preformed fragments, and the attenuation of ground shock in soils of different grain-size and moisture-content classes. Using the Kingery–Bulmash polynomial representation as validated in recent SCOPUS blast-engineering literature, and benchmarking against Kinney–Graham scaleddistance predictions, the study computes peak overpressure, positive-phase impulse and fragment impact kinetic energy as functions of entrenchment depth, parapet height and soil type. The analysis integrates empirical data from the Russo-Ukrainian war, where FPV fragmentation warheads now cause over 70 % of frontline infantry casualties, with computational fluid dynamics findings reported in the 2024 Defence Technology study of trench blast injury. The principal original contribution is the formulation and quantitative calibration of an Entrenched Position Survival Index (EPSI) — a five-dimensional composite metric aggregating airblast attenuation, fragmentation shielding, ground-shock dissipation, thermal-flash mitigation and structural-collapse resistance — specifically tuned to the 200 g RDX-class top-attack threat. Results indicate that legacy field-manual depths of 1.5–1.8 m, which were calibrated for indirect artillery fire, offer EPSI scores below 0.55 against FPV top-attack, whereas a 2.0–2.2 m trench with a 0.3–0.5 m sandbagequivalent overhead cover and a 0.5 m parapet raises the composite to 0.88–0.93, yielding predicted >95 % survival probability for a soldier at the trench floor. Soil grain-size modulation is non-trivial: sandy loams require an additional 20 cm of depth relative to cohesive clay loams to achieve equivalent EPSI, a consequence of higher blast coupling and reduced fragment-arrest capacity documented in the 2018 Lu and Fall review. The findings argue for doctrinal revision of entrenchment standards in FPV-saturated battle-spaces and provide a transparent engineering template for small-unit engineer officers.