## 4 Experimental Validation of the Geometrical Efficiency Calculation
### Polymer Fibre-Based Net Armour System  
The efficiency of the net is again given by the probability that the piezoelectric fuse of the warhead does not hit on the part of the net that will have sufficient resistance to trigger the detonation chain. Although this is similar to what was seen with other metallic cage armour systems, a straightforward determination of the efficiency of the net as a function of impact angle, such as for other cage armour systems, is not as evident. This is due to the possible slipping of the net over the fuse, even for a direct impact of the fuse on the part of the net. Nevertheless, it is clear that in the cases where the net does not function correctly (initiation of the shaped charge), this is again due to a too violent interaction of the fuse and a part of the net upon initial impact. This means that the net can still be modelled using a geometrical approach, such as in the case of other cage armour systems, but that the initial configuration and areal density of 'hard points' (which in other systems is any structural part of the system) is unknown. Assuming that the configuration and areal density of 'hard points', which is any point of the net giving sufficient resistance to trigger the fuse of the warhead, are constant (e.g., from one net to another, independent from the exact mounting conditions) and that these can be considered as being'spherical' in nature (at least as a first approximation), then the efficiency \(P_{\mathit{theo},\alpha}\) of the net as a function of impact angle \(\alpha\) can be described by:  
\[P_{\mathit{theo},\alpha}=P_{\mathit{theo},0^{\circ}}\cdot cos\alpha \tag{1}\] \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Case**} & \multirow{2}{*}{**Number of Shots**} & **Short-Circuited** & **Experimental** & **Theoretical** \\  & & **Fuzes** & **Probability (\%)** & **Probability (\%)** \\ \hline C & 51 & 25 & 49 & 55 \\ D & 52 & 27 & 52 & 59 \\ E & 4 & 2 & (50) & (46) \\ F & 3 & 2 & (67) & (57) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Experimental results for the four considered metallic net armour configurations and a comparison with their theoretical efficiencies. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Case**} & \multicolumn{3}{c}{**Net Armour Properties**} & \multicolumn{3}{c}{**Projectile Properties**} \\ \cline{2-7}  & \(D\) **(mm)** & \(D_{\mathit{min}}\) **(mm)** & \(D_{\mathit{max}}\) **(mm)** & **Type** & \(C_{\mathit{eff}}\) **(mm)** & \(P\) **(mm)** & **Impact Angle (\({}^{\circ}\))** \\ \hline C & 4 & 110 & 192 & PG-7 & 75 & 21.5 & 0 \\ D & 4 & 110 & 192 & PG-7L & 84 & 21.5 & 0 \\ E & 4 & 110 & 192 & PG-7 & 75 & 21.5 & 45 \\ F & 4 & 110 & 192 & PG-7L & 84 & 21.5 & 45 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Relevant metallic net armour and projectile properties. In order to verify this, an experimental campaign was performed were, in total, 58 PG-7M projectiles were fired against a polyethylene fibre-based net system. Thirty shots were done perpendicular to the net; the remaining 28 were done with a 45\({}^{\circ}\) impact angle. Although the 45\({}^{\circ}\) impact angle was in the vertical plane, a 45\({}^{\circ}\) impact angle in the horizontal plane would have given similar results due to the square meshing of the considered net system. The results for the two configurations are given in Table 5 and clearly indicate the validity of Equation (1). Just as for a metal-based net armour system, the efficiency of a polymer fibre-based net system will vary both with azimuth and elevation of the incoming projectile. It is important to note that the aforementioned analysis made some important assumptions. One of the factors that was not taken into account during this analysis but that may have a major influence on the outcome of the tests is the sensitivity of the fuse of the warhead and the variability of this sensitivity from one projectile to another or from one manufacturer to another.