The three explosive materials were modelled

The three explosive materials were modelled using JWL (Jones, Wilkins and Lee) equations of state; the parameters applied are listed in Table 3. Fig. 3 shows the GRIM modelling predictions of the two designs in both modes; the external groove or “Helical” design and the plastic insert and groove combination or “Hybrid” design. The predictions indicated the potential for significant differences in the size of the fragments between the two modes. Fig. 4 shows predictions from both Split-X and GRIM of the profile of fragment velocity along the length of the case. They both show a significant difference in fragment velocity for the low mode.
Gap testing Based on the gap tests (Table 4) a barrier thickness was selected at a nominal 22 mm; this thickness proved to be greater than that used in the blast only charges, due to a variation in the composition of the outer explosive layer and the steel case. The gap tests provided data to obtain an indicative shock level required to detonate the PBXN109 mimic (QRX-293-M6). By modelling the gap test, the shock level in the explosive was observed. This level in turn was then used to assess the charge designs using 2D models. These models showed a potential issue since the peak shock level is enhanced when it hit the steel case and then was further enhanced when it combined at the end of the charge with shock from the central explosive, Fig. 5. This assessment was used to modify the design of the charge by increasing the thickness of the inert layer at the Phenyl sulfate of the charge.
Charge manufacture The two charge designs chosen are detailed below and shown in Figs. 6 and 7: Fig. 8 shows the high mode version of both designs at the top with a full diameter disc of sheet explosive and the low mode of both designs at the bottom with a small disc of sheet explosive designed to only detonate the central core of explosive. Examples of the assembled charges are shown in Fig. 9; the right hand charge has the case painted black with a white grid applied to enable the case expansion to be calculated.
Trial setup Fig. 10 shows a view of the trial setup showing the blast gauges, velocity foils, steel witness plate, and fragment packs.
Helical cased charges The early case expansion of the helical cased charges during/following detonation is shown in Fig. 11. This figure shows a Helical cased charge operating in the high mode on the left and operating in low mode on the right; with the time post the firing trigger shown in each frame. The shape of the expansion of the case looked different when the two modes were compared, with the low mode showing a more barrelled shape. The helical cased charge formed strips in the high mode and in the first low mode firings. The case cracks appeared to form independently from the helical grooves (Fig. 12). For the second low mode helical cased firing the charge confinement was modified at the base to investigate/change a postulated reactive behaviour of the outer layer. This modification appeared to result in little or no significant difference to the fragment velocities or peak blast pressures. It did, however, show significant differences to the case fragmentation. Although the case still split into strips, some of the strips were partially defined by the grooves (Fig. 13). This was most likely due to the difference in confinement at the base of the charge.
Hybrid cased charge The early case expansion of the Hybrid cased charges during/following detonation is shown in Fig. 14. This figure shows a charge operating in the high mode on the left and in the low mode on the right, with the time post the firing trigger shown in each frame. The high mode charge showed a conical shaped expansion, whereas the low mode showed a more barrelled shape. The difference in early case expansion was an indication of the difference in explosive energy release rate. As was expected the low mode was shown to be more akin to a pressure burst, whereas the high mode showed a more typical conical shape with radial case displacement linked to detonation time. Given the low brisance of the outer layer and warhead geometry, snapshots of the early case shape were not expected to equate to a significantly different fragment scatter between the modes.