Shock Initiation Experiments on Low Density PBX 9501

New gas-gun experiments on PBX 9501 (95% weight HMX, 5% binder) were performed to study its shock initiation response at lower-than-normal densities, these being ~1.68 and ~1.75 g/cm³ corresponding to approximately 90 and 94% of theoretical maximum density (TMD) respectively. Four sustained-shock experiments were conducted at each density, with the growth of reaction to detonation monitored using embedded particle velocity and shock tracker gauges. These tests were supplemented by low-pressure Hugoniot and sound speed measurements to provide additional non-reactive data on low-density PBX 9501, thus providing a complete data set to help validate reactive burn models. Compared with near full-density PBX 9501 material (~1.830 g/cm³ or ~98.5% TMD), the results show an increase in shock sensitivity with increasing porosity as expected. The run distance to detonation data are compared to previously published low-density explosive data, and the differences discussed. The challenges for reactive burn models to accurately predict changes in shock sensitivity with starting density are also considered.