L-212 Use of Laboratory Setback Activator Tests to assess Suitability for Gun Launch

January 2017
Dr Ernest L. Baker

There is currently no agreed standard methodology for assessing the suitability of explosives for gun launch or for the determination of acceptance criteria for explosive fill defects.  Laboratory setback activator testing has been used as an assessment tool for investigating the suitability of explosives for gun launch.  Unfortunately, laboratory setback activator testing is not standardized and large variations exist in activator design, function and results between different laboratories.  However, it is the only currently available tool for assessing an explosives safety and suitability to launch-induced setback forces.  Observations indicate that field experience cannot be clearly correlated with the results of ignition sensitiveness results from laboratory setback activator tests. 

Both ignitability and explosiveness should be considered in assessing an explosive's resistance to launch-induced setback forces.  For this reason, explosives are often not rejected on the basis of exhibiting high ignition sensitiveness in the activators unless the reaction violence levels are also high.  The activators may be limited in their ability to appropriately measure explosiveness since they commonly generate significantly different pressurization rates and considerably shorter duration pressure pulses for a given maximum pressure than that produced in the launch environment.  This could inhibit violent reaction for explosives which do not burn rapidly either because insufficient surface area is produced during cavity collapse or because of relatively slow chemical kinetics and the premature relaxation of high pressures conditions. 

For the ignition sensitiveness comparison of brittle and soft explosives, ignition sensitiveness results are biased towards the strong brittle materials, often observed for melt pour explosives, which show a trend of lower ignition sensitiveness compared to softer cast cure or pressed explosives. Clearly, the laboratory setback actuator approach tells only part of the story since the applied stimulus level in the activator is independent of explosive mechanical properties and the testing may not reflect all the ways in which the mechanical properties influence the stimulus amplification mechanism.  This report provides a synopsis of laboratory setback activator testing technology, as well as descriptions of setback ignition phenomena. 


Emmanuel Schultz, Dr Michael Sharp, Martin Pope

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