Dependence of the blast on the shape of a high explosive charge

The high number of high explosive with very different compositions implies the need to compare them. That is the reason of the use of TNT equivalency. Even if this notion seems simple, it may lead to misuses, due to the physical differences between all compositions. The equivalent should be different for each effect, external blast, internal blast, ballistic capability, indentation, etc. For example, HMX is superior to TNT for external blast but inferior in internal blast.

Blast TNT equivalents are generally determined with overpressure gauges around an explosive charge. Even if a spherical shape ignited in its center is the best configuration to perform accurate measurements, the difficulty to machine a sphere lead often to use cylindrical charges. This charge is put on the ground surrounded by gauges at different distances on support at same height, leading to see the charge with different incidence. This setup is generally associated with the conclusion that TNT equivalent is a poor approach.

To assess the influence of the shape of the charge and of the incidence, 2D calculations have been performed with Ouranos hydrocode. Twenty numerical gauges were considered on each of 9 different directions, around cylinders of same mass and ratio length over diameter 1, 4, 10, 1/4 and 1/10 and compared to the pressure obtained for a sphere. The calculations show large differences on the time of arrival, the overpressure and the impulse. The difference for overpressure can be greater than a factor of 4. The influence of the ratio length over diameter, the incidence and the distance will be discussed.