248 SHOCK WAVE MEASUREMENTS 



The various measures of shock wave comparison are clearly related 

 to one another. For example, comparisons at equal weight and at equal 

 volume differ from one another because of differences in density of the 

 explosives and conversion from one basis to another can be made using 

 known values of density. The conversion also involves a knowledge of 

 the weight dependence of the parameter under consideration, which 

 must therefore be known. In many experiments, equal volumes of 

 explosives have been compared, and ratios for equal weights are derived 

 quantities, but the reverse procedure may equally well be followed. 

 Similar considerations apply to the other relative figures which have 

 been mentioned, and one can clearly devise a large number of such 

 figures all describing the same pair of explosives but from different 

 points of view. Systematic procedures for deriving a desired figure in 

 terms of another known one have been devised, for which reference 

 should be made to original reports. 



A decision as to what figure of explosive comparison is significant 

 depends of course on the reason for making the comparison. If, for 

 example, damage of structures of rapid time response by large explo- 

 sions is of interest, the peak pressure only may be of concern, and at the 

 opposite extreme the impulse may be the dominating factor. It may 

 equally well be that one is interested in the total energy release both 

 during emission of the shock wave and at later times, in which case 

 shock wave energy parameters are significant but only a part of the 

 story. Any discussion of the many possible applications is beyond the 

 scope of the present remarks, which have for their purpose only to list 

 some of the figures of comparison which have been obtained and to indi- 

 cate their relation to one another. 



7.6. Effects of Charge Shape and Orientation 



The discussion of preceding sections has been limited to the simplest 

 realistic case of shock waves possessing spherical, or nearly spherical, 

 symmetry. In the majority of practical cases, this situation should 

 ultimately be approached at sufficiently large distances from any charge 

 of finite dimensions, but it does not by any means follow that initial 

 differences from a spherical w^ave become insignificant within a few 

 charge dimensions. The case is, in fact, quite the opposite, for example, 

 differences in shock wave pressure-time curves around cylindrical depth 

 charges observed at distances of 20 feet (roughly 15 times the average 

 linear dimensions of the charge) are found at distances of 100 to 500 

 feet. These differences can clearly, therefore, be of practical as well as 

 fundamental importance, and representative experimental measure- 

 ments will be considered in this section. 



The idealization of a spherical charge detonated at its center is one 

 that is approached in practice only by special methods of preparation 



