2I^J^ SHOCK WAVE MEASUREMENTS 



nomena is given in section 7.9, but it may be said here that similarity 

 curves of the form discussed in this section and section 7.3 appear to 

 describe the pressures until they have fallen below 100 lb. /in. 2, corre- 

 sponding to distances of the order of 900 charge radii. 



7.5. Comparison of Shock Waves for Different Explosives 



A. Sumlarity curves. Similarity curves giving peak pressure, im- 

 pulse, and energy furnish the most comprehensive basis for the com- 

 parison of the shock waves. By their use, estimates can be made of 

 ratios of any of these parameters at any distance. As the approximate 

 formulas are written and curves are plotted, ratios can be most easily 

 compared for equal weights of explosive. If the equations are used, 



Table 7.4. Ratios of shock wave parameters for two explosives relative to an equal 

 weight of TNT at the same distance. 



it is to be remembered that they represent simple approximations which 

 give a reasonably good overall fit for the experimental ranges of W^'^/Rj 

 and hence are subject to some error. 



The peak pressure, impulse, and energy density ratios for equal 

 weights of explosive depend not only on the explosive but also on the 

 distance considered (because of differences in the distance decay laws). 

 A quantitative comparison for W^'^/R = 0.4 (corresponding to a dis- 

 tance of 17 feet from a 300 pound charge) is given in Table 7.4, the 

 values being obtained from the data used in obtaining the similarity 

 curves and formulas. Compared to TNT both Pentolite and tetryl 

 show rather small differences in peak pressure, but appreciable in- 

 creases in impulse and energy. Both these quantities depend on the 

 time integral of pressure, and the increases therefore reflect the in- 

 creased duration of the shock wave for equal weights of these explosives. 

 This effect is shown even more strikingly in more powerful explosives, 

 peak pressure increases of 20 per cent being accompanied by energy in- 

 creases of 50 per cent or more. 



The differences in increase of different parameters illustrate the 

 meaningless nature of a blanket statement without qualification that 

 loose tetryl, for example, is 10 per cent better than TNT as an under- 

 water explosive. If by better one means the impulse for the same value 

 of W^'^/R, the statement may be true, but the 10 per cent figure is 



