SHOCK WAVE MEASUREMENTS 253 



nearly the same shape, the slope being somewhat larger than the value 

 of one for a spherical acoustic wave. Near the charge, the pressure off 

 the side increases much less rapidly and gives indications of varying 

 more nearly as (ro)~^^^, which would correspond to a cylindrical acoustic 

 wave. This is qualitatively reasonable, for at nearby points on the side 

 the charge appears to be more nearly of infinite extent and the shock 

 front more nearly plane (see also Fig. 7.12). 



The pressure-time curves obtained off the side are very much more 

 like those for spherical charges in shape and duration than are the ones 

 obtained on the extended axis. The impulse at points off the side is 



7,000 LB/IN^ 2 



A**— 6»300 LB/ IN 

 r-«— 5,600 LB/IN^ 



l-i 200;USEC H H 200JUSEC H I" 200JUSEC H 



(a) OFF SIDE (b)OFF FAR END (c)OFF CAP END 



Fig. 7.15 Shock wave pressure-time curves for cylindrical charge (3.5 feet 

 from 0.55 pound loose tetryl, length-diameter ratio 1.4). 



some 30 per cent higher than values off the end. At a distance of 15 

 feet from the charge the initial rate of pressure decay corresponds to a 

 time constant of 0.32 msec, which increases to about 0.70 msec, at 58 

 feet, although the total duration is less than 1.4 msec. A nonacoustic 

 spreading of the wave profile thus occurs even though the total duration, 

 corresponding to the end of detonation, decreases. 



It is also evident from the observed pressure variations plotted in 

 Fig. 7.14 that at no point can the pressure decay with distance be con- 

 sidered acoustic. Any calculations based on this assumption are there- 

 fore very approximate, and it is perhaps surprising that so much of the 

 general behavior can be simply understood on this basis. 



B. Cylindrical charges. The asymmetry of charge shape of which 

 line charges are the extreme case is that of a cylindrical charge detonated 

 at one end. Many studies have been made of such charges for various 

 ratios of diameter to length. The general features of the shock wave 

 pressure-time curves at different points around the charge are broadly 

 similar in kind to those found for line charges, and can be qualitatively 

 understood in the same way. 



