^54 SHOCK WAVE MEASUREMENTS 



The qualitative correspondence of cylindrical to line charges is an 

 imperfect one, however, as illustrated by pressure-time curves of Fig. 

 7.15.^ These results were obtained at a distance of 3.5 feet from 0.55 

 pound loose tetryl charges with a length to diameter ratio of 1.4. As 

 for line charges, the peak pressure is highest off the side, but is higher 

 off the cap end than off the far end and the order of pressures for points 

 on the axis is thus reversed for the greater symmetry. 



The pressure-time curves off the side of the charge are found to be 

 very similar to those from spherical charges in that the initial peak is 

 followed by a smooth, roughly exponential decay. At points on the 

 axis, however, the curves are quite different. Not only are the peak 

 pressures somewhat lower, but they are followed by second peaks and 

 rather irregular decay, initially more nearly linear than exponential in 

 form. Off the detonated end, the separation of the two peaks is smaller 

 and the difference in shape less marked, but these effects are observable. 

 These multiple peaks of course make meaningless a single description 

 of the curves by peak pressure and time constant, and explicit descrip- 

 tion of such curves is much less simple. 



The simplest quantitative comparisons of curves such as those of 

 Fig. 7.15 are in terms of initial peak pressure (remembering its less clear 

 significance off the ends), impulse, and energy. Analysis of records for 

 various length to diameter ratios show that the impulses change very 

 much less with relative position than the peak pressures, the total dif- 

 ference in impulse being less than five per cent for the case of Fig. 7.15 

 while the difference in pressure is twenty per cent. The rate of decay 

 of pressure is thus affected in the opposite way as peak pressure, so as 

 very nearly to neutralize changes in impulse. This action is of course 

 reasonable from considerations of conservation of momentum, as the 

 vector momentum of the entire system must be zero (neglecting the 

 external force of gravity). The component of momentum parallel to 

 the charge axis, to be obtained by integration over the entire disturb- 

 ance, must also be zero, which implies the approximate equality of 

 measured impulse at opposite points. The energy density varies 

 roughly as the product of peak pressure and impulse and accordingly 

 is greatest for the directions of maximum initial pressin*e, being deter- 

 mined by the integrated square of pressure versus time. 



Rather complex pressure-time curves are observed for small cast 

 charges containing significant amounts of booster material and for 

 large charges with nonccntral boosters. The geometry and manner of 

 detonation is not simple in most service weapons and in most cases they 

 correspond only roughly to spherical or cylindrical symmetry. Meas- 

 urements on large cylindrical charges detonated near one end do, how- 

 ^From measurements by W. G. Schneider, reported in UE 15 (114). 



