1010 
20 
Investigation of the effect of shape of charge and point of detonation on shock wave 
and_bubble 
Photographs (reporduced in Figures 61 to 103) of the shock wave and bubble 
surrounding spherical, cylindrical and conical charges weighing approximately 1/2 1b. 
have been taken at intervals ranging from 10 to 200 microseconds after the initiation 
of detonation at various points in the charges. It has been found that the shock wave 
tends rapidly to become spherical after it leaves the charge. A graph (Figure 105) 
giving bubble size as a function of shock wave radius for spherical charges of this 
type is also included. 
The charges were all of cast pentolite and were bare except for a waterproof 
coating. The cylinders were cast in the following length-to-diameter ratios (keeping 
the weight constant): 1:1, 2:1, 4:1, 8:1. The cones were of the same weight as the 
cylinders and were equilateral. Timing of the detonation of the main charge and the 
flash charge was accomplished by the simultaneous firing of two No. 8 seismogr&aphic 
(SSS) DuPont caps connected in series, the delay interval being determined by a suitable 
length of primacord extending from one cap to the flash charge; the other cap was in- 
serted in a well provided in the main charge. One cylinder of each shape was detonated 
at the center and another at one end. Of the cones, one was detonated at the apex, 
another at the center of the bese and a third at the "center" defined as the point 
equidistant from the base and the conical wall. As control tests, photographs at the 
same time intervals were taken of cast pentolite spheres of the same weight and 
detonated from the center. 
The experimental set-up for these shots consisted simply of the main charge, the 
flash charge, the camera, and occasionally a translucent diffusing screen. The main 
charge was suspended in front of the camera by strings cast in the charge; the flash 
charge was mounted on the line from the camera to the main charge, and behind the 
main charge as viewed from the camera, The diffusing screen, when used, was mounted 
between the flash charge and the main charge. For the 10 and 50 microsecond series, 
the distance from the camera to the main charge was about 60 in.; the distance from 
the main charge to the flash charge was about 24 in. For the 100 microsecond series, 
the distances were about 76 in. and 32 in. respectively, and for the 200 microsecond 
series, the distances were about 112 in. and 46 in. respectively. 
In some of the spherical charge control shots, two straight steel rods 3/8" in 
diameter and about 6 ft. long were mounted perpendicular to the camera-charge axis 
and in the plane of the center of the main charge; the ends of these rods pointed 
toward the charge and were placed so as to be just outside the bubble at the instant 
of the photograph (Figures 62 - 64). Knowing the distance between the two ends, it 
was possible to obtain an estimate of the optical distortion of the bubble. The rods 
were sufficiently long so that the shock wave in the rod had not reached the far end 
of the rod at the instant of the photograph; hence, motion of the near end of the rod 
could not occur up to the instant of the photograph except by elastic compression of 
the rod. This amount is well under 0.1 in. in all cases. The ends of the rods were 
calculated from the photograph to be about 5% (2 to 8%) farther apart than they 
actually were which indicates that the diameter of the bubble must have been magnified 
by the shock wave by this amount. 
The photographs are reproduced in Figures 61 to 100. The 50, 100 and 200 micro- 
second series are complete; the 10 microsecond series is not complete, but the pictures 
which are available for this time interval are presented and also some other photographs 
which are of interest although they do not fall into any of the four time groups. The 
data for each figure appear on the page opposite the figure, and include lengths of the 
vertical and horizontal axes of the shock wave and bubble as determined from measure- 
ments on the original negatives and the lens equations. From previous optical tests 
(See III, 5, d), it is believed that the shock wave axes thus determined are accurate 
to within one inch. 
Each photograph is a double exposure due to the relatively weak flash of light 
given off by the main charge before detonation of the flash charge, the camera shutter 
being open during the whole interval. This causes an image of the original charge to 
appear on the filn. 
35 15518 
