SSI, SECONDARY PRESSURE WAVES 



migrational velocities due to gravity are smaller, assuming that the sur- 

 face pressure above the charge is atmospheric and not scaled to corre- 

 spond to large charge conditions. Small charge data therefore provide 

 the most favorable test of simple theory, and are experimentally much 

 easier to obtain with some precision. Investigations of this kind are 

 therefore considered first, although generally speaking these conditions 

 are of less practical interest, and investigations with larger charges are 

 taken up later. 



(1). Shallow 1 ounce charges. The pressure-time curves for 1 ounce 

 charges of Polar Ammon Gelignite at depths from 1.5 to 6 feet have 

 been measured by the Road Research Laboratory (96). In the first 

 series of measurements reported, gauges were mounted above, and at 

 the same depth as the charge at distances from 1.5 to 2.5 feet. At a 

 depth of 6 feet, the migration during the first pulsation was found to be 

 about 10 inches upward, as compared with the theoretical estimate from 

 Taylor's theory of 11 inches. The observed pressure pulse for this 

 condition of relatively small migration was found to be in good agree- 

 ment with one calculated from the approximate formula P — Po 

 = (po/r) d/dt (a^ da/dt) for the same weight of TNT, as shown in Fig. 

 9.6, which is reproduced from the RRL report. (These two explosives 

 are nearly equivalent, weight for weight, in calorific value.) This pres- 

 sure was measured at the side of the charge; measurements above it 

 indicated higher values, both because of the closer proximity of the 

 bubble as a result of migration and because at very close distances flow 

 pressures must be considered. Interestingly, the peak pressure at a 

 fixed point above the charge was found to be higher in the second pulse 

 than in the first, which further indicates the effect of bubble migration 

 and the need for considering its influence in explosion damage. 



For a charge depth of three feet, no migration effects were observed, 

 indicating the repulsion of the free surface, and the peak pressures in 

 the first pulse were some thirty per cent higher than at six feet, as is to 

 be expected for a motionless sphere at the time of maximum contraction. 

 In a second report, measurements were made at several depths which 

 indicated a maximum peak pressure at about three and one-half feet, 

 being smaller on either side of this value, while the impulse increased 

 somewhat with increasing depth. These results were for a gauge two 

 feet from the charge and at the same depth. Gauges mounted above 

 and below the charge showed somewhat different variations explainable 

 in terms of migration and surface reflections. 



(2). Charges up to 3^ pound in 24 feet of water. An extensive pro- 

 gram of measurements, carried out at Woods Hole in cooperation with 

 the Taylor Model Basin (2), has provided data on secondary bubble 

 pulse pressures from charges of tetryl (25, 50, 120, 300 gm.), cast TNT 

 (250 gm. plus 44 gm. tetryl booster), and other explosives which were 



