284 



MOTION OF THE GAS SPHERE 



stant for the range of depths and charge weights employed, except near 

 a bounding surface, as shown in Fig. 8.6. The vahies 0.7 to 0.8 indi- 

 cate that from 50 to 65 per cent of the energy of the first oscillation is 

 lost in the first contraction, and this order of magnitude is observed 

 generally for high explosives. These data, supplemented by theoretical 

 calculations of the detonation energy Q, also agree fairly closely in pre- 

 dicting that from 40 to 45 per cent of the initial energy of explosion 

 remains in the bubble motion during its first cycle. 



A further interesting application of bubble period measurements is 

 to comparison of explosives. For two charges of equal weight fired at 

 the same depth, Eq. (8.11) predicts that the energies Y remaining after 



0.9 



08 



g 0.7 



UJ 

 0. 



0.6 



10 

 DEPTH (ft.) 



15 



20 



Fig. 8.6 Ratio of first and second bubble periods for 0.66 pound tetryl charges. 



emission of the shock wave are in the ratio of the cubes of the observed 

 periods. This measurement has sometimes been used as an indication 

 of relative effectiveness of explosives. Data of this kind have been 

 obtained for a number of explosives. A comparison with calculations 

 of detonation energies and either calculated or measured values of shock 

 wave energies further permits at least a partial check on the consistency 

 of the various results. For example, if the fractional energy losses in 

 the shock wave, as expressed by n, are the same in two explosives, the 

 ratio of bubble energies Y should be the same as that of detonation 

 energies. The premise to this simple comparison has been found to be 

 approximately true in experimental data for a mixed explosive con- 

 siderably more powerful than TNT. The detonation energy relative 

 to TNT from calculations by S. R. Brinkley, Jr., agrees in this case 

 with the ratio of periods cubed to five per cent. That the premise and 

 comparison leased on it cannot always be justified is illustrated by 

 Pentolite. The bubble energy is (Table 8.1) virtually the same as for 

 TNT, but the comparison of experimental shock wave energies in Table 

 7.4 gives a value 27 per cent greater. Adequate data to test the energy 

 balance for Pentolite do not appear to be available at present. 



Perhaps the greatest uncertainty in energy comparisons is in the 



