655 



However, the advantages outweigh, at least for some considerations, 

 the disadvantages. They are 



(1) that the method can be applied to any size of charge with 

 equal precision, 



(2) that the cost of the recording equipment is only a fraction 

 of that needed for piezoelectric measurer onts, especieilly 

 where a drum type of camera is used, 



(3) that the analysis of the photographs is less onerous than 

 in some other methods, piezoelectric data for example, 



(a) that pressures can be determined for very high values of 

 W1/3/R. (This is the only experimental method of deter- 

 mining such high pressures with the exception of difficult 

 and costly optical methods), and 



(5) that the method rests on sound hydrodynamic theory, giving 

 a more neeirly absolute measure than some other methods. 



10. Value of the Method for Multiple Charge Effects 



It is seen by reference to Table IV that the precision of measuring 

 sympathetic detonation effects is satisfactory. While tWs application 

 was attempted only in the tank shots where various delay times could not 

 be tried, it would be quite possible to apply the method to full scale 

 charges with delay times between the firing of the two charges regulated 

 by such means as varying lengths of primacord. It is not in the province 

 of the present report to discuss the interpretation of sympathetic 

 detonation and/or multiple charge effects.^") Nevertheless, it can be 

 shewn by a simple calculation for orientation ^ in Table IV, that if the 

 pressure from D is simply added to that from A at the sxirface, the sum is 

 7900 lb/in. 2 which more than accoxints for the recorded value of 7770 lb/in. 2. 

 It would appear, therefore, that if the multiple charge effect really does 

 give an augmentation as is proposed by some, this can only be proved by a 

 careful study of energy as well as pressure data and cannot be accomplished 

 by the dome velocity method. 



11. The Surface Tension Lowering Problem 



The derivation of Eq. (2) does not include the surface tension of the 

 medium as a variable. It remains to be explained why V^ euid hence the 

 measured value of ?^ is elevated in our surface tension experiments. An 

 illuminating fact is observed if one assumes T - 6600 lb/in. 2 for the tank 

 shots. The sm'face tension results can then be placed on the corrected 

 and uncorrectec' curves (Fig. 15a). It will be noted that when plotted 

 on Fig. 15a all the surface tension points fall between the two curves, 

 indicating that the effect of lowering the surface tension is directed 

 towards lowering the value of T. 



It is difficult to sort out the role of surface tension from the com- 

 plex phenomena associated with dome formation. Nevertheless, since the size 

 of droplets foraed in a gravity field under essentially static conditions 

 Is dependent upon surface tension, it is not too difficult to assume a 

 dependence of the tensile strength T upon surface tension. Unfortunately 

 our experiments could not be performed under sufficiently controlled con- 

 ditions to point out an experiraentsd relation between these quantities 

 although one undoubtedly exists. If the surface tension results have no 

 other value they at least allow more reliance to be placed on the estimated 

 value of T. 



-It- 



