623 



Tnis relationshio t/etwe?n thp error in aprtn calculateo D> tne original dome analysis 

 mslhoo and the values of n ana V /v has been reojcea to a fonnula for a correction factor, /li. 



and ;i = 0.3 + 0.76 V/v + 0.012 n - 0.OCO12 n^ 



Th£ value of ^ has oeon calculated for ^{ict^ of tne results shL-wn in Figure 3a ind ?. revised 

 figure for the calculated depth. 



obtained in each instance. The error in dc r.a? then befm clotted in Figure 3b and it will be seen 

 at once that when ::llowance for the dane and olume factors is made in this way, very good agreement 

 between the calculatsJ an^ the true deoths is :Dtaine-, The majority of the results are within 

 ± 5» of the true ?r-oth and no result Is more than lOi in err,r. When it is realise: that all the 

 results at known :eoths available to date have been analysed and are shown in this figure and that 

 the results include charges both on and off the bottom it will be seen that the method cranises to 

 orovide a reliable me;:ns of deteriining the decth of charjes exsloding under water. How far the 

 method can be relied ucon cutsid'j tne range covt:rec by the results examined will be uncertain until 

 further tests are done. The ran.j? already covered (from little mere than 10 feet to over 60 feet 

 for a 300 lb. charge) is wide cnoujh for most aractical ourcoses. 



Some difficulty may 'irise in oractice with bombs fitted with a oistcl or fuze which detonates 

 at shallow oeoths. with these the cavity which the bent forms at entry may not be closed when the 

 bomb exolodts, a condition not reoresented in static tests. If the plume factor in these conditions 

 is based en the plume which apoears orematurely thr:ugh the still ooen cavity the factor will oe too 

 high. If it is baseo ,n the olume which aooears liter through the unbroken water it may be too 

 low because the force of the olume will have been soent in the ooen cavity. A method of correcting 

 the calculated deoth in these conditions could be based on tests with air launched charges drooped 

 in Shallow water of Known depth. The bombs would only require to be fitted with impact fuzes so as 

 to detonate on the bottom. "ending the results of such tests it will probable be best to neglect 

 the present correction in acolyino the dome analysis method to shallow firing bonos when the records 

 shew that the cavity is still ocen'. The depths associated with these conditions will be of the 

 order ;f 10 - 15 feet and since the uncorrected o^me analysis method may be expected te give such 

 o-'cths in jener^l within two feet or less, it will probably be sufficiently accurate. 



The relati pnship bctuecn dpyie and tlumc . 



Although the foregoing analysis has provided a practical method cf determining the depth of 

 detonation it does not explain the relationships which have been found. 



It has generally been assumed that the spray forming the doire is produced by the action cf 

 the detonation pressure pulse. Records obtained with plezo electric pressure gauges have indicated 

 that the detonation cressure (.ulse is very uniform in tne horizontal plane at the level of the 

 charge. That this d-es not appear t: conform with the large variations which cccur in the soray 

 velocity would be cxclained if similar variations in the oressure pulse were found to occur in the 

 region above the charge. This region does not appear to have been explored so far with pressure 

 gauges.. 



The value of v d for a 300 lb. charge which would be calculated from the peak oressure in 

 the detonation pulse based on the results of tests in the plane of the charge is 3,000 square feet 

 per second. In general therefore (see Table 1) the soray velocity is higher than would be expected 

 from the effect of the detonation pulse fcund in the horizontal plane although it is less in some 

 instances. 



The oresent analysis shows that the dome velocity is related to the olume velocity. As 

 might have been expected there is also evidence that the dome shaoe, i.e. the initial distribution 

 of velocity across the dome, is related tc the initial olume shaoe. The growth of the olume for 

 each of the oresent results is shown in Figure u. The results have been arranged in order frcm 

 thr; highest positive to the lowest negative value of the dome factor, n, irrespective of the other 

 conditions of the test. It will be seen at onct that there is a marked tendency for positive 



'values 



• 5ee also Note 1 in Appendix. 



