446 



28 



Camero 



Water 

 Surfoce 



Axis of 

 Symmetry 



Light Source 



Holding Ring 

 Mounting Ring 

 r-l" 



:v^ "I ■' l',i'^ w.ww.w. ■ .\ . .. . . .-. - 



7fg^ p^4^^^— t — ^ 



Diaptiragm 



Figure 21 



[] Chorge 



Schematic Diagram of Underwater Explosion Apparatus 

 Arranged for Streak Photography 



Observations of the motion of the diaphragms and the boat were made 

 by streak photography. As explained in Reference (10) such photographs show 

 the motion of spots of aluminum paint situated at various points on the dia- 

 phragm and boat. One spot called the center spot was at the center of each 

 diaphragm; two spots called the mid-spots were placed midway between the cen- 

 ter and edge on a diameter; and two called the edge spots were painted on the 

 holding ring at the extremities of the diameter on which the other spots were 

 located. The film was run at various speeds up to a maximum of about 1200 

 inches per second. If it was desired to obtain good resolution of the ini- 

 tial motion, which lasted about 2 milliseconds, it was obtained at high film 

 speeds, giving a record as shown in Figure 22a; if an overall picture of the 

 motion was desired it was obtained at low film speeds, giving a record like 

 that shown in Figure 22b. An intermediate speed yielded a streak photograph 

 such as that in Figure 22c. 



MOTION OF DIAPHRAGM AND HOLDING RING 



When the shock wave from a charge reaches a diaphragm it imparts to 

 the diaphragm and the holding ring an Initial velocity normal to the original 

 plane of the diaphragm. As far as can be determined from the streak pictures 

 with the present limit of resolution the action appears to be impulsive. When 

 the charge is sufficiently far from the diaphragm, that is, when its distance 

 exceeds 8 to 1 inches or one diameter, the shock wave reaches all parts of 

 the diaphragm at about the same instant, so that the initial velocity of the 

 diaphragm material is practically uniform. Except for the material near the 

 edge, which is Jerked to rest almost Immediately, the diaphragm seems to be- 

 have initially quite like a free plate. 



