Journal of the Society of Motion Picture and Television Engineers (1950-1954)

In the interest of brevity this paper is restricted to those phases of the explosion which involve the detonation of an uncased explosive charge and the shock which is observed in the immediate vicinity of that charge. The equipment and techniques discussed are those with which the author is familiar and those which, in his opinion, are applicable to the studies covered in this paper. Because of the requirements of military security there are many omissions of material and credits to other investigators engaged in similar work. Explosives and Explosive Reactions An explosive substance is one which, when subjected to the proper initiating conditions, undergoes a rapid chemical transformation to form more stable substances which are mostly gaseous and which have a combined volume (under normal atmospheric conditions) tremendously larger than that of the original substance. The chemical reaction is usually accompanied with temperatures in the range of 3500 K and pressures of several million pounds per square inch. The progress of the icaction or detonation front through the explosive is regular and well ordered. The detonation front moves with a speed of about 25,000 fps (feet per second), with the exact speed depending on the composition and density of the explosive. Large volumes of gas are liberated by the detonating charge in such a short interval of time that a rapidly moving surface of discontinuity, across which there is an abrupt change of pressure and temperature, is formed in the air which surrounds an uncased charge. The velocity of this "shock front" is initially even higher than the detonation rate in the explosive (about 30 times the speed of sound in air), but it decays initially very rapidly. As the shock velocity approaches the velocity of sound the decay rate becomes very gradual. It is this shock which inflicts the blast damage common to explosions. A special application of explosives based on the "Munroe Effect" (shaped charge) which involves the "focusing" of the explosive forces, and which is especially effective in the penetration of tank armor, has been described recently in many unclassified papers.2 Since several photographic studies discussed in this paper involve the shaped charge, it should be noted that when a cylindrical stick of explosive with a conical metal liner in one end is initiated at the opposite end, the detonation wave which passes over the metal cone causes it to collapse rapidly, and a "jet" of small metallic particles streams out at very high velocity. This jet can penetrate to amazing depths in common metals and, employing this effect, relatively small explosive charges have been used to produce holes through the walls of tanks. Photographic Instrumentation Comparison of Basic Types of Instruments for Photographing Explosions. Streak recording is the most common form of photography employed in the study of explosive reactions. This type of recording produces a plot of distance vs. time on a single strip of film by the relative motion of a narrow transverse slit along the film. Rotating mirrors and rotating drums have been used to produce the motion of the slit with respect to the film, and image speeds as high as 10,000 fps, which produce exposure times as short as 10~8 sec are readily obtainable with relatively simple equipment which can be fabricated at a reasonable cost in the average research laboratory. This type of recording has a serious shortcoming in that only the action which takes place in view of the narrow slit is recorded. The streak camera is blind to the events which occur out of the plane of vision of the slit. Single exposures of explosive events have been obtained with ordinary pressor professional-type sheet-film cameras Morton Sultanoff: Study of Explosives 147