COMMENTS ON THE RELATION OF THE PRECEDING REPORT TO RECENT OBSERVATIONS AT FORT BRAGG.
Recent experiments on the effect of ricochet and time fire of 105mm H.E. Shells against personnel, conducted by the Field Artillery Board at Fort Bragg, indicate an optimum altitude of burst as 45 feet instead of the 75 feet suggested by the computations of Dr. Chandrasekhar. This brings up the question of the reason for this experimental discrepancy.
The main reason appears to be that the computations of Dr. Chandrasekhar were presumably based on a tacit assumption of more deeply entrenched personnel, protected against fragments coming in at oblique angles, whereas the dummies used in the experiments at Fort Bragg came to within a few inches of the surface. This assumption would affect the computed optimum altitude of burst in two ways. First, obviously a very low burst could only injure personnel in a very small area. Thus whereas in the experiments at Fort Bragg, a burst at an altitude of 15-30 feet is found effective over a rather wide area, deeply entrenched horizontal personnel would be exposed to such a burst over a region perhaps 8 feet in diameter. And also, oblique hits have longer trajectories, and hence less striking velocity. This would explain the apparent decrease in striking velocity of small fragments from altitudes above 40 feet observed at Fort Bragg. It is usually supposed that this deceleration becomes important only for ranges of 70-100 feet; these would be consistent for fragments travelling obliquely at angles of 30°-45° with the horizontal.
Thus Dr. Chandrasekhar's conclusions should apply to entrenched personnel, screened against oblique hits.
Garrett Birkhoff
