GENERAL. It has been found that Luftwaffe employed a photoflash bomb containing powdered aluminum, which was scattered into the surrounding air by the explosion of the inner core of black powder. Due to the heat of the explosion, the aluminum powder bur-ned in the oxygen of the air and produced a rapid flash. The bomb weighed considerably more than the conventional type of photoflash bomb, which employs an intimate mixture of powdered oxidizing agent and magnesium or aluminum, and it produced a slightly lower peak candle power output. The time to reach peak intensity was also lower. However, the German bomb possessed a very great advantage in fact that ot was not sensitive to bullet impact, and was therefore much safer to employ.

DESCRIPTION. The BLC 50 photoflash bomb consists of a comparatively light steel ca-sing approximately 42.9 inches long and 8 inches in diameter. (See fig. 87.) The total weight of the bomb is 42 kg. There is a heavy concrete nose piece which acts as a bal-last to stabilize the flight of the bomb. The bomb itself consists of three approximately concentric tubes. The outer tube contains 15 kg of aluminum "pyroschliff", which is an extremely fine, flaked aluminum powder, having the following characteristics:

Metal content: 87 - 92%.

Another tube placed within the outer tube contains 3.5 kg of black powder, serving as the explosive which scatters and ignites the outer charge of aluminum powder. The black powder, "Marine-Geschütz" pulver, possesses the following characteristics:

Moisture content: Less than 1.3%.

Chemical composition:

Potassium nitrate: 75%

The black powder is exploded by means of an 80-cm length of detonation fuse which is placed inside a third tube passing through the black powder charge. The ends of the de-tonating fuse are dipped in a 4 percent solution of collodion, and allowed to dry.

The bomb is ignited by means of the standard type of electrically ignited delay fuze which is inserted in the fuze well in the side of the bomb.

REMARKS. In order to increase the usable light output of the BLC 50 photoflash bomb, the following researches were carried out recently:

A. The fine, flanker aluminum "pyroschliff" powder was replaced with an atomized alumi-num powder called "griess". The new, atomized powder possessed spherical particales, and therefore had approximately twice the apparent density of the flaked form. Approxi-mately 30 kg of the atomized powder could be placed in the volume formely occupied by 15 kg of aluminum "pyroschliff". This procedure increased the peak intensity from 450 to 800 million Hefner candles. However, the flash was very much slower, that is, the time to reach peak intensity was longer, and the total duration of the flash was longer. It was also stated that the results could not be reproduced, because of variations in the cha-racteristics of the atomized aluminum powder.

B. The 15 kg charge of flaked aluminum powder was replaced by 28 kg of pellets (13 mm diameter and 7 mm height) composed of the following composition:

Magnesium powder: 39%.

These pellets were packed in the outer tube of the bomb together with 9 kg of black powder and then a length of detonation fuse added.

This bomb, when exploded produced a peak intensity which was only 80 per-cent of that of the standard BLC 50 bomb. The time to reach peak intensity (100 milliseconds) was longer than that of the standard bomb, and the durations of the flash at one-half and one-tenth maximum intensity were much longer (230 and 550 milliseconds, respectively). In addition, this bomb was found to be sensitive to bullet impact.

C. The standard 15 kg aluminum powder charge was replaced by one huge pellet weigh-ing 24 kg. The composition of the pellet was as follows:

PTN: 50%.

The results of the firing test conducted with this bomb were very disappointing. They indicated a peak intensity only one-fifth of the standard bomb. The flash was very slow, since the durations at one-half and one-tenth maximum intensity were 500 and 1,000 milliseconds, respectively.

D. The standard 15-kg charge of aluminum powder was replaced by one composed of 28 pellets of the following composition:

Sodium nitrate: 45%.

Each pellet had hollow axial space, weighed 900 gm, and measured 60 mm diameter and 220 mm in height. The total weight of pellets placed in the bomb was 25.2 kg. A length of detonating fuze was passed through each pellet and all of the ends of the fuze bound together.

The results of the firing tests conducted with this bomb indicated that the total peak intensity was approximately the same as that of the standard bomb. However, the peak intensity measured through a yellow filter (which is considered important when photo-graphing at night in the presence of a slight ground haze) was 20 percent greater. Al-through the time intensity curve for the flash rose at the same rate as that of the stan-dard bomb, the duration of flash was very much longer, being 400 and 800 milliseconds at one-half and one-tenth maximum intensity respectively. This indicated that the total light output was very much greater than that of the standard bomb. However, this light would not be usable if short exposures were employed. In addition the bomb was found to be sensitive to bullet impact.

E. In order to decrease the time of the flash, and thereby raise the peak intensity and concentrate it in a small time interval, a bomb was loaded using the same general proce-dure as C, but pressing each pyrotechnic pellet into a separate steel case in order to in-crease the confinement. Detonating fuze was inserted through each pellet as before. The results of test with this bomb showed that the peak intensity could not be appre-ciably raised. The bomb was also found to be sensitive to bullet impact.

F. The results of all of the above test indicated that the peak intensity, and therefore the total usable light output, in a short exposure time could not be increased appreciab-ly. However, most of the experimantal bombs produced longer flash durations, with inten-sities consistently as high at that of the standard bomb. Therefore, efforts were directed toward the utilization of longer exposures times. A camera, known as the "E-5", employ-ing a moving film holder to compensate the movement of the airplane, was developed recently (Source B). The "E-5" camera make possible the use of very long exposure ti-mes, and thus permits the use of photoflash bombs producing a long, slow flash. In this manner the total usable light outpu can be multiplied three to four times.

Safe photoflash bombs, employing outer layers of large amounts of atomized magnesium or aluminum powder can then be used, because all of the light of the relatively slow flash is useful.

Figure 87 – BLC 50 A Photoflash Bomb