USEFUL LESSON

Waihi Daily Telegraph, Volume XXXI, Issue 8835, 21 October 1942, Page 3

USEFUL LESSON

EFFECT OF BOMBING AUSTRALIAN EXPERIENCE MAY BE EXPECTED IN RAID There is something of interest for our E.P.S. people in a survey -of the effects of bombing made recently by Mr Roy J. Bennie, .research engineer of the Melbourne Metropolitan Gas Company. The conclusions arrived at by Mr Bennie, aimed at the protection of such premises as the Melbourne gasworks, are as follow:

“We may expect accurately-aimed bombs if no anti-aircraft guns are available; but, if enemy aircraft are constrained to fly high by fear of guns or chasers, we may expect chance hits within the works. The weight of the bombs may be upwards of 10001 b. gross (5001 b. explosive), but more probably the largest would be 500 lb., containing not more than a 3001 b. charge.

“Warning of only 10" to 20 minutes can be expected. There is no way of protecting our plant from a direct hit, for the penetrating power is too great. “A first-class air raid shelter comprises a top layer of concrete four feet thick followed by 20 feet of earth, and a second four feet, of concrete. We can only endeavour to protect our plant and ourselves from the effects of splinters and missiles thrown out of the crater, and from the effects of blasts and from incendiary bombs.” Raid From Carrier Mr Bennie said that even though the Pacific Ocean might be policed by friendly navies, a squadron having an aircraft-carrier might slip in near the coast and make a surprise raid. In London the minimum warning might be ten minutes. In Melbourne it was desirable to assume a probable maximum of 20 minutes’ warning. The modern bomber travelled at 240 to 300 miles an hour, and 20 minutes would only correspond with their being first observed 80 miles away.

From this 20 minutes must be subtracted the unavoidable time in transmission of warning to local signal stations round the city. It would be well to allow at least five, and possibly 10, minutes for this, leaving a maximum time of only 10 to 15 minutes for making all necessary dispositions. Coastal cities might have less warning. The high-explosive bomb may be designed for extreme penetration with a delay-action fuse so as to pierce deep below exploding, says Mr Bennie. A 5001 b. bomb could seldom carry a charge exceeding 300 lb. weight. If this struck the open ground it would make a crater 30 feet across. This would occur if the bomb penetrated five feet into the ground. The terminal velocity of a bomb is usually between 600 and 700 feet per second, and the practical upper limit of 1200 when dropped from very high altitudes. Even at 400 feet per second a 5001 b. bomb would penetrate 20 feet in sand or gravel, but only 2ft. 6in. into concrete. The subsequent explosion would make further penetration and cause damage below this point. Power of Suction Two phenomena occur when a bomb explodes, Around the bomb is a sphere of expanding gas which produces its effects by the pressure exerted against obstacles and by the very high temperatures generated by the extreme pressures. Then there is the outer zone of “blast.” Blast is really an air wave. No translation of air takes place; only sufficient movement occurs to transmit outwards of pressure wave which is immediately followed by a suction wave, and then by minor oscillations until the wave is finally damped out. Blast was illustrated in Barcelona and Madrid, where the screen walls of steel and concrete buildings were ripped off and fell outwards when a bomb exploded sufficiently near to give the requisite intensity of airblast wave, but sufficiently remote not to exert direct explosive pressure. It was also shown in these cities that old brick buildings suffered destruction by the same means. The outer wall was pulled out and often followed by the collapse of the internal timber or cast iron columns. Modern concrete-cased steel frame buildings and reinforced buildings were shown to be far less liable to the effects of blast. Of course, the windows always burst outwards, and sometimes some of the lighter furnishings went out through the window, but the walls remained intact tinder conditions far more severe than the brick walls could withstand.