What Makes a Good Bomb Shelter?
STUDY THE PUBLICATIONS OF THE OFFICE OF CIVILIAN DEFENSE
By COLONEL GEORGE J. B. FISHER, Office Chief, Chemical Warfare Service, War Department, Washington, D. C.
Delivered on January 13, 1942, during the General Electric Science Forum Programfrom Schenectady, N. Y., over WGY
Vital Speeches of the Day , Vol. VII, pp. 285-286.
ANYONE who has heard the whine of an aerial bomb, falling at a speed that mounts to 600 miles per hour, has felt the urgent need of a bomb-proof shelter.
We learn, on excellent authority, that the music of falling bombs is now being heard with increasing frequency over Germany. And, as a result, the Nazis are at last beginning to take a very lively interest in air-raid shelters—this despite the fact that the British on their part are becoming less enthusiastic about large-group type shelters. This may very possibly denote a trend.
It is easy to build a shelter that will protect against 50 or even 100-pound bombs. What the designer has to worry about is a structure that will withstand the terrific force generated by an exploding 2000-pound bomb; a type, incidentally, which the R.A.F. appears to regard with increasing favor.
In 1939 and 1940 the typical high explosive bomb weighed 100 pounds. Occasionally, 500-pound bombs were used, but these were rare in the early stages of the war.
Today, however, 2000-pound bombs are not uncommon; and sizes up to three and even four thousand pounds are being dropped. As better bombers are developed, the proportion of larger bombs definitely increases.
We can best appreciate the structural strains imposed by these huge demolition bombs by considering the optimum effects they produce. These are three in number—all coming in split-second succession, yet each having its own direct influence on the design of protective shelters.
The first is impact—the shock imposed when a mass of a ton or more, moving at a speed of a thousand feet per second, meets a stationary body.
The tensions thus generated have no more than started when, within a tenth of a second, ordinarily, the detonation occurs.
This detonation is, technically, the reaction occurringwhen a suitable stimulus is applied to a relatively large quantity of trinitrotoluene, amatol, or other high explosive. The entire mass is converted, almost instantaneously, into other more stable substances, principally gases. The volume thus liberated by one cubic foot of explosive may equal one thousand cubic feet of gas.
The explosive reaction subjects the bomb case to heat as high as 20,000 degrees centigrade; it distends the case to one and a half times its normal size; and when the limit of expansion is reached, splits the case into a shower of sharp-edged splinters or fragments which may run, on the average, to about the size of a man's thumb.
Thus we have the second effect—fragmentation; and the third effect—blast.
The flying white-hot fragments are propelled initially at speeds as high as 5000 feet per second—twice the velocity of most military projectiles. But this velocity diminishes rapidly, as air-resistance brakes the irregularly-shaped fragments. Shelters fulfill their most useful purpose by absorbing the impact of these hot, sharp bomb splinters, which are hurled distances of say 1200 feet.
Blast effect, too, is readily protected against by a well-designed shelter. Yet as much as 55 per cent of the kinetic energy developed by the exploding bomb goes into blast—the ultra-rapid displacement of air to accommodate the transformation of high explosive into gas.
So powerful is blast effect against weakly designed structures that a special bomb—the aerial or land mine—has now been developed to apply an even larger proportion of force to this effect alone. The downward flight of the aerial mine is retarded by a parachute, so that its impact effect is negligible. The case is extremely thin, so that fragments are smaller; its powerful punch goes principally into a shattering air blast which demolishes all nearby structures that are not sturdily built.
We may say, then, that a perfect bomb-proof shelter is one that protects against impact, against fragments, against blast. As a practical proposition, protection against the impact of very large bombs in community shelters runs to high per capita costs. Protection against fragments, falling debris, and blast effects, on the other hand, is entirely feasible and need not be unduly expensive.
An impressive volume of scientific data had been developed, prior to this war, as to resisting media for protecting against bomb attacks. Some of these data were empirical. Some were compiled with a great deal of interpolation. Actually, there had been little in the way of critical evaluation of war experience. On the whole, however, scientific theory under which early air-raid shelters were built, has stood up remarkably well.
Yet most of the data in this field had been compiled abroad. Our War Department was not satisfied to accept it as entirely applicable to American conditions. So back in 1938—seven months before Hitler attacked Poland—the Chief of Engineers of the Army commenced an exhaustive scientific study of structural protection against aerial bombs, with particular relation to architectural and engineering practice in the United States.
By the fall of 1940 this project was far enough advanced to warrant the erection of type structures which in due time were subjected to actual bombing from service altitudes. These tests, conducted on the Edgewood Arsenal Military Reservation in Maryland, were substantially completed early last year—and the area (the only intenselybombed area of this sort in the United States) is now regularly visited for instructional purposes by students attending Civilian Defense Courses at the Chemical Warfare School.
Results of the Edgewood tests have now been published in a brochure issued just this month by the United States Office of Civilian Defense under the title, "Report of Bomb Tests on Materials and Structures."
Study of bomb proof shelters is, of course, continuing and will continue as long as necessary. In this work the War Department is being materially assisted by the "Committee on Passive Protection Against Bombing" of the National Academy of Sciences.
This committee—under the chairmanship, Dr. R. C. Tolman—has conducted numerous experiments of its own to develop resistance data and to establish general laws governing the penetration phenomenon. This scientific work is in all cases checked with Army and Navy experience and with data now continuously available from our allies.
We, in the Army, are sometimes accused of "passing the buck." That's exactly what I intend to do now. Instead of trying myself to explain how to make a good bomb shelter, let me advise everyone who is interested in this important subject to obtain and study the excellent publications of the Office of Civilian Defense—the agency through which the scientific data on protective shelters, that has been developed by our Government, is made available to the American public.