A 4,000 lb HC4000 British bomb was found on 30th August 2017 in Frankfurt during construction work. According to the news, the evacuation of 60,000 people has been scheduled. The Arms Control Center presents data about the bomb and simulates its explosion to illustrate its effects and help the German authorities take reasonable and safety measures which would not disrupt the economic and social life of the vicinity of the bomb at unnecessary distances. At the same time, our simulations and scenario will provide an educational video in the framework of the Arms Control Certificate awarded by the Arms Control Center to all its members. The total weight of the bomb found in Frankfurt is roughly 4000 pounds (i.e. 1942 kilograms) and it is a High Capacity bomb, hence the HC4000 name of the bomb. The High Capacity bombs were used for general bombardment purposes on operations where maximum blast damage was required.

The whole HC series included bombs of 2000, 4000, 8000 and 12000 pounds. Regarding the HC4000 bomb in question, there are six versions of the bomb Mark I, II, III, IV, V, VI. On the screen below we show Mark I.

Nose pistols were used for fuzing and in particular Mark I used one nose pistol (No 27, 42 or 44) while its other two side fuze pockets were not used. On the other hand, Mark II to VI used three nose pistols for fuzing of the same number (No 27, 42, 44). Here is a picture of Mark II-VI

The crucial issue here is what Mark the discovered bomb is so that we can specify the type and amount of explosives used for filling. By studying all the possible fillings we observe that the worst case scenario would involve a Mark VI HC4000 bomb filled with 3,294 pounds of Torpex explosive, whose relative effectiveness factor is 1.3 yielding a total explosive energy of 4,282 lb of TNT equivalent.

We have now established that a worst case scenario involves the explosion of 4,282 lb of TNT on a flat, rigid, clean and thermally reflective surface without any other sources of blast absorption or attenuation except for the targets, which in our case can be for example a human being or a building. This scenario will not only produce data regarding the safety measures that have to be taken but it will also provide a picture of the actual damage caused by such bombs in Frankfurt during the Second World War.

Using computer simulations we estimate that an effective overpressure of 0.5 psi causing glass fracture could hardly exceed a distance of 2000 ft (roughly 600 meters)
Moreover, an effective overpressure of 5 psi causing eardrum rupture is not expected to exceed a distance of 400 ft (roughly 122 meters)
In the same way, an effective overpressure of 30 psi which is the threshold for lung damage is not likely to exceed a distance of 180 ft (roughly 55 meters)
Finally, an effective overpressure of 100 psi which is the threshold overpressure for lethality cannot possibly exceed a distance of 100 ft (roughly 30 meters)

The above estimations are related to the effects of blast wave but there are other sources of injury such as primary and secondary fragments of the explosion or injuries due to the violent displacement of human bodies which can impact unyielding surfaces. These effects are not studied here in detail but they are not expected to be significant at distances where glass fracture is unlikely (i.e. larger than 600 meters). This is also supported by Christopherson’s formula for fragment range which gives for 4282 pounds of TNT equivalent yields a range of 1840 ft (roughly 560 meters).
We also show an old ballistics experiment video of the British Army which shows a note at 03:00 saying that fragments extend several hundred feet from the bomb which proves Christopherson’s formula to be very conservative and thus appropriate for our risk assessment

We can only approximate the potential explosion site using open information from various news websites but for the purpose of our scenario, we don’t need to know the precise potential ground zero. According to the news, the bomb was found at a construction site near Wismarer Street and Google Earth shows such a place, which we will choose as a potential explosion site for our educational scenario. Please note that this place is only selected for educational purposes and may not be related to the actual potential explosion site.

Please note that this video is concerned with blast and fragment lethality. Buildings can easily collapse at distances where blast simply causes eardrum rupture (i.e. 5psi ~400ft or 120 meters). At distances where overpressures are lower than 0.5 psi (glass fracture) buildings do not collapse. We studied the lethality of the blast and fragments on humans and we expect you to understand that if glass is not fractured buildings don’t collapse, either. Hence, a radius of 2000ft (~600 meters) is a reasonable evacuation radius to avoid direct blast injuries and the collapse of buildings (there is still some risk from glass fracture so a reasonable extension of the radius is justified). However, a radius of 1,5 km that the media reported is highly exaggerated! We should not forget that the bomb is now in a pit so the effects would be further reduced by the walls of the pit. According to the above analysis, the Arms Control Center believes that any evacuation zone larger than one kilometer is highly exaggerated and causes unnecessary disruption to the economic and social life of people.

Please note that the scenario studied in this webinar and the results of our simulations are only for educational purposes as they have been based on the available information appearing on the news. The citizens of Frankfurt and any other interested person or body should follow only the instructions of their governmental authorities and bomb disposal experts. The Arms Control Center will not be responsible for any decisions you make or you don’t make based on our report.

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