Regarding Dr. Griffin’s preferred theory, it should be pointed out that explosives rarely impart much momentum to solid objects, unless the explosive is actually contained – material making up a solid casing will be fragmented and sent at high velocity (i.e. shell fragments), but nearby solid objects will hardly move at all. This is because explosives create a pressure shock that moves at supersonic speeds. The explosive may exert a high pressure on nearby objects, but the pressure rapidly “washes over” those objects and thus does not have time to impart a large impulse.
Unless the pressure wave is somehow contained, the wave will rapidly move beyond nearby objects, at which time they are no longer accelerated. This effect is reminiscent of big-wave surfing – a truly large wave moves too fast for a surfer to gain much of a push from it and it will simply pass him by, unless he has either a longer, faster board or is towed into the wave by a jet ski.
For a worked example, Rememnikov [151] presents a typical charge of 100 kg TNT exploding at a distance of 15 meters. A series of objects placed at this distance would experience 272 kPa or just under 40 PSI, but would only experience the overpressure for 17.2 milliseconds, including the reflection of the blast, after which the pressure wave has passed the objects. Let’s assume we’re discussing a section of unattached, hollow square steel column 3 m high by 20 cm wide, with walls 4 cm thick. This object presents a maximum of 0.6 m2 to the blast front, so it experiences a maximum force of 272 kPa x 0.6 m2 = 163,200 N for 17.2 milliseconds, for a total impulse of 2807 Newton seconds.
It should be noted that the simplified calculation above grossly overestimates the total impulse, because we have assumed the peak pressure is sustained for the entire duration, when in reality a lower average value is expected. The actual expected impulse per facing area, seen in Table 1 of Rememnikov’s paper, is a mere 955 kPa-msec, or only 573 Newton seconds imparted to our column as above. We therefore are using a very generous estimate, almost five times higher than we actually expect. We will use our simplified estimate rather than the lower, more accurate number to silence any doubts that we have potentially underestimated the maximum imparted velocity.
The total impulse is equal to the mass of the object times the change in velocity. In this case, our column contains 256 cm2 x 3 m of steel or 76,800 cm3 of steel, for a mass of approximately 600 kg. The column would, therefore, be accelerated by 2807 N s / 600kg = 4.7 meters per second, or about 10 miles per hour – hardly a remarkable value compared to the ricochet scenario described above. In order to propel this column at the speed required, say 30 meters per second, we would need charges of at least 700 kg TNT equivalent – very large and clearly audible explosives indeed, even accepting our generous assumptions above.
