PHENOMENA IN OPEN WATER 
The general sequence of events 
5 Consider for simplicity an uncased spherical charge initiated from the 
centre. On initiation, a detonation wave will travel out radially from 
the centre with a velocity characteristic of the explosive. Within the 
region traversed by this wave, the explosive is converted into incandescent 
gas at a very high pressure. In front of the detonation wave the 
explosive remains unchanged, being "unaware" of the initiation. In T.N.T. 
for example, the detonation wave travels at approximately 23,000 ft. per 
sec. so that a charge of 400 lb. T.N.T. occupying a sphere of about one 
foot radius would be all converted into incandescent gas at a pressure of 
the order of several hundred tons per sq.in. in the very short time of 
about 4.0 microseconds. 
6. The high pressure gas bubble then trits to expand and the first result 
is a compression of the spherical layer of water immediately surrounding 
the charge. This layer in turn compresses a further layer and so on. 
In this way, a wave of compression is propagated through the water radially 
outwards from the explosion. This wave is usually termed the pressure 
pulse. 
7. The velocity of propagation of the pressure pulse at first decreases 
as the pulse travels outwards until, at a comparatively short distance fran 
the explosion, it becomes approximately constant at the normal velocity of 
sound about 5,000 ft. per sec. in sea water. Except for a small inner 
region surrounding the charge, the pressure pulse obeys the usual acoustic 
laws, the intensity decreasing steadily with increasing distance until, at 
very large distances, the pulse becomes simply a noisee The persistence of 
the pulse is demonstrated by the fact that the detonation of a charge of 
only 9 oz. guncotton can be detected at a distance of 40 miles. The 
characteristics or the pressure pulse will be considered in detail later, 
but one feature is that the pressure in the pulse can be treated as 
essentially positive, any subsequent suction in the pulse being unimportant. 
This is in marked contrast to blast in air where a phase of positive 
pressure is followed by an appreciable suction phase. 
8. While the pressure pulse is travelling outwards to large distances, the 
original gas bubble expands. The pressure in the gas bubble, therefore, 
decreases and becomes ultimately of the same order as the hydrostatic 
pressure in the water. Except in the initial stages, this expansion takes 
place relatively slowly and the motion of the surrounding water is in the 
nature of a general bodily flow, as opposed to the compression of successive 
layers associated with the pressure pulse. This outward flow involves a 
considerable mass of water with large kinetic energy and, as a result, the 
flow overshoots the equilibrium position in which the pressure in the gas 
bubble is equal to the hydrostatic pressure in the surrounding water. 
when the outward expansion ceases, therefore, the pressure in the bubble is 
less than the hydrostatic pressure for equilibrium and the bubble commences 
to contract with resulting inward flow of water. This return flow also 
over-shoots the equilibrium position and the bubble contracts to a small 
volume at a pressure which is fairly high, although not nearly as high as 
the original pressure at detonation. This small bubble then behaves 
effectively as a second explosion and the whole process is repeated, a 
second compression wave being sent out as the bubble commences to expand 
again. 
S. Simple theory predicts that the oscillation of the bubble would 
continue indefinitely in an unlimited mass of water, the original explosion 
producing in effect a series of successive explosions. In practice, 
however, energy is dissipated to the surrounding water and the pressure in 
succeeding pulses decays so that each explosion becomes intrinsically weaker 
than its . edecssor. Moreover, at a later stage, the bubble tends to break 
up into smaller bubbles with further dissipation of energy and the process 
terminates after a few oscillations. 
