A History of Research and a Review of Recent Developments

Underwater explosions 49
Table 2.5 Equivalent weights for explosives of the 1940s
not as great as in air or earth. Table 2.5 lists the equivalent weights for explosives
of the 1940s given by Christopherson [1.9].
The first underwater nuclear test, code-named Baker, was carried out at
the Bikini Atoll in 1946, when a nuclear bomb having a yield equivalent to 20
kilotons of TNT was detonated below the surface of the lagoon (200 feet
deep). As the shock wave initiated by the expanding hot gas bubble reached
the surface a rapidly expanding ring of darkened water was visible, followed
by a white area as the shock wave was reflected by the surface. A column of
water, the spray dome as it was called, was then formed over the point of
burst. Its upward velocity was proportional to the peak pressure in the shock
wave, and in the test it began to form 4 msec after the explosion. The upward
velocity was initially 2500 ft/sec. A few milliseconds later the gas bubble reached
the surface and a hollow column of gases vented through the spray dome. It is
said in ref. [2.9] that the column was 6000 ft high with a maximum diameter
of 2000 ft, and that 1 million tons of water were raised in the column. As the
column fell back a condensation cloud of mist developed at its base and surged
outwards. This bun-shaped cloud, known as the base surge, developed about
10 seconds after detonation, initially reaching a height of 900 ft. After about
4 minutes the height was double this, and the diameter about 3.5 miles. One
of the first deep underwater explosions (in 2000ft of water) took place in
1955, code-named Wigwam. During the pulsations of the gas bubble the water
surrounding it had a large upward velocity and momentum, and the spray
dome broke through the surface at a high speed. It is interesting to note that if
the surface breakthrough occurs when the bubble is in the ‘below ambient’
phase, the spray column breaks up into jets that disintegrate into spray not
unlike the vision of Moby Dick ‘blowing’.
The analysis of the action of the gas bubble, whether from a nuclear or
conventional underwater explosion, was a fruitful field of investigation during
the 1940s. In 1941 Edgerton photographed the pulsations of the bubbles
when detonator caps were exploded at various depths, and at about the
same time Swift [2.30] and others filmed bubble motion when 0.55 lb of the
explosive Tetryl was detonated 300 ft below the surface. The radius of the
gas sphere for this explosion is related to time in Figure 2.18, taken from the