A History of Research and a Review of Recent Developments

46
The detonation of explosive charges
were laid in all parts of the world—perhaps a quarter of a million—and by
the Second World War aerial mine laying had been perfected, as well as mine
laying from submarines and surface craft like destroyers, motor launches,
and specifically designed minelayers. The total mines laid by all combatants
in the Second World War is thought to approach a half million. The total of
merchant and naval vessels lost by all countries was probably approaching
2000—a worldwide sinking rate of about one ship per 250 mines.
The powered torpedo, with a name derived from the family name for electric
eels and rays, was first produced in its modern form by the English engineer
Robert Whitehead in 1868. It had depth and lateral controls, propellers driven
by compressed air, and a warhead of high explosive. Later the Americans
developed a method of heating the compressed air by burning alcohol. This
considerably raised the pressure available at the propellers and thereby increased
the range. Torpedoes were originally fired underwater, but were later ejected
from deck-mounted tubes and also launched from aircraft. By the end of the
Second World War the total weight of a single torpedo was about 3000 lb.
Mines and torpedoes usually explode on or after contact with the target,
but the other major naval weapon, the depth charge, was specifically designed
to explode at greater depths on or near the seabed in the vicinity of a submarine.
The design of depth charges therefore resulted in the scientific study of the
nature of explosions deep below the surface. Work in this field later escalated
with the coming of nuclear bombs and the need to assess the danger to naval
vessels of underwater nuclear explosions some distance away.
Much of the fundamental work in Great Britain began when outstanding
young scientists were mobilized to help the war effort in 1939 and 1940. Among
them was W.G.Penney, later to become Lord Penney, and one of his first tasks
was to examine the physics of underwater explosions on behalf of the Ministry
of Home Security Civil Defence Research Committee. There had been some
earlier work in this field at the end of the First World War, when H.W.Hilliar
[2.20] produced a report on the properties of pressure waves from underwater
explosions. He developed a piston gauge, similar in many ways to the crusher
gauge we discussed in the Introduction, but with the piston and crusher initially
at a set distance from the anvil, and not in contact with it. Hilliar found that the
deformation of the crusher as a result of piston impact was a function of the
energy of the piston alone, and using this he conducted 109 experiments to
determine the major features of underwater shock waves. It is generally accepted
that for its time Hilliar’s work was of great simplicity and brilliance, since he
was able to investigate the physics of shock waves, the principles of similarity
and the weight/distance laws at the same time. A quarter of a century later his
design of a multiple piston gauge was improved by G.K.Hartmann [2.21] in the
USA. This work is described in detail in a good book on underwater explosions
by R.H.Cole [2.22] first published in 1948. This monograph has proved to be
one of the best written works on the subject, and includes the hydrodynamic
analysis of the detonation process.