CALCULATIONS OF THE PEAK PRESSURE OF THE BLAST WAVE

CALCULATIONS OF THE PEAK PRESSURE OF THE
BLAST WAVE
Several ingenious methods were used by the various investigators
to determine, upon visiting the wrecked cities,
what had actually been the peak pressures exerted by the
atomic blasts. These pressures were computed for various
distances from X, and curves were then plotted which were

checked against the theoretical predictions of what the pressures
would be. A further check was afforded from the
readings obtained by the measuring instruments which were
dropped by parachute at each atomic attack. The peak pressure
figures gave a direct clue to the equivalent T.N.T. tonnage
of the atomic bombs, since the pressures developed
by any given amount of T.N.T. can be calculated easily.
One of the simplest methods of estimating the peak pressure
is from crushing of oil drums, gasoline cans, or any
other empty thin metal vessel with a small opening. The
assumption made is that the blast wave pressure comes on
instantaneously, the resulting pressure on the can is more
than the case can withstand, and the walls collapse inward.
The air inside is compressed adiabatically to such a point
that the pressure inside is less by a certain amount than the
pressure outside, this amount being the pressure difference
outside and in that the walls can stand in their crumpled
condition. The uncertainties involved are, first, that some
air rushes in through any opening that the can may have,
and thus helps to build up the pressure inside; and, second,
that as the pressure outside falls, the air inside cannot escape
sufficiently fast to avoid the walls of the can being
blown out again to some extent. These uncertainties are
such that estimates of pressure based on this method are
on the low side, i.e., they are underestimated.
Another method of calculating the peak-pressure is
through the bending of steel flagpoles, or lightning conductors,
away from the explosion. It is possible to calculate
the drag on a pole or rod in an airstream of a certain
density and velocity; by connecting this drag with the
strength of the pole in question, a determination of the
pressure wave may be obtained.
Still another method of estimating the peak pressure is
through the overturning of memorial stones, of which there
are a great quantity in Japan. The dimensions of the stones
can be used along with known data on the pressure exerted
by wind against flat surfaces, to calculate the desired
figure.