graphy. Consequently, an average charge radii of 376 feet were used, which compares
favorably with the average charge radii of 387 feet computed for the Ivy Mike surfacelevel data obtained with electronic gages. The pressure-distance curve for these equiv~
alent TNT charge radii was then scaled vertically by the NOL method for comparison
with measured data, ueing the observed ambient conditions at altitude. The uncertainty
of the measured data was such that it was not possible to correlate the vertical peak
overpressures with the theoretical curves derived from the surface-level peak overpres-~
sures in this manner. Consequently, it was not possible to determine the best method
of making an altitude correction to account for blast propagation through a nonhomogeneous atmosphere for high-yield bursts.
Those pressure data measured along the surface, obtained on Shots 1, 2, 4, and 6 by
using smoke-rocket and direct shock photography, are plotted in Figure 2.7. Gage data
from Jangle Surface and Ivy Mike have been included for comparison and correlation.
The data were normalized by scaling to 1 kt at standard sea-level conditions, so that
the composite free-air data scaled to 1 and 2 kt could be shown. A comparison to the
1- or 2-kt free-air curve for the purpose of determining a reflection factor for surface
bursts was not strictly valid, since the hydrodynamic determination of yield for these
shots involved an assumption of the factor of two. (Discussion of the surface-burst refiection factor was presented in Section 2.3.5.) Figure 2.8 shows scaled arrival-time
data obtained by smoke-rocket and direct shock photography, with the 1- and 2-kt composite free-air curve. Scaled data for both pressure and arrival time appear selfconsistent, as well as comparing favorably with Jangie and Ivy gage data. It seems
justified to conclude, then, that cube-root scaling of blast data from events in this yield
range te valid.
;
Part of the objective of the direct shock photography was to observe the formation
and growth of any precursor which might occur. At this time there was some doubtthat
the precursor would form on asurface shot. Actually, no precursor as such was noted;
however, anomalous wave forms were recorded by the pressure-time gages. Observa- _
tions made of the film exposed on Shots 4 and 5 disclosed a dense water cloud following
immediately behind the shock front. This cloud implies water droplets contained in the
shock front and may explain the anomaly.
2.4.2 Base Surge. Early planning provided for the determination of the characteristics of the base-surge phenomenon for each of the shots. It was hoped that from such a
study, scaling laws could be formulated to predict base-surge effects of surface shots
with yields different from those of Castle. The base surge becomes of military significance when it acts as a carrier of radioactive contamination to regiong beyond normal
fallo:-
The extent to which this could occur from surface bursts, as well as the general
dynamics of the phenomenon and the determination of scaling laws, were the objectives
of this study.
The experiment was almost entirely unsuccessful, since the primary analytical tool,
photography, was rendered useless when it was decided to schedule the shots before
sunrise. A minimum photographic effort was maintained throughout the series, from
which it was determined that a base surge probably did form on Shots 1 and 2.
This
limited material prevented any detailed study anticipated in the early objectives.
2.5
CLOSE-IN GROUND ACCELERATIONS
Study of ground motion produced by multimegaton devices detonated on the ground surface was planned for Castle to extend and supplement those data obtained from Ivy Mike.
31