Persistence as the study of almost any history of human events shows, has its merits. Or perhaps, it is frustration that leads to new challenges. In any case, the ground motion scientists joined forces with the airblast scientists in the search for a TNT replacement on military hardware tests dedicated to both sub surface and surface target investigations.
A three phase experimental program was planned and started in January 1975 using 1 lb to 5 ton charges. This development plan was executed under the technical supervision of CERF (the Eric H. Wang Civil Engineering Facility, now named NWMERI New Mexico Engineering Research Institute. The immediate objective of the first phase with 1 lb charges was to determine a suitable cylindrical charge geometry in terms of length to diameter ratio. Plastic C 4 explosive was used for the charges because it could be molded easily into the various geometries under consideration. Because axial symmetry of the hydrodynamic effects of the explosion is required or, at least highly desirable, thought had to be given as to the location of the initiation point (or points, as it soon became evident); for a spherical charge single point detonation at the center of the charge provides this symmetry.
The use of these small charges required rather a controlled environment so that even small differences in explosion cratering effects could be related to the differences in charge geometries and detonation points. The CERF field facility provided this environment with a 14 ft diameter pit into which well characterized commercial grade concrete sand was placed to provide a uniform test bed for all tests.
Concurrent with the experimental program, hydrodynamic calculations were being made by C. Needham, AFWL, for predicting the blast effects of Cylindrical charges. Swisdak, NSWC, provided detailed information on. the physical characteristics of commercially available ANFO and AN prills to M. Finger, LLL (Lawrence Livermore Laboratory) who equation of state calculations to characterize the detonation properties of the ANFO. This information provided the basis for Needhams calculations. His work and data available from an early 1960 study by J. Wisotski (Denver Research Institute) showed that the blast propagating off the sides of a right circular cylindrical charge is adversely affected by the rarefaction wave coming off the flat top of the charge. To prevent, or alleviate, these perturbations, the later shots of the Phase I program used cylindrical geometries with hemispherical caps.
At the completion of the one pound series, it was determined that for C4 charges a multiply initiated, tangent above, right circular cylinder with length to diameter ratio of .84 (measured on the cylindrical section) with hemispherical cap appeared to best meet the desired cratering and fireball shock expansion program objectives. In addition, it was observed that apparent crater volumes exhibited a +/- 10 percent variation in reproducibility in a well controlled test bed. Based on other field data, it is believed that this variation may be as large as 20 percent in a natural geologic medium. A nominal 20 percent variation in apparent crater volume was accepted as the uncertainty in determining cratering agreement fur the remainder of the program..
The second phase of the program with nominal 1000 1b TNT equivalent charges, was started and completed in March 1975. As part of this phase and based on calculations and the earlier phase results, a capped cylindrical ANFO charge with an L/D ratio of 0.84/1 and three point initiation was constructed. Bulk ANFO was used and contained in a thin case; the cylindrical portion of the charge was confined by a light cardboard form, the cap within a hemispherical Styrofoam mold. Using the information developed on the earlier ANFO tests, i.e., ANFO I, II, and III, 1200 lbs of ANFO were used to give a 1000lb TNT equivalence.
The shot was fired on 18 March 1975. The cratering results essentially duplicated on a scaled basis the results of the similarly configured 1 lb C 4 charge. Significant differences were noted in the fireball characteristics of the ANFO explosion and the control tangent sphere. 1000-lb TNT. shot. The ANFO fireball was short lived and largely white in color whereas the TNT fireball was of long duration and fiery red. Although these differences were noteworthy to the experimenters, they were observed and explained on the earlier ANFO I V series; they are attributable to the oxygen balance of the ANFO as contrasted to the oxygen deficiency of TNT. In an oxygen balanced explosive all the oxygen required to complete the combustion process is contained within the explosive compound or mixture. In an oxygen deficient explosive, the deficiency leads to afterburning, i.e., the utilization of atmospheric oxygen to continue and complete the detonation and combustion processes. Hence, the short, hot fireball for the ANFO explosion and the longer, cooler fireball for TNT.
The results of this phase of the program were indeed encouraging-a capped cylindrical charge of ANFO would replicate the effects of a surface tangent sphere of TNT if proper account is taken of the intrinsic differences in the explosive characteristics, charge geometries, and initiation points.
The requirement for the number and type of initiation points was studied further via hydrocodes by Needham, He found that the larger the number of points, the quicker would the detonation fronts within the charge coalesce to form a smooth outer contour before the front excited the charge; the smoother the front at this time, the more uniform would be the ensuing blast wave. So, for the Phase 3 shots with 5 ton TNT equivalencies, five and seven point initiation systems were used.
Multipoint initiation calls for special attention; to obtain the required smooth detonation front, all initiations have to take place simultaneously, lest skewed mach wave interactions between the several detonation fronts produce jetting within and outside the charge. Simultaneity was no mean feat, but it was successfully accomplished with the use of quick acting detonators and a well designed firing circuit.
Phase 3 operations started in the Spring of 1975 at the White Sands Missile Range, the prospective site for the main DICE THROW event. Three ANFO capped cylindrical charges were fired as well as a control, baseline establishing TNT tangent spherical charge. Based on the Phase 2 results, the first ANFO charge had an L/D ratio of 0.84/1, and as suggested by the initiation studies of Needham, five detonation points were used. The cylindrical portion of the container was constructed from thin sheets of fiberglass and the hemispherical cap was formed from thin nylon parachute fabric. A nominal 12,000 lbs of bulk ANFO was rained into the containment vessel to provide a 10,000 1b TNT equivalent yield. Relatively extensive instrumentation coverage was used on the test.
The ground motion results, and the airblast measurements were satisfactory and well within the normal spread of data from single explosions. However, the crater volume was about 30% smaller than the TNT control and high speed photographs showed several airblast anomalies.
A second capped cylinder was fired with an L/D ratio of 0.5/1. Again bulk ANFO was used to fill the form. This time, however, a heavy tarpaulin canvas was used to shape the hemispherical cap. The nylon used on the first shot of this series was flimsy so that a good hemispherical shape could not be attained; this was suggested as a cause for sane of the anomalies. The, charge exploded satisfactorily but because of the larger area of charge/ground contact with this 0.5/1 L/D ratio, a substantially large crater resulted on this shot than on the control TNT surface tangent sphere or the previous L/D 0.84/1 AMFO geometry. Also, blast anomalies were present again.
In this iterative experimental search for the most suitable ANFO shot geometry, the third and last 12,000 1b ANFO charge had an L/D ratio of 0.75/1. Perhaps remembering past history where it was observed that even lightly cased ANFO charges produced more anomalies than bag built charges, this charge was constructed with bagged ANFO. Careful attention was paid to bag placement so that a smooth periphery was obtained for the charge as on the 1970 ANFO I, II, and III series. In fact, because the relatively small diameter (6.28 ft) of the charge made it difficult to place the standard 50 lb bags of ANFO into a tight, bag butted against bag, configuration, the ANFO was repackaged into 15 lb sizes in nylon bags. Nylon was selected because the paper, canvas, and burlap bags investigated absorbed oil, some to the extent that the bags deteriorated and disintegrated. Changes were made in the detonation scheme also. Seven point initiation was used and. instead of spherical boosters, cylindrical C 4 boosters were arrayed along the axis of the cylindrical portion of the charge.
All these changes new L/D, bagged ANFO, seven point initiation - resulted in a very satisfactory charge performance and explosion effects. Crater size was close to the TNT standard crater, ground motions were well within the accepted standard scatter, airblast measurements were as predicted with little scatter and significantly, very few anomalies were evident. DNA and most of the testing community were ready to move on to DICE THROW, the 600 ton ANFO event. But prudence dictated an intermediate scale test first; a second series of tests was planned, pre DICE THROW II.
|Join the GlobalSecurity.org mailing list|