Pre-DICE THROW II
Pre DICE THROW II was arranged and conducted in August-September 1975. There were two shots, Event I, the control event for comparison purposes, a 100 ton TNT sphere tangent to the surface, and Event II, a 120 ton ANFO charge scaled to the highly successful pre DICE THROW, Phase 3, Event 4 shot. As with all the ANFO tests after the 0.82 TNT equivalence had been established, where direct comparisons are to be made between ANFO and TNT effects, the ANFO weight is approximately 1.2 larger than the TNT yield.
To develop a more effective and efficient combined HE blast and shock source, the PRE-DICE THROW II Charge Development Program was conducted by AFWL in 1976. This program, guided by previous large field trials, small shaped charge tests and hydrodynamic calculations consisted on 28 explosive detonations of C-4, ANFO, and TNT at 1-pound, 1/2-ton, and 5-ton yields. The objective was to develop a new charge configuration using ANFO that would provide blast and shock data compatible with the existing TNT surface tangent data base, while significantly reducing the associated fireball/air blast anomalies. The final charge design was a hemispherically capped 0.75:1 cylinder of bagged multiply detonated along the center line. Fireball/air blast anomalies were significantly reduced and air blast and ground shock records followed the TNT standard. While apparent crater profiles were in reasonable agreement, volumes were 23% high and crater morphology and subsurface deformations were visibly different.
Although test objectives were met for the dry sandy media used, questions remained with respect to larger yields and other media. Analysis of 10 events - PRE-DICE THROW II (100 tons on snad/clay with shallow water table, DICE THROW (500 tons on a dry gypsum lake bed), MISERS BLUFF II (7-100 tons events an a river valley alluvium with an intermediate water table) and MILL RACE (500 tons an dry desert alluvium) - provided the basis for assessing these questions.
Analysis shows air blast and air induced ground motions to be predictable, calculable and consistent with the TNT data base. In addition, these environments are noticeably more uniform with azimuth, probably the result of fewer fireball anomalies. Cratering efficiencies, even when normalized to TNT, increase with decreasing yields and wetter geologies, while differences between the TNT and ANFO craters persist with the latter exhibiting larger outward displacements near the surface. Crater and direct induced ground motions were consistent with the craters but magnitudes are generally lower than the TNT data base.
Pre DICE THROW II was a crucial operation; on the performance of this ANFO shot hinged the charge design selection for the main event, DICE THROW. The operation took on the aura and magnitude of the main event itself. Site selection was carefully made to meet the geologic requirements of the Air Force MX program; an area on the White Sands Missile Range, close to the pre DICE THROW I, Phase 3 site, was chosen. There were twenty two project agencies on the operation fielding twenty eight different projects. Some projects dealt with charge construction, initiation,, and diagnostics, air and ground shock measurements, gage development cratering, ejecta, ground displacements, technical photography, and prediction techniques for the phenomenology of concern. Other projects were directly concerned with military hardware items and detection systems. All in all, pre DICE THROW II was a big show with many participants, a large audience, and a concerned angel, DNA.
The block built TNT spherical charge was constructed under the direction of DRES personnel in a manner similar to previously built large TNT charges. The Event II charge with the domed cylindrical geometry was constructed under the supervision of Swisdak with 50 1b bags of ANFO obtained locally in New Mexico. A bag stacking plan similar to the one used on ANFO I and pre DICE THROW, Phase 3, Event 4, was used. As before, it was deemed important to obtain a smooth outer contour for the charge. For each layer of bags, the bag against bag arrangement was intended to get that smooth outer contour. To obtain some structural strength to the construction each layer had its, inner bags laid at abort a 90° angle to the bags below and above.
During construction of the stack, after about 40 tons had been emplaced, some of the dire concerns of DNA were realized; the stack collapsed partially during a heavy rainstorm. Although a tarpaulin had been used to protect the charge, rain water apparently penetrated to the ANFO resulting in a dissolution of some ANFO and a partial collapse of the charge. Also contributing to the collapse, it was postulated, was excessive personnel traffic on the rim of the charge during charge preparation and while emplacing the protective tarpaulin. And, the charge stacking plan was suspect. The outer bags particularly were essentially unsupported in the lateral direction and thus in a sort of unstable equilibrium; this design while successful for the smaller 12,000 lb charge, appeared untenable for the larger, almost 22 .ft high, charge.
A new stacking plan was quickly fashioned, one which followed tire pattern of the block TNT charge construction. All bags, including the peripheral ones, were interlocked to some extent, and additional structural strength to the stack was attained by rotating the bag stacking arrangement of each layer 900 with respect to the layer below. The smooth outer contour was sacrificed in this design; it was hoped that this variation from the ideal design could be tolerated that it would not lead to the generation of an excessive number of anomalies. The significance of a smoothly contoured ANFO charge versus a rough one had not been established by experiment or analysis, although no anomalies were noted on ANFO V even though it had a rough outer contour. In any case, the charge had to be constructed if DICE THROW was to continue on schedule; the theme was brawn before beauty. The charge, indeed, was successfully built and completed in three days without further mishap.
As with the prototype 12,000 lb ANFO shot, a seven point detonation scheme was used on this pre DICE THROW II 2 event. A new and sophisticated design was made by Swisdak in which arming, firing, and operational safety were the paramount considerations. The initiation and boostering system consisted of two parts, an MBA (Main Booster Assembly) emplaced along the axis of the stack during charge construction, and a BIS (Booster Initiation System), lowered into the MBA in order to arm the charge.
The MBA consisted of a 5 inch diameter (I.D.) PVC (Poly vinyl chloride) tube around which were affixed seven 29 lb, 5 inch thick, 12 inch diameter charges of 75/25 Octol, a rather insensitive explosive. A cardboard construction tube was placed around the MBA to protect it during ANFO bag stacking.
The BIS was the arming device. It consisted of a 4.5 inch diameter (O.D.) PVC tube into which were fixed seven pentolite explosive discs at spacings identical to the one of the Octol boosters. Each pentolite initiator was provided with two exploding bridge wire detonators (Reynolds Industry type RD 1), one as the primary, the other for redundancy.
To arm the charge, the BIS is lowered into the MBA (with the help of a crane) so that the pentolite and octol discs are aligned. Laboratory tests demonstrated the adequacy of this MBA/BIS system. The shock wave from the pentolite successfully bridged the air gap between the PVC tubes and through the tube wall thicknesses to detonate, high order, the octol boosters.
The somewhat flexible nature of the long PVC tubes presented a slight problem: difficulty was experienced at first in inserting fully the BIS into the MBA because the MBA PVC tube had a slight bend. With the help of a lot of grease and some little prayer, the BIS insertion was finally made satisfactorily.
On 12 August the Event I TNT charge was fired; on 22 September 1975, the Event II ANFO shot took place. The data obtained and their analyses resolved DNA's problems and questions relative to the selection of the charge for the DICE THROW main event; the 120 ton domed hemispherical bagged ANFO charge performed excellently, and in some important aspect, better than the control 100 ton block built spherical TNT charge. Both airblasts measure and fireball photography showed the absence of significant anomalies on the ANFO shot, while the TNT charge produced perhaps even more than its normal number of airblast perturbations. Ground motion particle velocities were similar in waveforms and amplitudes for the TNT and ANFO detonation. Adequate predictions could be made for blast and ground motion effects. The ANFO shot crater was larger than that of the TNT charge; this result was consistent with the results of the pre DICE THROW I tests. The TNT crater, however, was unexpectedly larger than predicted; this again points up the difficulties of trying to predict effects in a medium as inhomogeneous as ground.
The measurements unique to the ANFO went provided valuable self consistent and comforting information. The internal temperature probes in the ANFO charge showed 70° 75°F temperatures. The variations followed diurnal air temperature charges as there was no self heating of the charge.
The average detonation velocity of the charge, as measured by LLL with three rate sticks was 4,790 meters/second. This is higher than that measured in earlier tests. There apparently is a direct correlation between detonation velocity and charge size. It is probable that the bulk density of the ANFO in situ increases with the size of the charger the larger, i.e., taller, the charge the more the lower layers of prills are compacted by the weight of the upper layers. Some prill break up undoubtedly occurs; the fragmented prills fill the voids between the prills thus increasing the bulk density. With increased bulk density, increased detonation velocity can be expected as Finger's calculations indicate and earlier industry experiments have shown. (Note that the prill density need not change to get increased bulk density; it is the prill size distribution that determines to a large degree the bulk density. With prill density unchanged, the original stoichiometric ANFO proportions remai undisturbed on a prill basis.)
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