a CRLR - 283 - EVALUATION OF A THERMAL ABSORBING SMOKE SCREEN (WT-769). Elmer H. Engguist and Charles W. Forsthoff, February 1954. SECRET-RESTRICTED DATA The purpose of this experiment was to obtain a better under- anding of the method of formation and the characteristics of the precursor shock wave obtained in some prior atomic weapons tests. The modification of the shock wave at Shot 10 was emphasized through the presence of a heated-air layer provided by the absorption of thermal energy in 4 carbonsmoke screen. Supplementary measurements were made to determine the attenuation of thermal radiation by a thermal-absorbing smoke screen. Detailed analysis of the pressure curves by Projects 1l.la and 1.2 indicates that the blast wave was greatly modified by the presence of the carbon-smoke screen. The maximum pressure in the smoke screen was higher than in the clear area from approximately 900 to about 3,000 ft. The wave form was markedly modified from that of a normal shock wave in the precursor region, both in the smoke screen and in the clear area, though the distance over which this modification occurred varied between the smoke and clear areas. In the clear area, the normal shock front appeared at about 1,500 ft. The time of arrival of the initial shock disturbance at any position in precursor region was later, by as much as 20%, in the smoke-screen area than in the clear area. time were more nearly like what would be obtained from a nonthermally- tions. at us heated blast-reflecting surface. In the clear area, the blast data, because of the more pronounced precursor effect, differed considerably from the deta under the smoke screen. screen. The thermal flux was significantly reduced by the carbon-smoke The measured attenuation of thermal radiation at a ground station 2,640 ft from air zero was 97.4 + 0.3%. The normal thermal flux of U7, 0 + 4.0 cal/sq cm was reducedto 1.2 + 0.1 cal/sq cm. ne UKP-59. 4 preliminary report of this work appeared as CRLR 191 and i ahI ie BEL Me og The net effect of the smoke screen on the blast wave was to modify the precursor wave and reduce the range over which the precursor effect occurs. In general, the smoke screen reduced the thermal effect; and blest data such as the peak positive-pressure and shock arrival-