XW-35 was designed from the outset as a thermonuclear warhead for the first generation of ICBMs . Development was driven by the development of the Atlas missile , and when the accuracy of the Atlas was shown to be inferior to predictions the XW-35 design had to be altered to give a higher yield . By March 1958 the development of the XW-35-X1 was lagging . The XW-35-X1 was probably the device tested in the Koa shot of the Operation Hardtack I series ( May 1958 ) . Predicted yield was 1.75 megatons ; the actual yield was 1.37 megatons with the shortfall due to poor burning of the secondary . By this time the Air Force had designated the XW-49 , a simple modification of the TX-28 already successfully tested during the Redwing series . By August 1958 the first dedicated missile warhead design , the W-35 , had been cancelled in favor of the W-49
Operation Hardtack I
Operation Hardtack I
XW-35