 The first plutonium core , along with its polonium @-@ beryllium Urchin initiator , was transported in the custody of Project Alberta courier Raemer Schreiber in a magnesium @-@ field carrying @-@ case designed for the purpose by Philip Morrison . Magnesium was chosen because it does not act as a tamper . The core departed from Kirtland Army Air Field on a C @-@ 54 transport aircraft of the 509th Composite Group 's 320th Troop Carrier Squadron on 26 July , and arrived at North Field on Tinian on 28 July . Three Fat Man high @-@ explosive pre @-@ assemblies , designated F31 , F32 , and F33 , were picked up at Kirtland on 28 July by three B @-@ 29s ; two , Luke the Spook and Laggin ' Dragon , from the 509th Composite Group 's 393d Bombardment Squadron plus one from the 216th AAF Base Unit , and transported to North Field , arriving on 2 August . Upon arrival , F31 was partly disassembled in order to check all its components . F33 was expended near Tinian during a final rehearsal on 8 August , and F31 was the bomb dropped on Nagasaki . F32 presumably would have been used for a third attack or its rehearsal . 
