 Because of its complicated firing mechanism and the need for previously untested synchronization of explosives and precision design , it was thought that a full test of the concept was needed before the scientists and military representatives could be confident it would perform correctly under combat conditions . On 16 July 1945 , a Y @-@ 1561 model Fat Man , known as the Gadget for security reasons , was detonated in a test explosion at a remote site in New Mexico , known as the " Trinity " test . It gave a yield of about 20 kilotonnes ( 84 TJ ) . Some minor changes were made to the design as a result of the Trinity test . Philip Morrison recalled that " There were some changes of importance ... The fundamental thing was , of course , very much the same . " 
