The Air Force on Tuesday successfully tested the largest conventional bomb in the American arsenal in Florida, a munition so massive that its 18,000 pounds of high explosives must be dropped from the rear of a cargo plane, officials said. The bomb explodes a few feet above the ground, and is designed to send a devastating wave of fire and force hundreds of yards to kill troops, flatten trees, knock over structures, collapse cave entrances and, in general, demoralise those far beyond the impact zone.