The smoke bomb was dropped as proof of the concept that a man on a parachute might actually be able to land on the roof of this fortress. Bobby Shaftoe is, of course, the final and irrefutable test of this proposition. As he gets closer, and his head clears from the explosion, Shaftoe sees that the smoke bomb never actually reached the roof: its little chute got tangled up in the briar patch of antennas growing out of the top of the thing.