On the order ‘Buttercup’, he explained, all of the escort ships would turn outward from the convoy. They would accelerate to full speed, while letting loose star shells–an explosive that, like a dandelion puffball, released iridescent fragments that hung in the air on a parachute for up to sixty seconds and illuminated the ocean. If a U-boat was sighted, Walker would then mount a dogged pursuit, often ordering up to six of the nine ships in his group to stay with the vessel until it was destroyed.
