Now that I was looking to the sky, I noticed something else, something that I'd seen only a couple of times on the mainland. It was the contrail of a high-altitude plane, but this wasn't the geometrically perfect straight line of a highspeed civilian transport. No, this was like donuts on a rope—a central line contrail surrounded by evenly spaced torus-shaped loops. From what little I knew of aerospace technology—the kind of drek you pick up from scanning the popular press—the only kind of engine that could create that characteristic donut-on-a-rope structure was a pulse-detonation propulsion system. As far as I knew, pulse-detonation engines were used on only one kind of craft: hypersonic spy planes, Aurora class and up.