`   ==Dubious addition== :Caltrops are used inside some atomic bombs to center the uranium or plutonium ``pit`` inside the spherical tamper. The air gap allows the tamper more time to accelerate, which compresses the pit more tightly and leads to more efficient fission. I find this to be very unlikely  for something as highly-engineered as an atom bomb, I'd think they'd use something purpose-built.  `