They're useful for very hard targets, like dropping two 20Mt or so at the same time at each end of the access tunnel for the Cheyenne Mountain Complex (https://

They're useful for very hard targets, like dropping two 20Mt or so at the same time at each end of the access tunnel for the Cheyenne Mountain Complex (https://en.wikipedia.org/wiki/Cheyenne_Mountain_Complex https://en.wikipedia.org/wiki/Cheyenne_Mountain_Complex). Otherwise, at something in the 1 Mt plus or minus level, I forget, a lot of the energy is wasted by being thermally re-radiated out into space.