As discussed above, for destruction of cities and non-hardened targets, breaking the mass of a single missile payload down into smaller MIRV bombs, in order to spread the energy of the explosions into a "pancake" area, is far more efficient in terms of area-destruction per unit of bomb energy. This also applies to single bombs deliverable by cruise missile or other system, such as a bomber, resulting in most operational warheads in the U.S. program having yields of less than .