The lighter a structure launching into air, the better. That's one of the reasons why ostriches can't fly - because their bones are solid instead of hollow. It's also one of the reasons why researchers at HRL Laboratories created the lightest metal known to man.
The researchers collaborated with scientists at Caltech and UC Irvine to design metallic microlattice, a mesh lighter than styrofoam, for aerospace structural components. The material is so light it can sit atop a dandelion without crushing it.