The German army is developing “green” weaponry meant to reduce the pollution coming from guns and military rockets. In place of missiles and space rockets whose emissions contribute to acid rain and ozone-layer depletion, German scientists hope to produce explosives and missile fuels that emit nothing more than a puff of hot air. Thomas Klapotke, a chemistry professor at the University of Munich, said two greener explosives have already been patented. He added that cost could be an initial obstacle: Greener fuels are now 100 times more expensive than conventional fuels.